Crop Physiology

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Crop Physiology
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Andrea Monti
Editor
Switchgrass
A Valuable Biomass Crop for Energy
123
Andrea Monti
Department of Agroenvironmental Science
and Technology
University of Bologna
Viale Fanin 44
40127 Bologna
Italy
ISSN 1865-3529 e-ISSN 1865-3537

ISBN 978-1-4471-2902-8 e-ISBN 978-1-4471-2903-5
DOI 10.1007/978-1-4471-2903-5
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Foreword
Switchgrass (Panicum virgatum L.) is a warm-season (C4) perennial grass that is

being developed internationally into a biomass energy crop. It is native to the
prairies and steppes of North America. Switchgrass has been used in pastures and
for conservation plantings in the Great Plains and the Midwest, USA, since the
1940s. The research supporting its use as a pasture and conservation species was
initiated in the mid-1930s and was largely conducted by U.S. Department of
Agriculture (USDA) research programs, most notably the Agricultural Research
Service (ARS) project located at the University of Nebraska-Lincoln and USDA
Plant Materials Centers which are located throughout the United States. It was
often used in mixtures with other native warm-season grasses.
Interest in switchgrass as a bioenergy crop began with potential biomass energy
species screening trials conducted in the 1980s which were funded by the U.S.
Department of Energy (DOE) via the Oak Ridge National Laboratory. Switchgrass
was among the top two or three species for biomass yield in the majority of the
trials. In 1991, switchgrass was selected as the model perennial grass biomass
species by DOE. In addition to being a native species, other desirable attributes
were its known soil conservation benefits, it could be propagated by seed, there
was an existing seed industry, and it could be grown and harvested with available
forage equipment. Since 1992, there has been a steadily increasing amount of
research conducted on all aspects of switchgrass production and use as a biomass
energy crop both in the USA and in Europe. This funding was largely by gov-
ernment agencies until the early 2000s when commercial companies also began
investing in switchgrass research. Over $1 billion has been allocated in the USA to
biomass energy research since 2006 by both government and commercial com-
panies with much of the emphasis on perennial grass energy crops such as
switchgrass. Because of the investments that have been made in switchgrass
research, a large amount of information has and is emerging from the switchgrass
research pipeline and the pipeline itself is increasing in volume and delivery rate.
This book is the first devoted entirely to switchgrass. International authorities
on the development and use of switchgrass for bioenergy have reviewed and
summarized by major topic areas all the past and current scientific literature on
v
vi Foreword
switchgrass. Information is provided about the evolution of switchgrass as an

energy crop, its breeding, genetics, genomics, physiology, and management
including harvest and storage. The book also provides information about the
economics of switchgrass production for bioenergy and information about
switchgrass biomass biochemical and thermochemical conversion into biofuels.
In addition, the potential environmental impacts of switchgrass grown for bioen-
ergy are reviewed. This book, Switchgrass: a Valuable Biomass Crop for Energy is
very comprehensive and should prove to be an invaluable resource.
Kenneth P. Vogel
University of Nebraska, Lincoln
Preface
When Mr. Anthony Doyle suggested I put together a book on energy crops, I
immediately thought of switchgrass, followed by two questions: is our current
knowledge on switchgrass-for-energy substantial enough, and mature enough, to
merit such a publication? Is it appropriate that this initiative be undertaken by a
European with much less experience than his North American colleagues? The
first answer was certainly positive, while the second can only be answered by the
authors of the book and its future readers.
The reasons that led me to accept the challenge can be summed up in two
closely connected objectives. In the first place, the desire to help bring the
‘‘scholar’’ closer to the ‘‘teacher’’, in other words to combine the recent but sig-
nificant knowledge acquired in Europe with the more substantial and consolidated
North American knowledge. The other, more ambitious objective is to use the
‘‘energy’’ of switchgrass to create ‘‘synergy’’ in multidisciplinary communications
among scientists and stakeholders, as well as in parallel R&D programs in Europe,
North America, and elsewhere.
Undoubtedly, there are still many questions regarding science and technology
associated with the production and utilization of switchgrass, and the ambition of
this book is to offer a state-of-the-art overview on the knowledge and prospects of
switchgrass as a raw material for energy use, as well as suggestions for future
research programs. Nearly all the areas of lively current research which have
received ample attention, such as crop management, physiology, genetics and
genomics, logistic, economic and environmental assessment, and transformation
processes, are touched upon here.
In the introductory chapter, David Parrish and co-authors provide a fascinating
description of the evolution of switchgrass from its prehistoric origins to the late-
twentieth century efforts to develop it into an energy crop. In Chap. 2, Michael
Casler presents a brilliant review on the genetics and genomics of switchgrass,
showing how this species is still greatly undomesticated with a vast potential for
improvement of biofuel traits. Crop physiology is extensively discussed in Chap. 3
by Walter Zegada-Lizarazu and co-authors, who emphasize the considerable use
efficiency of natural resources by this crop, which indeed could be significantly
vii
viii Preface
improved through developing permanent switchgrass physiology programs. In

Chap. 4, Matt Sanderson and co-authors provide an in-depth overview on the
agronomy of switchgrass, and the specific management characteristics it requires if
yield and quality are to be maximized. Rob Mitchell and Marty Schmer reviewed
the importance of harvest and storage methods in providing consistent and high-
quality biomass to conversion plants. In Chap. 5, the authors clearly point out the
critical effects of harvest and storage management on feedstock characteristics as
landscape scale deployment of switchgrass for bioenergy moves forward. Whilst
switchgrass undoubtedly has potential advantages, it could have ecological or
environmental weaknesses in some situations. These issues are analyzed and
discussed in Chap. 6 by Howard Skinner and co-authors, who reviewed the major
environmental impacts of growing switchgrass as an energy crop, including the
current debate on land use change which has led to the fuel versus food debacle.
Several challenges still have to be overcome to produce biofuels and chemicals
from biomass in an economic and sustainable manner. Different processing steps
within the biochemical and thermochemical platforms are accurately discussed in
Chap. 7 by Venkatesh Balan and co-authors, while the analysis of economic
viability and conditions under which switchgrass becomes cost-competitive are
skilfully examined in Chap. 8 by Anthony Turhollow and Francis Epplin.
I am extremely grateful to the authors for preparing the excellent contributions
to this book of which I have the honor of being the editor, and to those whom I
could not invite due to the size of the book, I offer my apologies.
Andrea Monti
University of Bologna
Contents
1 The Evolution of Switchgrass as an Energy Crop . . . . . . . . . . . . . 1

David J. Parrish, Michael D. Casler and Andrea Monti
2 Switchgrass Breeding, Genetics, and Genomics . . . . . . . . . . . . . . . 29

Michael D. Casler
3 Crop Physiology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Walter Zegada-Lizarazu, Stan D. Wullschleger, S. Surendran Nair
and Andrea Monti
4 Crop Management of Switchgrass. . . . . . . . . . . . . . . . . . . . . . . . . 87

Matt A. Sanderson, Marty Schmer, Vance Owens, Pat Keyser
and Wolter Elbersen
5 Switchgrass Harvest and Storage . . . . . . . . . . . . . . . . . . . . . . . . . 113

Rob Mitchell and Marty Schmer
6 Environmental Impacts of Switchgrass Management

for Bioenergy Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
R. Howard Skinner, Walter Zegada-Lizarazu and John P. Schmidt
7 Biochemical and Thermochemical Conversion of Switchgrass

to Biofuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Venkatesh Balan, Sandeep Kumar, Bryan Bals, Shishir Chundawat,
Mingjie Jin and Bruce Dale
8 Estimating Region Specific Costs to Produce

and Deliver Switchgrass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Anthony Turhollow and Francis Epplin
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
ix
Chapter 1
The Evolution of Switchgrass
as an Energy Crop
David J. Parrish, Michael D. Casler and Andrea Monti
Abstract This chapter discusses the prehistoric origins of switchgrass, its

mid-twentieth century adoption as a crop, and late-twentieth century efforts to
develop it into an energy crop. The species probably first appeared about 2 million
years ago (MYA) and has continued to evolve since, producing two distinct ecotypes
and widely varying ploidy levels. We build the case that all existing switchgrass
lineages must be descended from plants that survived the most recent glaciation of
North America and then, in just 11,000 years, re-colonized the eastern two-thirds of
the continent. Moving to historic times, we discuss how switchgrass was first con-
sidered as a crop to be grown in monoculture only in the 1940s. Based on scientific
reports indexed in a well-known database, interest in switchgrass grew very slowly
from the 1940s until it began being considered by the US department of energy (DOE)
as a potential energy crop in the 1980s. The history of how switchgrass became DOE’s
‘model’ herbaceous energy crop species is recounted here. Also chronicled are the
early research efforts on switchgrass-for-energy in the US, Canada, and Europe and
the explosive growth in the last decade of publications discussing switchgrass as an
energy crop. If switchgrass—still very much a ‘wild’ species, especially compared to
several domesticated grasses—truly attains global status as a species of choice for
bioenergy technologies, it will have been a very remarkable evolution.
D. J. Parrish (&)
Department of Crop and Soil Environmental Sciences, Virginia Tech,
Blacksburg, VA 24061, USA
e-mail: dparrish@vt.edu
M. D. Casler
U.S. Dairy Forage Research Center, USDA-ARS, Madison, WI 53706, USA
e-mail: michael.casler@ars.usda.gov
A. Monti
Department of Agroenvironmental Science and Technology,
University of Bologna, Viale G. Fanin 44, 40127 Bologna, Italy
e-mail: a.monti@unibo.it
A. Monti (ed.), Switchgrass, Green Energy and Technology, 1

DOI: 10.1007/978-1-4471-2903-5_1, Springer-Verlag London 2012
2 D. J. Parrish et al.
1.1 Introduction
Beating swords into plowshares and spears into pruning hooks (Isaiah 2:4)
is a lofty goal. Not so noble but still laudable might be a figurative reshaping of
plows into oil derricks and coal tipples. In such a scenario, energy crops would
be grown on marginal or non-cropland for conversion into energy forms that
reduce dependence on petroleum and coal. The benefits would be multifold.
Energy cropping could offset fossil fuel use, thereby extending the supply of
non-renewable forms and reducing greenhouse gas emissions. Furthermore, a
mature, sustainable, biomass-based energy supply could offer economic and social
renewal to many rural areas. We do not reckon that making fuel from the grain of
maize (Zea mays L.) provides a sustainable path, but we hope that practice is
paving the way for a truly sustainable second generation of biomass-based energy
crops [1, 2].
Switchgrass (Panicum virgatum L.) has garnered much attention as an energy
crop in the past few years. This book was commissioned because the body of
knowledge on switchgrass-for-energy is implicitly now substantial enough and
mature enough to merit such a publication. We think that analysis will hold up
to scrutiny, but in this chapter we invite readers to consider that switchgrass is
still a very new crop—one that was not planted in monocultures until the mid
twentieth century. In a few decades, though, the species has catapulted from
obscurity to being the focus for a wealth of good science and to being fre-
quently cited as the feedstock of choice for a second generation biofuels
industry.
The rest of this book will deal with the wealth of good science focused on
switchgrass; this chapter will explain how switchgrass came to be that focus. We
will discuss first the species’ biological origins—its evolutionary relationship to
other members of the grass family. Then we will document its very recent ‘birth’
as a crop. Finally, we will discuss how this very new crop has come to be
considered the energy crop of choice by so many. The explanation is related
partially to the species’ biology and agronomy; but it also involves serendipity, a
bureaucratic decision, political and economic exigencies, and a ‘model’ that may
have been forgotten.
1.2 The Evolution of Switchgrass
Based on deposits of their distinctive pollen in the fossil record, grasses originated
55 to perhaps 70 million years ago (MYA) [3]. Since then, the grass family
(Poaceae or Gramineae) has evolved from rather humble beginnings into forms
that dominate significant portions of the planet. Since the much more recent
appearance of humans, the grass family’s connections to us have become extensive
1 The Evolution of Switchgrass as an Energy Crop 3
and, in some cases, essentially symbiotic. Human use and selection in the last
10,000 years have clearly reshaped some grass forms and functions [4], but grasses
may have had an even bigger impact on human development. The environment in
East African tall-grass savannas 2 MYA may have fostered the evolution of
bipedalism, tool-holding hands, and increased intellect in hominids [5]. That, of
course, is speculative, but we know without doubt that we are very highly
dependent on the grasses today. A few grasses—most notably those that became
domesticated during a hundred centuries of interaction with humans [6]—now
produce the great majority of the caloric energy consumed in human diets. These
might be considered the Olympians within the pantheon of valuable grasses. Some
other grasses are not so vital from a human perspective, but they could still be
described as belonging within the pantheon of valued grasses because they provide
feed for livestock and, hence, additional human dietary components as well as
draft power; and still other grasses are valued as sources of fiber, turf, and orna-
mentation. In this anthropocentric context, switchgrass is clearly already within the
pantheon, but it might be poised to join the Olympians—not as food from the gods
but as fire brought down from the sky.
1.2.1 Taxonomic and Phylogenetic Relationships

Within the Grasses
The approximately 10,000 grass species have been grouped into 600 to 800 genera
[7]. The number of genera is in some flux as taxonomists and systematists work to
make classification schemes more natural, i.e., to better reflect evolutionary
relationships—a task made perhaps especially difficult in the grasses by numerous
cases of parallel or convergent evolution [8]. Using morphological and cytogenetic
comparisons, the grass genera have been divided into six subfamilies, with various
numbers of tribes and subtribes in each [9]. In this classification scheme, Panicum
is the type genus of both its subfamily (Panicoideae) and its tribe (Paniceae).
Fellow subfamily members include Zea, Sorghum, and Miscanthus (each in a
different tribe), while the tribe Paniceae consists of about 100 C3 and C4 genera,
some with familiar names such as Echinochloa, Paspalum, and Setaria—all within
the same Setariinae subtribe as Panicum [9].
The Panicum genus is large and cosmopolitan, with over 450 rather hetero-
geneous species. The unifying trait within the genus is a distinctive spikelet
morphology, but little else seems to hold the taxon together; for example, five
different base chromosome numbers, ranging from 8 to 15, occur within the genus
[10]. To alleviate the unwieldiness—and likely unnaturalness—of the genus, some
taxonomists have subdivided Panicum into six or more subgenera and numerous
sections [10].
Systematics underwent a revolution in the last quarter of the twentieth century

with the advent of technologies that allow comparisons between and within species
at the gene level. Instead of focusing on morphological features, such as inflo-
rescences or embryos, this approach looks at DNA sequences. The logic is
straightforward: plants share genes that hark back to some putative original plant;
transcriptional ‘accidents’ infrequently but inevitably generate new, enduring
base-pair sequences within those genes (and non-coding DNA regions); during and
following speciation, varying sequences evolve into different taxa; and, over
evolutionary time, unique, new sequences are added to each evolving lineage.
Hence, when examining the variations in base-pair sequences of a gene, the more
similar the sequences are between two species, the more closely related are they
assumed to be. Increasing variation in DNA sequences for a gene indicates greater
evolutionary divergence. If molecular studies incorporate large enough stretches of
DNA and sufficient numbers of organisms, phylogenetic relationships generally
emerge [3].
In the late 1990s, a consortium of systematists from several institutions formed
the Grass Phylogeny Working Group [11]. They looked at 62 species representing
the breadth of the grass family. The 62 included switchgrass and several other
crops, as well as some lesser known but taxonomically important species. Base-
pair variations for six nuclear genes as well as chloroplast restriction site data were
compiled and analyzed to produce a ‘family tree’ that showed the most likely
genetic relationships of all 62 species. The findings suggested that the grass family
might reasonably be divided into 12 subfamilies, including the Panicoideae [11].
The GPWG phylogeny placed switchgrass as a close relative (adjacent branch) of
pearl millet (Pennesetum alopecuroides (L.) Spreng.), and that pair was next most
closely related to maize and Miscanthus japonicus Andersson. Of course, these
were comparisons only within the 62 species examined, but they suggested a more
natural classification for the grass family overall and retained switchgrass firmly
within its previously recognized taxa.
Molecular phylogenetic comparisons can be used to deduce when specific taxa
or traits first appeared in evolutionary time. The notion of a molecular clock is now
well appreciated [12]. Variations in base-pair sequences are assumed to accumulate
at some inexorable—but deducible—rate, which may vary among angiosperm
families [13]; and the number of variants observed can provide an indirect measure
of when they began to accumulate. The grass family is considered to be mono-
phyletic, i.e., all grasses are related to a putative original grass species [3]. The
molecular/genetic changes that led to the first event of grass speciation caused a
first branching point, or node, and each new branch since has followed its own path
of incremental changes and accrual of variations in base-pair sequences. If the
origin of the first grass can be placed in geological time, phylogeneticists can
reason when in evolutionary time various nodes appeared. As noted above, the
grass time line is reasoned from the fossil record to have begun 55 to 70 MYA. The
molecular clock suggests that the primordial grass lineage must have undergone a
total duplication of its genome almost immediately and then remained rather stable
until dividing into several subfamilies beginning about 50 MYA [14, 15]; although
some fossil evidence reveals morphological differences equating to some sub-

families may have appeared as early as 65 MYA [16].
For agronomists, one of grasses’ most important traits is the ability of species to
withstand drought and thrive in full sunlight. Interestingly, that is thought to be a
derived trait—not the ‘wild type’. Evidence suggests the early grasses were
adapted to forest margins and shade, just as some grass species still are [3]. Based
on molecular phylogenies, a tolerance of or preference for open habitats evolved in
grasses in at least two subfamilies but perhaps only after 20 million years as shade-
preferring plants [3, 11, 16].
The family connections between switchgrass and other grass species have been
investigated with molecular clock methods. One study looked at inter- and intra-
species variation in the DNA sequence of Acc-1, the gene that codes for plastid
acetyl-CoA carboxylase [17]. That study and more recent work [13, 18] conclude
that present-day switchgrass and maize diverged from a shared ancestor about
22 to 23 MYA. Switchgrass and pearl millet shared a common ancestor until about
16.5 MYA [13]. In a Huang et al. [17] study, while looking at Acc-1 in various
switchgrass cultivars, the authors concluded that the gene pool associated with the
species now recognized as switchgrass was assembled by about 2 MYA—initially
as a diploid—and that the now widespread polyploidization of the species has
occurred within the last 1 million years [17]. Based on chloroplast DNA (cpDNA)
sequences, Zhang et al. [18] estimated switchgrass diverged from two of its diploid
ancestors, P. hallii and P. capillari, about 5 to 10 MYA, suggesting that the
evolution of this tall-growing polyploid required many millions of years to evolve
out of a highly heterogeneous mix of diploid ancestors. This divergence occurred
as hybridization, natural selection, and polyploidization allowed switchgrass to
evolve into unique forms and habitats.
The appearance of switchgrass about 2 MYA was in the midst of the Pleisto-
cene Era, which began about 2.5 MYA and is often described as the ‘Last Ice
Age’. The new species perhaps originated during an interglacial period, but it
would have to endure many glacial episodes over the next 2 million years. We will
come back to this point soon.
1.2.2 Evolution of C4 Pathways
Because it is so important for both productivity and drought tolerance, the

evolution of C4 photosynthesis, which layers CO2-concentrating mechanisms onto
the archetypical C3 pathway, is of particular interest. Based on phylogenetic
observations and species-specific differences in C4 biochemistry [19], we can
conclude immediately that ‘the C4 pathway’ is actually multiple C4 pathways,
which have arisen multiple times within the angiosperms, i.e., their appearances
exemplify parallel or convergent evolution. A closer look at the grasses in
particular suggests that development of C4ness occurred multiple times just within
that family.
1.2.2.1 Where do C4 Pathways Appear Phylogenetically?
C4 pathways occur in at least 7,500 angiosperm species found in 19 families—16

eudicot and three monocot [19, 20]. Within taxa where the pathways occur, they
are not uniformly present—even within genera. Panicum, for example, has C3, C4,
and intermediate C3/C4 members. However, the very large Panicum genus is
thought by many to be polyphyletic, i.e., not to arise from a single Panicum
progenitor [3, 10, 20].
The number of separate convergent appearances of C4 pathways has recently
been tallied at 62 [20], with the majority of those lineages in the eudicots. But the
grasses have clearly capitalized most on the pathways, with at least 4,500 of the
7,500 C4 species being grasses [19, 20]. The molecular studies by the Grass
Phylogeny Working Group [11] revealed at least five separate appearances of the
C4 pathway just within the 62 species they examined [3]. A systematic comparison
of all 10,000 grasses suggests C4 pathways must have evolved at least 11 separate
times in the Poaceae (to include 7 times in just the Paniceae tribe), because C4
pathways appear in that many different tribes or subtribes with C3 ancestry [8, 19].
With the multiple, convergent appearances of C4 photosynthesis, there clearly
must be strong selective pressure to develop these pathways. The pathways may
provide survival value, or greater fitness, through increased CO2 fixation rates—
especially under brighter, warmer, drier, or saline conditions—and greater water
use efficiency. Nitrogen use efficiency also accrues to C4 species, since they
require less RuBisCO, the rather inefficient, sole CO2-fixing enzyme in the C3
pathway. Some have noted that C4 plants might be less subject to herbivory
because of their less favorable (for herbivores) protein content and C/N ratio [21].
In a very interesting review of the topic, Sage [19] suggests that the greatest
survival value of C4 pathways—and the reason they developed in so many
disparate families—may simply be the relief provided from photorespiratory C
losses, which stem from the nature of RuBisCO’s kinetics, especially its dual
carboxylation/oxygenation proclivity.
1.2.2.2 When did C4 Pathways Appear?
The distinctive 12C:13C isotopic fingerprint associated with C4 species begins to be

evident in geologic strata dating to 20 MYA; and, by 5 to 8 MYA, C4 species
apparently had become dominant producers in some parts of the globe [19]. This
supports the notion that C4 species (probably dominated by grasses, since they
were the pioneer C4s) were already common in some mid-Miocene ecosystems
[19]. Based on molecular clock methods, the first C4 pathway(s) appeared in the
grass family from 24 [14] to 30 [11] or 34 [14, 19] MYA. Based on the presence of
intermediate C3/C4 species and on genomic evidence from C3 plants where key
genes appear to be evolving in a direction that might favor C4ness [14], additional
species could be only a few steps, albeit perhaps many eons, away from C4 status.
But when did C4ness first appear in switchgrass’ ancestry? We know, for
example, maize and Sorghum spp. are in the all-C4 Andropogoneae tribe and have
a presumptive shared C4 ancestor that lived 12 to 15 MYA [14]. The origin of
C4ness is not so clear with Panicum’s ancestry. Because Panicum has species with
C3, C4, and C3/C4 intermediate pathways, we might assume that C4 evolved within
the genus sometime after the original Panicum appeared. This makes very relevant
the earlier speculation that the genus—as it is usually constituted—is not mono-
phyletic, i.e., it does not arise from a single original Panicum.
A consensus would appear to be developing that Panicum, as currently con-
stituted with its 450+ species, is polyphyletic [3, 10]—even ‘highly’ polyphyletic
[20]. By definition, then, it is impossible to place its various species into definitive
lineages and date the origin(s) of the C4ness. Based on molecular data, it has been
proposed that Panicum virgatum be retained within the ‘true’ Panicum (sensu
stricto) along with a few other strictly C4 Panicum spp. [10]. Still open to question
is whether C4ness developed de novo within a smaller, truly monophyletic
Panicum genus or was inherited from a non-Panicum progenitor. The GPWG
survey of base-pair sequences in six genes within 62 grass species suggested
switchgrass and pearl millet arose from a shared C4 ancestor [3], but the GPWG
analysis was admittedly limited—not surveying any other species within Panicum
or Pennisetum.
In short, we do not know when C4 first appeared in switchgrass’s lineage.
The answer to that question must await a suitable parsing of the lineage, which
must include a sorting out of the Panicum genus.
1.2.3 Center of Switchgrass’s Origin and Spread Across North

America
Switchgrass is a New World species. Its range when Europeans arrived included
Central America and eastern North America [22]. It could be found in a wide range
of habitats nearly anywhere east of the 100th meridian. After the species arose
some 2 MYA [17], it likely radiated and adapted across major portions of the
North American continent. However, a priori reasoning suggests that periods of
glaciation in the last 2 million years would have driven most of those lineages into
extinction or into more southern, ice-free climates. The survivors would
presumably have followed the ice northward during interglacial periods, only to
repeat the retreat/re-colonize cycle again and again [18, 23].
McMillan [24] posited switchgrass (and other prairie grasses) retreated to
refugia during the ice ages and then moved poleward again as the climate warmed.
He posited more specifically three regional refugia arose during the most recent
glaciation: Lowland (or Southern) Great Plains, Eastern Gulf Coast, and Upland
Plains. Recent molecular marker studies examining simple sequence repeats
(SSRs) of 18 switchgrass cultivars and accessions [25] have provided tantalizing
support for this three-refugia theory. The latter work provides strong evidence that
most—perhaps all—of today’s cultivars can be sorted into three groups based on
SSRs, with each group harking back to one of the three putative refugia. To follow
the line of reasoning, we must first look more closely at the notion of switchgrass
‘ecotypes’.
1.2.4 Upland and Lowland Origins, Distinctions, and Connections
Essentially all cultivars, lines, or accessions of switchgrass can be placed into one of
two categories: upland or lowland. A few ‘intermediate’ or ‘ambiguous’ types,
which are not readily assigned to one of these two categories, may represent archaic
natural hybrids [23]. The upland and lowland groups are usually described as
‘ecotypes’, a term from evolutionary ecology connoting genetic variations within a
species that allow the ecotypes to be better adapted to particular geographies or
habitats. Ecotypes—sometimes also described as subspecies—typically differ in
morphology or physiology in ways that make them better suited for different
environments, but they are able to interbreed and produce fertile offspring—meeting
that classical criterion for a species. More recently, these groupings have also been
termed upland and lowland ‘cytotypes’, referring to the diagnostic DNA sequence
data carried in their plastids [26].
Within Panicum virgatum, genotypes belonging to the upland ecotype are
typically finer stemmed and shorter than those identified as lowlands. As the
upland designation might imply, these lines are also generally better adapted to
drier and colder habitats, while the lowland ecotype tends to thrive in warmer,
wetter habitats. Indeed, most of the lowland lines, e.g., Alamo, are derived from
accessions from the southern USA; and the upland genotypes are more generally
associated with the northern Great Plains. All identified cultivars from the lowland
ecotype are tetraploid (2n = 4x = 36), whereas the upland ecotype consists of
genotypes that are both tetraploid and octoploid (2n = 8x = 72) [25]. Only
recently have possible octoploid lowland plants been discovered in a small number
of accessions [18, 23]. The two ecotypes, which were initially distinguished by
their phenotypes, can now also be grouped into upland and lowland genetic
clusters, or cytotypes, using various molecular markers [25, 27]. While crosses
between octoploid (upland) and tetraploid (lowland) genotypes are incompatible,
tetraploid cultivars from each of the two ecotypes have been crossed and produced
fertile offspring, exhibiting significant hybrid vigor [27].
Using molecular clock calculations based on cpDNA sequences, estimates of
the upland–lowland divergence range from 0.5 to 1.3 MYA [18, 28]. Because
octoploids are extremely rare within the lowland ecotype, it is likely that poly-
ploidization from 4x to 8x occurred after the upland–lowland divergence. Indeed,
there is evidence for multiple polyploidization events within the upland lineage,
suggesting that this process has occurred frequently. Clear separation of tetraploid
and octoploid lineages within the upland ecotype suggests that some of these
octoploid lineages are indeed very ancient [18]. Because 2n gametes are very
common in the Poaceae, polyploidization from the 4x to 8x level could have
occurred many times in many different lineages of switchgrass. It must be noted
that 2n gametes have not specifically been identified in switchgrass, so the
mechanism of polyploidization is still unknown.
Key to understanding the evolution of switchgrass is the massive impact that
Ice Age cycles have had on habitats that we tend to think of as permanent and
immobile. During the past 2 million years, there have been approximately 16 to 20
continental glaciation events in North America, each sufficient to force the com-
plete relocation of the tall-grass prairie and savanna habitats toward warmer cli-
mates, e.g., the Gulf Coast. Individual lineages of switchgrass that had evolved to
become adapted to more northern areas would have survived by migrating
southward (via pollen or seed), or they would have gone extinct. The polyploid
nature of switchgrass would have been a key factor in helping lineages to survive,
preserving vast amounts of genetic variability within populations, individual
plants, seeds, and even individual pollen grains.
Lineages of switchgrass that would have survived the Ice Ages would be those
that were endemic to or immigrated southward to areas that allowed their survival
during many centuries of glaciation. As suggested by McMillan [24], in the most
recent period of glaciation, three areas may have provided ice-free and sufficiently
warm growing seasons to serve as refugia for many grassland species. McMillan’s
logic, which was built on an understanding of climatic geography during the last
glacial period, suggested the Lowland (or Southern) Great Plains, the Eastern Gulf
Coast, and an area in the Upland Plains were three places that—even in the midst
of the glaciation—would have had growing seasons suitable for many of the plants
that eventually re-colonized the Great Plains.
Casler and colleagues have looked carefully at the distribution of North
American populations of the two switchgrass ecotypes and the morphological and
genetic similarities and differences between and within those populations [18, 23,
25, 29, 30]. Other labs (e.g., [31]) have provided similar or additional evidence that
the current populations of North American switchgrasses can be placed into a few
groups based on molecular markers and that those groups are associated with
particular geographies, or provenances.
Zalapa et al. [25] examined SSRs in 18 switchgrass cultivars: 7 lowland (all
tetraploid) and 11 upland (two tetraploid and the remainder octoploid). The work
found alleles unique to, i.e., diagnostic for, each ecotype and also found alleles that
distinguished tetraploid from octoploid members of the upland populations. The
analysis revealed also clusters of allelic similarities, or genetic pools, within each
of the ecotypes; and, perhaps not surprisingly, those groupings reflected geography
of origin. Accordingly, lowland cultivars were grouped by allelic similarities into
two clusters; cultivars in one cluster all came from the Eastern Gulf Coast region,
and those in the other were all from the Southern Great Plains. The nine octoploid
upland cultivars fell into three allelic clusters, or genetic pools, each with a unique
provenance: those associated with the Central Great Plains, the Northern Great
Plains, and the Eastern Savannah [25]. Zalapa et al. [25] suggest their findings may
provide support for the three Ice Age refugia posited by McMillan [24].1 Zalapa
et al. [25] hypothesized that each of the two lowland allelic (and geographic)
genetic pools noted above is descended from McMillan’s similarly named
refugium, i.e., Lowland/Southern Great Plains and Eastern Gulf Coast. They
suggested also that at least two of the upland genetic pools may be the descendants
of plants that survived in the Upland Plains refugium. The Zalapa et al. [25] work
also offers a reasonable model for arriving at the current situation where octoploids
are the more frequent ploidy level for upland cultivars. It builds on the notion that
the duplicated genome offers more grist for the evolutionary mill, a notion
reflected in the writings of others (e.g., [13, 16]).
More recent studies have identified multiple upland and lowland lineages
within the eastern USA [18, 23]. The observation of obvious geographic patterning
among upland lineages in the northern USA, combined with a general lack of
patterning among lowland lineages in the southern USA, suggests that evolu-
tionary forces have acted on the nuclear genomes of migratory switchgrass,
allowing these populations to adapt to a wide range of habitats and climates during
the 11,000 years since the last glacial period. Indeed, the allelic patterns of SSR
markers identified by Zalapa et al. [25] are sufficiently specific to geographic
regions that Zhang et al. [18] were able to identify two 8x upland accessions that
were inadvertently transported by the US Army to remote regions of the USA,
eventually becoming established and many years later incorrectly identified as
‘local’ switchgrass accessions.
One more evolutionary consequence of the Ice Ages was the periodic juxta-
position (in refugia) of upland and lowland lineages for tens of thousands of years,
resulting in upland–lowland matings and the establishment of mixed or hybrid
lineages, some of which completely defy simple classification [23]. These hybrid
lineages are an additional mechanism by which switchgrass enriches and preserves
genetic variability to be utilized during and after post-glacial migrations, creating
phenotypic variations in flowering time, cold tolerance, and heat tolerance [30, 32]
that have allowed it to adapt to such a wide range of habitats.
In sum, we can suggest that our ‘modern’ switchgrasses, i.e., those that emerged
from and radiated after the last Ice Age, may have come from a relatively small
number of survivors. Those survivors included a few—maybe only two—groups
representing the lowland genetic pool and perhaps a few more groups carrying the
upland gene set. What we see today reflects the rather remarkable ability of those
few survivors/pioneers to radiate, adapt, and re-colonize two-thirds of a continent
in a scant 11,000 years; but 2 million years of switchgrass evolution (which
included repeated winnowings and forgings on the anvil of continental glaciation)
and development of two ecotypes (with some representatives possessing a
quadrupled genome size) clearly set the stage well for a rapid reclaiming of the
North American landscape once it was again habitable.
1
Casler et al. [30, 32] had adumbrated earlier the colonization of prairie ecosystems by remnants
from southern refugia.
1.3 The Agronomic History of Switchgrass
Switchgrass has been a ‘crop’ in the usual sense of that word for only a few
decades. Unlike maize, wheat (Triticum aestivum L.), rice (Oryza sativa L.), and
some other grasses that prehistoric humans co-opted into domestication [6],
switchgrass has only very recently even been planted or studied in monoculture.
Panicum virgatum preexisted Homo sapiens, of course, but only recently have we
begun to take note of it and adapt it to human purposes.
One way to document the rise of switchgrass into human consciousness—or
human technology—is to survey the history of publications about the species. We
have done that using CAB Direct, the bibliographic database of Commonwealth
Agriculture Bureau, which indexes over 9 million entries from applied life sci-
ences fields—entries from 1900 to the present. We searched it for the occurrences
of Panicum virgatum, switchgrass, or switch grass in ‘all fields’, i.e., title, abstract,
key words, or CAB’s coding descriptors and identifiers. As a result, some articles
indexing to switchgrass mention it rather coincidentally, e.g., not a host for an
aphid, or as one among many species in mixed swards or in multi-species
screenings. Along with refereed journal publications, the canvass returns a number
of brief abstracts and non-refereed proceedings from agronomy, animal science,
and weed science conferences, as well as agricultural experiment station bulletins.
On the other hand, some published reports that deal with switchgrass rather
extensively (e.g., [1, 33]) do not index to switchgrass, because they do not mention
switchgrass in their abstract and the indexer has not included the species as an
‘organism descriptor’. Or, in other cases, switchgrass reports are published in a
source—often a book or monograph such as this one—that is not cataloged by
CAB (e.g., [34, 35]). So, we know our survey is not an exhaustive or compre-
hensive list of publications dealing with switchgrass, but we feel confident that it
provides a good indication of the overall trend or trajectory for such publications.
As part of our survey, we perused each abstract (and a few full articles) to
determine in what context switchgrass was discussed. Was switchgrass a primary
focus of the work? For what use/purpose was it being considered? Figure 1.1 plots
the total number of CAB-indexed reports referring to switchgrass, the number
looking only at switchgrass (or comparing it with only one other species), and the
number mentioning switchgrass as a potential energy crop. Accordingly, it doc-
uments the ‘birth’ of switchgrass as a crop: first appearing as a subject in scientific
investigations about a century ago, exhibiting a long ‘lag phase’, and then entering
a vigorous ‘growth phase’ in just the last 20 years.
The volume of work on switchgrass is still very small compared to many other
crops. For example, canvassing CAB Direct for citations in 1940 produces 713 hits
for Zea mays, 395 for Avena sativa (oats), and only six for Panicum virgatum. The
number of publications in 2010 indexing to switchgrass is 165, but that barely
outdistances the number of hits for maize in 1929 (and maize provides 5,610 hits
in 2010). On the other hand, the 165 switchgrass citations in 2010 compare with
just 15 for big bluestem (Andropogon gerardii Vitman), a tall-grass prairie species
Fig. 1.1 Annual (a) and

cumulative (b) number of
CAB Direct citations
mentioning switchgrass (SG),
having SG as their major
focus, and/or discussing SG
as an energy crop
that has much in common with switchgrass historically and ecologically; and in
only two of those 15 citations is big bluestem a primary focus of the work.
1.3.1 ‘Prairie Grass’ Origins
The first indexed occurrence of switchgrass in the CAB database comes in 1914,
where the species is mentioned as not being a host for the aphid about which the
article was written. The next appearance is in 1931, in the quaintly named ‘Who’s
who among the prairie grasses’ [36], where switchgrass is mentioned as occupying
‘less desirable lowland soils’. That publication and most of those few that followed
over the next 20 years allude to switchgrass as one of the species in ‘tall-grass
prairie’, ‘prairie grasses’, ‘prairie hay’, ‘native grasses’, ‘range grasses’, ‘mixed
grasses’, ‘warm-season grasses’, etc.
Those early papers discussing switchgrass’s contribution to grass mixtures
include a few peer-reviewed articles and numerous agricultural experiment
station bulletins and annual reports. Also appearing at this time are reports on the
natural occurrence of switchgrass in various ecosystems. One such report, coming
in 1932 from Massachusetts, is the first CAB-indexed citation where switchgrass

is the primary or sole species of interest [37].
1.3.2 Early Studies and Uses as a Monoculture, and the Growth

of Reports on Switchgrass
Switchgrass begins to emerge from the anonymity of being ‘just’ a prairie grass in
the 1940s. An article in 1941 looks at differences among various accessions in
susceptibility to rusts and is the second paper published with switchgrass as the
primary or sole subject of the investigation [38]. A 1947 agricultural experiment
station report refers to studies of switchgrass and other prairie grasses done on pure
stands established in 1937 [39]. During the late 1940s and 1950s, reports on
selection and breeding studies with switchgrass appear in a few agricultural
experiment station annual reports. Overall, though, the species receives scant
attention. Indeed, through 1960, a total of 123 CAB-indexed reports mention
switchgrass, and many of those simply mention its occurrence in various
ecosystems.
The first CAB-indexed paper dedicated solely to switchgrass physiology (and
only the third where the species is the primary focus) appears in 1947; it sought
relationships between ploidy level and winter survival (but found none) [40]. The
total number of indexed studies with switchgrass as their major focus grows
slowly. By 1960, 14 such studies have accumulated. By 1970, there are 30; and by
1980, 55. Those reports focusing on switchgrass as a monoculture, i.e., a ‘true’
crop, deal with a range of topics. Some are reports of cultivar releases, e.g.,
Blackwell, Caddo, Summer, Pathfinder, and then Kanlow. A 1953 publication
provides pioneering data on chemical composition [41]. Some as early as the
1940s discuss switchgrass for erosion control in waterways, and several in the
1960s considered the species’ value in reclamation. However, most of the
switchgrass-focused reports deal with the species as a forage crop either from an
agronomic perspective or from an animal nutrition perspective. All of the cultivar
release reports noted above discuss forage value.
Beginning in the 1980s, we observe an up-tick in the study of switchgrass.
In that decade, 65 indexed reports appear dealing primarily or solely with
switchgrass—more than doubling the previous 50 years’ cumulative for this sta-
tistic. The focus is still heavily on forage value and breeding, but a few reports deal
with reclamation, erosion control, and diseases. At the close of the decade comes
the first peer-reviewed article written on switchgrass as an energy crop [42]. Some
more background on that publication and further discussion of the trajectory in
research studies on switchgrass as an energy crop will be given in Sect. 1.4.1.1.
After a plateau in the early 1990s, interest in switchgrass (as conveyed by
indexed publications at least) increases noticeably in the second half of the decade.
Reports dealing solely or primarily with switchgrass average eight per year from
1990 through 1994, nearly the same rate as in the 1980s; but the second half of the
decade sees an average of 16 articles per year focused on switchgrass. As will be
discussed below, that burst of activity is driven largely by the increasingly frequent
appearance of reports on switchgrass as an energy crop, but the species continues
to be studied for forage and other purposes as well.
In sum, for this section, based on indexed reports in the scientific literature, the
history of switchgrass as a crop is very short. Only during the second half of the
twentieth century did the species move clearly from being one of the ‘prairie
grasses’ to being a crop grown in monoculture. For the first 40 years of its very
short agronomic history, the volume of work on switchgrass was small, averaging
only about five CAB-indexed mentions per year and averaging less than one report
per year dedicated primarily or solely to it. From 1930 to 2010, more than 1,600
reports that index to switchgrass have been published, with more than half of those
appearing after 1997. This might suggest that the crop is in the process of joining
the Olympian list of ‘most useful grasses’, but let us hold that judgment in
abeyance until we have looked at some other matters.
1.3.3 Current and Proposed Uses
The caveat about ‘most useful’ status notwithstanding, we can say without reser-
vation that switchgrass now serves us very well in several roles, i.e., it belongs in the
grass pantheon. Its initial adoption as a forage species was probably a logical
extension of its millennia-long role as food for ungulates on the Great Plains of North
America. In addition to that use, though, it has been adopted or is under consideration
for a broad range of other purposes [43], which we will simply summarize:
Established roles/uses for switchgrass:
• Forage for grazing, hay, or haylage;
• Erosion control in waterways, levees, stream margins, etc.;
• Vegetative filter strips (to reduce runoff of soil and nutrients);
• Reclamation/stabilization of sand dunes and disturbed areas;
• Wildlife habitat.
Other roles/purposes under study (or in early adoption):
• Energy feedstock for:
– Combustion;
– Conversion to liquid or gaseous forms.
• Fiber or pulp for paper;
• Phytoremediation to include smelter and mining sites;
• Pharmaceuticals, biomaterials, plastics, etc.;
• Value-added ‘by-products’ from biorefineries;
• Substrate for mushroom culture.
1.4 The Origins of Switchgrass as an Energy Crop
A few published articles have discussed the brief history of switchgrass as an

energy crop. One [44] is by individuals in the US Department of Energy (DOE)
funding agency that initiated studies on switchgrass as an energy crop, but its
account begins essentially after the decision has been made to focus on switch-
grass. An internal DOE report [45] and a subsequent journal publication [46]
provide more of the ‘back story’ of how switchgrass came to be essentially the sole
focus in DOE’s efforts on herbaceous energy crops. One of us (DJP) was a par-
ticipant in the discussions that first brought switchgrass to the attention of DOE,
and he has written about the selection of switchgrass [34, 43, 47]. We will
summarize and expand on all of these sources in the sections that follow; and some
personal observations are provided because we hope they will show how seren-
dipity, pragmatic decision-making, and politics—as well as science, of course—
have shaped the narrative of the switchgrass story.
We shall take up our account of the switchgrass-for-energy ‘story’ beginning in
1984 with DOE’s early work on herbaceous biomass species. However, interest in
biomass as an energy source certainly predates that. Indeed, biomass use for
energy is prehistoric, with wood remaining a primary energy source until the mid
nineteenth century. Since then, we have become increasingly dependent on fossil
fuels, but many countries—especially in war time—revert to biomass energy
sources. In one of the more recent occurrences, the ‘energy crisis’ following the
embargo imposed by oil producing and exporting countries (OPEC) in 1973
spurred interest and work on energy cropping and biomass as a feedstock in the
USA, the EC [48], and the UN [49]. Out of those efforts came many reports,
including one in the USA that mentions switchgrass as a possible biomass source
[50]. However, in our view, there is a loss of continuity (or certainly of
momentum) in the biomass-for-energy narrative, when interest in biofuels flagged
in the late 1970s, as oil prices returned to ‘pre-crisis’ levels.
1.4.1 In North America
It is fitting that the first studies of switchgrass as an energy crop were done in North
America, but not every energy crop candidate has been first studied in its region or
country of origin. For example, miscanthus from southern Asia was first studied as a
possible energy crop in England (see Sect. 1.4.2.2). But for switchgrass, the impetus
to consider it as an energy crop was as native as the species itself.
1.4.1.1 DOE Extramural Screening Studies
In 1982, the Oak Ridge National Laboratory (ORNL) of the US DOE assumed
control of a young program looking at woody species for energy purposes [46].
In 1984, ORNL expanded their biomass-for-energy program and issued a request for
proposals (RFP) to screen herbaceous species as energy crops, i.e., species that might
produce significant amounts of lignocellulosic biomass. The RFP further stipulated
that the work must be done on ‘marginal croplands’ [46]. In 1985, the first five
subcontracts were awarded for what became known as the Herbaceous Energy Crops
Program (HECP); and, in the first few years of the HECP, both the ‘woody’ and
‘herbaceous’ subcontractors met together periodically to compare biomass
production data.
After the five initial HECP subcontractors were identified, ORNL called them
together in April 1985. They came from Alabama (Auburn), Indiana (Purdue),
Ohio (a private research firm), New York (Cornell), and Virginia (Virginia Tech).
Two more subcontractors—Iowa State and North Dakota State—were added to the
screening study in 1988 [45]. At that April meeting, each of the five groups shared
their list of species to be screened. Each list was appropriate to the region in which
the work was to be done, but no species was common to all lists. No benchmark
species was there to allow cross-region comparisons of biomass productivity of the
over 30 disparate species that would be grown at over 30 disparate locations, each
of which was marginal for disparate reasons.2
The eight species proposed by Virginia Tech included switchgrass. Their
proposal noted that switchgrass is a native that will ‘produce better growth and
cover on droughty, infertile, eroded soils [which characterized the marginal sites
proposed for studies in Virginia] than most introduced grasses’. Dale Wolf, the
forage scientist who chose switchgrass for Virginia Tech’s proposal, suggested to
those present at the 1985 meeting of subcontractors and administrators that the
wide natural occurrence of switchgrass should allow it to serve well as the desired
benchmark species. His suggestion was adopted, and he subsequently supplied
Cave-in-Rock seed from a single source for all subcontractors. For the later-added
subcontractors, switchgrass was stipulated as a candidate/benchmark species. So,
switchgrass appeared in all seven subcontractors’ screening studies—but only after
it was added to most. By contrast, 17 candidates from the screening studies were
on only one of the seven lists.
The initial round of subcontracts called for a 5-year study to allow each of the
screened species to come to full production and to experience a range of growing
seasons.3 With the addition of two more subcontractors in 1988, the total number
of species screened grew to 36 plus two polycultures, and the total number of sites
was 31 [45]. When the final reports of the screening studies were compiled,
switchgrass had proven itself to be one of the most prolific producers of biomass
across most of the locations.4 It, in fact, did well soon enough in the 5-year cycle
2
Each group would plant their list of candidates at from two to eight sites. (Data in this and the
next few paragraphs were compiled from Wright [45]).
3
Most were perennials, but that was not a requirement of the RFP.
4
Switchgrass did not fare so well in Ohio perhaps because the marginal sites there were poorly
drained.
that some of the subcontractors did switchgrass-specific studies looking at man-

agement techniques (Virginia Tech) and screening cultivars (Auburn) [45].
Partly because of the long-term nature of the screening studies and perhaps
because biomass production for energy purposes was not yet a ‘standard’ topic for
editors of agronomic publications, the first papers on switchgrass as a model
species were slow to appear. Only two switchgrass-based and DOE-credited
reports appeared during that first 5-year cycle. Both came out of Purdue and
included the paper mentioned above [42]. However, annual reports and final
reports from all of the subcontractors and ORNL HECP staff were submitted in a
timely fashion.5
1.4.1.2 Intramural Efforts at ORNL and Other DOE Agencies
In the early 1990s, HECP was subsumed into the Bioenergy Feedstock
Development Program (BFDP), reflecting some reorganization within ORNL and
merging woody and herbaceous biomass programs under this new name and
management. As of this writing, feedstock development efforts in DOE remain
based in ORNL’s Environmental Sciences Division (part of ORNL’s Energy and
Environmental Sciences Directorate) with a program name of Renewable Energy
Systems. In addition, work on microbial conversion of biomass into biofuels is
housed in the directorate’s Biosciences Division as the Bioconversion Science and
Technology Program.6
A very significant body of work on switchgrass has and continues to come
from ORNL scientists. Besides intramural studies on microbial conversions of
switchgrass biomass, ORNL staff have examined molecular markers and basic
physiology of switchgrass and the species’ potential for sequestering carbon.
Several on the ORNL staff have also looked at the economics of large-scale
switchgrass production. Much of that body of work—as well as annual and final
reports from subcontract work—can be accessed at ORNL’s website.7
Other DOE laboratories outside of Oak Ridge are also engaged in work on
switchgrass as well as other biomass species. Much of the biomass conversion
work has been done at the Solar Energy Research Institute (SERI), which was
formed in 1977 and was reorganized and renamed the National Renewable Energy
Lab (NREL) in 1991. NREL is the home of the National Bioenergy Center; and,
along with ORNL and three other national DOE laboratories, it supports the efforts
of DOE’s umbrella Biomass Program. As the name implies, NREL deals with
more than biofuels, but their portfolio includes efforts aimed at development and
commercialization of biomass conversion technologies, i.e., biorefineries.
5
http://www.ornl.gov/info/reports/
6
http://www.ornl.gov/sci/ees/organization.shtml
7
http://www.ornl.gov/info/reports/
1.4.1.3 DOE Extramural ‘Model Species’ Studies
Switchgrass, which was somewhat serendipitously chosen as the benchmark

species for HECP’s initial 5-year, 7-state, 36-species screening studies, did so well
in those studies that ORNL’s BFDP subsequently invited proposals to study only
switchgrass. The RFP described switchgrass as a ‘model species’ [44–46]—per-
haps an implicit reservation or caveat that switchgrass might ultimately prove less
productive than some of the thousands of species that had not been screened.
However, it was a very reasonable notion that lessons learned from studies of a
model species could be applied or adapted to other promising biomass species
when they might appear. The decision to focus on a single herbaceous species was
made at a time when ORNL’s budgets for biofuels work were shrinking, sug-
gesting that the decision to focus on a single herbaceous species was perhaps at
least partially a pragmatic one, based on fiscal constraints [45, 46]. ORNL nar-
rowed their focus on woody biomass to a single model species at this time also
[45]. The upshot of the switchgrass-as-a-model-species decision was that DOE
essentially stopped looking for new herbaceous energy crops after the first 5-year
screening study.
A second and then a third 5-year round of DOE-funded extramural subcontract
work—now focused solely on switchgrass—began in 1992 and 1997. In 1992,
several long-term varietal and management studies were initiated, some of which
ultimately received 10 years of DOE support (e.g., [51, 52]). Field studies on
cultivar selection, improving establishment, and management for biomass pro-
duction (especially fertilization and harvest management when grown for biomass)
were performed at Auburn, Iowa State, Texas A&M, and Virginia Tech [44].
During DOE’s ‘model species’ funding cycles, switchgrass breeding efforts were
supported at Oklahoma State and the University of Georgia, as were tissue culture
and transgenic work at the University of Tennessee [44]. Also included in these
rounds of DOE/ORNL funding were collaborations with USDA personnel based at
various public universities and USDA facilities, included the very productive
program at the University of Nebraska [44].
During this era, papers discussing switchgrass as an energy crop authored by
DOE subcontractors, ORNL scientists, and collaborating USDA personnel began
to appear with increasing frequency (Fig. 1.1). These papers represented the bulk
of papers being published on herbaceous biomass species at the time, and they
typically discussed switchgrass as a ‘model species’, ‘energy crop candidate’,
‘potential energy crop’ etc.; but the ‘model’ designation seemed to fade from
consciousness (along with ‘marginal croplands’) as more and more reports
appeared on switchgrass—especially as non-DOE efforts increased. Essentially all
of the first several papers dealing with switchgrass as an energy crop can be traced
to DOE/ORNL efforts and support, but that would change quickly at the beginning
of the twenty-first century.
1.4.1.4 Transition of Support from DOE to DOT, USDA, and the Private
Sector
Shifting US politics and administrations cause the switchgrass story to take a right-
hand turn in 2002. The DOE/ORNL/BFDP program had issued a new RFP for
switchgrass studies in 2001 and was in discussion with potential subcontractors
when funding was withdrawn based on ‘decisions made within DOE’ [44]. Work
continued within DOE, but no more funding went to outside parties. Interestingly,
towards the end of the same administration, switchgrass was given a boost into the
public consciousness when it was mentioned in the 2006 State of the Union
message to the US Congress and citizens. That reference triggered much interest
from the news media, resulting in a flurry of telephone calls and e-mails to the
relatively small fraternity of scientists then working on switchgrass; and it prob-
ably brought first knowledge of the species and its bioenergy potential to millions.
It was almost certainly the impetus for a spate of magazine and newspaper articles.
Following the loss of DOE funding for extramural research on switchgrass-
for-energy, the US Department of Ariculture (USDA) began slowly and then more
vigorously to assume leadership. For example, the USDA Agricultural Research
Service (ARS) developed a national intramural program on Bioenergy and Energy
Alternatives that includes major studies with switchgrass at several USDA
facilities.8
Some of the initial post-DOE funding for efforts on switchgrass came through
the Sun Grant Program, which was enacted legislatively in 2002 and overseen by
the US Department of Transportation (DOT) with substantial inputs from both
USDA and DOE. Various regional studies on switchgrass and other bioenergy
species were developed and funded (and continue to be funded) by Sun Grant.9
Also stepping into the biofuels arena increasingly in the first decade of the
twenty-first century has been the private sector. Some major petroleum companies
have invested in biofuels research, in some cases via centers established at uni-
versities. A number of new companies that hope to capitalize on switchgrass’s and
other species’ bioenergy potential have also appeared. Most of them have their
own cadre of research scientists, but they have also contracted work out to
scientists at various public and private institutions. Another major participant
in switchgrass-for-energy studies has been the private, not-for-profit Noble
Foundation in Oklahoma, which has expanded its long-standing efforts on forages
into studies aimed specifically at the energy crop potential of switchgrass.10
During this time of change in funding sources and administrative oversight of
bioenergy efforts, there were also quantitative and qualitative changes in the tra-
jectory of publications on switchgrass. Since the first two switchgrass-for-energy
citations in 1989, the number of reports dealing with that topic has grown rapidly.
8
http://www.ars.usda.gov/research/programs/programs.htm?np_code=307
9
http://www.sungrant.org/
10
http://www.noble.org/Research/Biofuels/index.html
In the 1990s, 57 appear; and from 2000 to 2010, another 429 are added to the CAB
database. Not all of those are dealing solely with switchgrass; some only compare
it briefly with another species of interest; but it seems in many cases that
switchgrass is the standard—the benchmark again—against which other biomass
species are being compared. Interest in switchgrass for other purposes certainly
does not go away during this time, but the great majority of reports with
switchgrass as the main focus are looking at it for its bioenergy potential. For
example, as of this writing, over 100 CAB Direct entries in 2011 index to
switchgrass, and three-quarters of them mention it as an energy crop.
1.4.1.5 Canadian Efforts
Switchgrass is native to southern portions of Canada, and Canadian workers

became involved in switchgrass-for-energy studies early on, but it was largely a
one-institution project based at McGill University. In 1993, workers at McGill
planted a screening study that looked at five species of warm-season grasses, to
include 12 cultivars or lines of switchgrass [53]. They followed phenology and
yields for two post-establishment seasons and reported biomass on a per-plant
basis. They concluded that several cultivars of switchgrass and one of cord grass
(Spartina pectinata L.) were the most productive. A simultaneous study at the
same location looked at phenology and allometry of nine switchgrass cultivars and
one line of big bluestem [54]. It showed again potential for these warm-season
grasses in a short-season locale. Those screening studies were followed by several
studies focusing specifically on switchgrass: looking at management, seed physi-
ology, energy yield, and chemical composition. Taken together, these works
represent a significant body of knowledge on the potential of switchgrass-for
energy (or other) purposes in southern Quebec.
After the flurry of studies done at McGill, Canadian work on switchgrass-
for-energy was taken up and expanded to an international scale by scientists at
Resource Efficient Agricultural Production (REAP) Canada, which is located on
the McGill campus. That organization has championed in Canada and elsewhere
the adoption of switchgrass as an energy crop. They are particularly interested
in—and are fostering commercial ventures that employ—the concept of growing
switchgrass for conversion into densified units (e.g., pellets) that can be used for
heating [55]. Their web site11 has a comprehensive list of their work and
recommendations, which include growing guides and information about pelletizing
and burning switchgrass.
11
http://www.reap-canada.com/
1.4.2 In Europe and Elsewhere
1.4.2.1 A Brief History of Switchgrass in Europe
If the North American history of switchgrass as an energy crop can be considered

brief, the same is all the more true in Europe. It is notable that the first European
studies on switchgrass-for-energy were done at the famed Institute of Arable Crops
Research (IACR)—formerly known as Rothamsted Experimental Station—when
scientists there planted plots of upland and lowland switchgrass in 1993 [56, 57].
One year later, A. Biotec, an Italian private research institute located in Cervia
(southern Italy) undertook pioneering studies on switchgrass in the Mediterranean
region. These English and Italian studies revealed an adaptability of switchgrass to
both northern and southern European conditions. However, because of lower
productivity compared to other biomass crops, switchgrass was initially deemed to
be a less suitable crop for energy conversion than some other species. In southern
Europe, switchgrass produced significantly lower biomass yields than giant reed
(Arundo donax L.) and sorghum (Sorghum bicolor L.), while in northern Europe it
was less productive than miscanthus and some short-rotation coppices such as
poplar (Populus spp.) and willow (Salix spp.).
Nevertheless, the promising results with switchgrass beginning to come from
North America in the 1990s paved the way for a 1998 to 2001 European project
specifically focused on switchgrass: ‘Switchgrass as an alternative energy crop in
Europe’ [58]. This project effectively coordinated research activities on switch-
grass in Europe and extended trials over a wide range of European latitudes, soils,
and climates. In addition to trials in the Netherlands and UK, studies were
established in southern Europe: Trisaia, Italy (40.09’ N, 16:38’ E), Aliartos,
Greece (38.22’ N, 23.10’ E) [59], and Bologna, Italy (44.43’ N, 11:47’ E) [60].
In general, the results from this first pan-European switchgrass project con-
firmed a yield advantage of lowland over upland ecotypes; but, in some cases, it
also revealed lowlands’ susceptibility to cold stress, which considerably limited
their productivity. Some lowland plantings failed, especially in Germany, western
UK, and Ireland. This was likely due to a particularly harsh winter, since there
were other cases of successful lowland plantings with high yields in more northern
countries.
Other studies carried on as part of this first European switchgrass project
assessed economic and environmental impacts of switchgrass [61, 62], variety
choice [63], nutrient composition [58], modeling [64], thermal conversion (com-
bustion, gasification, and pyrolysis), ethanol production [61, 65], and industrial
non-energy uses [66, 67]. Studies by Monti et al. [68] and Minelli et al. [69]
examined tillage and weed control methods for improving switchgrass establish-
ment and provided sustainability strategies aimed at the use of machinery and
herbicides.
Collectively, these projects led to several conclusions: (1) switchgrass can be
grown over a wide range of European latitudes, although the level of biomass
production may not always be satisfactory; (2) production of up to 25 Mg biomass

ha-1 is possible, with higher yields generally seen when planting lowland cultivars
in more southern latitudes; (3) seed propagation is valued by farmers, as it
significantly reduces their investments compared to vegetatively propagated
grasses and short rotation coppices; (4) switchgrass is particularly vulnerable
during establishment, but, once established, it requires relatively low use of
chemicals and farm resources.
A weakness of the first European (and American) switchgrass projects was the
absence of farm-scale studies under truly operational conditions. Nearly all pro-
duction data—and subsequent economic and environmental analyses—were in
fact extrapolated from plot trials, which it is generally conceded often produce
inflated yields. Studies at farm and field scales were therefore undertaken in a
subsequent 2001 to 2005 project, ‘Bio-energy chains from perennial crops in
southern Europe’ [70], which compared switchgrass, miscanthus, giant reed, and
cardoon (Cynara cardunculus L.) grown in the Mediterranean region. The
University of Bologna led the work on switchgrass and planted Europe’s first field-
scale experiment (ca. 9 ha), where interdisciplinary studies were carried out, e.g.,
geo-statistical [71], economic [72], agronomic [73], and agro-environmental [74].
One of the main findings of this Italian work was that, under real-world
operational conditions (using common farm machinery), up to 30% of switchgrass
biomass can be left in the field. However, much of that loss could be reduced
through simple machinery adjustments. Therefore, switchgrass generally achieved
acceptable biomass yields, and its management required very few investments in
terms of additional machinery or implements. Along with these important oper-
ational advantages, switchgrass could also provide significant environmental and
economic benefits compared to other energy crops [74]. The advantages seen in
Italy, however, were not always seen in other Mediterranean countries. For
example, in Greece the productivity of switchgrass was much lower than that of
giant reed, and in Spain the productivity of cardoon was clearly higher than
switchgrass. Therefore, no clear picture emerged on which crop should be used for
energy in the Mediterranean area. Indeed, just as in the USA, it may well be that
some species will out-perform switchgrass as biomass producers in specific
locales.
Following the first two multi-year European projects, a number of new national
(e.g., LignoGuide in France, and BioSeGen in Italy) and multi-country projects
were launched (Fig. 1.2) to look at switchgrass—in many cases along with other
species. Agronomic studies at different scales or levels increased, especially in
western Europe, but they moved eastward as well (Fig. 1.2). Some European
projects that have emphasized the possibilities of switchgrass as an energy crop
are: (1) On-Cultivos (2005–2012), which has set out to define, promote, and
develop a sustainable market of energy crops; (2) Babilafuente (2007–2022),
which hopes to demonstrate the commercial feasibility of a second-generation
ethanol plant in Spain; (3) 4FCROPS (2009–2011), which has analyzed potential
land allocation and prospects for switchgrass in Europe; (4) BIOLYFE
(2010–2013), which has addressed second-generation cellulosic ethanol, including
Fig. 1.2 Switchgrass trials in Europe. Data gathered from the literature and by personal
communications. Numbers inside each country identify which of the following European projects
that country has participated in: 1 Switchgrass (1998–2001); 2 Bioenergy chains (2002–2005); 3
On-cultivos (2005–2012); 4 Babilafuente (2007–2022); 5 4FCrops (2009–2011); 6 BIOLYFE
(2010–2013); 7 Pellets-for-Power (2010–2013); 8 OPTIMA (2011–2014)
upstream and down-stream processes, such as pretreatment steps; (5) Pellets-

for-Power (2010–2013), which is aimed at developing switchgrass on 1 to 5 million
currently underutilized hectares in the Ukraine and other countries of eastern Europe:
(6) OPTIMA (2011–2014), which is dealing with the development of perennial
grasses in the Mediterranean basin, particularly on marginal lands.
These continuing research efforts and the reports coming from them have
‘raised the profile’ of this non-native species in Europe, such that switchgrass has
become an increasingly frequent subject for scientists, farmers, and entrepreneurs.
By virtue of its frequent appearance in scientific and popular reports, switchgrass
has come to be considered as one of the most important energy crops in Europe—
much as happened in the US.
To summarize, the results to date from various European studies suggest that
switchgrass is broadly adapted to many of Europe’s countries. However, there is
still great uncertainty on whether lowland or upland ecotypes should be used in
northern European countries. Biomass productivity is clearly the most important
determinant in selecting energy crops in Europe. For this reason, the expectations
for switchgrass as an energy crop are still significantly lower compared to other
perennial and annual grasses which may out-yield it: giant reed, sorghum, and
miscanthus in southern Europe and miscanthus in northern Europe. The advantage
of switchgrass compared to other competing perennial grasses mainly lies in its
integrated assessment, i.e., by weighing all the operational, economic, and
environmental aspects. In its favor, switchgrass is propagated by seed and requires
very little investment in terms of farm machinery and agricultural inputs. In
comparing several biomass crops, Monti et al. [74] and Fazio and Monti [75]
found that the environmental loads and the annual equivalent cost per unit biomass
were the lowest in switchgrass. The ongoing projects will likely contribute to
raising the awareness of switchgrass benefits in Europe by emphasizing the inte-
grated assessment in terms of farming systems and economic and environmental
sustainability.
1.4.2.2 Switchgrass Studies in Other Countries
The number of non-North American and non-European countries in which

switchgrass has been studied is growing. In a tally done in 2005, the species had
been investigated or was reported as in use in 11 countries [43]. The list now
stands at more than 20. Among the first reports of adoption of the species outside
of North America was one from Australia considering switchgrass-for soil con-
servation uses [76]. Besides the North American and European nations already
noted, the countries producing studies on switchgrass include: Argentina,
Australia, China, Colombia, Japan, Korea, Mexico, Pakistan, Poland, Sudan, and
Venezuela. In most cases, especially for the more recent citations, the studies deal
with switchgrass-for-energy.
1.5 Conclusions
The story of switchgrass, which began 2 MYA in the first quarter of the Pleisto-
cene epoch (the Ice Ages), does not intersect with human science and technology
until the middle of the twentieth century. Initially the species was of interest to us
primarily as a member of prairie ecosystems, but it began slowly to gain attention
as a potential forage crop and then for other uses when grown in monoculture. Less
than three decades ago, we began to consider it for bioenergy purposes.
Switchgrass came out of the Ice Ages’ climatic upheavals and into our scientific
era as two distinct, polyploid ecotypes, each possessing a range of morphologic,
physiologic, and genetic differences. The species’ legacy of having endured

repeated glacial and interglacial episodes combined with the greater genetic
plasticity afforded by its polyploidy likely explain switchgrass’s ability to rapidly
re-colonize North America after the last glacial episode; and those same factors
likely produced a deeper, wider genetic pool from which we can now draw traits to
serve us for a variety of purposes today.
The history of switchgrass-for-energy begins in 1985 with its selection as the
benchmark species for a US DOE herbaceous energy crop screening study.
Switchgrass proved to be among the top biomass producers of the 36 crops con-
sidered, and in 1991 DOE designated it a ‘model species’ for further study. Since
DOE was the primary US agency supporting biomass-for-energy work at that time,
the next decade produced a significant amount of work on switchgrass as a model
energy crop, while no other herbaceous lignocellulosic species were being con-
sidered in a systematic way. When DOE funding for extramural work on
switchgrass ceased in 2002, other public, as well as private, organizations kept the
work moving and even accelerating. But in the transition, the notion of ‘model’
may have become blurred or even lost altogether. As a result, switchgrass has
perhaps become a de facto biomass species of choice. At the very least, it is
regularly held up as a benchmark against which other energy crop candidates are
being considered.
Incontrovertible data show a remarkable rise of interest in switchgrass as a
bioenergy crop in the last quarter century. If that trajectory is maintained,
switchgrass could well become what some have already ascribed to it—the bio-
mass species of choice for many systems. We feel that its biology, especially its
phenotypic and genetic variability, may well make it more than just a good model
species. In time, it may be grown over millions of hectares to serve as a trans-
former of the sun’s energy into forms that reduce our dependency on fossil fuels.
If that happens, it should, indeed, be allowed to join the handful of species that
constitute the Olympians within the grass pantheon.
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Chapter 2
Switchgrass Breeding, Genetics,
and Genomics
Michael D. Casler
Abstract Switchgrass was one of the dominant species of the North American
tallgrass prairie and savanna ecosystems that once dominated a large portion of
the continent. It is currently used for pasture, hay production, soil conservation,
and biomass production for conversion to energy. Switchgrass was selected in
1992 as the herbaceous model species to develop dedicated cellulosic bioenergy
crops. Breeding and genetics studies began on switchgrass in the 1950s, focused
on utilization in livestock agriculture. Recent developments have rapidly increased
the rate of gain for biomass yield, largely by increasing the focus and intensity of
selection and improving the choice of germplasm and selection methods. Modern
genomics tools are rapidly being incorporated into switchgrass breeding programs
to increase the rate of gain for important agronomic and bioenergy traits, as well as
to create new variability that can be captured in commercial cultivars.
2.1 Introduction
Switchgrass is a highly versatile grass, used for soil and water conservation,
livestock production, and biomass production for conversion to energy. The spe-
cies is native to North America, east of the 100th meridian, ranging from southern
Canada to northern Mexico. It was once one of the dominant species of the
tallgrass prairie and associated ecosystems that included savanna, sand barrens,
forest margins, and grassland–wetland transition zones. The most significant
taxonomic division within switchgrass occurs at the ecotype level and is related to
M. D. Casler (&)
U.S. Dairy Forage Research Center, USDA-ARS, Madison, WI 53706, USA
e-mail: michael.casler@ars.usda.gov

DOI: 10.1007/978-1-4471-2903-5_2, Ó Springer-Verlag London 2012
30 M. D. Casler
habitat. Upland and lowland ecotypes were named largely for an obvious phe-
notypic differentiation that was originally associated with habitat. Upland ecotypes
were found on upland sites that were subject to occasional or frequent droughts,
while lowland ecotypes were found on lowland sites that were prone to seasonally
wet soils. The upland–lowland taxonomic division figures prominently in nearly
all the cultivation and breeding history of this species.
2.2 Biogeography
Switchgrass germplasm has been preserved in remnant prairies and associated

ecosystems throughout its historic range (Fig. 2.1). While less than 1% of the
tallgrass prairie and associated savanna ecosystems have been preserved, the species
native to these habitats have been preserved in thousands of remnant sites [1]. Most
of these sites have never been developed or plowed, but some represent abandoned
farmlands, which have been allowed to return to their native habitat and species
assemblage. Presumably the native species on these sites have been restored to these
sites as a result of viable seed banks. In some cases, human intervention has been
used to assist recovery, in the form of seeds introduced from other sites.
Prairie and savanna remnants in North America have been preserved under the
leadership and custodianship of a wide range of organizations, including the U.S.
Forest Service, many state agencies responsible for preservation and use of natural
resources (e.g. State of Wisconsin Department of Natural Resources), non-gov-
ernmental agencies such as The Nature Conservancy, and numerous private
organizations that include railroad right-of-ways, rural cemeteries, and private
conservationists. The size of these remnants ranges from tiny family cemeteries of
several hundred square meters up to several national grasslands, some of which
exceed 0.5 M ha in size (e.g. Cimarron, Comanche, Rita Blanca, and Black Kettle
National Grasslands). These grasslands are highly variable in species composition,
due to variations in climate and soil type, with switchgrass occupying a range of
positions from the dominant species to rare or absent in many cases.
Switchgrass is a highly polymorphic species with considerable morphological
and physiological variation that is closely related to climatic factors. It is highly
photoperiodic along its north–south adaptation range. In their native habitat,
northern accessions may flower as early as late June or early July after only
3–4 phytomers have been produced. Conversely, accessions from the extreme
southern portion of the range may flower as late as mid-October after production of
7–10 phytomers. Photoperiodism and extreme flowering times have created a
strong adaptation gradient associated with both photoperiod and temperature. Most
recommendations are to move switchgrass germplasm no more than one hardiness
zone (5°C increments) north or south of its origin to avoid stand loss. Northern
germplasm is insufficiently heat tolerant and too early in flowering to be pro-
ductive at southern locations, while southern germplasm may lack sufficient cold
tolerance to survive at northern locations [2–4].
2 Switchgrass Breeding, Genetics, and Genomics 31
Fig. 2.1 Historical range of switchgrass in North America ([92], reprinted with permission)
Two ecotypes form the principal taxonomic division within switchgrass

(Fig. 2.2). These two ecotypes were originally described based on observations of
a distinct polymorphism associated with upland and lowland habitats, hence their
names: upland and lowland ecotypes. Upland ecotypes are widely adapted north of
34°N latitude, extending into much of eastern Canada, but extremely rare at lat-
itudes below 34°N. Lowland ecotypes are widely adapted up to approximately
42°N in the western portion of the range, but can be found as far north as 45°N in
eastern North America due to climate-moderating oceanic effects. Upland and
lowland ecotypes have generally been differentiated on the basis of plant pheno-
type: lowland plants are taller, have fewer and larger tillers, longer and wider
leaves, thicker stems, and are later in flowering than upland plants (Table 2.1).
Most lowland ecotypes also have a distinct blue coloring on stems and leaves,
believed to be due to a waxy bloom on the epidermis. The blue hue is easily
removed from some genotypes by touching or rubbing plant tissue between two
fingers. As suggested by their names, upland ecotypes tend to be more drought
tolerant than lowland ecotypes [5].
Upland and lowland switchgrass diverged on the evolutionary tree of life
approximately 0.8–1.0 Mya [6–8]. Since that time, there have been approximately
32 M. D. Casler
Fig. 2.2 Native ranges of upland and lowland switchgrass ecotypes in North America ([27],
reprinted with permission)
Table 2.1 Summary of the most common range of phenotypic values for upland and lowland
switchgrass plants grown in direct-comparison experiments in Wisconsin and New Jersey
(40–42°N latitude)a
Ecotype Heading Plant Flag Flag Number of Stem CIE CIE y-scale
dateb height leaf leaf tillers (# diameter x-scale colorb
(doy) (m) length width plant-1) (mm) colorb
(cm) (mm)
Upland 180–195 0.9–1.7 32–48 9–11 150–300 3–5 x \ 0.4 0.4 \ y \ 0.8
Lowland 205–220 1.9–2.2 50–58 12–14 40–90 5–7 x \ 0.2 0.2 \ y \ 0.4
a
Cortese et al. 2010 [18]; Casler et al. 2010, unpublished data
b
Heading date = day of year. Color reference: McLaren [94]; http://www.colorbasics.com/
CIESystem/
12–15 major ice age cycles [9] that have compressed the native range of
switchgrass into a relatively narrow band along the current coastline of the Gulf of
Mexico [10]. Ice ages forced upland and lowland switchgrasses to occupy a rel-
atively narrow region for tens of thousands of years, allowing upland and lowland
ecotypes to occasionally mate with each other.
These matings probably occurred at a relatively low frequency, due to differ-

ential flowering time, but resulted in significant and measureable gene flow
between the two ecotypes [8, 11]. As such, visual or morphometric assessments of
plant phenotype are no longer reliable as a mechanism of classifying plants into
the upland or lowland ecotype taxa [12]. The most reliable classification method is
based on sequence-based marker analysis of plastid DNA, specifically chloroplast
DNA, which evolves very slowly over time, much more accurately reflecting the
ancient division between the two ecotypes [6–8].
Following each ice age, gradual and punctual warming of the earth’s climate
resulted in a slow, gradual, and highly discordant northward migration of species
to repopulate their former habitats. Because plants are sessile organisms and
former habitats became buried under many meters of glacial outwash and sedi-
ment, northward migrations of switchgrass and other species required assistance
from birds and mammals. Deep sampling of pollen from lakebed sediments has
shown that this process required thousands of years and followed a progression of
tundra, taiga, boreal forest, deciduous or mixed forest, and grassland. Minor cli-
matic shifts were often sufficient to cause local extinctions and short-term cyclic
changes between dominant habitats during the early and mid-Holocene period
[13–15]. Because switchgrass and other species were wholly dependent on animals
for transport to new sites, it should not be surprising to observe multiple genetic
lineages of switchgrass at any particular site; indeed, this is a common occurrence
[6, 8, 11, 12].
McMillan [16] proposed that switchgrass was preserved in three refugia during
the ice ages, corresponding largely to the eastern Gulf Coast, western Gulf Coast,
and western montane (dryland) regions. His research was based on the most
extensive collection and evaluation of switchgrass accessions ever conducted,
representing nearly the entire range within the USA. Molecular marker analyses of
both nuclear and plastid DNA have confirmed that Calvin McMillan was largely
correct, but suggest that his ‘‘three’’ refugia might not have been isolated from
each other to the extent that he hypothesized. During the Holocene period that
followed the Pleistocene glaciation, tallgrass prairie and savanna establishment
was largely completed by approximately 2–3,000 year BP. Since that time, climate
and animals have continued to mold the genetic landscape of switchgrass, due
largely to its highly outcrossing nature and the dominance of wind-aided polli-
nation. Individual accessions of switchgrass, samples of plants collected from a
single remnant site, typically possess between 65 and 80% of the genetic vari-
ability observable within the species, based on DNA markers [8, 11, 12, 17–22].
Genetic differentiation occurs on a fine-scale in some cases, e.g. changes in soil
type or microclimate, but is largely associated with the photoperiod-temperature
cline and an east–west cline of humidity and/or precipitation. McMillan hypoth-
esized that the western montane refuge was largely the source of switchgrass
genotypes that populated the western portion of its range, where dryland envi-
ronments and drought tend to be more frequent than in eastern North America.
More recent phenotypic studies across a range of sites have confirmed a tendency
for eastern accessions to have relatively poor performance at western sites and vice
34 M. D. Casler
Fig. 2.3 Proposed gene pools for deployment of regionally adapted switchgrass germplasm and
cultivars for use in breeding programs or in conservation and restoration projects PP prairie
parkland, GPS great plains steppe, LMF laurentian mixed forest, EBF eastern broadleaf forest
[23]; HZ USDA hardiness zone [93] ([92], reprinted with permission)
versa [2]. Anecdotal observations have suggested that eastern accessions lack the
drought tolerance to perform well in the western regions, while western accessions
lack the disease resistance to perform well in the more humid eastern regions.
One net result of these studies has been the development of a concept of gene
pools for switchgrass (Fig. 2.3). Each of the proposed gene pools spans a region
that includes two neighboring hardiness zones, with a range in mean temperature
of no more than 10°C. The east–west division approximately follows the
Mississippi River Valley, splitting the range according to historic tallgrass prairie
versus historic savanna ecosystems [23], Sanderson et al. [24] has estimated that
cultivar recommendations that follow this regional gene pool concept are
responsible for approximately a 20–25% increase in local biomass yields, simply
associated with choosing appropriately adapted cultivars. There is currently at
least one switchgrass breeder located in each of the eight regions shown in
Fig. 2.3, creating opportunities to develop regionally adapted cultivars that take
advantage of the significant genotype x environment interactions that are common
to this species.
2.3 Genetics and Cytology
Switchgrass has a basic chromosome number of x = 9, with a wide range of

chromosome numbers for somatic cells ranging from n = 18 to 108 [25, 26].
Lowland ecotypes of switchgrass are largely tetraploid with 2n = 4x = 36 chro-
mosomes, while octoploids with 2n = 8x = 72 chromosomes are very rare [11].
Conversely, upland ecotypes exist at both tetraploid and octoploid levels, with the
octoploid form approximately two to three times more abundant than the tetraploid
form. Tetraploids contain approximately 3.1 pg DNA per nucleus with a haploid
genome size of *1.5 Gb [27]. True hexaploids, 2n = 6x = 54, are extremely rare
but have been observed within remnant sites that possess both tetraploid and
octoploid plants, suggesting their potential role in interploidy gene flow [11].
Recent results based on genotype-by-sequencing suggest that hexaploids likely
arise by union of a normal gamete and an unreduced (2n) gamete (Costich et al.
2011, unpublished data). Although 2n gametes are common in grasses [28], they
have yet to be verified in switchgrass. Aneuploids appear to be common in
switchgrass, particularly at the octoploid level, and largely characterized by
chromosome loss from the normal euploid number [29].
Switchgrass is a disomic polyploid, or allopolyploid, with largely diploid
inheritance at the tetraploid level [30]. Tetraploid switchgrass has 18 linkage
groups arranged in two highly homologous sets that are highly conserved with
other C4 grasses, e.g. Sorghum and Setaria [30]. Meiosis of both tetraploid and
octoploid individuals is largely characterized by normal bivalent pairing [31, 32].
A high frequency of DNA markers are characterized by segregation distortion and
multi-locus interactions that could be caused by low frequencies of quadrivalent
pairing including homeologous chromosomes [30], which could occur in a recent
polyploid such as switchgrass.
Switchgrass is predominantly cross-pollinated with a gametophytic self-
incompatibility system similar to the S-Z system found in many grasses [33].
Pollen is dispersed by wind and early reports suggested that the percentage of
selfed seed was low in most genotypes, generally \1% [33, 34]. More recent
reports suggest that some genotypes are capable of producing selfed seed in fre-
quencies as high as 50% (Buckler et al. 2011, unpublished data; [35]).
Upland and lowland ecotypes of switchgrass can be easily crossed with each
other at the tetraploid level, reflecting their relatively recent divergence on the
evolutionary scale [33]. A single hybrid between a random upland and a random
lowland plant produced a cross with an average of *35% high-parent heterosis,
suggesting that the evolutionary divergence between the two ecotypes has been
sufficient to create genetic divergence and allelic complementarity. A post-fertil-
ization incompatibility system between ploidy levels minimizes the opportunity
for interploidy crosses and gene flow. The existence of 2n gametes in switchgrass
would be an effective mechanism to bridge this barrier. Vogel [36] suggested that
tetraploid and octoploid plants that are sympatric within a single remnant prairie
site are effectively members of different interbreeding populations.
36 M. D. Casler
2.4 Germplasm, Ecotypes, and Early Use
Early cultivars of switchgrass are exclusively represented by seed increases from

source-identified remnant prairies (Table 2.2). These natural-track cultivars have
generally undergone no direct selection for agronomic traits or been subjected to any
plant improvement efforts. Most of these cultivars were given a name that reflects the
geographic location of the original accession. Most of these cultivars were collected
and evaluated by personnel of the plant materials centers (PMC) of the Soil
Conservation Service (SCS), which later became the Natural Resource and Conser-
vation Service (NRCS), an agency of the USDA. Many evaluations included com-
mon-garden agronomic evaluations of numerous accessions, allowing personnel to
chose only one or two of the best populations for use in each region. There are 26 PMC
locations within NRCS, 15 of which have been involved in collection and/or release of
switchgrass cultivars using this approach. Because these populations represent little
or no breeding history, they represent the natural genetic diversity within specific
regions of the switchgrass range. Nevertheless, there are exceptions to this, in which
different seed lots of a single cultivar have been shown to have diverged from each
other, most likely due to seed increase under different environmental conditions [19].
Natural-track cultivars of switchgrass formed the basis for development of a
switchgrass seed industry, shared between a small number of private companies
and public organizations such as the Nebraska and South Dakota Crop Improve-
ment Associations. The principal use of switchgrass since the 1940s has been for
pasture and rangeland, largely in the Great Plains region of the USA, but only
sparsely in the eastern portions of North America. In addition, switchgrass has
been a component of seed mixtures for prairie and savanna restoration projects
since the mid-twentieth century. Early agronomic trials of unimproved cultivars
quickly established the presence of large amounts of ecotypic variation, suggesting
that some sense of local adaptation should be used in developing recommendations
for the geographic range of individual cultivars [36].
With the choice of switchgrass as a herbaceous model species for the U.S.
Department of Energy (DOE) Biofuel Feedstock Development Program (BFDP) in
1992, public and commercial interest in switchgrass rapidly increased. Because the
seed industry was largely based on unimproved cultivars that represented source-
identified collections, these cultivars came under rapid and high demand as source
material for agronomic field trials and demonstration plots for studying conversion of
herbaceous biomass into biofuel. This interest led to an expansion of the switchgrass
seed industry and the establishment of hundreds of field experiments across the native
range of switchgrass in the USA and Canada. Many of these field experiments were
highly valuable in helping to define the limits of adaptation of individual cultivars. Even
though they were not centrally coordinated or conducted under uniform conditions,
these experiments were responsible for identifying Cave-in-Rock and Alamo as cul-
tivars with remarkably broad adaptation. Alamo, from central Texas, can be success-
fully grown throughout the southeastern USA and along the Atlantic Seaboard into
southern New England [37]. Cave-in-Rock is broadly adapted in hardiness zones 4
Table 2.2 Switchgrass cultivars and released germplasm populations representing various
habitats in the central and eastern USA, largely representing local ecotypes with minimal or no
selection for plant traits
Cultivar PI numbera Ecotype Ploidy Year of Geographic origin USDA
release hardiness
zonesb
Alamo 422006 Lowland 4x 1978 Southern Texas 6, 7, 8, 9
Kanlow 421521 Lowland 4x 1963 Northern Oklahoma 6, 7
Pangburn Lowland 4x NAd Arkansas 6, 7
Penn Center Lowland NA 2010 Coastal South Carolina 8
Stuart 422001 Lowland 4x 1996 Southern coastal Florida 9, 10
Timber Lowland 4x 2009 Unknown mixturee 6, 7, 8
Miami 421901 Up/Lowc 4x 1996 Southern Florida 9, 10
Wabasso 422000 Up/Low 4x 1996 Southern coastal Florida 9, 10
Dacotah 537588 Upland 4x 1989 Southern North Dakota 2, 3, 4
Falcon 642190 Upland 4x 1963 New Mexico 4, 5, 6
Grenville 414066 Upland NA 1940 Northeastern New Mexico 4, 5, 6
High Tide Upland NA 2007 Northeastern Maryland 5, 6, 7
KY1625 431575 Upland 4x 1987 Southern West Virginia 5, 6, 7
Blackwell 421520 Upland 8x 1944 Northern Oklahoma 5, 6, 7
Caddo 476297 Upland 8x 1955 Central Oklahoma 6, 7
Carthage 421138 Upland 8x 2006 North Carolina 5, 6, 7
Cave-in-Rock 469228 Upland 8x 1973 Southern Illinois 4, 5, 6, 7
Central Iowa 657600 Upland NA 2000 Central Iowa 4, 5
Forestburg 478001 Upland 8x 1987 Eastern South Dakota 3, 4
Nebraska 28 477003 Upland 8x 1949 Northeast Nebraska 3, 4
Shelter Upland 8x 1986 Central West Virginia 4, 5, 6
Southlow 642395 Upland NA 2003 Southern Michigan 4, 5, 6
a
GRIN accession number (http://www.ars-grin.gov/). Empty cells indicate that an accession is
not available through GRIN; b USDA Hardiness Zones are defined in approximately 5°C
increments of mean annual minimum temperature (http://www.usna.usda.gov/Hardzone/
ushzmap.html); c Upland cytoplasm, but lowland phenotype and nuclear DNA, suggesting an
ancient hybrid origin [12]; d NA information not available; e DNA marker analyses suggest a
mixture of germplasm from the southern Great Plains and the southeastern USA [8, 11]
through 7, covering much of the eastern USA and Canada north of 35°N latitude, but is
poorly adapted to dryland regions [38]. Most other cultivars have significantly narrower
ranges of adaptation, more aligned with the regional gene pools shown in Fig. 2.3.
Numerous collections of switchgrass have been generated throughout the spe-
cies range. The official USDA collection of switchgrass accessions is located at
Griffin, GA, part of the national plant germplasm system (NPGS) and germplasm
resources information network (GRIN). At the time of this writing, the GRIN
collection consists of 497 historical accessions, of which 174 are currently
available for distribution. A small number of seeds are made available to anyone
anywhere, upon request through the web link.1
1
http://www.ars-grin.gov/
38 M. D. Casler
There are thousands of additional accessions that are currently being stored in
collections made by both public and private organizations involved in restoration,
conservation, production, breeding, and genetics. Regardless of where they are
housed, these collections are all essentially private, because they are not made
broadly available to the public. Any accessions can be donated to GRIN, simply by
contacting the switchgrass curator via the GRIN web link. Either seed or living
tillers can be donated, but seed is preferred because it requires less urgency for
care and handling. Source-identified accessions are preferred and basic passport or
descriptive information about the collection site is highly desirable as a link
between each accession and its natural environment. Donations can be made of
switchgrass germplasm at any stage of development, ranging from wild popula-
tions to highly bred cultivars.
2.5 Genetic Improvement
2.5.1 Breeding Objectives
Switchgrass breeding was initiated at the University of Nebraska in the 1950s,

largely based on regional seed collections as the basis for breeding populations
[39]. Early objectives were focused largely in improving forage quality for
livestock production systems, as well as fundamental research to develop a
more thorough understanding of reproductive biology and breeding behavior of
switchgrass.
Development of a high-throughput in vitro dry matter digestibility (IVDMD)
assay [40] was one of the major research breakthroughs that led to some of the
most significant breeding gains in switchgrass. Three cycles of selection increased
IVDMD by 5% over the original population mean. Simultaneous improvements in
both IVDMD and forage yield are possible, but at a reduced rate of gain for both
traits due to the reduced individual-trait selection pressure required for multi-trait
selection [34, 41–43]. Several improved cultivars have been derived from these
efforts to improve forage quality of switchgrass (Table 2.3).
Genetic improvements in forage quality traits such as IVDMD are remarkably
stable across environmental conditions and management systems [44, 45]. This
principle allowed the rapid cycling of high-IVDMD switchgrasses from breeding
nurseries into small-plot trials of agronomic traits and large-plot grazing trials to
measure livestock performance. Early grazing trials demonstrated that a 40 g kg-1
increase in IVDMD was realized as an increase in daily live-weight gains 0.15 kg
animal unit-1, an increase in beef cattle production of 67 kg ha-1, and an increase
in profit of $59 ha-1, measured in 1998 $US [46]. Documentation of improved
livestock production, directly associated with increases in IVDMD, was respon-
sible for rapid adoption and success of these cultivars on many thousands of
hectares in the years following release of these improved cultivars.
Table 2.3 Improved switchgrass cultivars and germplasm releases representing significant
breeding and selection activities
Cultivar PI Ecotype Ploidy Year of Principal traits selected USDA
numbera release during cultivar developmentb hardiness
zonesc
EG2101 Upland 8x 2009 Biomass yield, spring vigor, 4, 5, 6
rust resistance
Pathfinder 642192 Upland 8x 1967 Biomass yield and vigor 4, 5
Shawnee 591824 Upland 8x 1996 IVDMD, biomass yield 5, 6, 7
Sunburst 598136 Upland 8x 1998 Large seed size and mass 3, 4, 5
Trailblazer 549094 Upland 8x 1984 IVDMD, biomass yield 4, 5
Summer 642191 Upland 4x 1963 Earliness, rust resistance 4, 5
BoMaster 645256 Lowland 4x 2006 IVDMD, biomass yield 6, 7, 8
Cimarron Lowland 4x 2008 Biomass yield 6, 7, 8
Colony 658520 Lowland 4x 2009 IVDMD, biomass yield 6, 7, 8
EG1101 Lowland 4x 2009 Biomass yield, spring vigor, 8, 9, 10
rust resistance
EG1102 Lowland 4x 2009 Biomass yield, spring vigor, 6, 7, 8
rust resistance
Performer 644818 Lowland 4x 2006 IVDMD, biomass yield 6, 7, 8
TEM-LoDorm 636468 Lowland 4x 2007 Reduced post-harvest seed 6, 7, 8
dormancy
a
GRIN accession number (http://www.ars-grin.gov/). Empty cells indicate that a cultivar is not
available through GRIN; b IVDMD in vitro dry matter digestibility; c USDA Hardiness Zones
are defined in approximately 5°C increments of mean annual minimum temperature (http://
www.usna.usda.gov/Hardzone/ushzmap.html)
Increases in IVDMD have been largely associated with reductions in lignin

concentration [44] and reduced ratios of p-coumaric/ferulic acid [47]. Low-lignin
switchgrass genotypes have also significantly reduced cortical fiber and secondary
wall thickenings in stem tissues, compared to high-lignin genotypes [48]. These
low-lignin genotypes have significant potential to improve conversion efficiency of
switchgrass biomass to bioenergy in a fermentation system where lignin is one of
the chief factors limiting ethanol production [49, 50].
Three different approaches have been used in efforts to improve establishment
capacity of switchgrass. Selection for large seed size was highly effective in
increasing mean seed mass by up to 50% compared to other cultivars, resulting in
double the emergence rate and 6-week seedling height [51]. Selection for high
seedling shoot mass was highly successful, but did not translate into consistent
improvement of seedling vigor, root growth, or establishment capacity in field
studies [52]. Finally, divergent selection for elevated versus reduced crown node
position failed to affect seedling vigor or establishment capacity [53]. Given the
persistent establishment problems associated with switchgrass, especially as a
monoculture for bioenergy production systems, there has been surprisingly little
research conducted on breeding approaches or directed breeding efforts to improve
establishment capacity. Instead, research efforts have largely focused on estab-
lishment methods to reduce weed competition (see Chap. 4).
40 M. D. Casler
Seed dormancy is a significant problem in many natural populations of

switchgrass, but every cycle of selection in a breeding program moves one step
closer to reducing seed dormancy problems for improved cultivars, simply by the
process of eliminating seeds that do not germinate. Gradual elimination of seed
dormancy problems in switchgrass is currently the first of several wild traits to be
eliminated on the road to domestication of this species.
A number of pathogens and herbivorous or boring insects utilize switchgrass as
a host in various portions of its range [36, 37]. Breeding for resistance to many of
these pests is a high priority for most switchgrass breeders, but is largely con-
ducted as a secondary objective in field-based breeding programs. There are no
reports of pest screens conducted under highly controlled or uniform conditions of
artificial inoculation. Rather, breeders have tended to rely heavily on natural
inoculum in field nurseries, which can often be highly variable, inconsistent, and
unreliable. Susceptibility to pests is one of the principal sets of traits that are used
to eliminate plants from breeding nurseries prior to measurement of traits related
to productivity and quality. Genetic variation is known to exist for some pests
(e.g. [54]) and disease epiphytotics and insect infestations will likely elevate pest
resistances to higher priorities in the future. There are several recent reports of
widespread insect pests that are capable of drastically reducing biomass yields and/
or seed yields of switchgrass [55, 56]. Smut, caused by Tilletia maclaganii, has
been reported in several switchgrass fields in Iowa, USA, already causing up to
50% reduction in biomass yield with high infection rates [57].
The global emphasis on development of herbaceous energy crops has led to
increased emphasis on biomass yield as the principal breeding objective for most
switchgrass breeding programs. Economic studies have clearly indicated that
biomass yield of on-farm production systems is the most important factor limiting
economic viability of switchgrass as a bioenergy crop [58]. Significant improve-
ments, directly associated with selection and breeding, have been demonstrated in
a diverse array of environments and breeding populations [41, 42, 59–61]. Typical
rates of gain have averaged about 1–2% year-1 and some have been sustained for
multiple cycles of selection. Realistically, these results suggest that switchgrass
breeders have likely increased biomass yields by an average of 20–30% since the
inception of the U.S. DOE BFDP in 1992. Many of these gains are yet to be
realized or quantified in agronomic or field-scale demonstration trials due to
the 8–10 year lag required to complete evaluations, cultivar releases, and seed
multiplications.
While the use of locally adapted and representative germplasm is a cornerstone
of restoration and conservation applications for switchgrass, breeders are bounded
only by what germplasm will survive or can be modified to survive in their target
environments. Switchgrass breeders are beginning to take advantage of southern
germplasm, collecting and evaluating germplasm from as far south as possible in
an effort to extend the effective growing season. Biomass yield of switchgrass
peaks near anthesis [43], often leaving 6–8 weeks of unutilized growing season in
northern climates. Lowland ecotypes, which can be 4–6 weeks later than upland
ecotypes in flowering time, are currently being selected and bred for improved
Fig. 2.4 Relationship between lowland-ecotype biomass-yield advantage and mean minimum
annual temperature (hardiness zone definitions—[93]) for 23 cultivar-evaluation trials conducted
under varying climatic conditions in the USA. Each point is represented by a mean of at least two
upland and two lowland cultivars and the difference is expressed as a percentage of the upland
mean. Data were collected from [3] (Arlington and Spooner, WI; Mead, NE; Manhattan, KS;
Stillwater, OK), [95] (Hope, AR; College Station, Dallas, and Stephenville, TX), [96] (Princeton,
KY; Raleigh, NC; Jackson and Knoxville, TN; Blacksburg and Orange, VA; Morgantown, WV),
[97] (Chariton, IA); and [4] (Beeville, College Station, Dallas, Stephenville, and Temple, TX)
cold tolerance and biomass yield at northern latitudes. A meta-analysis of 23 field

trials conducted in a wide range of climatic conditions shows a strong relationship
between lowland-ecotype biomass yield advantage and mean minimum tempera-
ture (Fig. 2.4). Lowland cultivars are generally at least twice as productive as
upland cultivars at the most southern locations, but this advantage gradually
declines with increasing latitude. Lowland cultivars can be expected to have
biomass yields 30–50% higher than upland cultivars in the transition zone where
both are well adapted. At the most extreme northern locations, biomass yield of
lowland cultivars is limited by their inability to survive multiple winters [3].
2.5.2 Breeding Methods
By far the most common breeding method used on switchgrass is some form of
phenotypic recurrent selection, mostly using one or more restrictions as proposed
by Burton [62] and described in detail by Vogel and Pedersen [63] and Burson
[64]. Breeding begins with the assembly of germplasm to be evaluated for
inclusion in adapted populations. Nearly all switchgrass breeders conduct initial
switchgrass germplasm screens as spaced-plant nurseries at a single location.
Because most breeding programs are focused on regional adaptation (Fig. 2.3), a
single representative location is typically sufficient to make gains. Spaced plant-
ings are used to conduct efficient evaluations of individual genotypes over 2 or
42 M. D. Casler
3 years, allowing each genotype to express itself without competition. Many

breeders use some form of mental ideotype in these nurseries, selecting for a range
of traits that provide the morphological form desired for the region and the pro-
duction system. For example, several studies of both upland and lowland ecotypes
[65–67] have suggested that tiller density may be the most efficient indirect
spaced-plant selection criterion for improving biomass yield.
Although spaced-plant nurseries are generally ineffective toward improving
biomass yield of C3 forage crops [45], they have been highly useful for improving
biomass yield of switchgrass. All of the gains cited in the previous section were
based on selection in spaced-plant nurseries. Controlling spatial variation within
spaced-planted nurseries is critical to ensure a moderate to high heritability for
biomass yield. Missaoui et al. [60] accomplished this using a moving-mean or
Papadakis-type analysis, similar to nearest-neighbor analysis. Casler [59] con-
trolled spatial variation by conducting all selection within 10-plant blocks, forcing
each block to be a mini-selection nursery. Rose et al. [61] evaluated both low- and
high-yield environments for selection nurseries and observed greater genetic gain
for biomass yield in the low-yield environments.
Spaced plantings are highly efficient for improving quality traits related to
feeding value for ruminants or conversion to bioenergy. Traits such as IVDMD
and lignin concentration have sufficiently high heritabilities for unreplicated
spaced plants that gains can easily exceed 2% cycle-1 [44, 68]. Quality traits also
possess high genetic correlations between spaced-plant and sward-plot conditions,
allowing gains made on spaced plantings to be quickly transferred to real-world
pastures and hay fields where performance is measured in terms of livestock
production [69, 70].
Duration of spaced plant nurseries can be a critical factor in the success of a
breeding program. Reducing cycle time to speed up gains can result in insufficient
time in the field to evaluate critical traits such as cold tolerance, heat tolerance,
pest resistance, and tiller production. Three cycles of selection for high IVDMD
were highly successful, but the third cycle resulted in catastrophic winter injury in
the Cycle-4 nursery (\10% survival), largely attributed to selection based 3-year-
old plants that had survived only two winters of cold stress [43]. Selection of
survivors from this nursery, combined with an additional winter of observation
prior to final selection decisions has been attributed to reversing this loss in plant
fitness [43].
A typical spaced planting in a switchgrass nursery consists of 1,000–10,000
plants. Once selections are made from a spaced planting, plants are intercrossed to
create seed to begin the next cycle of selection. Intercrossing can be conducted in
situ leaving the unreplicated plants in place [59], by transplanting selections to an
isolated crossing block [43], or by vegetatively propagating the genotypes and
transplanting them into a replicated polycross block [63]. If the final data col-
lection and selection decisions are made late in autumn, plants can be transplanted
once they are dormant, or transplanting can take place very early in spring without
compromising the ability of plants to intercross and produce seed. Some breeding
programs intercross selections in the glasshouse, which can be effective following
selection and transplanting from the field at numerous times during the growing
season. Switchgrass does not require vernalization to flower, so the most critical
factor is to obtain clonal ramets that possess sufficient numbers of tillers or tiller
buds to generate inflorescences. Photoperiod adjustment, including low-irradiance
24 h photoperiod, can be used to promote flowering in the glasshouse and to
synchronize flowering among genotypes of widely different origins [71].
Family-based selection methods have become commonplace in switchgrass
breeding programs. Half-sib families are the most common type of family, largely
because they are simple and efficient to produce. Family-based selection methods
strive to utilize as much genetic variability as possible by conducting selection
among and within families. Both of the USDA-ARS breeding programs in Lincoln,
NE and Madison, WI rely heavily on half-sib family selection, using family rows of
spaced plants. Both programs attempt to create a competitive environment for
individual plants using two different methods. In Lincoln, highly rhizomatous
plants are spaced 1.2 m apart, but their spread is constrained to 0.5 9 0.5 m2 by
frequent tillage [72]. Plants are harvested and biomass yield is expressed on a unit-
area basis. In Madison, family rows are created with a plant spacing of 0.3 m within
rows and 0.9 m between rows, allowing plants to begin competing with each other
in the second year [59]. If half-sib family seeds are produced in sufficient quantity
in the field, half-sib family breeding methods can utilize drill-seeded plots to more
accurately simulate a realistic agricultural production system [59].
Cycle time for switchgrass breeding programs ranges from 2–7 years,
depending on the objectives and specific methods. Theoretically, each cycle could
spin off a new and improved population that could move into candidate-cultivar
status [63]. Most breeding programs establish new field trials of candidate cultivars
every 2–4 years, depending on timing and resources. These field trials are more
extensively replicated than selection nurseries, utilizing multiple locations
throughout the target region of environments, replicated and randomized experi-
mental designs, drill-pots that can provide accurate biomass yield assessments, and
2–4 years of agronomic-trait measurements. Because many breeders do not have
access to a wide array of test sites, some switchgrass breeders collaborate by
pooling resources and sharing test sites for candidate-cultivar field trials, partic-
ularly when some of the candidate cultivars may have expected adaptation to
broader regions than the breeder’s range of test sites.
Hybrid breeding is expected to be a significant activity in the future. The
evolutionary divergence between upland and lowland ecotypes has been sufficient
to create significant allelic differentiation for a wide range of DNA markers and
sequences throughout the genome (previously discussed). This allelic diversity has
created a certain level of complementarity between uplands and lowlands, such
that F1 hybrids have significantly superior performance to their parents [72, 73].
This allelic complementation, manifested as hybrid vigor or heterosis, does not
occur in F1 hybrids within either the upland or lowland ecotype. The observation
of 30–35% heterosis, superiority of the F1 hybrid to the best of the two parents,
lends great optimism to this approach, particularly since the parents were selected
more-or-less at random, without any selection for specific combining ability.
44 M. D. Casler
Commercial production of F1 switchgrass hybrids is still many years from

reality, due to two massive logistical problems. Simply crossing upland and
lowland ecotypes with each other is extremely difficult, due to the differences in
flowering time of 4–6 weeks (Table 2.1). Flowering time can be relatively easily
manipulated in glasshouse or growth chamber environments using temperature,
light intensity, and day length [37]. However, commercial hybrids will require
field-scale seed production between two clonal genotypes or two inbred lines.
Growth regulators, such as the families of compounds that are used to reduce or
delay flowering in grasses [74], might be a viable mechanism to promote field-
scale interpollination between upland and lowland parents. Field-scale propagation
of the parents poses a second problem. Current approaches to F1-hybrid devel-
opment are based on heterozygous genotypes as parents, requiring some form of
efficient and high-throughput vegetative propagation methodology, e.g. somatic
embryogenesis or micropropagation [75]. Both parents would be propagated in the
laboratory and transplanted in alternate rows to a hybrid-seed production field
using existing transplanting technologies (Fig. 2.5). Commercial development will
require parents to be screened for specific combining ability (genetic capacity for
hybrid vigor in the progeny, complementing the other parent) and competency for
large-scale vegetative propagation.
Ideally, and in the long term, such a system should strive to replace hetero-
zygous clonal parents with inbred lines. However, inbred lines are many years
from realization in switchgrass, due to self-incompatibility and inbreeding
depression. Self-incompatibility is genetically controlled, so it can be modified by
selection and breeding for selfing competence. Inbreeding depression can also
be partially overcome by long-term selection for vigor and seed production during
the inbreeding process, a proven phenomenon in maize breeding, allowing the
development of inbred lines capable of seed propagation. This is an extremely
long-term goal in switchgrass that is theoretically possible, based on the maize
model, but has no precedent in perennial plants.
2.5.3 Impact of Genomics on Breeding
Bi-parental linkage-mapping populations have provided the most definitive evi-

dence that tetraploid switchgrass is characterized primarily by disomic inheritance,
acting largely as a diploid organism with 18 pairs of chromosomes [30, 76].
These populations consist of two highly heterozygous genotypes from contrasting
populations, crossed under controlled conditions to create a pseudo testcross
population. Each parent serves as a tester for markers and quantitative-trait loci
(QTL) that are heterozygous in the other parent. Second-generation linkage pop-
ulations, which would allow detection of markers and QTL that are homozygous
for different alleles in the two parents, have yet to be reported in switchgrass. The
18 linkage groups of switchgrass consist of two highly homologous sets of nine
chromosomes that are highly collinear with Sorghum and Setaria [30].
Fig. 2.5 Seed production scheme to develop F1 hybrids between vegetatively propagated
genotypes of upland and lowland switchgrass ecotypes
Numerous DNA marker systems have been adapted for use in phylogenetic
studies of switchgrass and development of DNA markers that can be used in
switchgrass breeding [27]. Based on these marker systems, both wild switchgrass
populations and bred cultivars contain levels of variability equivalent to 65–85%
of that found across the range of the species. The remaining variability is asso-
ciated with the two major taxa (upland and lowland), geographic differentiation
46 M. D. Casler
[8, 12], and fine-scale differentiation due to natural selection [6]. The consistent
similarity of wild populations and bred cultivars indicates that the limited number
of generations of selection in breeding current switchgrass cultivars has not created
significant bottlenecks to impact the overall genetic diversity within the cultivars.
Selection and breeding has likely impacted a relatively small number of genes
scattered throughout the switchgrass genome, preserving genetic variability at the
genomic level.
A reference genome sequence is not available for switchgrass, largely due to the
complexity and expense involved in sequence assembly of such a complex
polyploid. Current efforts by DOE Joint Genome Institute (JGI), Walnut Creek,
CA are focused on deep sequencing the parents and 192 progeny from the AP13 9
VS16 (Alamo 9 Summer) bi-parental cross. An existing linkage map of this
population will be used to localize genomic scaffolds to one of the 18 linkage
groups, creating a reference map that can be used to order and localize future
sequence data from other genomic resources of switchgrass, including bacterial
artificial chromosomes (BAC, [77]), expressed sequence tags (EST, [78]), and
exome sequences that are currently under development [27].
Marker-trait associations have yet to be specifically identified in switchgrass.
The AP13 9 VS16 bi-parental cross has undergone phenotypic evaluation for
several morphological and agronomic traits in southern Oklahoma, but data have
yet to be analyzed at the time of this writing. Two association panels of switch-
grass were assembled and established in field studies. A northern panel, consisting
largely of upland accessions, consists of 10 genotypes each of 60 populations and
has been evaluated for single nucleotide polymorphic (SNP) markers and phe-
notypic traits at Ithaca, NY. A southern panel, consisting largely of lowland
accessions, consists of 10 genotypes each of 48 populations and has been evaluated
for phenotypic traits at Athens, GA and Ardmore, OK. Future plans will include
genome-wide association studies (GWAS) of marker-trait associations within each
panel and across the two panels [27].
Finally, genomic selection (GS) offers considerable potential to increase the
rate of gain for important traits such as biomass yield [79]. Genomic selection
offers two mechanisms to increase selection efficiency and rate of gain. Once a
training population of genotypes and families is established, predictive equations
are developed and validated to predict phenotype from genotype. This predictive
equation can be applied to seedlings to apply indirect selection pressure to
important agronomic traits prior to establishment of field-based nurseries, dra-
matically increasing the proportion of genetic variation utilized in selection [80].
Second, recurrent selection could be altered to eliminate the field-based evaluation
in some cycles, relying on short-term maintenance of linkage disequilibrium in the
population following only one or two recombination events. For example, Cycle 1
could consist of a field evaluation of biomass yield, equation development and
validation, and within-family selection. Cycle 2 (and possibly Cycle 3) could
proceed with simple phenotypic selection using the predicted breeding values
based on seedling DNA marker analyses, prior to returning to the field for another
cycle of field evaluation and recalibration of the predictive equations. Seedling
selection offers huge time advantages because it can be accomplished within

1 year, as opposed to a minimum of 5 years for a typical field-based selection
cycle. Validation and computer simulation studies will be critical in determining
the number of seedling selection cycles, if any, that can be utilized to speed up the
selection process.
Current efforts in Wisconsin USA are focused on development of three training
populations: an upland population selected for biomass yield, a lowland population
selected for winter survival, and a hybrid population. All populations are struc-
tured as half-sib families and the basic selection method will involve selection
among families for field-based biomass yield in plots replicated across locations
and years and selection of seedlings within families for breeding values predicted
from DNA markers. If the accuracy of breeding-value prediction is [0.3, GS with
10% within-family selection intensity has [35% higher expected gains than any
phenotype-based selection protocol [80]. If genotyping costs are sufficiently low to
increase within-family selection intensity to 1%, expected gains for GS are 70%
greater than phenotypic selection.
2.5.4 Genetic Engineering and Risk Assessment
Switchgrass can be transformed with the addition of specific functional genes from
other organisms using one of two methods. Agrobacterium-mediated transforma-
tion uses a biological vector whereas particle bombardment uses a non-biological
vector to insert new genes into the switchgrass genome [81]. Agrobacterium-
mediated transformation tends to result in lower copy number and fewer genomic
rearrangements than particle bombardment [82–84]. While efficient, high-
throughput genetic transformation systems have been developed for switchgrass,
these systems are currently genotype-dependent. Certain genotypes are more
responsive to both the tissue culture and transformation phases of this process
(Fu and ZY Wang, 2011, personal communication), creating a need for more
genotype-independent methodologies.
Genetic transformation of switchgrass to reduce recalcitrance of biomass for
conversion to energy has received the greatest amount of attention. Manipulation
of one gene is sufficient to create measurable and significant changes to cell-wall
composition and structure, impacting sugar release and downstream processing of
biomass to energy [85–87]. Reductions in lignin concentration or modifications to
lignin structure can positively impact the availability of cell-wall carbohydrates in
a fermentation system to produce liquid fuels and in a livestock-production system
where switchgrass is a livestock feed [50]. Such changes not only increase energy
that is available to microorganisms that conduct fermentation, but also create
opportunities to significantly reduce input costs of production, allowing reduced
pretreatment severity and enzyme requirements in the case of biofuel fermentation
systems [85].
48 M. D. Casler
Genetically modified (GM) organisms have traditionally been highly regulated,

requiring several years of extensive testing and risk assessment studies [88].
Scientific issues that have potential to impact the regulatory process include the
effects of transgenes on plant fitness under a wide range of environmental con-
ditions and their potential to be transmitted to non-transgenic switchgrass. In the
case of switchgrass, transmission can occur via either pollen or seed. Pollen-flow
studies have not been conducted on switchgrass, but pollen has been known to
travel as far as 21 km in Agrostis [89]. Seed is literally unbounded in its ability to
travel across the landscape, facilitated by birds, mammals, and humans [8]. Risk
assessment is further complicated by the need for most assessments to be designed
specifically for the gene in question (related to its function in the plant and both
proposed and unintended potential functions in the environment), the biology of
the species, the likelihood and frequency with which natural stands of compatible
relatives exist within the agricultural range of the species, and the range of
environmental conditions under which the species will be utilized [90].
Based on its broad natural range, the existence of thousands of native prairie
and savanna remnants, and the range of management systems and environmental
conditions under which switchgrass can be grown for biomass or forage, dereg-
ulation of any switchgrass transgene is guaranteed to result in its dissemination and
introduction into natural populations. Switchgrass is a wild plant that still contains
many traits common to wild species, including seed dormancy, seed dispersal by
shattering, variability in flowering time within individual panicles and plants, and
small seeds that can easily escape containment, each of which can contribute to
switchgrass seed dispersal and viability over space and time. The scientific com-
munity is highly dichotomous over the potential implications of genetic
improvement of switchgrass, the agricultural community generally in favor of
speeding up domestication and incorporating useful traits (e.g. [85]) and the
ecological community more cautious and concerned about limiting impacts outside
of agricultural fields (e.g. [91]). In the USA, genetically modified switchgrass is
regulated by the Animal and Plant Health Inspection Service (APHIS) of the
USDA. APHIS is currently experiencing pressure from the U.S. DOE which has
invested millions of dollars into development of new GM concepts and technol-
ogies, aimed at improving production and processing of switchgrass biomass for
energy production.
2.6 Conclusions
Genetic resources of switchgrass are vast and untapped. Modern cultivars repre-
sent no more than five or six cycles of selection removed from wild germplasm,
insufficient to create significant genetic bottlenecks. Rates of gain have historically
been modest, largely focused on one or two principal traits and likely based on
relatively low numbers of genes. As such, switchgrass is still an undomesticated
plant with vast potential for improvement of agronomic and biofuel traits.
Huge stores of genetic variability exist for adaptive traits such as pest resistances,
stress tolerances, biomass yield and quality traits, and phenological traits, pro-
viding a basis for utilizing genetic resources from a broad geographic area to
generate highly targeted improvements within regions suitable for biomass pro-
duction. Genomic resources are rapidly being developed to create opportunities for
increasing selection efficiency and rate of gain. Development of a formal
switchgrass research community, already underway in 2010, is expected to
improve communications among researchers of diverse interests and disciplines
and provide mechanisms for researchers to keep up with rapidly developing and
changing technologies.
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23:433–442
Chapter 3
Crop Physiology
Walter Zegada-Lizarazu, Stan D. Wullschleger, S. Surendran Nair

and Andrea Monti
Abstract In this chapter, we review the physiology of switchgrass from seed

dormancy till the effects of water and nutrients stress on grown plants. These
characteristics are presented and discussed mainly at the canopy and whole-plant
level with emphasis on the agro-physiology of the species in view of the possible
contribution of crop physiology to agricultural development. Switchgrass is noted
for the variable degrees of seed dormancy regulated by endogenous and exogenous
factors that determine the successful seedling establishment. Plant growth rates are
determined by temperature while the reproductive phase is controlled mainly by
photoperiod. There is also evidence that some physiological attributes, such as
photosynthesis, transpiration, and water use efficiency differ between tetraploid,
hexaploid, and octoploid ecotypes. But despite these differences, in general
switchgrass combines important attributes of efficient use of nutrients and
water with high yields thanks to its ability to acquire resources from extended
soil volumes, especially at deep layers. Moreover at canopy level, resources
capture and conservation are determined by morpho-physiological characteristics
W. Zegada-Lizarazu (&) A. Monti

University of Bologna, Bologna, Italy
e-mail: walter.zegadalizarazu@unibo.it
A. Monti
e-mail: a.monti@unibo.it
S. D. Wullschleger
Environmental Sciences Division,
Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
e-mail: wullschlegsd@ornl.gov
S. Surendran Nair
Agricultural and Resource Economics, University of Tennessee,
2621 Morgan Circle Drive, Knoxville, TN 37996, USA
e-mail: ssurendr@utk.edu

56 W. Zegada-Lizarazu et al.
(C4 photosynthetic pathway, stomatal control of transpiration, high leaf area index,
low light extinction coefficient) that enhance radiation use efficiency and reduce
carbon losses. However, specific information on switchgrass physiology is still
missing, in particular deeper understanding of physiological principles controlling
the water and nutrients acquisition mechanisms and allocation under suboptimal
growing conditions. The physiology of tillering and root respiration are also fac-
tors that need further investigation.
3.1 Introduction
Switchgrass (Panicum virgatum L.) is a perennial grass of temperate zones that has
evolved into a forage crop and more recently into an energy crop. The species
originates from the central plains of North America as a component of the tall
grass prairie [1]. Basically switchgrass is a warm season, deep-rooted, photosen-
sitive, C4-type metabolism species with a high adaptability to a wide geographical
range and soil types. Considering habitat preferences and other plant character-
istics, switchgrass is classified into upland and lowland ecotypes associated with
latitudinal origin (northern and southern ecotypes) [2, 3]. In general switchgrass
presents a high interspecific variability in physiological characteristics, with its
own response adaptations to photoperiod, temperature, water logging or drought,
and other stresses. These traits influence the carbon fixation efficiency and
therefore crop production potential.
Surprisingly, a comprehensive review on switchgrass physiology as a crop
species is lacking. In most of the cases, switchgrass physiology is covered along
that of other forage grasses. Sanderson et al. [4], for example, provide a general
overview of the morphological and physiological response to stress of forage
grasses, but specific information on switchgrass is limited. Moreover, in a 10-year
research program designed by the US Department of Energy to evaluate and
develop switchgrass as an energy crop and that involved a large network of research
sites, universities, laboratories, and US Department of Agriculture facilities,
only two institutions were listed as interested in switchgrass physiology [5].
Hence, most of the information available on switchgrass physiology comes from
previous studies that focused on its forage end use. Topics were wide ranging from
seed dormancy physiology to the crop growth determinants. Across the world there
is an increasing interest in switchgrass as a multipurpose crop species, but the
establishment of a permanent switchgrass physiology program as an important part
of agricultural research is needed.
In this chapter, we review the physiology of seed dormancy, seedling estab-
lishment, above- and belowground biomass development, resource use efficiency,
and the effects of water and nutrient stress. These characteristics are presented and
discussed mainly at the canopy and whole-plant level with emphasis on the agro-
physiology of the species in view of the possible contribution of crop physiology
to agricultural development. Organ and cellular levels are not presented here.
3 Crop Physiology 57
3.2 Seed Germination and Seedling Establishment
3.2.1 Physiology of Seed Dormancy
Switchgrass seeds, even within the same lot of seeds, have variable degrees of
dormancy, which is an optimum strategy to survive in the wild and trough periods
of environmental stress but at the same time is a major obstacle for its wide-spread
cultivation as a forage or biomass crop [6, 7]. At harvest, more than 90% of the
seeds of some cultivars could be dormant [6, 8]. Although dormancy declines
naturally with time, the mechanisms of dormancy in switchgrass are not well
understood. Several authors indicate that a combination of physical and physio-
logical factors may be involved and, to a lesser extent, morphological factors too
[9–13]. The seed coat is in part responsible for switchgrass dormancy. These outer
layer coverings act as barriers for water and oxygen uptake, produce and encap-
sulate germination inhibitors, modify the light reaching to the embryos, and act as
physical barriers that inhibit germination [10].
Since the embryos of switchgrass are fully developed at harvest time, it is
suggested that after-ripening is not a major factor inducing dormancy break in
switchgrass [9, 13]. The aforementioned authors reached such a conclusion based
on their observations that after injuring, cutting, or completely removing the
embryos, endosperms, and/or the seed coat, unspecified germination inhibitors
were released, thus the germination percent increased significantly. Hence, pri-
mary dormancy in switchgrass may not be related to underdeveloped embryos but
to dormancy mechanisms within the embryo and the required quiescence period of
the seeds [6, 9, 13]. Under natural conditions this period could last months if not
years [7, 13]. In order to accelerate the decay of dormancy, several artificial methods
can be used. Mechanical and chemical scarification of switchgrass seeds, for
example, resulted in 73% and 61% increased germination, respectively [10, 14].
Such a process may weaken the fiber tissue in the lemma, allowing more gas
exchange and water uptake, and eliminate or weaken the physical barrier posed by
the lemma and palea that impede the embryo expansion [10].
Impermeable membranes to oxygen but permeable to water in switchgrass
seeds seem to be also responsible for dormancy. Under suboptimal temperatures,
these membranes prevent the respiration of the stored energy within the seed and
therefore delay or inhibit germination. However, by exposing seeds to cool tem-
peratures and moisture (stratification), oxygen can be absorbed by the seeds and
therefore dormancy reduced or broken. For example, Sanderson et al. [8] indicated
that naturally (e.g., cool and wet conditions prevail in early springtime) and
artificially stratified seeds germinated well and provided good stands and yields.
Moreover, Shen et al. [6] showed evidence that germination of Cave-in-Rock
switchgrass seeds could be increased up to 80% within a 14-day stratification
period at 5°C. However, dormancy break by stratification is not straightforward
and unidirectional process, and it depends on undetermined factors within the
seed and the surrounding environment. Shen et al. [6] indicated that stratified
switchgrass seeds could become dormant again after the seeds are dried for
mechanical planting. This tendency toward secondary dormancy was termed as
reversibility. The authors suggested that this reversibility was linked to a physi-
ological continuum that the seeds enter when the right environmental conditions
occur for germination. In case such conditions do not occur or stop, the seed goes
dormant again, which is called by some authors residual dormancy [12]. However,
reversibility depends on the degree on which dormancy was removed. Shen et al.
[6] suggested that a 42-day period of stratification would be enough to completely
remove dormancy, but the length of such a period will depend on how well the
seeds were after-ripened or aged, as stratification and after-ripening have additive
effects. Moreover, residual dormancy is responsive to the modification of endog-
enous levels of nitric oxide (NO) and/or reactive oxygen species (ROS) [12].
In general, environmental stresses (e.g., drought, temperature, light, etc.) can cause
mutations in the genes responsible for germination. Such mutations can be asso-
ciated with the degree of seed maturation and with the biosynthesis of dormancy-
regulator hormones [15, 16].
Toward seed maturity, the concentration of germination inhibitors, such as
abscisic acids (ABA) in switchgrass, as in other dormant grass species, remains
high in comparison to growth promoters, such as gibberellic acid (GA); therefore,
dormancy prevails [17, 18]. The balance between ABA and GA seems to be
affected by endogenous and micro-environmental factors; however, the mecha-
nisms of such shifts are not fully understood [12]. These authors, however,
indicated that NO and ROS are important reactive pathways to perceive ABA.
Then these receptors and cognate proteins drive signaling cascades resulting in
biological outputs [17, 18]. For example, exogenously applied ROS, as H2O2,
inhibited the effects of ABA probably by overriding the ABA-dependent signals,
resulting in enhanced switchgrass germination [17, 18]. Even though the exog-
enously applied NO was not able to overcome the ABA-dependent signals,
probably because of a NO scavenger, Sarath et al. [11, 17, 18] indicated that high
levels of endogenous NO are required for germination. Exogenously applied H2O2
may stimulate the production of endogenous NO in the aleurone layer, the main
site for NO synthesis in switchgrass [17, 18], and therefore overcome the ABA
inhibition of germination.
3.2.2 Seedling Establishment
Seedling establishment comprises the germination and emergence phase, and the
adventitious root development phase [19]. A seed is considered germinated and
emerged when the radicle protrudes from the seed coat and when the coleoptiles
become visible; radicle extension precedes the coleoptile emergence [11, 12].
A rapid initial development of roots enables seedlings to acquire the necessary
water and nutrients for growth. When the coleoptile emerges from the soil surface,
subcoleoptile internode elongation stops, the coleoptile opens, and shoot growth
Fig. 3.1 Switchgrass

seedling showing the
placement of the crown, sub-
coleoptile elongation zone,
and seminal roots. Photo by
Andrea Monti
and chlorophyll synthesis begins ([20]; Fig. 3.1). The speed and rate of germi-
nation and emergence are affected by environmental factors such as water, tem-
perature, and light. In general, it is indicated that the base temperature for
germination is between 8.1 and 10.3°C, and optimum is between 25 and 30°C,
with maximum germination occurring after 72 h of imbibition [21, 22]. Maximum
temperature for germination may be as high as 45°C [22], but all of these con-
ditions seem to be cultivar dependent. Germination rate is affected by the degree of
dormancy, imbibition rate, and respiration, which are temperature-dependent
factors, while maximum seed germination and emergence is mainly affected by the
degree of water uptake [22, 23].
Since the seeds of switchgrass are very small (Table 3.1), the amount of water
required for germination is also very small, especially at the hydration phase.
The water requirements will increase as the seedling develops and its juvenile root
system (composed of the primary root, seminal roots, and subcoleoptile internode
roots) starts growing and functioning [24]. Radicle protrusion coincides with
radicle emergence, which is characterized by short duration. Some authors con-
sider it to be a negligible part of a switchgrass seedling [25]. However, it plays a
fundamental role in the early establishment phase of the young seedling, a role that
has not yet been clearly defined [24]. Proper soil moisture is essential at this stage
Table 3.1 Ploidy levels, origin and average seed weight of the principal cultivars of switchgrass
Cultivar Ecotype Ploidy Origin of Maturity Seed weight References
level germplasm (mg 100
(latitude) seeds-1)
Alamo Lowland Tetraploid Texas (28°) Very late 94 [35, 36, 119]
Blackwell Upland Octoploid Oklahoma (37°) Mid/late 142 [35, 36, 119]
Caddo Upland Octoploid Southern Great Late 159 [35, 36, 119]
Plains (35°)
Carthage Upland Octoploid North Carolina Late 148 [100, 119]
(35°)
Cave-in- Upland Octoploid Illinois (38°) Mid/late 166 [35, 36, 119]
Rock
Dacotah Upland Tetraploid North Dakota (46°) Very early 148 [35, 119]
Expresso Lowland Tetraploid Missisippi ? ? [22]
Forestburg Upland Octoploid South Dakota Early 146 [35, 119]
(44°)
Kanlow Lowland Tetraploid Oklahoma (35°) Very late 85 [35, 36, 119]
Nebraska Upland Octoploid Nebraska (28°) Early/mid 162 [36, 119]
28
Pangburn Lowland Tetraploid Arkansas (34°) ? 96 [36]
Pathfinder Upland Octoploid Kansas (40°) Mid/late 187 [35, 36, 119]
Shelter Upland Octoploid Virginia (40°) Mid 179 [35, 36, 119]
Stuart Lowland Tetraploid Florida (29°) Late ? [36]
Summer Upland Tetraploid Nebraska (41°) Late/mid 114 [35, 36, 119]
Sunburst Upland Octoploid Dakota (44°) Mid 198 [35, 36]
Trailblazer Upland Octoploid Nebraska (40°) Mid 185 [35, 36]
Tusca Lowland Tetraploid Mississippi ? ? [22]
Wabasso Intermediate Tetraploid Florida (27°) Very late ? [36]
NL 93-1a Lowland Tetraploid ? ? 121 [42]
NU 94-2a Upland Octoploid ? ? 173 [42]
SL 93-2a Lowland Tetraploid ? 87 [42]
SL 93-3a Lowland Tetraploid ? ? 140 [42]
SL 94-1a Lowland Tetraploid ? ? 142 [42]
SU 94-1a Upland Octoploid Oklahoma ? 183 [42]
a
Cross from different genotypes of diverse origin.
Source Alderson and Sharp [119]; Gunter et al. [35]; Hopkins et al. [36]; Stout et al. [100]; Seepaul
et al. [22]; Taliaferro and Hopkins [42]
as the establishment of a viable seedling is not yet ensured. However, information

on switchgrass water requirements for germination and seedling establishment is
scarce. The few data available indicate that under drought stress (-0.3 MPa) only
2.5% of all sowed seeds emerged, representing 5% survival of the germinable
seeds [26]. Hence, these results suggest that the lower limit of moisture availability
for switchgrass germination might be around -0.3 MPa, regardless of the sowing
depth and soil texture.
Adequate soil surface moisture is also essential for the formation of adven-
titious roots. The long-term survival of the seedling will be determined by the
development of robust adventitious roots as they will become the major root
system of the seedlings ([1, 19]; Fig. 3.2). In any case, Smart and Moser [27]
suggested that few long adventitious roots that reach moist subsurface soil layers
are enough for the successful early establishment of seedlings. Moreover,
Newman and Moser [19] showed that in switchgrass the number of adventitious
roots increased rapidly between 4 and 8 weeks after planting following 4 or
more days of consecutive rain. However, during the first 4 weeks after planting,
there were few adventitious roots even under adequate soil surface moisture
conditions. In fact, switchgrass starts to develop adventitious roots by the third-
leaf stage [24]. After this period, water flow may be preferential to the growing
shoot, as it is suggested to happen in blue grama [28]. Xu et al. [29] found that
switchgrass seedlings (fifth- to sixth-leaf stage) exposed to continuous soil
dehydration increase by 11% their allocation of carbohydrates to the roots. Such
a change in carbon partitioning may be a useful strategy for the seedling
survival.
Adventitious roots develop in clusters from the coleoptilar node or seedling
crown ([19]; Fig. 3.2). Since the crown node is pushed to the soil surface by the
elongating subcoleoptile internode until a certain light level is sensed, the location
of the seedling crown and that of the adventitious roots will be close to the soil
surface regardless the sowing depth [19, 20]. At greater soil depths, moisture and
temperature conditions are more favorable for the successful development and
functioning of the adventitious roots, which in turn will secure the seedling
establishment and survival. On the other hand, if the seedling’s crown is at or close
to the soil surface, its exposition to faster soil desiccation may be abortive or
limiting for the development of adventitious roots, especially in drier environ-
ments [1, 19]. In general, switchgrass has excessive crown node elevation, which
makes its successful establishment difficult. However, Elbersen et al. [20] dem-
onstrated that populations with low crown placement can be selected and that this
trait is heritable. Such genotypes have shorter subcoleoptile internodes which
facilitate water flow toward the coleoptile and transpiring leaves and therefore
accelerate emergence and establishment. Thus, these genotypes are better able to
withstand drought conditions before adventitious roots are developed [30]. In fact,
in field trials with alternating wet and dry periods, it was shown that the selected
genotype for low crown placement had greater seedling germination and emer-
gence rates [30].
Selection for seed size can also improve switchgrass seedling establishment.
Several authors indicated that larger seeds accelerated the germination, emer-
gence, growth rates, and development of adventitious roots, but all these advan-
tages associated with seed size were no longer evident at later growth stages, even
when soil moisture was suboptimal [1, 27, 31, 32].
Fig. 3.2 The origin of first-

and second-generation tillers,
and formation point of
adventitious roots and
rhizomes. Photo by Andrea
Monti
3.3 Canopy and Root Development
3.3.1 Phenological Stages
Based on habitat, morphological characteristics, and ploidy level, switchgrass has

been classified into two ecotypes: lowland (mainly grown in lower and wetter
environments) and upland (mainly grown in mesic environments). Each ecotype is
further subdivided according to its geographical origin into southern and northern
ecotypes ([33, 34]; Table 3.1). Casler et al. [2] indicated that latitude is the largest
determinant of switchgrass productivity and survival. Lowland ecotypes are usu-
ally tetraploid (2n = 4x = 36 chromosomes), while the upland types range from
tetraploid to hexaploid (2n = 6x = 54 chromosomes) to octoploid (2n = 8x = 72
chromosomes) [35–38]. This large genetic diversity result in morphologically and
physiologically different plants. In general, the lowland ecotypes are taller, with
thicker stems, longer bluish-green leaves, have larger panicles, and produce higher
yields but have lower initial growth rates than upland ecotypes [1, 39]. When
lowland/southern ecotypes are moved to northern latitudes, they produce higher
biomass yields because they remain vegetative for a longer period of time with a
longer photosynthetically active period, but they may not reach maturity and form
seeds. In fact, in order to survive the winter, switchgrass should acquire adequate
dormancy and translocate storage carbohydrates to the roots, but due to the
extended photoperiod, southern ecotypes may start this process too late when
moved to northern latitudes, and their survival may be compromised. Moreover,
the harvested biomass from immature plants can have high moisture and ash
contents due to the partial translocation of nutrients to the rhizomes, which is
unfavorable for energy purposes. On the other hand, upland/northern cultivars
moved southward have lower biomass yields and are more susceptible to diseases
[2, 33, 40, 41] because they are exposed to shorter-than-normal days during the
summer, which leads to early flowering. So far, hybridization attempts between

these two ecotypes have been unsuccessful [42], and therefore at the moment, the
functional advantages of one ecotype cannot be introduced into the other. In order
to study the growth and development of switchgrass as function of environmental
and management variables, descriptive indices of its phenological stages have
been developed (Table 3.2). The general method used for the study of perennial
grasses proposed by Moore et al. [43] and specifically adapted by Sanderson [34]
for switchgrass indicates five main phenological stages: (a) emergence, (b) veg-
etative/leaf development (c) stem elongation, (d) reproductive/floral development,
and (e) seed development and ripening. Each stage is sub-divided into sub-stages,
which are identified by numerical codes that go from 0.5 to 35. These codes
provide a descriptive attribute of the physiological and ecological status of the
plant and enable researchers to quantitatively assess the successive growth stages
and statistically determine deviations from the normal growth. Under natural
prairie conditions, seed shattering and complete dormancy could also be consid-
ered as additional growth stages during the life cycle of switchgrass [40]. In
switchgrass as in other perennial grasses, the vegetative growth stages are discrete,
so leaf growth continues even when the stem elongation stage has started [44].
Moreover, the floral development starts when there are still some leaf primordia
and unemerged leaves on the apex [45].
3.3.2 Growth Analysis
The physiological development of switchgrass is typical of that of perennial

grasses and follows the general growth sequence indicated above (Table 3.2).
Al-though the duration of each stage is cultivar dependent, the vegetative growth
of switchgrass (including leaf development and internode elongation) is strongly
influenced by temperature [46]. Sanderson and Wolf [44], for example, indicated
that when Cave-in-Rock (northern/upland) and Alamo (southern/lowland) culti-
vars that are grown close to their place of origin require 200 and 430 growing
degree-days (GDD) above a base temperature of 10°C to complete the leaf
elongation stage and 378 and 1,020 GDD for the stem elongation phase. On the
other hand, the reproductive phase of switchgrass is controlled mainly by the day
length (photoperiod), regardless of temperature and moisture availability [41],
suggesting a facultative short-day response, though the influence of other factors
cannot be excluded completely. The effect of photoperiod extension was studied
by Van Esbroeck et al. [41] on two switchgrass cultivars; when Cave-in-Rock was
subjected to long days (16 hr), panicle emergence was delayed by 18 days and the
time for panicle exsertion was increased by 243% as compared to the control
treatment (12-hr photoperiod). Such delay was associated with an increase in the
phyllochron (the time between the appearance of two successive leaves) and in
leaf size, while the number of leaves was not altered. In the case of the Alamo
cultivar, the number of leaves and their size decreased. In both cases, however, the
64
Table 3.2 Descriptive indices of switchgrass phenological stages based on Moore et al. [43] and Sanderson [34]
Moore et al. Sanderson
Growth stage1 Index Description Index Description
G0 0.0 Dry seed
G1 0.1 Imbibition
G2 0.3 Radicle emergence
G3 0.5 Coleoptile emergence
G4 0.7 Mesocotyl and/or coleoptile elongation
G5 0.9 Coleoptile emergence from soil 0.5 Emergence
V0 1.0 Emergence of first leaf 1–10 Leaf development
V1 (1/N) ? 0.9 First leaf collared
V2 (2/N) ? 0.9 Second leaf collared
Vn (n/N) ? 0.9 Nth leaf collared
E0 2.0 Onset of stem elongation 11–19 Stem elongation (n. internode [ 1 cm Es 14 = 4 internode [ 1 cm)
E1 (1/N) ? 1.9 First node palpable/visible
E2 (2/N) ? 1.9 Second node palpable/visible
En (n/N) ? 1.9 Nth node palpable/visible
R0 3.0 Boot stage 20 Boot stage
R1 3.1 Inflorescence emergence/lst spikelet visible
R2 3.3 Spikelets fully emerged/peduncle not emerged 21–29 Percent of inflorescence visible
R3 3.5 Inflorescence emerged/peduncle fully elongated 30 Spikelets visible with peduncle
R4 3.7 Anther emergence/anthesis 31 Beginning of anthesis
R5 3.9 Post-anthesis/fertilization 32 End of anthesis
S0 4.0 Caryopsis visible
S1 4.1 Milk 33 Milk/dough stage
S2 4.3 Soft dough
S3 4.5 Hard dough
S4 4.7 Endosperm hard/physiological maturity 34 Physiological maturity
S5 4.9 Endosperm dry/seed ripe 35 Seed shattering
1
G0–G5, Germination and emergence; V0–Vn, Vegetative-Leaf development; E0–En, Elongation-Stem elongation; R0–R5, Reproductive-Floral development; S0–S5,
Seed development and ripening
W. Zegada-Lizarazu et al.
duration of the panicle exsertion was extended. Then, an increase in the duration of
the panicle development could maximize seed production, but its effects on bio-
mass accumulation remain unclear. On the other hand, early flowering results in
fewer leaves, reduced photosynthetic capacity, and lower yields.
In switchgrass, the leaf appearance rate (LAR) is somehow also related to
photoperiod. The LAR decreases when days are long and increases when days are
short. The faster LAR is associated with a short period between floral initiation and
floral emergence [41, 45], thus reducing the vegetative period and potential bio-
mass yield. In early maturing cultivars from northern latitudes, the phyllochron
was almost double than that of southern late cultivars, suggesting an important role
of latitude in controlling maturity time [45]. In general the lamina extension rate
(LER) range from 0.20 to 0.30 cm GDD-1, with the longest leaves located near
the middle of the canopy up to the seventh leaf. Even though leaf growth continues
until the leaf collar has emerged [47], when panicle development begins (around
1,000–1,200 GDD) LERs decline and shorter leaves are formed on the top [45],
probably due to the increased sink force of the emerging panicles. Among several
switchgrass cultivars, the final number of leaves on spring-emerged tillers range
from 9 to 11, while from summer-emerged tillers the range was from 6 to 8 [45].
The same authors indicated that leaf formation in spring tillers, and thus biomass
accumulation, continues until environmental conditions induce floral development.
Flowering in switchgrass is induced by decreases in day length following the
summer solstice. However, the photoperiod requirements of the diverse cultivars
change depending on the latitude of origin of each ecotype.
Beaty et al. [48] indicated that switchgrass tillers behave as true biennale tillers;
that is, the first-year tillers remain as rhizomes buds. Then in the coming spring,
when temperatures are adequate, a flush of tillers emerges (Fig. 3.2). The physi-
ological mechanisms responsible for tillering initiation in switchgrass have not
been fully studied. Perhaps, as is suggested for other perennial grasses, the
antagonistic actions of hormones such as auxin produced in the apical meristems,
and cytokinin and strigolactones produced in the roots, together with resource
availability (e.g., nutrients, water) and photosensitivity to red and far red light play
an important and decisive role in the growth of axillary meristems [49], but spe-
cific information on such mechanisms is still lacking. Lower internodes begin to
elongate after some leaves have been produced and continue until the inflores-
cence has emerged [48, 50], at which stage the carbohydrate reserves in the stem
bases are the lowest [51]. Elongation rates can range from 1.4 to 2.8 cm d-1
depending on the cultivar and environmental conditions, with the more southern-
origin ecotypes having greater growth rates [2, 46]. Upland ecotypes can reach
1.5–2 m in height, while lowland ecotypes are 3–4 m tall [52]. In general, tiller
density during the vegetative growth stage is high but declines with advancement
towards the following growth stages [53]. The final tiller density is, however,
highly variable in number and physiological stage, depending on cultivar and
environmental conditions. For example, 3-year-old stands of Cave-in-Rock and
Dacotah cultivars, grown between 43o N (Arlington, WI) and 44o N (Brookings,
SD) in the USA had tiller densities of 677 and 1,355 tillers m-2, respectively,
while the reproductive tiller fraction, averaged across cultivars, was 81 and 8% at
Arlington and Brookings [54].
The tillering capacity is an important component of switchgrass plasticity and
its ability to respond to environmental stimuli in time and space. The general
issues concerning the physiology of tillering in crop plants may apply to switch-
grass, but specific information is not available. For example, in dense grass can-
opies, tiller elongation rate is stimulated and apical dominance is enhanced due to
the low red–far red light ratio and low blue light perceived by the phytochromes
[55]. However, information on this subject is still lacking in the case of
switchgrass.
In addition to the genotype, photoperiod, and temperature, other factors such as
plant density, irradiance, water, and nutrition may influence tiller initiation. Muir
et al. [56] indicated that nitrogen fertilization and row spacing have a direct effect
on the number of productive tillers, which play an important role in increased
productivity. However, the same authors indicated increased tiller mass rather than
an increased number of tillers is the main mechanism by which biomass yield is
increased. Moreover, as in other grasses, new switchgrass tillers may obtain water
from the mother plant until functional adventitious roots are developed; otherwise,
the new tiller may die [57].
3.3.3 Root Growth and Function
Initial root growth of switchgrass is very rapid, especially during the first 3 weeks
after sowing, and then slows down gradually but remains the major carbohydrate
sink at least for the first 15 weeks after sowing [58]. The root-to-shoot ratio of
3-week-old seedlings was 5.5, while at 15 weeks of age the ratio was reduced to
2.0 [58]. Such a large allocation of carbohydrates and rapid initial root growth rate
are fundamental to the successful establishment of switchgrass. The C4 physiology
of switchgrass may allow a large and well-structured root system to develop that
would ensure more active and efficient acquisition of soil resources and increase
the nutrient storage capacity. Unfortunately there is no information on the root
growth patterns and carbon partitioning of older plants, especially at what growth
stage the tillers would become the major carbon sink. Actually most of the
information available is for plants that have already reached maturity (4 or more
years old), but in general mature plants follow a similar pattern to the one
described above. The whole function of the plant is then determined by the canopy
architecture and carbohydrate allocation [5]. For example, Garten et al. [59]
indicated that the root-to-shoot ratio of a 4-year-old switchgrass stand averaged
over four cultivars changed from 5.8 at the beginning of the growing season
(April) to 0.76 at mid season (July) and to 0.77 at the end of the season (October).
In contrast to the initial growth stages of switchgrass seedlings where most of the
seed reserves are allocated to the development of a vigorous root system, in the
case of mature plants a well-developed root system is already present where most
of the nutrient reserves, mainly N and nonstructural carbohydrates, are accumu-

lated and therefore able to sustain a rapid growth of the aboveground parts of the
plant in the following season. In mature plants, starch is the primary and most
dynamic nonstructural carbohydrate stored in the roots. Sucrose is secondary in
importance [60].
The dynamic root growth throughout a growing season is heavily influenced by
the annual harvest/clipping practices of the aboveground biomass, but typically the
root system of switchgrass continues to grow even until advanced autumn, prob-
ably due to the continuous production of rhizomes. In fact, the highest root bio-
mass production occurs at the end of the growing season (from midsummer to
autumn; [59]), while aboveground biomass production is maximized from spring
to midsummer. In central Iowa, the largest mass of fine (0–2 mm diameter) and
probably small roots too (2–5 mm diameter) were found between August and
October at 6- and 7-year-old plantations, while the lowest root mass was found in
May [61]. Similarly, Xu et al. [62] found uninterrupted growth of root mass
throughout the season (from April to November) and throughout the whole soil
profile (from 0 to 1.50 m depth) in a 5-year-old switchgrass plantation in North-
west China. These observations suggest that an increased root system of switch-
grass is not limited to shallower layers and that root growth terminates well beyond
the flowering stage.
Roots of mature switchgrass plants can reach more than 3 m in depth [63], but the
bulk of the roots is commonly found in the upper 1 m of the soil profile [61, 63, 64].
In a few of the studies available that measured the root length density (RLD) of
switchgrass, Monti and Zatta [64] reported a RLD of 311 cm cm-3 and that only
35% of the root mass was located in the top 0.35 m of the soil, while at lower layers,
up to 1.2 m, roots were more uniformly distributed. In the study of Ma et al. [63],
however, more than 68% of the roots were found in the top 0.15 m of the soil.
Similarly, Bolinder et al. [65] indicated that at least 78% of the roots were located in
the top 0.15 m of the soil at the peak standing crop growth stage. Garten and
Wullschleger [66] indicated that up to 94% of coarse root mass ([2 mm diameter)
was located in the upper 0.4 m of soil. These results differ from those of Monti and
Zatta [64] possibly due to the different soil types and cultivars used in each study.
Soil respiration at a switchgrass plantation, as with any other species, can be
related to live roots that directly contribute to soil respiration and dead roots and
exudates that provide energy and nutrients for microbial respiration. However,
there is limited information on switchgrass root respiration and turnover rates.
Tufekcioglu et al. [67] and Frank et al. [68] reported similar soil respiration
patterns throughout the growing season of several switchgrass cultivars; they
concluded that soil respiration increased rapidly from winter to midsummer and
then decreased rapidly toward the autumn. Such seasonal changes were highly
related to temperature changes and, to a lower degree, to soil moisture changes.
Moreover, Tufekcioglu et al. [67] indicated that the annual soil respiration rates
were strongly related to the production fine roots (\2 mm) and soil organic car-
bon, suggesting that the large resources allocated to produce an extensive fine root
system not only increase the resource capture capacity of the plant but also the soil
carbon inputs through root turnover. Frank et al. [68] indicated that about half of
the carbohydrates captured in plant biomass during a growing season is lost
through soil respiration. Garten and Wullschleger [69] estimated the range of
turnover to be between 2.4 and 4.3 years for particulate organic matter and
between 26 and 40 years for more recalcitrant mineral-associated organic matter.
Water uptake capacity and efficiency of switchgrass roots seem to be directly
related with RLD but independent of root distribution along the soil profile ([64];
Fig. 3.3). Thus water may passively move into switchgrass roots in response to
water potential gradients, rather than actively pumping solutes in order to create an
osmotic gradient, in the cell-to-cell pathway as the wetting front moves down-
wards. A general feature of drought-resistant crops is a deep root system which
facilitates access to deep-moist soil layers. Eggemeyer et al. [70], using the stable
isotopes of hydrogen and oxygen, determined the water sources of switchgrass in
the Sandhills grasslands of Nebraska. In agreement with the results shown in
Fig. 3.3, Eggemeyer et al. [70] found that throughout the growing season
switchgrass mainly extracts water from the upper 0.5 m of the soil profile. During
the dry period (August), they found that water uptake increased at deeper layers
but the amount acquired was insignificant in relation to the total amount of water
used by the crop. The reason for the limited contribution was not discussed, but
reduced hydraulic conductivity due to lignification of deep roots may be excluded
since Garten et al. [59] indicated that lignin concentration in deep coarse and fine
live switchgrass roots was lower than that in shallower layers. Hence, the amount
of available water at deep layers might be the limiting factor.
In general, switchgrass has low nutrient amendment requirements compared to
annual cereals, mainly because a pool of nutrients is recycled/conserved annually.
The long-lived rhizomes (up to 10 years) may act as sites of nitrogen and carbon
storage [71]. Several authors indicated that nutrients and nonstructural carbohy-
drates are translocated from the canopy to the crown/root system at the end of each
growing season (after anthesis but before killing frost) and vice versa during
resprouting [60, 72–74]. This is corroborated by Reynolds et al. [75], who found a
higher nitrogen concentration in aboveground biomass of diverse switchgrass
genotypes when harvested in mid season than when harvested during the fall.
Moreover, in the fall harvest more nitrogen was present in the aboveground bio-
mass in a double harvest system than in a single harvest system, probably because
in the double harvest system the re-grown tillers were at a younger stage, and
translocation of nutrients to roots was interrupted because they did not reach full
senescence [75]. In a single harvest system, Garten et al. [59] indicated that
nitrogen reserves in the root system declined to 1.4 g N m-2 due to acropetal
translocation during the period of fast growth of the canopy. On the other hand,
about 50% of the nitrogen fixed in the aboveground biomass was translocated to
the roots by the time the plants had become dormant [59]. Lemus et al. [72]
estimated that the total amount of nitrogen remobilized from roots to shoots and
vice versa may range from 40 to 100 kg N ha-1. Griffin and Jung [76] reported that
phosphorus levels in switchgrass and big bluestem decreased with maturity. They
found that stem tissue phosphorus content declined from an average of
Fig. 3.3 Root water uptake

efficiency coefficient
(k) determined as a function
of root length density and soil
moisture content at different
soil depths. The higher the
coefficient, the faster the soil
water depletion for a given
root. Standard errors are
indicated by the horizontal
bars
0.24–0.14% with increasing maturity. Smith and Greenfield [77] reported

the highest phosphorus accumulation in the inflorescence. Radiotis et al. [78]
reported 0.12% phosphorus in switchgrass tissues at the reproductive stage, and it
declined to 0.04% when it was left to overwinter. Seasonal recycling of phos-
phorus, potassium, and other nutrients follow more or less a similar pattern to
nitrogen [5, 73], but instead of translocation, the main recycling mechanisms may
be related to leaching from senesced/dead leaves within the switchgrass extensive
rooting zone. In either case, nutrients are returned to the soil during the winter,
helping to maintaining soil fertility and reducing fertilizer requirements.
3.3.4 Crop Modeling
Field trials for the herbaceous energy crop switchgrass are beginning to provide
valuable insights into the climatic, genetic, soil, and management practices that
govern the production of biomass for this species [79–81]. Bioenergy crop models
are a useful tool for summarizing information gained through field studies and for
understanding the potential supply, resource utilization, and environmental
impacts associated with the large-scale expansion of bioenergy crops [82].
Bioenergy models for many first- and second-generation energy crops,
including switchgrass, can be broadly classified into empirical and mechanistic
models. Empirical models are developed using statistical methods that establish
relationships between biomass yield and biophysical and agronomic variables.
Wullschleger et al. [83] developed an empirical biomass yield model for switch-
grass using 39 field trials conducted across the United States. A nonlinear para-
metric model was used to determine the relationship between biomass yields, with
physical, climatic, and management variables such as precipitation, temperature,
nitrogen fertilization, and ecotype (lowland and upland cultivars). Results showed
that lowland cultivars produced 1.5 times more biomass than did upland cultivars.
Temperature, precipitation, and nitrogen application during the season showed a

significant and positive effect on biomass yield. Jager et al. [80] further explored
the determinants of biomass yield in switchgrass cultivars by developing an
ecotype-specific empirical model based on a slightly expanded dataset from that
used by Wullschleger et al. [83]. The results showed that the responses to several
biophysical and management variables were different for lowland and upland
ecotypes. The impact of growing season temperature and precipitation on the
production of biomass was greater for lowland than for upland ecotypes. The
minimum winter temperature showed a positive response on biomass yield of both
ecotypes but was highly significant only for biomass yield of upland ecotypes.
Applied nitrogen also showed a positive and significant response on biomass yield
of lowland ecotypes. A significant and positive response to soil moisture was
found for upland but not for lowland ecotypes.
In contrast to empirical models, mechanistic models provide details of under-
lying physiological and morphological processes and their interactions on crop
yield. Two models in particular are widely used in switchgrass, ALMANAC and
EPIC. ALMANAC was developed to understand crop growth and yield across
varied environments by accounting for competition from other crops/weeds and
abiotic stresses [84], while EPIC was developed to account for the environmental
impact of production practices along with biomass yield estimation [85, 86]. EPIC,
which simulates switchgrass growth and development, is a process-based model
capable of simulating a wide array of ecosystem processes including plant growth,
crop yield, water and nutrient balances and soil erosion. ALMANAC is related to
EPIC in many ways; ALMANAC uses biophysical subroutines and process
descriptions from EPIC with additional details for plant growth processes and is
capable of simulating several crops including switchgrass [87]. Inputs for both
models are consistent with field measurements gathered for field crops including
leaf area index, radiation use efficiency, carbon gain and allocation, and pheno-
logical stages of development. Thus, crop models for switchgrass are relatively
easy to parameterize and interpret. Biomass produced under a range of scenarios
can be derived using EPIC or ALMANAC at individual sites [88, 89] or larger
spatial scales more suitable for regional analyses [82, 90].
Although mechanistic models require more extensive parameterization, they
are widely used to forecast yields at spatial scales from local to regional level and,
in some instances, to relate the production of biomass to other environmental
consequences including soil erosion and water quality. This information is
essential for developing a technically feasible, environmentally sound, and
economically viable supply of bioenergy. Models that allow users to obtain reli-
able estimates of bioenergy crop production provide an essential framework to
understand site-specific information that in turn can facilitate pragmatic decisions
regarding the suitability of certain regions where bioenergy crops can be
sustainably produced and any environmental benefits or consequences associated
with that production.
3.4 Resource Use Efficiency
3.4.1 Radiation Use Efficiency
Radiation use efficiency (RUE) can be defined as the biomass production per unit
light interception. Monteith [91] was the first to provide a strong and convincing
theoretical foundation for this parameter by demonstrating experimentally a robust
relationship between light interception and stress-free biomass production for
several agricultural crops. Since then, RUE has been considered a crop-specific
parameter and a widely used efficiency measure for comparing plant productivity
across different crops and management practices.
Field experiment-based RUE measurements for switchgrass are very limited,
and any available data covers only North America. In general, RUE is higher for
switchgrass as compared to other traditional cultivated crops. A mean RUE value
of 4.7 g MJ-1 intercepted photosynthetically active radiation (IPAR) was reported
for Alamo switchgrass and 3.7 g MJ-1 IPAR was reported for maize in Texas
[92]. This is not surprising because switchgrass possesses ideal qualities that
support high RUE, such as C4 photosynthesis, a high leaf area index, and a low
light extinction coefficient [93]. In general octoploid switchgrass cultivars have
higher leaf gas exchange rates than tetraploid ones (Table 3.3), and this is
attributed to the greater activity of RuBP carboxylase, PEP carboxylase, and NAD-
malic enzymes and concentration of biochemical constituents and smaller cell size
[94]. However, Wullschleger et al. [38] indicated that, more than ploidy level,
photosynthetic rates in either type are determined by seasonal changes and/or
water stress.
RUE measurements vary widely across switchgrass cultivars, growing loca-
tions, growing seasons, and management practices. Mean RUE value for Alamo
switchgrass ranged from 3.04 g MJ-1 IPAR for the high plains of Texas to
5.05 g MJ-1 for Missouri [93]. The same switchgrass cultivar had different RUE
values for two different growth periods: 3.2 g MJ-1 for 1995–1997 and
4.4 g MJ-1 IPAR for 2008–2010 [93, 95]. Madakadze et al. [96] reported that
RUE values vary across different upland switchgrass cultivars in Canada:
1.98 g MJ-1 IPAR was reported for the cultivar Sunburst to 2.38 g MJ-1 IPAR
for the cultivar Cave-in-Rock. Heaton et al. [97] showed that under North America
conditions (IL), RUE for Cave-in-Rock can be as low as 1.2 g MJ-1 IPAR. Kiniry
et al. [93] conducted a comparative study of different cultivars for RUE mea-
surements in Missouri. Results of this study revealed that mean RUE for the
cultivar Cave-in-Rock was 3.17 g MJ-1 IPAR, which is below the mean for the
lowland cultivar Alamo (4.3 g MJ-1 IPAR) but noticeably higher than the earlier
report for Cave-in-Rock from IL by Heaton et al. [79]. Management practices also
play a major role for different RUE values for the same switchgrass cultivar. Under
irrigated conditions, Alamo exhibited higher RUE as compared to the water deficit
condition, and the relative reduction in RUE under a water deficit environment was
greatest for fields with higher plant density than lower plant density [93, 95].
72
Table 3.3 Some physiological characteristics of switchgrass determined across the United Statesa and on the Loess Plateau of Chinab
Cultivar Photosynthesis (lmol Transpiration (mmol Stomatal conductance Instantaneous water use efficiency Dark respiration (lmol
m-2 s-1) m-2 s-1) (mol m-2 s-1) (lmol m-2 s-1) m-2 s-1)
Range of 25 17.5–30.8 6.2–13.0 0.16–0.30 2.08–3.77 1.76–2.24
cultivars[a]
Alamo[a] 27.90 8.2 0.23 3.60 1.98
Cave-in-rock[b] 10.60 3.1 – 4.01 –
Blackwell[b] 10.79 3.0 – 4.43 –
Dakota[b] 8.48 2.2 – 3.91 –
Forestberg[b] 9.93 2.5 – 4.38 –
Nebraska 28[b] 10.35 1.8 – 5.74 –
Pathfinder[b] 9.71 2.2 – 4.53 –
Sunburst[b] 8.88 2.4 – 3.87 –
Source a Wullschleger et al. [38, 134]
b
Ma et al. [106]
Fig. 3.4 Average water use

efficiency of 25 upland and
14 lowland switchgrass
cultivars grown across the
United States
Modeling based on RUE is considered to be the most preferred approach for crop
growth modeling because of the sheer simplicity in factors needed and its
straightforward implementation [98]. Two commonly applied crop simulation
models for switchgrass such as EPIC and ALMANAC are based on a RUE approach
for biomass simulation. In ALMANAC, a RUE value of 4.7 g MJ-1 IPAR was used
for simulating switchgrass biomass at several sites across the United States [87–89].
3.4.2 Water Use Efficiency
Switchgrass is often described as a drought-tolerant, warm season perennial due to

its deep roots and C4 metabolism. Physiologically, C4 plants are known to efficiently
use water through stomatal control of transpiration. As indicated before, root sys-
tems of perennials, especially C4 grasses, can access water stored deep within the
soil profile and thus extract more water at depth than annual crops [99]. Although
this rooting characteristic could translate to higher rates of water use for switchgrass,
few studies report estimates of either water use or water use efficiency (WUE) for
this emerging bioenergy feedstock. Early investigations by Stout et al. [100] and
Stout [101] measured WUE for switchgrass cultivated in the northeastern United
States. In their studies, WUE for the cultivar Cave-in-Rock was 25.0 kg ha-1 mm-1
for summer-growth switchgrass. More recently, Hickman et al. [102] compared
rates of evapotranspiration, water use, and WUE for switchgrass, maize, and mi-
scanthus grown in Illinois. Rates of water use exceeded 750 mm whereas WUE was
9.7 kg ha-1 mm-1 for the cultivar Cave-in-Rock grown in Illinois.
While information on water use and WUE is sparse, the database compiled by
Wullschleger et al. [83] can be used to derive estimates of WUE for a large
number of lowland and upland cultivars of switchgrass. Here annual biomass
production can be analyzed along with growing season precipitation to obtain a
reasonable estimate of WUE. Across the entire database, which includes almost
1,200 observations of biomass and precipitation for 25 upland and 14 lowland
cultivars, WUE averaged 21.6 with a range from 2.3 to 103.1 kg ha-1 mm-1
(Fig. 3.4). Sixty-eight percent of the observations fell within the range of 10 to
Fig. 3.5 Water use

efficiency of upland and
lowland switchgrass cultivars
30 kg ha-1 mm-1 interval. Separation of the data revealed that WUE was, on
average, higher for lowland than for upland cultivars (Fig. 3.5). WUE for lowland
cultivars averaged 25.6 (Fig. 3.5a), whereas WUE for upland cultivars averaged
16.2 kg ha-1 mm-1 (Fig. 3.5b).
3.4.3 Nutrient Use Efficiency
Nitrogen use efficiency (NUE) is dependent on many factors including soil

nitrogen availability, uptake and assimilation, and carbon–nitrogen flux and is one
of the major limiting factors in increasing crop productivity.
Although NUE can be calculated in a number of ways [103], a simple yet useful
measure is biomass yield per unit of nitrogen applied to the soil. More specifically,
NUE can be calculated from a series of nitrogen addition plots where annual
biomass yield is determined for a range of soil nitrogen additions, including a
control where no supplemental soil nitrogen is applied: NUE = (yield of Nx–yield
at N0)/kg of nitrogen applied; where Nx = N rate [ 0, and N0 = no N applied.
Although the definition of NUE is simple, and although the response of biomass
yield to applied nitrogen has been repeatedly studied for switchgrass [1], NUE for
bioenergy crops including switchgrass has not been well quantified (Table 3.4).
While annuals depend more on acquired nutrients for growth, perennial crops, as
indicated before, may derive benefits through traits such as remobilization of
carbon and nitrogen reserves in the spring that can then support growth from
overwintering rhizomes or roots. Thus, perennial plants have a higher NUE than
annual crops [104]. In addition, switchgrass has a higher NUE than traditional
annual crops in part due to differences in harvest time and management, which
allow higher rates of translocation of nitrogen to storage organs like stems and
roots. Based on field trials conducted at various locations, Staley et al. [105] and
Lemus et al. [72] have reported the most thorough analysis of NUE for switchgrass
to date. In those studies, these authors examined the NUE, nitrogen concentration,
total nitrogen uptake, and apparent nitrogen recovery for switchgrass fertilized
with 0, 90, 180, or 270 kg nitrogen per hectare. Field data collected over the years
revealed a diminishing return or inefficiency in NUE with higher rates of nitrogen
(Table 3.4). Averaged across all treatments in the study of Lemus et al. [72], there
was a yield advantage with nitrogen fertilization of about 9 kg of biomass per kg
of applied nitrogen per year. These findings suggest that applying B90 kg nitrogen
per hectare per year would provide good yields for switchgrass produced with two
cuttings. In a subsequent study by these same investigators [73], it was shown that
nitrogen removal exceeded the amounts of nitrogen applied in both one- and two-
cut management, suggesting that nitrogen was being supplied via mineralization or
other processes. Others have obtained similar results, leading Parrish and Fike [1]
to conclude that switchgrass is quite efficient and inherently thrifty in its use of
applied nitrogen with a capacity to obtain nitrogen from sources that other crops
cannot tap. Studies are just now beginning to examine the potential shifts in
microbial community composition beneath bioenergy crops and the potential
exists for unknown associations of microbes that facilitate nitrogen acquisition and
uptake for energy crops like switchgrass and miscanthus [106].
The high NUE of switchgrass is also in part attributed to its deep root system
and its symbiotic associations with mycorrhiza. Huang et al. [107] indicated that
about 22% of the total nitrogen required by the crop could be supplied by deep
roots (deeper than 1.2 m). Moreover, the capacity of the root system to recover/use
deep nitrogen sources changes with the season, with the maximum recovery
occurring just before/during anthesis; afterwards a significant reduction was reg-
istered due to shoot senescence. In general switchgrass can uptake between 1.49
and 2.63 kg nitrogen ha-1 d-1, depending on soil nitrogen levels and nitrogen
fertilization [108].
3.5 Yield Gap
Evaluations of existing commercial varieties and new cultivars have served to

identify the most productive ones and some management practices that would
improve productivity. Yield improvements of up to 50% were already obtained
76
Table 3.4 Switchgrass (cv. NJ-50) yield, nitrogen concentration, nitrogen use efficiency, total uptake, and percentage derived from nitrogen fertilizer at
different rates and locations. The water holding capacities at Klinnesville, Calvin, and Leck Kill (Northern Appalachian ridge) were 4.9, 14.4, and 25.3,
respectivelya
Site Nitrogen rate Yield (Mg ha-1) Nitrogen concentration (g kg-1) Total nitrogen uptake (kg ha-1) Nitrogen from fertilizer (%)
Cut 1 Cut 2 Tot NUEb Cut 1 Cut 2 Cut 1 Cut 2 Tot Cut 1 Cut 2
Klinnesville 0 3.47 0.63 3.98 – 9.3 8.5 31.6 5.2 35.0 0.0 0.0
90 6.63 1.03 7.31 37.0 11.3 9.1 71.7 8.7 77.5 35.0 19.8
180 7.80 1.55 8.83 26.9 13.6 12.0 104 17.1 115 39.3 27.4
Calvin 0 5.52 0.99 6.18 – 8.9 9.7 48.7 9.5 55.0 0.0 0.0
90 8.86 1.41 9.80 40.2 11.1 9.4 98.5 13.3 107 27.8 18.2
180 9.76 1.81 11.0 26.8 13.0 11.5 127 20.2 140 33.2 30.4
Leck Kill 0 5.68 1.25 6.52 – 9.0 9.6 49.0 12.2 57.2 0.0 0.0
90 8.66 1.73 9.82 36.7 11.7 9.6 104 16.8 115 24.1 13.5
180 – – – – – – – – – – –
Average 0 4.89 0.96 5.56 9.07 9.27 43.10 8.97 49.07 0.00 0.00
90 8.05 1.39 8.98 38.0 11.37 9.37 91.40 12.93 99.83 28.97 17.17
180 8.78 1.68 9.92 24.2 13.30 11.75 115.50 18.65 127.50 36.25 28.90
Source: a Staley et al. [105]
b
Nitrogen use efficiency (NUE), NUE = (yield at Nx–yield at N0)/(kg N applied), where Nx = N rate [0,
and N0 = no N applied
with the identification of the most suitable varieties to determined agroecological

zones [90]. However, within a production system, switchgrass is usually thought of
as being suitable for or allocated to marginal areas where soil resources such as
water, nutrients, etc., are available at minimum levels. Such stresses reduce the
crop’s growth and potential yield; therefore, increased scientific knowledge on
how stresses limit the productivity of switchgrass is needed in order to move
forward and further improve the productivity and sustainability of switchgrass.
3.5.1 Limitation of Productivity by Water and Salinity
Under drought conditions, switchgrass yield losses can be limited by a combination

of drought avoidance and tolerance mechanisms, as with most perennial grasses [4].
Such mechanisms include the development of deep roots, a high leaf area index,
pubescent and waxy leaves, changes in leaf orientation, leaf senescence, and osmotic
adjustment. However, in upland and lowland ecotypes, some acting mechanisms
may not be the same. For example, an interesting difference that might contribute to
the reduction of transpiration losses in upland cultivars and upland x lowland
switchgrass reciprocal hybrids is the presence of different amounts of pubescent hairs
on the leaf blades [3, 109]. Leaf blades in lowland cultivars are more bluish and with
waxy covers [95]. Adaxial leaf rolling is another defense mechanism in both eco-
types that reduces the leaf surface area (stomata) exposed to sunlight and therefore
reduces the radiation load on the leaves [110, 111]. This may allow a reduction in the
water stress level while remaining photosynthetically active.
Barney et al. [26] indicated that biomass production, plant height, number of
tillers, leaf area, and specific leaf area were significantly reduced (up to 80%) by
severe drought and extreme drought (-4.0 and -11.0 MPa) in both upland and
lowland ecotypes. In general, upland ecotypes are considered to be more drought
tolerant [112], but more conclusive evidence is needed as Stroup et al. [113] did not
find any significant reduction in biomass production of both ecotypes when the
drought stress was less severe (-1 MPa). However, upland ecotypes were somewhat
less affected than lowland ecotypes under such stress level, indicating the generic
capacity of switchgrass to tolerate drought. In response to drought, both ecotypes
increase the proportion of leaves with respect to the total dry matter produced [113],
probably due to shorter tillers. On the other hand, even though lowland ecotypes
outperformed upland ecotypes under waterlogged conditions, the reduced tiller
numbers and length, leaf area, and biomass of both ecotypes were not that large,
suggesting the wetland facultative properties of switchgrass [26, 114].
Several studies indicate that photosynthetic rates and leaf water potential of
switchgrass are reduced to varying degrees depending on the water stress level. For
example, Sanderson and Reed [115] indicated that at a soil water tension lower than
-45 kPa, photosynthesis and xylem water potential are reduced by 10% and 48%,
respectively. However, transpiration efficiency of diverse switchgrass cultivars was
not affected by drought probably because the leaf components, mainly proteins, and
enhanced transpiration efficiency [116]. Knapp [117] indicated that the photosyn-
thetic activity of switchgrass was virtually stopped during the driest period of the
season, but after a substantial rainfall, photosynthetic rates recovered to about pre-
drought period values. The decreased photosynthesis in switchgrass is accompanied
with a decrease in stomatal conductance and significant osmotic adjustment. The
capacity of switchgrass to adjust osmotically reflects its capacity to recover from
drought. Apart from that, drought-induced reductions in photosynthesis are associ-
ated with shoot nitrogen retranslocation to the roots as a probable mechanism to
ensure the availability of resources for growth and survival after drought [118]. Even
though variations in photosynthetic rates and ploidy levels were identified, it is not
yet clear how these differences affect productivity [38].
In switchgrass, physiological growth stages are delayed by drought stress at the
primary and regrowth stages [113]. Depending on the stand age and growth stage of
the drought occurrence, yields would be variably affected. Sanderson and Reed [115]
suggested that switchgrass is more sensitive to water stress at the seeding year than
when the plants are already fully established. Moreover, in a dry year, the typical
30–37% of the total biomass concentrated in the roots can be increased up to 60–73%
[95] as a response mechanism to limited water availability. Then the increased root
growth may ensure a better plant water status and improved nutrient acquisition.
In general, salt stress reduces seed germination, stand establishment, and yield
of perennial grasses, such as switchgrass, to varying degrees [4]. Information on
the salt tolerance of switchgrass ecotypes, however, is almost nonexistent. Most of
the limited available information is focused on germination and seedlings estab-
lishment but not on mature plants, except for a study (as far as we know) that
found lowland Alamo to have moderate tolerance to salinity [119]. Aboveground
biomass yield of switchgrass was only 29% of the control when NaCl was applied
in a 2.65 M solution for 5 weeks in pots [120]. Dkhili and Anderson [121] tested
the effects of soil salinity (1.1, 6.5, 9.8, and 14.9 dS/m) in combination with
different amounts saline irrigation water (0, 4, and 8 dS/m) on pathfinder
switchgrass seedlings growth. Their results showed that switchgrass seedlings
cannot survive soil salinity levels of 14.9 dS/m or irrigation water with an electric
conductivity of 8 dS/m. Moreover, even slight soil salinity levels (6.5 dS/m)
delayed emergence, decreased percentage of emergence, reduced seedling height,
and reduced dry matter production of aboveground and belowground organs.
However, the interactions between saline irrigation water and soil salinity
decreased the salt effects as the amount of irrigation increased. Similarly, Kim
et al. [122] indicated that the growth and development of Cave-in Rock switch-
grass started to show the effects of salinity and ion imbalances in plant tissues at
around an electric conductivity of 5 dS/m.
Although switchgrass increases the size of its stomata and develops salt glands
to excrete salt excess, these response mechanisms to salinity seem to not function
well as large amounts of sodium accumulate in the roots and shoots, even when
exposed to moderate salinity levels [122].
3.5.2 Limitation of Productivity by Nutrients
Plant nutrients are essential for the growth and development of the different plant
parts and for their correct functioning. Nitrogen is mostly involved in enzymatic
processes and in proteins. Biomass productivity of upland and lowland switchgrass
ecotypes is mainly determined by nitrogen availability rather than by water [113].
The same authors indicated that at low levels of nitrogen (10 kg ha-1) the plants
were so small that their water requirement never reached stressful levels even
though their water supply was limited. Similarly, Sanderson and Reed [115]
indicated that well-watered nitrogen-deficient plants developed a higher soil water
tension than droughted high-nitrogen-fertilized plants, probably because the
smaller transpiring canopy of nitrogen-deficient plants. Other studies, however,
indicate that shortage of water may be the most important limiting factor for
switchgrass growth in semiarid regions [123, 124] probably because nitrogen
mobility decreases substantially as soil moisture decrease. In any case, Stout et al.
[123] showed evidence that when precipitation was evenly distributed, nitrogen
level accounted for 80% of the variation in yield and water use efficiency.
Nitrogen-deficient plants are chlorotic, with lower photosynthetic rates, lower
growth rates, and lower aboveground and belowground resources acquisition
capacity [4, 115, 113]. Suplick et al. [125] reported that the LAR and LER in
switchgrass respond to increasing nitrogen fertilization in a quadratic fashion, with
the highest rates around 164 kg nitrogen ha-1. The lower LAR and LER rates at
lower nitrogen levels than the aforementioned threshold could be attributed to
reduced cell division rather than reduced cell elongation [126]. Because nitrogen
deposition in the growing zone (cell division zone) of elongating leaves is reduced
at low nitrogen levels, LER and therefore yield are reduced due to lower number of
cells produced [127]. Moreover, Suplick et al. [125] found that LER was highly
correlated with the total dry biomass production. However, Stroup et al. [113]
suggested that the reduced partitioning of carbohydrates to stems and sheaths,
commonly associated with nitrogen limitation, is the main reason for reduced
yields at low levels of nitrogen. On the other hand, high nitrogen fertilization rates
result in increased weight and number of tillers [113].
Apart from storing nitrogen in the roots for regrowth the following spring,
perennial grasses such as switchgrass remobilize nitrogen to the roots when avail-
ability of external sources decline [128]. However, this nitrogen reserve could be
completely depleted if additional nitrogen sources are not made available. Although
no signs of deficiency were reported, Lemus et al. [72] demonstrated that in the
course of 3 years the internal root-stored nitrogen reserves accumulated during the
first year decreased from 1.05 to 0.50% in the following years in the absence of
fertilization following crop establishment. They also speculated that 0.50% may be
the lower limit before inadequate nitrogen levels start to affect productivity.
Currently there is not much information on the effects of phosphorus deficiency on
switchgrass productivity. Brejda [129] and Muir et al. [56], among others, reported
little or no yield response of switchgrass to phosphorus fertilization, while Parrish
and Fike [1] indicated that switchgrass is inherently thrifty in the use of applied
phosphorus. Phosphorus provides plants with, among other things, a means of using
the energy harnessed by photosynthesis to drive its metabolism [130]. According to
Mills and Jones [131], concentrations between 0.8 and 1.7 g kg-1 of phosphorus are
sufficient for optimal switchgrass growth. In general, phosphorus is a relatively
immobile element in the soil, and the majority of the phosphorus absorbed is via
diffusion; therefore, roots have to grow toward where the pool of phosphorus is
located or other factors, such as mycorrhizae and exudation of hydroxyl ions and
organic acids [132], have to intervene to make it available for the plant. Research
indicates that switchgrass phosphorus uptake increased by 37 times when mycor-
rhizae were present [133]. The higher phosphorus uptake may be related to the
enlarged root surface absorption by the symbiotic association with mycorrhizae.
Understanding the basic processes of crop resource capture (nutrients, water,
etc.) and allocation have been invaluable tools for designing and evaluating
agronomic management practices to improve crop resistance to stress. However,
there are still many unrevealed agro-physiological characteristics at canopy and
root level that may contribute to improve switchgrass resource use efficiency and
to enrich its agronomic outcome. The establishment of a permanent switchgrass
physiology program as an important part of agricultural research is urgently
needed in ordered to incorporate the already acquired knowledge and further
develop the scientific understanding of physiological mechanisms underpinning
the control of growth and plant resources use.
Acknowledgments Support for Stan D. Wullschleger and S. Surendran Nair was provided by
the U.S. Department of Energy, Office of Science, Biological and Environmental Research. Oak
Ridge National Laboratory is managed by UT-Battelle, LLC for the U.S. Department of Energy
under contract DE-05-00OR22725.
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plant growth of prairie cordgrass and switchgrass. Bioenerg Res. doi:10.1007/s12155-011-
9145-3
123. Stout WL, Jung GA, Shaffer JA (1998) Effects of soil and nitrogen on water use efficiency
of tall fescue and switchgrass under humid conditions. Soil Sci Soc Am J 52:429–434
124. Thomason WE, Raun WR, Johnson GV et al (2004) Switchgrass response to harvest
frequency and time and rate of applied nitrogen. J Plant Nutr 27:1199–1226
125. Suplick MR, Read JC, Matuson MA, Johnson JP (2002) Switchgrass leaf appearance and
lamina extension rates in response to fertilizer nitrogen. J Plant Nutr 25:2115–2127
126. MacAdam JW, Volenec JJ, Nelson CJ (1989) Effects of nitrogen on mesophyll cell division
and epidermal cell elongation in tall fescue leaf blades. Plant Physiol 89:549–556
127. Gastal F, Nelson CJ (1994) Nitrogen use within the growing leaf blade of tall fescue. Plant
Physiol 105:191–197
128. Engels C, Marschner H (1995) Plant uptake and utilization of nitrogen. In: Bacon PE (ed)
Nitrogen fertilization in the environment. Dekker, New York
129. Brejda JJ (2000) Fertilization of native warm-season grasses. In: Anderson BE, Moore KJ
(eds) CSSA special pub no. 30. Native warm-season grasses: research trends and issues,
Crop Science Society of America, Madison
130. Lemus R (2004) Switchgrass as an energy crop: fertilization, cultivar, and cutting
management. PhD Dissertation, Virginia Polytechnic Institute and State University,
Blacksburg, Virginia
131. Mills HA, Jones JB Jr (1996) Plant analysis handbook II. Micro Macro Publishing, Athens
132. Marschner H (1995) Nutrition of higher plants, 2nd edn. Academic Press, San Diego
133. Brejda JJ, Moser LE, Vogel KP (1998) Evaluation of switchgrass rhizosphere microflora for
enhancing seedling yield and nutrient uptake. Agron J 90:753–758
134. Wullschleger SD, Sanderson MA, McLaughlin SB, Biradar DP, Rayburn AL (1996)
Photosynthetic rates and ploidy levels among populations of switchgrass. Crop Sci
36:306–312
Chapter 4
Crop Management of Switchgrass
Matt A. Sanderson, Marty Schmer, Vance Owens,

Pat Keyser and Wolter Elbersen
Abstract Management of switchgrass for bioenergy and forage share some

commonalities, of particular interest in bioenergy crop production is: (1) rapid
establishment of switchgrass to generate harvestable biomass in the seeding year,
(2) highly efficient management of soil and fertilizer N to minimize external
energy inputs, and (3) harvest management to maximize yields of lignocellulose.
Bioenergy cropping may entail management for multiple services in addition to
biomass yield including soil C sequestration, wildlife habitat, landscape man-
agement, and water quality protection. Management is a critical factor especially
as land classified as marginal or idle land will be emphasized for bioenergy
M. A. Sanderson (&)
Northern Great Plains Research Laboratory, USDA-ARS,
459 Mandan, ND 58554, USA
e-mail: Matt.sanderson@ars.usda.gov
M. Schmer
Agroecosystem Management Research Unit, USDA-ARS,
131 Keim Hall University of Nebraska, Lincoln, NE 68583, USA
e-mail: Marty.schmer@ars.usda.gov
V. Owens
Plant Science Department, South Dakota State University,
244C NPB, 2140-C Brookings, SD 57007, USA
e-mail: Vance.owens@sdstate.edu
P. Keyser
Department of Forestry, Wildlife, and Fisheries, University of Tennessee,
274 Ellington Plant Sciences Bldg, Knoxville, TN, USA
e-mail: pkeyser@utk.edu
W. Elbersen
Food and Biobased Research, Wageningen UR, Bornse Weilanden 9,
Building 118, 6708 WG, Wageningen 17, 6700 AA, Wageningen, The Netherlands
e-mail: wolter.elbersen@wur.nl

88 M. A. Sanderson et al.
production to reduce conflicts with food production. Marginal land may also be
more risky. To date, there has been no long-term commercial production of
switchgrass on a large scale and there is little in the way of hands-on, practical
farm experience with switchgrass managed as a bioenergy crop. In this chapter, we
lay out the key best management practices for switchgrass as a bioenergy crop
including establishment, soil fertility, and pest management.
4.1 Introduction
Switchgrass management as a bioenergy crop is relatively new. Early in the devel-

opment of switchgrass as a bio-energy crop it was assumed that management for
bioenergy would be similar to forage management [1]. For example, it was assumed
that establishment methods and weed management guidelines for switchgrass as a
forage crop would work as well for its use as a bioenergy crop. Although manage-
ment for bioenergy and forage share some commonalities, of particular interest in
bioenergy crop production is: (1) rapid establishment of switchgrass to generate
harvestable biomass in the seeding year (2) highly efficient management of soil and
fertilizer N to minimize external energy inputs, and (3) harvest management to
maximize yields of lignocellulose. Harvest management may differ the most
between bioenergy and forage cropping because the goal is to maximize yields of
lignocellulose in bioenergy rather than to optimize forage yield and forage quality.
However, extension material for establishing and managing switchgrass for bioen-
ergy has been developed based on the best-available information [2].
Bioenergy cropping may entail management for multiple services in addition to
biomass yield including soil C sequestration, wildlife habitat, landscape man-
agement, and water quality protection. Management is a critical factor especially
as land classified as marginal or idle land will be emphasized for bioenergy pro-
duction to reduce conflicts with food production. Marginal land may also be more
risky.
Switchgrass has been grown for grazing, hay, and conservation uses for decades
and a sizable body of scientific and practical information has been accumulated
[3–5]. To date, there has been no long-term commercial production of switchgrass
on a large scale and there is little in the way of hands-on, practical farm experience
with switchgrass managed as a bioenergy crop. Most of the technical information
on the management of switchgrass as a bioenergy crop has come from small-plot
research studies, pilot-scale demonstration projects of a few acres, and a handful of
field- and farm-scale demonstration projects, such as the Chariton River Valley
project in southern Iowa and the Tennessee Biofuels Initiative,1 which includes
6,000 acres (2,400 ha) of switchgrass in eastern Tennessee.
1
http://www.iowaswitchgrass.com/; http://renewablecarbon.tennessee.edu/Partners.html
4 Crop Management of Switchgrass 89
In this chapter, we lay out the key best management practices for switchgrass as
a bioenergy crop including establishment, soil fertility, and pest management.
Where management protocols translate well from and are firmly founded in forage
management, we briefly summarize and direct the reader to the appropriate
authoritative source.
4.2 Strategic Planning for Bioenergy Crop Management
As mentioned at the outset, there is no large-scale production of switchgrass for

bioenergy. When bioenergy cropping matures, however, farmers will need to do
some strategic planning regarding production systems. For example, the first
planning decision the farmer may face is whether or not to grow switchgrass for
bioenergy. Economic considerations (e.g., price paid for biomass, costs of pro-
duction) will certainly drive that decision but other considerations, such as flexi-
bility to grow other crops in rotation or to manage switchgrass as a forage crop to
take advantage of other markets, will affect decisions as well. Knowledge of
switchgrass production will also influence decision making. A survey of Tennessee
farmers in 2005 indicated that farmers were unfamiliar with switchgrass; however,
nearly a third of those surveyed would grow switchgrass if it were profitable. Some
of their concerns included the need for technical assistance to grow and manage
switchgrass and the lack of developed markets [6]. In a survey of Illinois farmers,
most respondents indicated that growing perennial grasses for bioenergy may be
worthwhile, however, they had limited knowledge of the grasses and how to grow
or manage them [7]. Some of their concerns regarding production included lack of
a local market for the biomass and a hesitancy to replace annual row crops with
perennial grasses. Farmers who rented land were reluctant to consider perennial
grasses because their landlord may not approve of the cropping system change.
Most respondents stated that perennial grasses would fit best on their marginal
cropland.
The option to use switchgrass both as cattle forage and biomass for energy
could offer an attractive incentive for farmers wherever livestock production is an
important enterprise. Flexibility in its use could also provide an incentive to adopt
switchgrass production as a new enterprise [8].
Strategic planning also includes considerations for site selection for growing
switchgrass. Site selection criteria may include suitability of soils for switchgrass
production among other factors. Planning for establishment should begin at least
one or more years in advance so that soil deficiencies (e.g. low pH) can be
corrected or to control weed populations through appropriate crop rotation along
with herbicide use.
Other important considerations in the planning process are to consider if the
proposed crop rotation has the flexibility to respond to markets or climate with
alternative crop choices. Producers should also consider risk assessment and the
probability of success and to be completely aware of potential to realize all
outcomes. Risk management practices such as participation in government

incentive programs [e.g., the biomass crop assistance program (BCAP)], and the
availability of crop insurance should also be considered. In the USA, the BCAP
program provides eligible producers reimbursement of up to 75% of the cost of
establishing a perennial bioenergy crop along with assistance (up to $45 per ton)
for harvest, transport, and storage of bioenergy crops. Risk management also
applies to harvest decisions. Harvesting switchgrass early to obtain quality forage
for livestock and using regrowth for biomass may provide flexibility for the pro-
ducer but can risk stand longevity and future productivity. Managing for a single
autumn harvest can maximize yield, whereas allowing the stand to remain in the
field over winter to reduce mineral and water levels for efficient harvest and
storage risks yield loss.
Proper management is needed to realize the best yields possible for a given land
area so as not to replace (or reduce) or compete with land for food production [9].
The importance of proper management was illustrated by Lemus et al. [10] who
showed that simply improving agronomic management by controlling weeds and
implementing a standard harvest schedule (without N fertilizer) increased yields
from 2.3 to 5 Mg ha-1.
Implementing appropriate management practices can be an important part of
reducing production risk. Appropriate establishment techniques protect against
risk of stand failure. Use of best management practices positions the farmer for the
best yield response in favorable growing conditions. Similarly, implementing best
practices can protect against losses from pests, diseases, and abiotic stresses.
4.3 Switchgrass Establishment
4.3.1 Site Selection and Preparation
Like most perennial species, switchgrass establishes best on well-drained, fertile

soils but it also can establish and persist under highly variable soil conditions.
Switchgrass used in bioenergy is likely to be grown on land that is unsuitable for
crop production either based on erosion potential or low crop productivity.
Switchgrass establishment practices for bioenergy plantings will require rapid
establishment and effective weed control.
Proper planning is important for establishment success. Key factors to consider
for successful establishment are measuring soil fertility status, proper field
preparation, proper cultivar selection, and using high quality seed. Soil sampling
before planting is recommended to determine soil nutrient status with the number
of soil samples required being dependent on the soil heterogeneity of the land to be
converted. At a minimum, soil should be tested for P, K, and pH before planting
with subsequent soil sampling every three to five years thereafter. Optimal seed
germination occurs between pH of 6–8 [11] but switchgrass seedlings can tolerate
pH values between 3.7 and 7.6 under field conditions [5, 12, 13]. The overall
effectiveness of liming before switchgrass establishment has been mixed but
liming is likely beneficial on soils with pH values below 5.0 [13].
Planting switchgrass into existing cropland, pastureland, or conservation land
requires land management preparation one to two years in advance. For example,
scouting for and controlling perennial weeds within fields before switchgrass
establishment will minimize stand failures. Allelopathic effects from previous
crops on switchgrass establishment have not been well documented but certain
crops can increase or decrease the likelihood of successful establishment
depending on weed suppression, herbicide carryover, and residue amount.
Herbicide-resistant crops in general provide good weed suppression the following
year for effective establishment. For example, soybean (Glycine max L. Merr.)
provides a firm seed bed, minimal residue, and good weed control which are
important attributes for successful establishment of switchgrass. High-residue
crops, such as maize (Zea mays L.) may require heavy duty drills to plant
switchgrass under no-till conditions or tillage practices for effective seed-to-soil
placement and uniform seeding depth. Establishing switchgrass in former pas-
tureland or conservation land is more challenging and requires multiple manage-
ment tools such as non-selective herbicides, tillage, and burning.
4.3.2 Cultivar Selection
Selecting the proper cultivar is critical for both establishment and persistence.
Switchgrass cultivars are morphologically divided into upland and lowland ecotypes
(see Chap. 1). Lowland ecotypes are taller, have longer more bluish-green leaves,
have longer ligules, are higher yielding, grow like a bunchgrass, are more rust
(Puccinia emaculata Schwein.; Puccinia graminis Pers.; Uromyces gramincola
Burrill) resistant, and have a coarser stem than upland ecotypes. Upland ecotypes are
shorter growing, have a finer stem, and are more tolerant of dry climatic conditions
than lowland ecotypes [4]. Within ecotypes there are northern upland, southern
upland, northern lowland, and southern lowland strains based on responses to
latitudinal effects [14]. Viable switchgrass seed can be produced when lowland
ecotypes and upland ecotypes are crossed with the same chromosome number [15].
Development of F1 hybrids derived from upland and lowland ecotypes has shown
increased biomass yields compared with the parental lines [16].
Estimates of economic yields of switchgrass biomass vary, but McLaughlin et al.
[17] indicated that 9 Mg ha-1 average annual yield across all production areas was
economic in the USA. In a meta-analysis of switchgrass biomass yields from 39
sites in 17 states of the USA, lowland ecotypes of switchgrass averaged
12.0 ± 5.9 Mg ha-1 and upland ecotypes averaged 8.7 ± 4.2 Mg ha-1 (Fig. 4.1;
[18]). Based on their empirical model of switchgrass yield, greatest biomass yields
were predicted to occur in a region from the mid-Atlantic region to eastern Kansas
and Oklahoma.
Fig. 4.1 Variation in biomass yields of upland and lowland switchgrasses at several locations in
the USA (from Wullschleger et al. [18], with permission, copyright American Society of
Agronomy)
European research also indicates that lowland ecotypes yielded more than
upland ecotypes. Averaged over four years and two systems of cutting in Italy,
lowland ecotypes averaged 14.9 Mg ha-1, whereas upland ecotypes averaged
11.7 Mg ha-1 [19]. Yields for several switchgrass varieties ranged from 7.1 to
21.3 Mg ha-1 during four years in Greece and 0.9–26.1 Mg ha-1 in Italy [20].
Switchgrass is photoperiod sensitive, requiring short days to induce flowering
[21]; however, there is variation among switchgrass cultivars in photoperiod
response [22, 23]. When switchgrass is grown north of the original adaptation area, it
is exposed to longer photoperiods resulting in a longer vegetative stage and more
biomass is produced than existing populations originating within that latitudinal
environment. Switchgrass populations moved south of their original adaptation area
produce less biomass than in their original adaptation area because floral initiation
begins earlier. Switchgrass populations moved too far north in a temperate climate do
not survive the winter because the plants do not cold harden before the onset of
freezing temperatures. Switchgrass populations that originated in high latitudes or
low latitudes within the United States have the most defined plant responses to
latitude [14]. Generally, switchgrass cultivars should not be planted more than one
hardiness zone (Fig. 4.2) north of their area of origin [4, 24]. Longitudinal response
by switchgrass populations is less defined than latitudinal responses and is variety
specific [24].
The optimal latitude for growing a specific switchgrass variety will also differ
between Europe and North America. For example, the cultivar Cave-in-Rock,
which originates in southern Illinois USA (38.30° North), is probably best adapted
to northwest Europe (Netherlands and United Kingdom *52° North) [25]. When
cultivars are grown too far north, they may not survive winter or have reduced
Fig. 4.2 Expected relative yields of varieties from low and high latitude of origin when grown at
a Northern location (52° north, the Netherlands) and at a southern location (38°, Greece) (from
Elbersen, with permission)
Fig. 4.3 USDA plant hardiness zones for the 48 contiguous states in the USA (http://
www.usna.usda.gov/Hardzone/ushzmap.html). Switchgrass cultivars should not be planted more
than one hardiness zone north of their area of origin. The average minimum annual temperature
(oC) ranges by zone: zone 1, \-45.6; zone 2, -45.5 to -40; zone 3, -39.9 to -34.5; zone 4,
-34.5 to -28.9; zone 5, -28.8 to -23.4; zone 6, -23.3 to -17.8; zone 7, -17.7 to -12.3; zone
8, -12.2 to -6.7; zone 9, -6.6 to -1.2; zone 10, -1.1 to 4.4; zone 11, [4.5
stand life. A generalized relationship between latitude of origin of a cultivar and

the yield at a southern and northern site in Europe is illustrated in Fig. 4.3.
Cultivars originating from the semi-arid Great Plains may be more susceptible
to disease when established in more humid locations, whereas cultivars originating
in humid climates may be less adapted to the drought conditions of semi-arid
Fig. 4.4 Plant adaptation region map for the 48 contiguous states of the USA (from [26]). Plant
adaptation regions are derived from combining ecoregion and plant hardiness zone classification
systems. Example plant adaptation regions labeled include the Great Plains Steppe hardiness
zones 4 and 5 (331-4, 331-5, 331-5, and 332-5) and the Prairie Parkland Temperate hardiness
zones 4 and 5 (251-4 and 251-5)
climates. Plant adaptation regions define suitable environments for leading

switchgrass populations (Fig. 4.4, [26]).
The majority of released switchgrass cultivars were developed for conservation
or traits desirable for pasture grazing. Current switchgrass varieties tend to be from
(1) seed increases from random plants originating from a remnant prairie or
(2) improved cultivars based on selection of defined traits such as yield, digest-
ibility, seed dormancy, or disease resistance [27], see Chap. 2]. Most improved
switchgrass cultivars have been developed in the Central Plains of the USA.
Breeding for specific bioenergy traits (e.g. low lignin, high cellulose) will be
dependent on the conversion process used within a given region [1].
4.3.3 Seed Quality
Seed quality attributes include viability, purity, cleanliness, and vigor [28]. Seed
vigor relates to the ability of a seed to germinate and establish a viable seedling
under field conditions (see Chap. 3). Growing conditions, harvest timing, seed
drying, cleaning and processing procedures, storage conditions, field sanitation,
diseases, and insects influence switchgrass seed quality [28]. Agronomic practices
that improve seed quality include N fertilization, use of row cultivation, spring
burning, and control of smut (Tilletia maclaganii (Berk.) G.P. Clinton) and rust.
Selection for increased seed size can increase viability and vigor. Average seed
weight for switchgrass is 850 seeds g-1 but variation exists among and within
cultivars for seed size [5]. High germination rates and greater emergence have
been reported for switchgrass seed with higher than average seed weights [29, 30].
Under field conditions, switchgrass seedlings from heavy seed had greater ger-
mination and earlier growth than lighter seed but no growth differences were
detected 8–10 week after emergence [31].
4.3.3.1 Seed Quality Tests
Standard germination and purity tests have limited utility in the direct calculation
of planting rates for switchgrass. Pure live seed (PLS) is used in expressing seed
quality and determining recommended field seeding rates. Pure live seed is cal-
culated as: PLS (%) = [seed purity (%) 9 seed germination (%)]/100. Switch-
grass germination testing protocols from the Association of Official Seed Analysts
include a 14 day cold stratification (5°C) period before germination testing, which
reduces the amount of primary dormancy found in switchgrass. The germination
percentage found on seed tags tends to overestimate the percent of viable seeds,
which complicates determining proper field seeding rates [13]. Germination trials
without cold stratification are recommended if seed dormancy is expected [5].
Seed vigor tests that evaluate emergence from depth or accelerated aging tests
have been recommended as alternative methods in determining field seeding rates
[32, 33]. Switchgrass seed lots with the same germination percentage but different
seedling vigor have resulted in emergence differences of more than 40% [28].
4.3.3.2 Dormancy
Seed dormancy reduces seedling vigor and establishment. Dormant seed can be
defined as seed that is unable to germinate even when subjected to suitable con-
ditions [34]. The mechanisms for seed dormancy of switchgrass are complex but
the expression of seed dormancy is caused by structures that surround the embryo
and mechanisms within the embryo [35]. Dormancy is likely caused by genetics
and environmental effects during seed production, harvesting, and processing.
Genetic selection for low dormancy seed has been shown to lower overall primary
dormancy in lowland ecotypes [36]. Primary dormancy of switchgrass seed
can generally be broken by an after-ripening period or by cold stratification [4].
After-ripened switchgrass seed is generally one-year-old or more. Switchgrass
seed that is stored for three or more years at room temperature may have poor
seedling vigor and reduced establishment [28]. Secondary or latent dormancy
occurs when viable seed becomes dormant after unfavorable environmental con-
ditions [35]. Environmental or chemical methods can be used to break secondary
dormancy. Seeds that have undergone secondary dormancy-breaking techniques
but still demonstrate low germination have residual dormancy [37]. Residual
dormancy can be reduced in switchgrass when endogenous levels of nitric oxide
(NO) and reactive oxygen species (ROS) are altered [38]. High abscisic acid
(ABA) levels can increase dormancy in monocot seeds [39]. Exogenous ABA and
diphenyleneiodonium levels in switchgrass seeds are believed to block germina-
tion by restricting ROS and NO activity [38]. Gibberellic acid, sodium nitro-
prusside, potassium ferrocyanide, potassium nitrate, polyethylene glycol, and
hydrogen peroxide have been used to reduce residual dormancy in switchgrass but
overall success of each treatment is cultivar-specific [40, 41].
4.3.3.3 Seed Treatments
Several seed treatments have been investigated for their ability to increase
switchgrass germination and establishment. Seed priming, an osmotic process
where seed is hydrated to a level where metabolic activity begins but radicle
emergence does not occur, may enhance switchgrass germination [42, 43]. Seed
water uptake is regulated when priming media such as synthetic calcium silicate is
used as a water source until equilibrium between the seed and media is reached.
Hydrogen peroxide treatment of non-dormant switchgrass seeds increased seed
germination and emergence along with more uniform development of seedlings
[38]. Sodium nitroprusside, a NO donor, promoted germination of the lowland
ecotype Kanlow [37]. Field conditions at planting influenced overall effectiveness
of primed seed [42]. Blending primed and non-primed seed may reduce overall
seeding costs and increase stand establishment under variable field conditions [44].
Karrikinolide [3-methyl-2H-furo[2,3-c]pyran-2-one], a compound from smoke
that has been found to promote germination and seedling establishment in several
native species, did not increase switchgrass germination or seedling vigor [45].
Switchgrass seeds coated with fungicides have been used in humid climates to
increase seedling emergence; however, it is unclear if fungicide application limits
the symbiotic relationship between switchgrass and arbuscular mycorrhizal fungi.
Insecticides have also been shown to improve establishment [46] and can be used
as a seed coating applied before planting.
4.3.4 Seedbed Preparation
Switchgrass can grow under variable soil conditions ranging from sand to clay
loam [5]. Switchgrass has been successfully established under various tillage
practices but side-by-side tillage studies and preceding crop comparisons have
been limited [13]. Switchgrass is mainly established through direct seeding using a
culti-packer seeder, grain drill, or no-till drill. Seed-drill calibration is necessary to
ensure proper seeding rates. Broadcast seeding also has been used in conservation
plantings but lack of stand uniformity may limit its potential use for bioenergy
plantings. Seedbed preparation likely will be predicated on equipment accessi-
bility, soil erosion concerns, preceding crop, and initial soil moisture conditions.
A firm seed bed is recommended for proper seed placement regardless of planting
method since switchgrass is planted at a shallow depth. Planting switchgrass using
conventional tillage methods is a common practice for effective establishment.
A grain drill with a small-seed box and small-seed tube attachment or a culti-
packer seeder is effective in establishing switchgrass under conventional tillage
practices. Conventional tillage can control or reduce cool-season weed populations
before seeding as well as reduce residue from previous cropping systems that may
interfere with proper seed placement. Soil temperatures will be higher under
conventional tillage than no-tillage practices during a spring seeding. Following a
tillage practice, soil clods need to be reduced or eliminated by successive tillage,
packing, or firming the soil to provide good seed-to-soil contact at time of
planting. Soil firming before seeding has been more effective in switchgrass
establishment than soil firming after seeding [47]. Conventional tillage practices
are not recommended on fields with steep slopes because of the risk of soil erosion.
Soil carbon loss via CO2 emissions from tilled soil is also a concern, especially on
land that was previously in set-aside programs or other perennial grassland sys-
tems which tend to maintain high levels of soil carbon [48]. No-tillage seeding of
switchgrass has also been effective under variable climate conditions and previous
cropping systems. No-tillage practices have been successful in establishing
switchgrass in existing grasslands and are highly effective on soybean stubble [49].
A no-till drill is recommended when planting in sod or heavy residue because the
drill has coulters to remove residue before seed placement and it is heavier than a
conventional grain drill. No-tillage seeding provides greater water conservation
benefits than conventional tillage especially near the soil surface. For bioenergy
purposes, both pre-emergent and post-emergent herbicides are critical under
no-tillage practices to control or reduce weed populations during the establishment
year.
4.3.5 Fertilization
Nitrogen fertilizer is not recommended during the planting year because N will
encourage weed growth, increase competition for establishing seedlings, increase
establishment cost, and increase economic risk associated with establishment if
stands should fail [49]. Sanderson and Reed [50] found that there was no biomass
yield response to N at rates of 22 or 112 kg ha-1 during the establishment year of
switchgrass and indicated that switchgrass was able to use the available N found in
the soil during the establishment year. Starter fertilizer (9 kg N ha-1 and 27 kg P
ha-1) applied at planting did not improve switchgrass establishment or initial
yields [51]. As stated earlier, soil tests (P, K, pH) are recommended before
planting. Phosphorus levels (Bray & Kurtz #1 method) should exceed 25 mg kg-1
when establishing warm-season grasses [52, 53]. Phosphorus and K levels are
generally adequate for switchgrass growth in most agricultural fields.
4.3.6 When to Plant
Successful establishment of switchgrass seedlings is determined by seeding depth, soil

texture, soil moisture, and soil temperature [13]. Soil temperatures above 20°C are
required for switchgrass germination and seedling growth [54, 55]. Optimal germi-
nation of several switchgrass cultivars was found to be between 27 and 30°C [56] but
germination tolerance to temperature varies by cultivar [57]. Switchgrass has a
panicoid type of seedling root development (see Chap. 3) which places the crown node
(from which adventitious roots develop) near the soil surface by elongation of the sub-
coleptile internode or mesocotyl [58]. The subcoleoptile internode has a relatively
small xylem cross-sectional area in most warm-season grasses. This limits the amount
of water and nutrients that can be transported from the primary root system to
developing leaf area and contributes to seed establishment failure in warm-season
grasses before adventitious roots develop [59]. Selection for low crown node place-
ment (i.e., a shorter subcoleoptile internode) increased switchgrass seedling survival in
the southern Great Plains [60]. Greater planting depths reduce warm-season seedling
emergence [61]. Recommended planting depths for switchgrass are 0.5–2 cm [4] but
seeding depths of 3 cm have been recommended on coarse textured soils [13].
The most important factor for switchgrass to develop adventitious roots is
adequate soil surface moisture over a period of time [62]. Rainfall frequencies
between 7 and 10 days were found to be crucial for seedling survival under south
central US climate conditions [63]. Under favorable environmental conditions,
warm-season grasses initiate adventitious roots at 2–4 weeks after emergence [59].
Switchgrass will typically have one adventitious root by the three-leaf emergence
stage [62]. Adventitious roots have a larger xylem cross-sectional area than the
subcoleoptile internode resulting in higher water uptake and nutrient absorption
than the primary root system [59].
Seeding dates for warm-season grasses are recommended in early spring when
soil moisture is not limiting and soil temperatures are low enough to cold-stratify
seeds. For the Northeast United States, switchgrass planting is recommended
3 weeks before and 3 weeks after corn planting [64]. In Iowa, switchgrass planted
in mid-April to early May had higher dry matter production than when planted
later in the season [65]. Planting during late April to mid-May was optimum for
warm-season grasses in mid-latitude areas of the United States [54, 55].
In Nebraska, planting in March resulted in higher standing crop biomass than April
or May plantings [66]. Dormant planting (late autumn) or frost seeding (late
winter) may be used to establish switchgrass for conservation purposes.
4.3.7 Seeding Rate
Recommended seeding rates for switchgrass are 200–400 pure live seeds (PLS) m-2.
With excellent weed control, however, a seeding rate of 107 PLS m-2 gave adequate
stands for conservation plantings [67]. Seeding rates of 4.48–11.20 kg ha-1 resulted
in similar biomass yields during post-establishment years in the southeast US [68].

Lower seeding rates are recommended for regions with limited rainfall but this
recommendation is based on conservation plantings and may not be appropriate for
bioenergy-specific plantings. Switchgrass swards appear to regulate plant density
depending on the climate conditions and initial seedling density with low seedling
density tending to increase with sward age and high seedling density tending to
decrease until equilibrium has been reached for a particular site.
4.3.8 Mixed Species Planting
Mixed species plantings are common for conservation and forage purposes and may
be useful for bioenergy purposes as well. The use of low-input high-diversity
mixtures has been proposed as a sustainable way to produce bioenergy on degraded
land [69]. Species and cultivar selection is important for long-term success and
should have similar growth characteristics, seed vigor, forage quality characteristics,
maturity dates, and tolerance to selected herbicides. Warm-season grass species of
big bluestem (Andropogon gerardii Vitman), indiangrass [Sorghastrum nutans
(L.) Nash], switchgrass and little bluestem [Schizachyrium scoparium (Michx.)
Nash] were the dominant species of tallgrass prairie in North America. These species
are common in mixtures for set-aside plantings within the central US. A positive
relationship has been found between soil heterogeneity and plant diversity for
grassland ecosystems. Planting mixed species would likely increase establishment
success on areas with variable soil types or topography by allowing certain species to
grow based on broad or narrow niche requirements. When switchgrass is planted as a
mixture with other perennial grasses, it is recommended that no more than 20% of the
seeds should be switchgrass [5].
In a multilocation study in Minnesota, biomass yield increased by 28% when
species richness of mixtures increased from 1 to 8 species; however, there was no
further yield increase at species richness levels of 8–24 species [70]. In most
instances, only a few species produced most of the biomass yield. A comparison of
warm-season grass monocultures with polycultures of 4–16 species of grasses and
forbs demonstrated that monocultures produced more biomass more economically
than polycultures [71]. Observational research on conservation grasslands in the
northeastern USA indicated a negative relationship between plant species diversity
and biomass production [72].
Companion crops have been used successfully to establish perennial grasses
especially when herbicide options are limited. Advantages to companion crops
include reduced erosion potential under conventional tillage along with a potential
cash crop that can be harvested, which may reduce overall establishment costs
especially if a switchgrass stand failure occurs. Companion crops also reduce weed
populations during switchgrass establishment but management practices are
important to ensure proper establishment. A disadvantage to using companion
crops is that in certain regions, an establishment year switchgrass harvest is not
possible because of limited biomass. Switchgrass has been successfully established

under maize and sorghum-sudangrass (Sorghum bicolor 9 sudanense) [73, 74].
Winter wheat (Triticum aestivum L.), spring oat (Avena sativa L.) and other annual
cool-season grasses managed as a hay crop have been successfully used for
establishing perennial grasses like switchgrass. Soybean planted in wide rows
(76 cm) can be used in cooler regions where row canopy closure is delayed until
late-summer. Seeding densities for the companion crop should be reduced from
optimal monoculture plantings to allow more light to reach the switchgrass
seedling.
Establishing switchgrass on land formerly in set-aside programs or pastures has
been done using either conventional tillage or no-tillage methods [13]. Manage-
ment practices for effective establishment may include autumn or spring hay
harvest, prescribed burning in autumn or spring, pre- and post-herbicide treat-
ments, tillage, and the use of companion crops. Planting set-aside land or pastures
with herbicide-resistant soybeans or spring-planted cover crop mixtures one to two
years before switchgrass establishment may also be an effective management tool
for seedbed preparation.
4.3.9 Weed and Pest Control During the Establishment Year
Weed competition is a major reason for switchgrass stand failure during estab-
lishment. Acceptable switchgrass production can be delayed by one or more years
by weed competition and poor stand establishment [75]. The most common
weeds in establishing warm-season grasses are annual grasses such as crabgrass
[Digitaria sanguinalis (L.) Scop.], green foxtail [Setaria viridis (L.) Beauv.],
yellow foxtail [Setaria glauca (L.) Beauv.], autumn panicum (Pancicum dicho-
tomiflorum Michx.), and barnyardgrass [Echinochloa crusgalli (L.) Beauv.] [76].
The recommended practice of controlling weeds in fields planted with switchgrass
is with the use of pre-emergent herbicides particularly for annual grass control.
Non-selective herbicides, such as glyphosate [N-(phosphonomethyl) glycine] are
effective in weed control before switchgrass emergence especially under no-till
plantings. It is important to follow all herbicide regulations, label directions, and
safety precautions. Extension staff or professional advisors should be consulted
regarding proper use and application of all herbicides.
Atrazine [2-chloro-N-ethyl-N’-(1-methylethyl)-1, 3, 5-triazine-2, 4-diamine]
has been an effective herbicide during switchgrass establishment controlling
mainly cool-season annual grasses and broadleaf weeds [65, 77]. Switchgrass
biomass yields were higher with atrazine application than without [52, 77].
Quinclorac (3, 7-dichloro-8-quinolinecarboxylic acid) is another effective herbi-
cide in establishing switchgrass [2, 78]. Quinclorac controls warm-season annual
grasses such as giant foxtail [Setaria faberi (L.) Beauv.], yellow foxtail, green
foxtail, and barnyardgrass along with a limited number of broadleaf species [79].
Switchgrass treated with a pre-emergent combination of quinclorac and atrazine
had greater biomass yields and comparable switchgrass stand frequencies compared
with switchgrass treated with atrazine or quinclorac alone and both herbicides were
equally effective on upland and lowland ecotypes [80]. Post-emergent application of
quinclorac reduced switchgrass yields compared with atrazine but was highly
effective in controlling annual grasses [81]. Pre-emergent applications of imazeth-
apyr {2-[4,5-dihydro-4-methyl-4-(1-methlethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-
3-pyridine-carboxylic acid} has been effective in switchgrass establishment [82].
Post-emergent sulfosulfuron [1-(4,6-dimethoxypyrimidin-2-yl)-3-(2-ethylsulfony-
limidazo[1,2-a]pyridin-3-yl)sulfonylurea] applications on switchgrass are more
effective in controlling smooth pigweed (Amaranthus hybridus L.) than quinclorac
but are less effective on annual grasses [78]. The use of 2,4-D (2,4-dic-
hlorophenoxyacteic acid) is cost effective for broadleaf weed control when applied
post-emergence at the 4- or 5-leaf stage. Broadleaf weed control using a mechanical
treatment (mowing) can be successful when broadleaf weeds are taller than
switchgrass and the mowing application can be done to minimize switchgrass leaf
loss [83]. After successful establishment, only limited herbicide use should be
necessary.
Research on pest and disease control on switchgrass grown for bioenergy
has been limited. Establishment year insecticide applications like carbofuran
(2,3-dihydro-2,2-dimethly-7-bensofuranyl) have had variable success on switch-
grass yield and stands [13, 84]. Insect injury on switchgrass seedlings is dependent
on climatic conditions, seeding dates, and weed populations. Outbreaks of rust and
smut can occur during the establishment year but are generally more likely to
occur post-establishment.
4.3.10 How to Evaluate Establishment Success
A grass seedling is considered fully established when adventitious roots are

formed, able to overwinter, and survive the following growing season [85, 86].
Switchgrass seedlings must develop two or more tillers to survive winter [87].
At the field-scale, switchgrass stands are evaluated based on morphological
development, weed control, and seedling density. Integrating a global position
system receiver and the use of geographic information systems (GIS) with ground-
based scouting tools can assist in quantifying stand densities and weed populations
across fields. Maps can be generated using GIS-derived data to identify areas
within fields for possible reseeding or weed control measures. Switchgrass will
typically not reach full yield potential until 1–2 years after the establishment year.
Under normal weather conditions and proper agronomic management, switchgrass
can achieve C50% of full yield potential during the establishment year [75].
Harvesting switchgrass in the establishment year reduces farm-gate production
costs and improves economic returns [88].
The definition of an acceptable stand density is dependent on the desired

management goal. Stand densities of C10 seedlings m-2 are acceptable for con-
servation purposes [89–91], whereas switchgrass managed for grazing or bioenergy
purposes will require higher initial stand densities. Establishment year stands
of [20 switchgrass seedlings m-2 with minimal weed pressure are considered to
be fully successful, whereas stands between 10 and 20 seedlings m-2 are con-
sidered adequate, and stands below 10 seedlings m-2 may require reseeding.
Switchgrass has the ability to produce similar biomass yield under different
seedling densities as a result of compensatory responses to tiller number and sizes
[50]. When comparing recommended seeding rates (200–400 PLS m-2) and
desired seedling densities, planted seed to established seedling success rate is
generally \20%.
Stand frequency measurements have been used to quantify seeding establish-
ment and assess grassland improvements [92, 93]. Frequency is determined by the
number of samples in which a species occurs, within a given area, and is expressed
as a percentage of the total. Vogel and Masters [93] developed a frequency grid
tool that can be used to assess plant populations at numerous sampling areas within
a field and provide a conservative estimate of stand density. In a study conducted
in the northern Great Plains USA on 10 farms, switchgrass fields with stand
frequency of 40% or greater provided a successful establishment year stand
threshold for subsequent post planting year biomass yields [75]. The frequency
grid tool was designed for monitoring plantings under culti-pack drop seeders or
conventional seed drills. Switchgrass planted into wider drill rows ([38 cm) may
require alternative methods to quantify establishment success, such as a frequency
rod or line transects [77, 94].
4.4 Soil Fertility and Crop Fertilization

in the Established Stand
Fertilizer requirements of switchgrass managed for bioenergy depend on yield

potential, soil productivity, management practices, and weather. Most switchgrass
fertility research has focused on N because it is often the most limiting nutrient
[95–97], and before 1990 the primary goal was to better understand the effect of
fertility on switchgrass grown for forage rather than bioenergy [98]. This is an
important consideration because a biomass harvest, especially in northern envi-
ronments, is often done around a killing frost in the autumn when some nutrients
will have been translocated to underground tissue [99].
4.4.1 Nitrogen
Optimum N rates for switchgrass managed for bioenergy vary by geographic

region, environmental and weather conditions (specifically precipitation),
N-supplying capability of the soil, and harvest frequency [97, 100–103].

Maximum yields of ‘Kanlow’ switchgrass were achieved with 448 kg N ha-1 applied
in April in a 3-cut system in Oklahoma [97]. Multiple harvests each year and high
N application rates, however, had a greater negative effect on average annual biomass
production than a single harvest system over a four-year period at one of two study sites
and was not economically feasible. Aravindhakshan et al. [100] concluded that har-
vesting once annually and applying only 69 kg N ha-1 was most economical in a
similar environment. In a comparison of a 1- or 2-cut system with added N (50 kg ha-1
with one cut or 100 kg ha-1 with two cuts), Lemus et al. [102] noted that N removal
was significantly greater in the 2-cut system because of the high concentration of
N in the mid-summer harvest. Although mean switchgrass yields were slightly higher
in the 2-cut, 100 kg N ha-1 system than in a 1-cut, 50 kg N ha-1 system, they con-
cluded that the cost of additional inputs and greater N removal rates would not be worth
the small yield increase. Applying 56 kg N ha-1 increased switchgrass biomass
production in several locations in South Dakota, USA, but application rates above this
amount resulted in no further yield advantage and increased weed pressure [103].
Switchgrass response to N levels up to 220 kg ha-1 was linear or quadratic in Iowa,
USA [104].
Applying 10–12 kg N ha-1 for each Mg ha-1 biomass removed under a single
late-summer or autumn harvest is recommended in the Midwest USA [99]. This
allows for replacement of N removed in biomass because switchgrass typically
contains 60–120 kg N Mg-1. Regardless of amount applied, N is typically applied
after green-up in the spring to minimize weed encroachment, particularly from
invasive cool-season grasses.
Nitrogen response of switchgrass may depend on harvest management. For
example, in western Europe, switchgrass generally is harvested after senescence in
winter (i.e., delayed harvest) and yield response to N is minimal, even if the crop is
grown for many years with no fertilizer N [105, 106]. The low N removal (because
of N recycling within the plant) combined with high rates of atmospheric
N deposition and generally high soil N levels probably contribute to the lack of a
N response in delayed harvest switchgrass in Europe.
4.4.2 Phosphorus and Potassium
Less research has been done relative to P and K fertilization of switchgrass for
biomass or forage. Recommendations for P and K application are based on soil test
levels and soil characteristics, [101]. There was no response of switchgrass to
P application at two locations in Texas, USA over a 3 or 7 year period [96].
Switchgrass production increased when P and N or P, K, and N were applied
together with lime compared to N alone on five different soils in Louisiana, USA
[107]; however, the authors speculated that response to P fertilization would be
limited without N.
Switchgrass grown on acid soils (pH \ 5) may be subjected to P deficiencies.

Switchgrass performs better on acid soils when grown in symbioses with arbus-
cular mycorrhizal fungi [108, 109]. Phosphorus may be more available to plants
with this association because arbuscular mycorrhizal fungi are better able to access
more soil P.
4.4.3 Lime
Switchgrass has limited response to lime. In a greenhouse pot experiment with five
acidic soils, yield did not increase when soil pH was brought to 6.5 with lime
[107]. A yield response was noted, however, when N and P or N, P, and K were
co-applied with lime. On a strongly acid (pH 4.3–4.9) soil in Pennsylvania, USA,
untreated switchgrass yielded approximately 50% of that receiving the high lime
and fertilizer rate [110].
4.4.4 Manure
Cattle manure may be used as a source of nutrients on switchgrass. Lee et al. [111]
compared three equivalent N rates (0, 112, and 224 kg ha-1) of cattle manure or
ammonium nitrate and found that switchgrass yields increased with the medium
application rate of either N source. However, ammonium nitrate had a greater
deleterious effect on switchgrass stand persistence and weed encroachment than
the equivalent rate of manure. They speculated that this may have been due to the
slow release of N from manure compared to the rapid availability of N from
ammonium nitrate. Switchgrass biomass yields increased linearly with dairy
manure applied at rates of 0–600 kg N ha-1 [112]. Switchgrass filter strips
effectively reduced concentrations of nutrients and pollutants in runoff water from
the dairy manure-treated plots.
4.5 Pest and Weed Management in the Established Stand
4.5.1 Diseases
A number of diseases have been reported in the literature for switchgrass. Disease
pressure will likely increase if large scale production of switchgrass for bioenergy
is realized [13]. Rust (Puccinia emaculata) has been found on switchgrass in
central and eastern South Dakota. Rust symptoms have been more severe on
cultivars of northern origin; however, heritability exists for rust resistance [113].
Other diseases reported in a review by Vogel [5] include anthracnose
[Colletotrichum graminicola (CES) G.W. Wils; now suggested to be caused by

Colletotricum navitas J.A. Crouch [114]], smuts [T. maclaganii (Berk.) G.P.
Clinton], Phoma leaf spot (Phoma spp.), and Fursarium root rot (Fusarium spp.).
Trailblazer switchgrass seemed to be more susceptible to anthracnose than Cave-
in-Rock in Pennsylvania [115]. Reduced switchgrass biomass and seed yields were
attributed to smut in several production fields in Iowa, USA in the late 1990s
[116]. Smut was found in 15 of 17 fields surveyed, and the authors estimated that
50–82% of the area in switchgrass production was infested with T. maclaganii.
Fields with smut incidence [50% yielded less than half of the expected biomass
and some seed production fields were a total loss in 1999 [116]. Thomsen et al.
[117] estimated yield losses from T. maclaganii of 2–40% in switchgrass fields
sampled in Iowa, USA, and noted the critical need for research on disease man-
agement approaches if switchgrass is to be a successful biomass crop.
4.5.2 Insects
Few insects have been identified as potential pests of switchgrass; however, damage
from insects may increase if or when switchgrass monocultures are grown on
large production fields. Distribution and symptoms of a stem-boring caterpillar
(Blastobasis repartella Dietz.) were described by Prasifka et al. [118]. In this survey
B. repartella was consistently found in cultivated and natural switchgrass stands in
eight northern states. In the four northern states (Illinois, Nebraska, South
Dakota, and Wisconsin), sampling indicated that 1–7% of tillers were damaged by
B. repartella [118]. A new species of gall midge [Chilophaga virgate Gagne
(Diptera: Cecidomyiidae)] was recently discovered in South Dakota, USA [119].
Proportion of tillers infested with the gall midge in 10 switchgrass genotypes ranged
from 7 to 22%, mass of infested tillers was 35% lower than normal tillers, and
infested tillers produced no appreciable seed. Grasshoppers (Orthoptera) are known
to feed on switchgrass, but the extent of the damage has not been quantified [13].
4.5.3 Weeds
In established stands of switchgrass, weed pressure during the second growing

season is often worse than in subsequent years if there was poor site occupancy by
switchgrass seedlings during the seeding year. In that scenario, a number of weeds
can become established during or after the first summer. The problem is often
compounded because where site occupancy may not have been high during the
seeding year, stand vigor still may be reduced in the second year and, therefore,
vulnerability to weed competition may be increased. With adequate weed control
during the first two years of a stand, subsequent problems with competition can be
limited in switchgrass managed as a biofuel feedstock.
Cool-season annual weeds usually are not a concern in switchgrass unless the
infestation is severe. In such cases, use of either a broadleaf formulation (2,4-D,
dicamba {3,6-dichloro-2-methoxybenzoic acid}, picloram {4-amino-3,5,6-tri-
chloro-2-pyridinecarboxylic acid}, metsulfuron {2-[[[[(4-methoxy-6-methyl-1,3,5-
triazin-2-yl)amino]carbonyl]amino]sulfonyl]benzoic acid}, sulfosulfuron, or am-
inopyraild {4-amino-3,6-dichloro-2-pyridinecarboxylic acid} active ingredients)
or a broad-spectrum herbicide, such as glyphosate, will provide effective control;
where grasses are the concern, the latter is the proper choice. A burn-down
chemical, such as paraquat {1,10 -dimethyl-4,40 -bipyridinium}, is also an option in
that instance. It is important to apply non-selective herbicides before the switch-
grass breaks dormancy to avoid crop injury.
It is important to follow all herbicide regulations, label directions, and safety
precautions. Extension staff or professional advisors should be consulted
regarding proper use and application of all herbicides.
Cool-season perennial weeds are relatively easy to control in switchgrass
because their growing season differs from that of switchgrass. Dormant-season
applications of glyphosate can be used to control cool-season grasses. Switchgrass
has modest tolerance to glyphosate early in the growing season [120], which
allows some flexibility in terms of timing spring treatments. Regardless, it is best
to use glyphosate before spring dormancy break or in autumn after switchgrass is
fully dormant.
Summer annual weeds are usually not a problem unless the switchgrass stand
density is low. Aggressive competitors such as crabgrass or seedling johnsongrass
[Sorghum halapense L. (Pers)] usually will not establish and compete after
switchgrass has developed a full canopy and is able to overtop these species.
However, where grass weeds do persist imazethapyr [for crabgrass and signalgrass
(Brachiaria platyphylla (Munro ex C. Wright) Nash)], quinclorac [for foxtails
(Setaria species) and (Echinochloa crus-galli (L.) P. Beauv.)], or sulfosulfuron
(for johnsongrass) can be useful, but generally must be applied when weeds are
small (6–20 cm, depending on the weed species). Warm-season broadleaf weeds
may be controlled with the same herbicides used during the cool-season, except
that broad-spectrum chemicals should not be used once switchgrass is growing
actively.
Low-growing summer perennial weeds often are not able to compete with well-
established stands of switchgrass. Johnsongrass, on the other hand, can persist in
switchgrass stands because of its tall growth habit. Lowland varieties of switch-
grass typically will outcompete johnsongrass. Where that does not occur, and
control is still necessary, sulfosulfuron can be effective. Perennial warm-season
broadleaf weeds can be controlled with the same herbicides that are used to control
cool- or warm-season annuals. With perennials, attention must be paid to stage of
plant development because it affects application timing and rates.
As with any use of herbicides, attention should be paid to crops that may be
planted on the site in the next 12 months because they may be sensitive to some
herbicides. Also, there are some scenarios in which producers may want to use
some part of the switchgrass crop otherwise intended for biofuels for livestock
forage. In such cases, it is important to consider animal feeding restrictions stated

on the herbicide label in deciding which product to use for weed control.
Early-season prescribed fire can also be effective for weed control [76].
Switchgrass should not be burned after mid-April, for the southerly and middle
latitudes of the USA, so as not to reduce biomass yield [120]. There must be
adequate fuel to carry a fire. In some cases, dense stands of early spring weeds, and
the large amount of green vegetation they may produce, can hamper burning.
Burning earlier (early to mid-March) may be effective before the weed cover
becomes too dense. Clipping or mowing may be useful but will reduce yields if it
occurs beyond the very early portion of the growing season. If there is a risk of a
particularly undesirable weed going to seed or weeds otherwise reducing stand
vigor or quality, clipping may be the desirable choice despite potential short-term
reduction in yield.
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Chapter 5
Switchgrass Harvest and Storage
Rob Mitchell and Marty Schmer
Abstract The feedstock characteristics of the conversion platform will influence

the optimal harvest and post harvest management practices for switchgrass.
However, many of the harvest management practices are tied to plant phenology
and will be similar across platforms. Proper harvest and storage of switchgrass will
help provide a consistent and high-quality feedstock to the biorefinery. Bioenergy-
specific switchgrass strains are high-yielding and in most cases can be harvested
and baled with commercially available haying equipment. Many options are
available for packaging switchgrass for storage and transportation, but large round
bales or large rectangular bales are the most readily available and are in use on
farms. Large round bales tend to have less storage losses than large rectangular
bales when stored outside, but rectangular bales tend to be easier to handle and
load a truck for transport without road width restrictions. Although there is limited
large-scale experience with harvesting and storing switchgrass for bioenergy,
extensive research, as well as a history of harvesting hay crops for livestock in
many agroecoregions, makes harvesting and preserving switchgrass for bioenergy
feasible at the landscape scale.
R. Mitchell (&)
Grain, Forage, and Bioenergy Research Unit, USDA-ARS,
135 Keim Hall, University of Nebraska, Lincoln, NE 68583, USA
e-mail: Rob.mitchell@ars.usda.gov
M. Schmer
Agroecosystem Management Research Unit, USDA-ARS,
131 Keim Hall, University of Nebraska, Lincoln, NE 68583, USA
e-mail: Marty.schmer@ars.usda.gov

114 R. Mitchell and M. Schmer
5.1 Introduction
Switchgrass is not a new crop and switchgrass research is not a new phenomenon.
The USDA location in Lincoln, Nebraska, USA has been conducting switchgrass
research continually since 1936. Although the first 50 years of research focused
on switchgrass for livestock and conservation, the research since 1990 within
USDA-ARS, numerous universities, and more recently private industry, has
emphasized bioenergy [1, 2]; see also Chap. 1). Although there is limited
large-scale experience with harvesting and storing switchgrass for bioenergy, more
than 20 years of bioenergy research from small plots to on-farm trials provides
experience and critical insights.
Switchgrass is native to the North American tallgrass prairie and is broadly
adapted to habitats east of the Rocky Mountains and south of 55°N latitude [3].
Switchgrass plants are generally caespitose or with short rhizomes and reproduce
both sexually and asexually. Switchgrass has two primary ecotypes (upland and
lowland) and two primary ploidy levels (tetraploid and octoploid) [2]. Switchgrass
genotypes are largely self-incompatible and seed production results from cross-
pollination by wind [1]. Switchgrass generally grows 1–3 m tall depending on
location and genetic background and can develop extensive root systems to a depth
of 3 m [1]. The aboveground growth and root structure makes switchgrass
well-suited for dual use as a biomass crop and vegetative filter strips which have
removed 47–76% of the total reactive P in surface runoff water in areas treated
with manure [4].
Morphology and phenological development are important to understand when
managing switchgrass for bioenergy. The growth form of both the caespitose and
rhizomatous plants is erect with leaf blade length ranging from 10 to 60 cm
depending on genotype, environment, and location within the plant [2]. Switchgrass
plants tend to be less prone to lodging than other warm-season grasses. Switchgrass
is photoperiod sensitive and requires shortening day length for floral induction,
which helps explain why switchgrass morphology is strongly correlated to day of the
year (DOY) and growing degree days (GDD) [5]. Switchgrass has a determinate
growth habit where most vegetative growth terminates with inflorescence devel-
opment [5, 6], which has implications for regrowth following harvest. Following
floral induction, tillers advance to the seed ripening stages, growth stops, and tiller
senescence occurs. In switchgrass swards in eastern Nebraska, there were no
vegetative tillers present by DOY 196 and 100% of the tillers had elevated apical
meristems [7]. Any regrowth following a harvest at or after this stage will occur only
from retillering. In eastern Nebraska, sufficient regrowth to warrant a second harvest
after a killing frost occurs about one year out of four [8]. For a more complete review
of the morphology and tiller dynamics of warm-season grass swards, see Mitchell
and Moser [9].
Canopy architecture affects the physiology of growing plants and compositional
characteristics of harvested biomass [10] and breeding for increased biomass
and digestibility changed the canopy architecture of switchgrass [11]. Canopy
5 Switchgrass Harvest and Storage 115
architectural traits such as tiller density, phenology, and leaf area index (LAI)
are in a continual state of flux and functions of tiller morphology and the growth
stage distribution of tillers within the tiller population [11, 12]. In Trailblazer
switchgrass, there was an inverse relationship between advancing phenology and
tiller density, with tiller density declining by an average of 9.4 tillers m-2 d-1 and
an average tiller density of 1,525 tillers m-2 during the 2 year study [7]. Quan-
tifying the phenology of tiller populations provides information for understanding
these architectural changes in the grass sward. For example, switchgrass phenol-
ogy advanced linearly with DOY and GDD across six environments in Nebraska
and Kansas [5]. The predictability of switchgrass development in response to DOY
and GDD indicates switchgrass management recommendations for adapted culti-
vars may be made based on DOY within a region [5]. Switchgrass LAI increased
as phenology advanced and varied across years with maximum LAI ranging from
4.9 to 7.7, with at least 95% of the variation in LAI explained by DOY [7]. If the
selected conversion platform targets feedstock material harvested after senescence,
there will be less variability in the phenologic stage of the swards at harvest and
may provide a more uniform product to the biorefinery. However, the morpho-
logical status during the growing season will have implications for other man-
agement decisions.
5.2 Harvest Management
The bioenergy conversion platform likely will determine the optimal harvest and
post harvest management practices for switchgrass [2]. However, many of the
harvest management practices will be similar for all conversion platforms. Many
agroecoregions in the US have a history of harvesting and preserving hay for
livestock, so making adjustments to harvesting for bioenergy production will be an
easy transition. Due to the extensive research conducted on switchgrass, best
management practices and extension guidelines have been developed for many
regions [8, 13, 14]. High-yielding switchgrass fields ([12 Mg ha-1) can be har-
vested and baled with commercially available haying equipment, but some
important items must be considered [8]. For example, self-propelled swathers with
rotary heads (disc mowers) will be required to optimize efficiency and handle the
volume of material harvested from switchgrass bioenergy production fields [8].
Cutting height is easily adjusted and in most cases will be 10–15 cm, which keeps
the windrows elevated above the soil surface to facilitate air movement and more
rapid drying to less than 20% moisture content prior to baling [2]. After harvest,
switchgrass can be packaged for storage and transportation in large round bales or
large rectangular bales [2, 8]. Large round bales tend to have less storage losses
than large rectangular bales when stored outside, but rectangular bales tend to be
easier to handle and load a truck for transport without road width restrictions [8].
These technologies are in use on farms to harvest and package forages for live-
stock and are discussed in more detail in later sections.
5.2.1 Timing and Frequency
Maximum biomass yield with high lignocellulose content is the primary objective of
most herbaceous bioenergy feedstock harvests [8, 15]. Depending on ecoregion,
switchgrass biomass can be maximized with a one-cut or multi-harvest system
[13, 16–18]. Most research supports a single annual harvest for optimizing biomass
and energy inputs, as well as maintaining stands. For example, Sanderson et al. [17]
concluded a single harvest near DOY 260 maximized biomass yield in the south-
central USA. In most rainfed environments of the Great Plains and Midwest USA,
maximum first-cut yields and long-term stand maintenance can be achieved by
harvesting switchgrass once during the growing season to a 10-cm stubble height
when panicles are fully emerged to the post-anthesis stage, near DOY 215 [8, 18,
19]. However, harvesting after frost minimizes nutrient removal, especially N [13].
With upland ecotypes, plant material senesces rapidly and is completely dormant
within 7 days of killing frosts. However, lowland ecotypes grown in northern lati-
tudes senesce and enter dormancy slowly after exposure to killing frost. This dif-
ference in response by ecotype is illustrated by upland and lowland plants harvested
27 days after the first killing frost and exposed to low temperatures of less than 0°C
on 17 of the 27 days. The completely dormant material is Shawnee, whereas the
material with green stem bases is a lowland strain selected from Kanlow (Fig. 5.1).
This delay in entering dormancy may be one explanation for the winter injury
susceptibility of lowland ecotypes. However, harvest strategies for upland and
lowland ecotypes have not been compared in agro-ecoregions where both ecotypes
occur, so harvest strategies may vary [8]. Proper harvest timing, cutting height and
maintaining adequate N fertility are important management practices required to
maximize yield and ensure persistent switchgrass stands [2, 8]. As previously
mentioned, time of harvest research generally indicates a single harvest at post-
anthesis maximizes yield, but harvesting after a killing frost ensures stand persis-
tence and productivity, especially during drought [2, 8]. For example, Vogel et al.
[18] reported switchgrass biomass increases up to anthesis, then decreases by
10–20% until killed by frost. This fits well with recommendations by Mitchell et al.
[8] who reported switchgrass should not be harvested within 6 weeks of the first
killing frost or below a 10-cm stubble height to ensure carbohydrate translocation to
the plant crowns for setting new tiller buds and maintaining stand productivity.
Wullschleger et al. [20] compiled a database comprised of switchgrass biomass
production studies conducted at 39 field sites in 17 states which supported the single
harvest for bioenergy. They reported the switchgrass mean biomass yield across all
locations was 8.7 ± 4.2 Mg ha-1 for upland cultivars and 12.9 ± 5.9 Mg ha-1 for
lowland cultivars and the yield difference between ecotypes was significant.
Additionally, they reported that there was no evidence that small plots biased
switchgrass yield when compared to field-scale sites and stressed the importance of
single harvest systems for biomass energy.
Several studies throughout the Great Plains and Midwest have evaluated
switchgrass harvest management. Phenologic stage at first harvest did not affect
Fig. 5.1 Upland and lowland ecotypes of switchgrass enter dormancy at different rates when
grown in the same environment. Both photographs were taken following field-scale switchgrass
harvest in eastern Nebraska on 15 November, 2011. Notice that the upland cultivar a ‘Shawnee’
is completely dormant, whereas the lowland experimental strain b still has green stem bases
(photos by Rob Mitchell)
switchgrass persistence, but regrowth potential decreased as first harvest was

delayed to later stages of development and later DOY [21]. Harvesting switchgrass
two or three times each year resulted in the greatest stand reductions [22].
Switchgrass harvested once at anthesis in Nebraska and Iowa had greater biomass
than areas harvested twice [18]. Biomass was maximized with a single harvest
during anthesis and yields ranged from 10.5 to 12.6 Mg ha-1 yr-1 with no stand
reduction [18]. In Texas, Sanderson et al. [17] harvested several switchgrass
strains once or twice per growing season from multiple environments and con-
cluded that a single harvest in autumn maximized biomass and maintained stands.
In general, delaying harvest until after a killing frost reduces yield, but
ensures stand productivity and persistence, especially during drought, and reduces
N fertilizer requirements for the following year by about 30% [2, 8]. Post-frost
harvests allow N and other nutrients to be mobilized into roots for storage during
winter and use for new growth the following spring, but will reduce the amount
of snow captured during winter and will limit winter wildlife habitat value [8].
Harvesting after a killing frost is a logical management decision for thermo-chemical
conversion platforms and biopower because N, Ca, and other plant nutrients that
function as contaminants in the thermo-chemical process are minimized in the plant
tissue. Another alternative harvest time is to leave switchgrass standing in the field
over winter and harvest the following spring [23]. Delaying harvest until spring
reduced yield by 20–40% compared with harvesting in autumn after a killing frost, but
had no effect on gasification energy yield [23]. Yield losses associated with delaying
harvest until spring may be acceptable if wildlife cover during winter is critical [23].
These studies from a broad geographic range in the USA support a single
annual harvest will maximize biomass and maintain stand persistence, but harvest
timing needs to be considered for optimizing biofuel production. Additionally, the
conversion process is an important consideration when determining the optimum
harvest date. With good harvest and fertility management, productive stands can
be maintained indefinitely and certainly for more than 10 years [8].
5.2.2 Nutrient Removal
Harvesting biomass, whether crop residue or dedicated herbaceous perennial

feedstocks such as switchgrass, removes large quantities of nutrients from the
system [19]. In most agro-ecoregions, nitrogen (N) is the most limiting nutrient
for switchgrass production and is the most expensive annual production input.
Consequently, reducing N removal from the switchgrass production system
has a positive effect on the economic and environmental sustainability of the
system. For example, harvesting 10 Mg ha-1 of switchgrass DM with whole-plant
N concentration of 1% will remove 100 kg of N ha-1, whereas if harvest is delayed
until after senescence, N concentration can decline to 0.6%, resulting in the removal
of only 60 kg of N ha-1. Depending on conversion platform and a predictable
harvest window in autumn or winter, this 40 kg of N ha-1 reduction in N removal
may be an acceptable trade-off for the yield losses associated with delaying harvest.
Collins et al. [24] reported the average yield for three switchgrass cultivars
irrigated in the Pacific Northwest ranged from 14.5 to 20.4 Mg dry matter ha-1
yr-1. They reported each kg of N produced 83 kg of biomass and the macronu-
trient export averaged 214 kg N ha-1, 40 kg P ha-1, 350 kg K ha-1, 15 kg S
ha-1, 60 kg Ca ha-1, 38 kg Mg ha-1, and 6 kg Fe ha-1. Averaged across culti-
vars, switchgrass removed less than 1 kg ha-1 of B, Mn, Cu, and Zn. Additionally,
delaying harvest until spring reduced ash content and leached nutrients from the
vegetation [23]. Although management of all nutrients in the system is important,
N is the most expensive, has the greatest potential for environmental contamina-
tion, and has the greatest influence on life cycle assessment.
Nitrogen removal in switchgrass production systems is a function of biomass
yield and N concentration, with biomass N concentration increasing as N
fertilization rates increase [18]. In a multi-environment study evaluating numerous
N rates and harvest dates, biomass was optimized when switchgrass was harvested
at the boot to post-anthesis stage and fertilized with 120 kg N ha-1 [18]. At this
harvest date and fertility level, the amount of N removed at harvest was similar to
the amount of N applied, and soil NO3-N did not increase throughout the study
[18]. Consequently, it is important to consider the interaction of N rate and harvest
date to only replace the N needed for the production system to prevent over-
fertilization and soil N accumulation.
5.2.3 Soil Carbon
As mentioned previously, switchgrass has an extensive perennial root system

which protects soil from erosion and sequesters carbon (C) in the soil profile [25].
Soil organic carbon (SOC) typically increases rapidly when annual cropland is
converted to switchgrass [26, 27]. The amount of C sequestered depends on the
climate, soil type, original soil C content, time, and placement depth of C [28, 29].
For example, switchgrass grown and managed for bioenergy on three marginally
productive cropland sites in Nebraska resulted in an average SOC increase of
2.9 Mg C ha-1 yr-1 in the top 1.2 m of soil in just 5 years [30]. In South Dakota,
switchgrass grown in former cropland enrolled in CRP stored SOC at a rate of
2.4–4.0 Mg ha-1 yr-1 at the 0–90 cm depth [31]. McLaughlin et al. [32] reported
an average of 1.7 Mg C ha-1 yr-1 sequestered in the Southeast U.S. on switch-
grass experimental plots. Soil carbon levels on low-input switchgrass fields have
been shown to increase over time, across soil depths, and are higher than adjacent
cropland fields in the Northern Plains [25]. A similar result was found between
switchgrass and a corn (Zea mays L.)-soybean (Glycine max (L.) Merr.)-alfalfa
(Medicago sativa L.) rotational system in Iowa [33]. Switchgrass managed for
bioenergy on multiple soil types in the Northern Plains was carbon-negative,
sequestering 4.42 Mg C ha-1 yr-1 into the soil profile [34]. In the Southeast
U.S.A., an estimated 0.17–0.21 Mg C ha-1 yr-1 was sequestered on switchgrass
plots, managed as a bioenergy crop, based on SOC that was near steady state [35].
Nitrogen applications on switchgrass plots did not alter root C storage when
compared with non-fertilized plots in a 2 year study [36]. However, fertilization of
grasslands increased the amount of C sequestered by 0.30 Mg ha-1 yr-1 on 42
studies throughout the world [28]. Microbial biomass carbon increased after
establishment of switchgrass and carbon mineralization increased by 112 and
254% at depths of 0–0.15 m and 0.15–0.30 m, respectively [36]. Soil organic C
increased at rates ranging from 1.7 to 10.1 Mg C ha-1 yr-1 after switchgrass
establishment throughout North America [31, 34, 35, 37].
5.3 Storage Management
Substantial amounts of switchgrass biomass will need to be safely stored on a

year-around basis to supply a cellulosic biorefinery. Cellulosic biorefineries in the
U.S. are expected to keep only a 72 h feedstock inventory with the remaining
feedstock inventory at the edge of field or at satellite storage facilities [38].
At present, there is uncertainty on the overall capacity of cellulosic biorefineries
but techno-economic models have evaluated refinery sizes ranging from 535 to
8,000 dry Mg feedstock per day [39–41]. Offsite storage management will be
critical to maintain desirable composition characteristics and to ensure feedstock
access under variable weather conditions. Storage infrastructure requirements will
need to be cost effective, maintain desirable quality characteristics depending on
conversion technology, provide an aerobically stable environment, and have
flexible delivery schedules depending on regional weather factors [42].
Storage requirements and management will be dependent on how switchgrass is
harvested. In the near-term switchgrass will be harvested and baled using com-
mercial hay equipment. Self-propelled mower/conditioners (swather) with rotary
heads are effective in harvesting high-yielding ([12 Mg ha-1) switchgrass fields
(U.S. [43]). The conditioner component on a swather accelerates switchgrass
drying by crushing plant stems but not altering plant structure and consolidates the
switchgrass into a windrow [38]. After harvest, the baling step bundles switchgrass
into a more condensed form to ease handling, transport and storage. Variable
chamber round balers or rectangular balers will likely be used to consolidate and
bundle switchgrass. Round balers will typically make a bale that is 1.2–1.8 m in
diameter and 1.2–1.8 m in length. Large rectangular bale size ranges from 0.9 to
1.2 m in height and width and 1.8–2.4 m in length. Round balers and large
rectangular balers require switchgrass moisture levels to be B18 and B16%,
respectively, at time of baling to reduce storage losses. Bale moisture content in
excess of these respective values may result in composition degradation or
spontaneous combustion. Field drying prior to baling is required to meet safe
moisture levels for baling which may be hindered depending on the region and
harvest date. Balers can be modified to spray preservatives (e.g. propionic acid)
onto hay limiting microbial growth and removing excess moisture for hay with
20–25% moisture content [44].
The density of a round bale or a large rectangular bale will vary depending on
harvest period with anthesis harvest bales having a greater density than post-killing
frost harvest bales. There are advantages and disadvantages for the round baling or
large rectangular baling methods but both are capable of processing switchgrass
and are commercially available to producers. The round baler is one-fourth to
one-third the capital cost as a large rectangular baler [45] but the field capacity of a
round baler is lower because the baler needs to be stopped to wrap and release the
bale. Large rectangular balers continuously bales without the need for stopping
and is estimated to cost less per unit of harvested area [46]. Smaller bioenergy
producers may opt for the round baler methods because of the lower capital costs
or may outsource harvest and baling to custom harvesting enterprises that are
equipped with large rectangular balers. Rectangular bales need to be removed from
the field soon after baling and protected from precipitation events because the flat
surface of the bale does not shed water and resultant DM losses can be large [44].
Commercially available self-propelled or pull-type round or rectangular bale
stacking equipment collect bales within the field and are able to place bales at the
edge of field for short-term or long-term storage until feedstock delivery to a
biorefinery. These stacking systems significantly lower energy use and increase
field capacity efficiency when compared with a single bale loader system.
Switchgrass round bales have less storage losses than large rectangular bales when
stored outside as they are less prone to water penetration especially when net
wrapped [38]. Net wrapped round bales had 60–70% lower DM losses when
compared with round bales tied with plastic twine [47]. Rectangular bales tend to
be easier to handle and load a truck for transport without road width restrictions.
The time required to load bales onto semi trailers is double for round bales than it
is for rectangular bales [38]. Unless cellulosic biorefineries stipulate a certain
baling method or alternative harvest method, both baling methods will likely occur
for a given region.
Consolidation methods other than baling may be implemented in regions where
weather conditions or existing infrastructure enterprises allow for alternative
harvesting scenarios [19, 48–51]. Wet storage methods have been proposed for
switchgrass in regions where drying conditions for baling operations are not
possible because of high relative humidity and increased chance of a precipitation
event after harvest [50]. Switchgrass harvested using wet storage methods include
either a swather harvest and then chopped using a self-propelled forage harvester
with a windrow pickup or directly cut with a self-propelled forage harvester with
an attached rotary head that blows the material into adjacent semi bulk trailers.
Moisture content for switchgrass at time of pickup under wet storage methods
are[40%. Advantages to wet storage methods include reduced harvest costs, lower
DM losses during storage, improved switchgrass cell wall recovery during enzymatic
hydrolysis and lower potential risk of fire during storage [50]. Disadvantages for the
wet storage method include higher equipment and storage structure costs than a
conventional baling system [44]. The wet storage method was found to be more
expensive than other collection and storage methods for cellulosic refinery sizes
greater than 1,500 Mg switchgrass per day because of the high cost of the ensiling pit
and transportation of wet material by truck [51].
Regions where silage harvesting is common would likely have increased partic-
ipation in storing switchgrass under wet conditions. Field chopping using a forage
harvester can be done at moisture levels similar to baling in less humid regions. Field
chopping has an added advantage to baling in that particle size is much smaller which
may eliminate a preprocessing step at the biorefinery [19]. Estimated delivery costs
for chopped switchgrass biomass are less than for a conventional baling system [51].
Chopped biomass requires specific storage areas either at farm site or at a satellite
storage facility. Chopped biomass has the lowest bulk density and densification may
be an issue for long-term storing and transporting the material [19]. Southeastern
U.S. researchers have proposed increased densification of chopped switchgrass by
using modulizing technology developed for the cotton industry [48, 49].
A loafer stacker system has also been proposed as a cost effective method to
collect switchgrass for biomass production [52]. The loafing system is similar to
the field chopping system (dry storage) with the exception that instead of blowing
switchgrass material into a semi trailer the loaf stacker picks up switchgrass from
the windrow and makes a biomass stack approximately 2.4 m wide, 6 m long, and
3.6 m high [51]. The roof of the loafer stacker has a dome shape which creates a
biomass stack that resembles a bread loaf and is designed to shed water. Field
capacity of a loafer stacker is lower than either conventional baling system or a
forage chopper. Once the loafer stacker is full, the operator needs to immediately
transport the biomass stack to the edge of field or use specialized trailers to
transport the biomass stack after harvest. Biomass stacks are also susceptible to
large biomass loss in regions with significant wind velocities if placed perpen-
dicular to prominent wind direction.
The U.S. Department of Energy has proposed a uniform-stacking feedstock
supply design that can pre-process switchgrass and other cellulosic materials
regardless of collection method for use in a large-scale cellulosic biorefinery
(C4,535 Mg d-1) which would increase regional and producer flexibility to har-
vest and collect switchgrass [38].
5.3.1 Desirable Storage Characteristics
The ideal storage management procedures are to preserve switchgrass so that it

enters and leaves the storage phase in an unaltered state [53]. Key factors in
minimizing storage loss for bales are to ensure low moisture levels prior to storage
and protection from moisture during the storage phase. Low relative humidity and
low ambient temperatures during storage also reduce DM loss and composition
degradation. Maintaining low biological activity during storage to reduce micro-
bial growth and subsequent storage loss is also important.
Switchgrass with higher levels of N or with increased soluble sugars have
increased potential for microbial growth and degradation during bale storage [38].
Harvest dates determine overall N and soluble sugar content in switchgrass [54].
Storage conditions that reduce the potential for spreading crop diseases, low rodent
populations, and mold spore formation are also desirable [38].
5.3.2 Storage Platforms
Although there is limited research on switchgrass storage platforms for specific

bioenergy purposes, there is significant storage research on forages that offer
insights into the advantages and disadvantage of different storage options.
Near-term storage strategies include placing bales outside on well-drained surfaces
(i.e. gravel, crushed rock), tarping, bale wrapping in plastic and indoor placement.
Optimal storage platforms are dependent on expected bale storage losses and
projected storage costs to offset these losses. For example, enclosed buildings are
the most expensive storage platform but also ensure the greatest switchgrass value
and lowest storage loss [55]. Proper storage of wet material or ensiling has been
well documented for a number of feedstocks including switchgrass [38, 44, 56].
Pre-processing steps such as pelletizing or briquetting switchgrass provides
decreased storage losses and decreased transportation costs [57]. Estimated capital
costs for a pellet mill or briquettor, however, potentially offset any near-term
savings in storage or transportation costs [41].
5.3.3 Storage Losses
Limited research has been conducted on DM losses during switchgrass storage

with most research evaluating storage loss using the baling method. In Texas, DM
losses for large, round bales ranged from 1 to 5%, with larger losses occurring with
drier material [58]. Switchgrass round bales stored for 6–12 months inside had
0–2% DM losses, whereas bales stored outside lost 5–13% of the original bale
weight [58]. In southeastern U.S., round bales with higher initial moisture content
and longer storage times caused increased DM loss when stored outside [59].
In Indiana, switchgrass round bales wrapped in twine had 13% DM loss on sod but
bales stored on crushed rock had 5% DM loss after six months [60]. Switchgrass
round bales stored outside on either sod or gravel showed similar DM losses
12 months after baling in Texas [58]. Estimated DM storage losses in excess of
16% are required to cover the initial cost of storage sites using crushed rock for
improved drainage [38]. In southern Europe, switchgrass round and rectangular
bales showed minimal storage loss and no visible microbial activity when stored
under a sheltered roof [61]. Storage loss was found to be greater for tarped large
rectangular bales than for tarped round bales and that delivery costs increased with
larger storage times due to increased storage losses [62]. Tarped and untarped
large rectangular bales had DM losses of 7% and up to 25%, respectively, six
months after harvest in Nebraska [8]. Water and temperature together determines
microbial damage for storage systems with regions having high relative humidity
and having temperatures results in increased storage degradation on portions of
biomass in direct contact with air [38]. In general, biomass stored dry should be
kept at moisture levels below 15% to prevent biomass degradation by filamentous
fungi and bacteria [63]. Additional physical factors that cause storage loses include
wind erosion or handling losses, moisture partitioning, bulk settling, and dust
accumulation [38].
5.3.4 Changes in Composition During Storage
Composition changes during storage will likely be more unfavorable for bio-
chemical conversion than either thermo-chemical conversion technology or direct
combustion for electrical generation. Switchgrass round bales stored unprotected
outside lost up to 11% of ethanol extractables, which could significantly reduce
conversion to ethanol [64]. Biomass quality heterogeneity will occur within bales
with portions of the bale showing no signs of degradation while other portions
having significant spoilage or composition degradation. Round bales can be
segmented to four portions based on the potential for deterioration [65].
Approximately 33% of a round bale circumference contacts the ground after
settling which can absorb moisture and result in spoilage [38]. The first round bale
portion is where the round bale contacts the ground up to 15 cm. A transitional
area above this portion (15–30 cm) can also be degraded depending on moisture
conditions and length of storage. Sanderson et al. [58] noted that switchgrass round
bales stored on sod had a large, black layer where the bale was in contact with sod
whereas round bales stored on gravel did not have this layer of spoilage indicating
outside storage method will influence composition heterogeneity of bales. The
third portion of the round bale is the outer 15 cm of the round bale not in contact
with the ground. This portion can also have compositional changes depending on
weather factors, length of storage, and wrapping methods (i.e. plastic twine, plastic
net-wrap). The final portion of the round bale is the core which is the least likely to
Fig. 5.2 Large rectangular

bales are susceptible to
spoiling on the top and
bottom of the bale if not
stored properly. This bale
was cut in half to expose the
spoilage on the bale interior
(photo by Rob Mitchell)
have compositional changes during storage. Biomass degradation from weather

and microbial activity can be as high as 42% by volume for a round bale [65].
For large rectangular bales, moisture can penetrate at the top of the bale or can be
absorbed at the bottom of the bale. A large rectangular bale is comprised of a
number of layers of switchgrass compressed together. Water channeling can occur
within these layers causing heterogeneous spoilage ([38]; Fig. 5.2).
Large rectangular bales are typically stacked so water channels between layers
can cause biomass degradation to adjacent bales as well. Chopped switchgrass in a
dry form will have the most compositional changes around the outer surface layer
(0–0.8 m) with the interior portions unaltered. The amount of compositional
changes by volume from dry chopped switchgrass piles is a result of the overall
stack size. Sulfuric acid pretreatment on switchgrass stored under wet conditions
inhibited microbial activity and resulted in 7% higher ethanol conversion effi-
ciency than untreated switchgrass [50].
5.4 Conclusions
This brief overview has scratched the surface of switchgrass harvest and storage
management. Proper harvest and storage management is paramount to providing a
consistent and high-quality feedstock to the biorefinery. Although the bioenergy
conversion platform will guide the switchgrass harvest and post harvest
management practices, proper handling will ensure optimum biofuel recovery.
Continued research on the effects of harvest and storage management on feedstock
characteristics is critical as landscape scale deployment of switchgrass for
bioenergy moves forward. Important areas for continued research include the
effects of compositional changes during storage on biofuel production; harvest
timing effects on ecosystem services, especially SOC sequestration, wildlife, and
pollinator habitat, and GHG emissions and mitigation; and long-term research
evaluating harvesting effects on macronutrient and micronutrient removal, as well
as developing strategies for maintaining soil nutrient status.
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Chapter 6
Environmental Impacts of Switchgrass
Management for Bioenergy Production
R. Howard Skinner, Walter Zegada-Lizarazu and John P. Schmidt
Abstract In this chapter, we review major environmental impacts of growing

switchgrass as a bioenergy crop, including effects on carbon sequestration,
greenhouse gas emissions, soil erosion, nutrient leaching, and runoff. Information
from life cycle analyses, including the effects of indirect land use change (iLUC),
is examined to quantify the full impact of migration to bioenergy cropping systems
on both managed and natural ecosystems. Information on the environmental
impacts of switchgrass cultivation is scarce and there exists a critical need for
additional research. What limited information there is suggests that switchgrass
provides multiple environmental benefits compared to annual crop cultivation.
However, benefits generally appear to be similar to other perennial crops.
6.1 Introduction
An evaluation of the environmental impacts of switchgrass (Panicum virgatum L.)

depends on contrasts to alternative crop species or cropping systems that
switchgrass will potentially displace or to which switchgrass might be preferred.
R. Howard Skinner (&)

Pasture Systems and Watershed Management Research Unit,
USDA-ARS, University Park, PA 16802, USA
e-mail: howard.skinner@ars.usda.gov
W. Zegada-Lizarazu
University of Bologna, Viale G. Fanin 44, 40127 Bologna, Italy
e-mail: walter.zegadalizarazu@unibo.it
J. P. Schmidt
Pioneer Hi-Bred International, Inc., A Dupont Business,
Champaign Research Center, 985 County Road 300 E, Ivesdale, IL 61851, USA
e-mail: john.schmidt@pioneer.com

130 R. Howard Skinner et al.
In the Midwestern United States, an area dominated by maize (Zea mays L.) and
soybean [Glycine max (L.) Merr.] production, an important contrast will be based
on impacts relative to these crops. Alternative perennial crops could be various
cool-season (C3) grasses, forage legumes, or another C4 grass, Miscanthus
x giganteus Greef & Deuter ex Hodkinson & Renvoize (hereafter referred to as
miscanthus). The most relevant contrasts are those that represent realistic alter-
natives. If switchgrass is grown as a bioenergy crop, it will likely compete for a
place on the landscape with maize, soybean, cool-season perennials, and
miscanthus.
Switchgrass was selected as a model bioenergy crop in the U.S. because it is a
native plant, produces substantial above-ground biomass (20 Mg ha-1 yr-1), and
has an extensive areal range in North America. The development of this selection
is described by Wright and Turhollow [1]. Another bioenergy crop, miscanthus,
offers a particularly interesting alternative to switchgrass. Miscanthus has been the
focus of bioenergy research in Europe because it produces as much as
40 Mg ha-1 yr-1 above-ground biomass with similar or fewer N fertilizer inputs
as switchgrass [2]. Both miscanthus and switchgrass are desirable bioenergy crops
because: (1) biomass yield is high; (2) they are perennial rhizomatous plants that
cycle nutrients seasonally between the above- and below-ground portions of the
plant, thus minimizing fertilizer requirements and corresponding environmental
impacts; (3) they provide a clean burning fuel when they are harvested after
senescence; (4) planting is required only once, so minimal fuel costs associated
with tillage and planting are incurred; and (5) they are both C4 plants, which are
photosynthetically more efficient than C3 species [2]. A potential advantage of
switchgrass over miscanthus is that it is reproduced by seeds, whereas miscanthus
reproduction is vegetative with accompanying higher establishment costs and need
for specialized equipment.
In this chapter, we review major environmental impacts of growing switchgrass
as a bioenergy crop including effects on carbon sequestration, greenhouse gas
emissions, soil erosion, nutrient leaching, and runoff. Where available, information
from life cycle analyses, including the effects of indirect land use change (iLUC),
will be examined to quantify the full impact of bioenergy crops on both managed
and natural ecosystems.
6.2 Climate Change
6.2.1 Carbon Sequestration
One important impact of growing switchgrass or other bioenergy crops will be the
potential for C sequestration or loss of C from the soil. The number of studies
looking at C sequestration in switchgrass stands is limited, but those that exist have
shown that replacing annual crops with perennials such as switchgrass increases C
sequestration. In an analysis of published estimates of soil organic carbon (SOC)
6 Environmental Impacts of Switchgrass Management 131
changes following conversion of natural or agricultural lands to biofuel crops,

Anderson et al. [3] found that removing maize grain and residue as a bioenergy
feedstock led to rapid loss of SOC at rates up to 4.2 Mg ha-1 yr-1 compared with
management for grain removal only. They calculated that 10 years of maize
biomass removal resulted in a loss of about 3 Mg ha-1 yr-1 at 25% residue
removal and about 8 Mg ha-1 yr-1 at 100% removal. In contrast, SOC accumu-
lation under switchgrass ranged from about 0.4 to 0.7 Mg ha-1 yr-1 depending on
the statistical model used to estimate changes.
Anderson et al. [3] reviewed the existing literature and identified three general
principles governing the C balance of biofuel crops. First, conversion of unculti-
vated soil to biofuel crops initially entails a loss of SOC; second, crops differ in the
ability to sequester C with perennial crops outperforming annuals such as maize;
and third, a tradeoff exists between biomass removal and C sequestration. In
another review of the literature, Blanco-Canqui [4] also concluded that residue
removal reduced SOC concentration, whereas, planting warm-season grasses such
as switchgrass increased C sequestration.
In a paired comparison of 120 cm deep soil samples from 42 switchgrass/
cropland sites, Liebig et al. [5] found that SOC was greater in switchgrass stands at
soil depths of 0–5, 30–60, and 60–90 cm and that the differences in SOC were
especially pronounced at deeper soil depths. They attributed the difference at depth
to the greater switchgrass root biomass below 30 cm compared with cropland sites.
In a modeling study, simulations with the DAYCENT model [6] predicted an
increase in SOC of 45–300% after 15 years of switchgrass growth compared to
cotton production. However, another modeling exercise using DAYCENT sug-
gested that maize had slightly greater SOC than switchgrass [7] and that little
change in SOC was predicted to occur over a 10 year period for either cropping
system.
In contrast to the studies cited above, Tolbert et al. [8] found that both no-till
maize and switchgrass had accumulated SOC after three growing seasons, with no
significant difference in accumulation rate between the two. However, SOC was
numerically greater under switchgrass even though differences were not signifi-
cant. In a Canadian study [9] switchgrass increased SOC by 3 Mg ha-1 yr-1
compared with maize at a higher fertility site, whereas, there was no significant
difference after 4 years between switchgrass and maize on a rocky shallow-soil.
Because SOC content changes relatively slowly against a large background
pool, long-term studies are often needed before differences in accumulation rates
are translated into significant differences in the magnitude of SOC pools. Most of
the switchgrass studies cited here, and reviewed by Anderson et al. [3] lasted less
than 5 years, highlighting the critical need for more long-term studies.
Comparisons between switchgrass C sequestration and other perennial systems
generally revealed little difference between systems, or even lower C sequestration
potential for switchgrass sites. Non-significant differences in SOC or C sequestration
rates were observed when switchgrass was compared with forests or cool-season
grasses [10], smooth bromegrass (Bromus inermis Leyss.) [11], or short-rotation
poplar plantations [12]. When Chamberlain et al. [6] simulated land use conversion
from unmanaged grasses to switchgrass, SOC decreased if the switchgrass was not
fertilized, was unchanged when 45 kg N ha-1 was applied, and increased when 90
and 135 kg N ha-1 were applied. Omonode and Vyn [13] observed little difference
in surface SOC content between switchgrass and mixed native warm-season grasses
but when SOC mass was calculated to a depth of 1.0 m, SOC was 8% higher under
switchgrass than under the native mixture.
Two modeling studies have suggested that C sequestration would be less for
switchgrass compared with other perennial species. Growing willow (Salix alba x
glatfelteri L.) was calculated to increase SOC by 9.0–9.5 Mg ha-1 yr-1 compared
with 3.0–3.5 Mg ha-1 yr-1 for switchgrass [9]. Davis et al. [7] estimated that SOC
under switchgrass would be similar to native prairie but that both would be sig-
nificantly less than miscanthus. While switchgrass appears to have greater C
sequestration potential than annual crops, including maize grown as a biofuel,
there is no indication that it has any greater C sequestration potential than other
perennial systems.
The potential to sequester C depends on a number of factors including initial soil C
content, prevailing soils and climate, and management practices. Sequestration is
generally greater when existing SOC pools have been depleted, in cool compared
with warm climates, in fine-textured compared with course-textured soils, and where
soil fertility is high [14]. Perhaps the most widely studied variable is the effect of N
fertility. In a switchgrass study in the southern USA, Ma et al. [15] found no dif-
ference in SOC among N application rates of 0, 112, and 224 kg N ha-1. They
attributed the lack of N effect to the short, 3 year, duration of the study. However, in a
study of similar duration in the northern Great Plains, Lee et al. [16, 17] applied 112
or 224 kg N ha-1 as either ammonium nitrate or as manure to mature switchgrass
stands. Applying N as manure increased SOC accumulation rate to a depth of 90 cm
by 33–125% compared with mineral fertilizer, probably because of the additional C
input from the manure. All fertilizer rates and sources increased C sequestration
compared with no fertilization but there was no difference between the 112 and
224 kg N ha-1 application rates. Averaged across N rates, C sequestration was
2.4 Mg ha-1 yr-1 for plots receiving mineral fertilizer compared with
4.0 Mg ha-1 yr-1 when manure was applied.
In the simplest terms, C sequestration represents the net balance between C inputs
into the system, mainly from photosynthesis but potentially from application of
organic sources such as manure or crop residues, and C outputs, mostly from soil and
plant respiration but also from removal of harvested material and potentially from
runoff or leaching. An assessment of CO2 fluxes during the first 4 years of switch-
grass establishment by Skinner and Adler [18] found that photosynthetic inputs
varied little from year to year, ranging from 9.2 to 9.4 Mg-C ha-1 yr-1. In contrast,
ecosystem respiration ranged from 6.8 to 9.1 Mg-C ha-1 yr-1, and harvested bio-
mass removal ranged from 0 to 2.4 Mg-C ha-1 yr-1. Mean C sequestration over the
4 years was 0.4 Mg-C ha-1 yr-1, and clearly depended more on processes affecting
C loss than on C uptake. Similar dependence of sequestration on C loss rather than
uptake were observed for cool-season pastures in the northeastern U.S. [19] and for
forests along a north–south transect in Europe [20].
Several factors have been found to affect the C dynamics of switchgrass sys-
tems. Stepwise regression analysis by Lee et al. [16, 17] found that soil temper-
ature was highly correlated with soil CO2 flux, whereas, soil moisture was not.
Garten and Wullschleger [21] also observed slower decomposition rates in cooler
climates. In contrast to the results from Lee et al. [16, 17], Frank et al. [22]
measured lower soil CO2 flux during a drought year compared to CO2 fluxes
during 2 years of above average precipitation.
Lee et al. [16, 17] also found that manure application increased soil respiration
but ammonium nitrate application did not. The manure effects were due to
increased soil microbial biomass C and potentially mineralizable C. Soil texture
may also exert some control over dynamic soil C fractions such as microbial
biomass C and thus affect soil respiration. In turn, microbial biomass C will be
affected by C input from roots coupled with the influences of soil moisture and
temperature [23]. Ma et al. [23] also found that harvest frequency affected soil
respiration and attributed the results to the effect of harvest frequency on root
lifespan.
Establishment of any new crop, including switchgrass, usually entails an initial
loss of soil C, incurring a ‘‘carbon debt’’ that must be repaid before net C
sequestration can occur. Corre et al. [10] reported that conversion from cool-
season grass to switchgrass initially resulted in a loss of SOC, and that it took
16–18 years after planting for SOC under switchgrass to approach that under the
undisturbed cool-season grass. It has been suggested that growing perennial
grasses on former cropland soils might result in little or no carbon debt, whereas,
replacing uncultivated land could incur a debt that might require decades or even
centuries to repay [3, 24]. Whatever the magnitude of the debt, it is important that
initial C losses be considered when evaluating the C sequestration potential when
replacing existing vegetation with switchgrass or other bioenergy crops.
6.2.2 Nitrous Oxide
Among carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), the three
primary greenhouse gases associated with agricultural production systems, the
latter has the greatest global warming potential. Although N2O is found at lower
atmospheric concentrations, its global warming potential can be as high as 296
times that of CO2 over a 100 year period [25]. In nature, soil and oceans are
sources of N2O, where it is produced by microbial processes of nitrification and
denitrification. Nitrification is the aerobic oxidation of ammonium to nitrate, and
denitrification is the anaerobic reduction of nitrate to nitrogen gas. N2O is a
gaseous intermediate in the reaction sequence of denitrification and a minor
by-product of nitrification [26].
Almost all agricultural systems are a significant source of direct (from
agricultural lands) and indirect (from volatilization/deposition and leaching/runoff)
N2O emissions through the application of N fertilizers and animal manures.
In general annual crops produce about three times more emissions than unmanaged
successional lands and perennial crops such as poplar [27]. According to these
authors, the major determinant of N2O emissions is the amount of nitrogen
available in the soil. However, Stehfest and Bouwman [28] indicate that nitrogen
fertilization rate, crop type, fertilizer type, soil organic content, soil pH, and
texture also play an important role in controlling the activity of nitrifiers and
denitrifiers and thus the N2O emissions from agricultural fields. Therefore, crop-
ping systems and crops such as switchgrass with lower nutrient demands or more
efficient utilization of fertilizer inputs are likely to have a greater potential to
reduce N2O emissions and be more profitable to the farmer.
In general, the well developed root systems of grasses like switchgrass have a
great capacity for N uptake and large amounts of inorganic N seldom accumulate
in soils where they are grown [29]. Moreover, Bransby et al. [30] indicated that
switchgrass has the ability to recover about 66% of applied N, which is about 16%
higher than an established standard of wheat (Triticum aestivum L.) or maize,
confirming its high potential to reduce GHG emissions compared to annual crops.
Moreover, a comprehensive review of switchgrass and miscanthus agronomy
indicated that switchgrass has a stronger response to N fertilization than miscan-
thus [2]. This higher response to fertilization could be one of the reasons for the
75% lower N2O emission from switchgrass than miscanthus as reported by
Zeri et al. [31] in one of the few side-by-side comparisons of N2O fluxes from
these grasses.
The fertilization rates for switchgrass vary widely; several authors indicated that
economically and energetically viable yields can be obtained with 0–100 kg ha-1
of N fertilization, depending on site-specific soil conditions, water availability, and
crop management [32–34]. Moreover, for optimum biomass yields, Vogel [35]
indicated that switchgrass requires between 10 and 12 kg N ha-1 for each Mg ha-1
of biomass produced.
Since Bransby et al. [30] showed evidence that the N recovery capacity of
switchgrass does not change with varieties or harvesting times but only with the
yield levels, it is assumed that N2O emission could be decreased by increasing
switchgrass productivity. However, due to the sometimes quadratic [36] depen-
dency between increased fertilization and yields, the effective potential to reduce
GHG emission can be counteracted by N2O emissions that may or may not be
proportional with the amount of nitrogen fertilization. For example, Heggenstaller
et al. [36] indicated that total plant N content with fertilization rates of
220 kg N ha-1 was less than with 140 kg N ha-1, indicating that at higher fer-
tilization rates more N remained in the soil with a greater potential for N loss from
the system. The N loss was probably by a combination of volatilization, denitri-
fication, and/or leaching. However, because information is lacking on the relative
contribution of each N loss pathway, an exact determination of the greater N2O
emission potential at the higher fertilization rate is difficult.
Actual quantification of N2O emissions from switchgrass fields is almost
nonexistent, not only because installing the measuring chambers is costly, but also
because of the high spatial and temporal variability in N2O fluxes [25]. Some
studies have shown that the highest N2O fluxes occur just after N fertilizer
application and/or after large rainfall events [37, 38], making it difficult for spot
measurements with small chambers to be representative of total GHG emissions.
Therefore, most of the emission values reported in the literature are estimated
based on emission factors and calculation guidelines developed by the IPCC [26]
and life cycle analysis (LCA) studies such as Qin et al. [39], Adler et al. [40],
Crutzen et al. [41], among others. Discrepancies and uncertainties between
reported emissions depend on how they were calculated and expressed in the
respective LCAs.
Adoption of fertilizer best management practices is one strategy that could
reduce N2O emissions by 30–40% [42]. The appropriate amount, timing, and
placement of fertilizers are examples of best management practices [30, 37, 42, 43],
but the particular response of switchgrass to such practices depend on climatic,
management, and mycorrhizal symbiotic relations [35].
Other agronomic practices such as intercropping with legumes contribute to
reduce emissions, although the decomposition of organic residues may contribute
to postharvest N2O emissions. In any case, the limited available results suggest
that switchgrass, when compared to other crops, is particularly good at mitigating
the soil N2O emissions associated with N fertilizer applications. However, more
studies based on actual measurements of N2O fluxes are needed to confirm or
provide more precise emission factors to be used in LCA and other studies because
the general figure that 70% of GHG emission from agricultural activities comes
from N2O emissions seems to be an underestimation [41, 44]. If N2O emissions are
higher than the IPCC estimations, its mitigation will become a priority or at least
of equal importance as C sequestration [45].
6.2.3 Methane
CH4 is a greenhouse gas with global warming potential equivalent to 21 times that of
CO2 [46]. Lately, its atmospheric concentration has increased significantly mainly
due to agricultural activities and the use of fossil fuels [44]. Soils can act as sources
or sinks for CH4, depending on land use and climatic conditions [25, 46, 47]. Soil
temperature, moisture, pH, and soil N status are factors affecting the capacity of a
soil to act as a CH4 sink [48]. Moreover, forest soils and grasslands are net con-
sumers of CH4 and have a greater sink potential than cultivated soils, as agronomic
and fertilization practices reduce the sink potential of the soil [46–50]. For example,
Mosier et al. [51] indicated that annual fertilization increases N2O fluxes and at the
same time decreases CH4 uptake in the soil by 41%.
In mid and late unmanaged successional forests, N2O emissions were almost
completely offset by CH4 oxidation [27]. Moreover, in unfertilized and undis-
turbed grasslands CH4 uptake was 1.4 and 2 times higher than that in fallow lands
and cultivated wheat fields [51]. Since switchgrass is a typical perennial grass with
low fertilization and tillage requirements, CH4 emissions from this crop may be
Table 6.1 N2O and CH4 emission factors from switchgrass feedstock production in the power
generation chain
Source of emission by activity Emissions (kg ha-1)
N2O CH4
Land preparation 2.22E-4 1.23E-2
Crop growth 1.11E-3 5.93E-2
Crop harvest 2.72E-3 1.23E-1
Transport harvested material 2.72E-2 1.41E-0
Production and use of fertilizers and atrazine 5.01E-0 1.6E-0
Use of lime 2.47E-4 1.23E-2
Biomass degradation losses 0 6.10E+1
Combustion in boilers 2.22E-0 3.46E-0
Post combustion activities 4.20E-5 2.07E-3
Data source Qin et al. [39], assumed switchgrass biomass yield 25 Mg ha-1, stand life 10 years,
transport distance 40 km
close to zero, or there may be significant CH4 uptake. For example, Adler et al.
[40] indicated that CH4 uptake by switchgrass was 1.41 g CO2-eq m-2 yr-1.
However, another study estimated that during the agronomic practices to establish
switchgrass the total CH4 emission were 23 g CO2-eq m-2 and that during the
harvesting operations (mowing, bailing, etc.) emissions were 17.4 g CO2-eq m-2 [52].
Currently, however, limited information is available on CH4 flux contributions
to net GHG emission from switchgrass. Qin et al. [39] in a LCA study estimated
that the largest CH4 emissions are produced during the processing/combustion
phase of switchgrass (Table 6.1), but even then they remained of low significance.
As far as we know actual CH4 flux measurements in a switchgrass stand are
nonexistent, probably because most studies do not consider it relevant to include
these measurements because of the assumed small effect on GHG emissions.
Therefore, CH4 flux based on field measurements are urgently needed to precisely
determine the most impacting phases (cultivation, transformation, etc.) of
switchgrass when used as a feedstock for diverse purposes.
6.2.4 Life Cycle Assessment
In theory, LCA is an all-inclusive account of the inputs and outputs of a pro-

ductions cycle [53]. Inputs and outputs can include energy requirement and yield,
economic cost and benefit, and environmental impacts whether positive or nega-
tive. However, the meaning of ‘all-inclusive’ can be somewhat nebulous, and a
clear definition of comparable system boundaries, both for alternative and tradi-
tional fuel sources, is necessary but potentially difficult to achieve when con-
ducting a LCA. The purpose of this review is not to evaluate the appropriateness of
various LCAs, but analysis boundaries must be kept in mind when evaluating LCA
results.
Because the use of biofuels was prompted by the recognitions of human
impacts on global warming and the need to reduce GHG emissions, an appropriate
starting point for LCA would be to examine total GHG emissions from various
bioenergy systems. In an early LCA comparing switchgrass with other bioenergy
crops, Adler et al. [40] found producing ethanol and biodiesel from switchgrass
and hybrid poplar reduced GHG emissions by 115% compared with gasoline and
diesel. In comparison, maize rotations reduced GHG emissions by 40% and reed
canary grass by 85%. They found that displaced fossil fuels were the largest GHG
sink, followed by soil C sequestration. They also concluded that GHG reductions
resulting from biomass gasification for electricity generation were greater than for
biomass conversion to ethanol.
Other studies have found smaller GHG savings from ethanol production from
switchgrass. Thus, Cherubini and Jungmeier [43] calculated that the use of
switchgrass in a biorefinery reduced GHG emissions by 79% with soil C seques-
tration responsible for a large part of the GHG benefit. Bai et al. [54] found a 65%
reduction in GHG emissions with switchgrass ethanol fuels, and Hsu et al. [55]
suggested a 43–57% reduction compared with cars operating on conventional
gasoline.
The LCA by Adler et al. [40] suggested that N2O emissions were the largest
GHG source. According to Qin et al. [39] the largest source of N2O emissions
during the crop production phase are: the production and use of fertilizer and other
chemicals, the transport, harvest, and growth stages, in that order of importance
(Table 6.1). When comparing a biorefinery fed with switchgrass biomass with a
traditional fossil fuel refinery, Cherubini and Jungmeier [43] indicated that during
the first 20 years of operation of the biorefinery the use of switchgrass had a net
reduction in GHG emissions, but that the emissions of N2O were about 10 times
higher than in the fossil fuel refinery. This was due to N2O emissions from the N
fertilizer (112 kg N ha-1) applied to the soil and possibly because of the
decomposition of the soil organic matter and dead roots but it seems that this point
is not taken into account by the authors. According to their computations, the
production phase of switchgrass was responsible for 80% of the GHG emissions
and from that 40% were N2O emissions.
Several other studies also indicated that the crop production phase is the main
source of N2O emissions [40, 56, 57]. Therefore, one of the best options to reduce
the large impact of fertilization in GHG emissions would be to minimize the use N
fertilizers, or to use and develop more efficient N-use strategies. This would also
be the case when manures are the fertilizer source because losses of ammonia to
the atmosphere and nitrate to groundwater are larger with manures than from
synthetic inorganic fertilizers [45].
It is important to also consider other environmental costs and benefits when
evaluating bioenergy production systems. In one such analysis, Harto et al. [58]
investigated the life cycle water use of biofuel and other low-carbon transport
systems. They found that adoption of electric vehicles and some algae-based and
switchgrass systems could contribute to the decarbonization of transportation

systems with little additional water consumption. However, use of irrigated biofuel
crops could have a significant potential impact on water resources. Whereas, non-
irrigated cellulosic ethanol production would require less than 10 gallons of water
per gallon of fuel produced, irrigated maize or cellulosic ethanol would consume
more than 400 gallons of water per gallon of fuel. They concluded that using
irrigated switchgrass to provide 10% of transportation fuel demand in the US
would require 7% of total consumptive water demand. Supplying 50% would
require 37% of total national annual consumption. Non-irrigated switchgrass, on
the other hand, would only consume 1.4% of annual water use to supply 50% of
the national fuel demand.
In addition to global warming potential, Bai et al. [54] conducted LCAs for
bioenergy production effects on abiotic depletion, eutrophication, photochemical
oxidation, ozone layer depletion, human toxicity, eco-toxicity, and acidification.
Results were mixed, with bioenergy production reducing global warming, abiotic
depletion, and ozone layer depletion, but increasing human toxicity, eco-toxicity,
acidification, photochemical oxidation, and eutrophication. Reduced emissions
from crude oil production caused the reduction in ozone layer depletion, whereas,
abiotic resource depletion decreased due to reduced use of crude oil. They
attributed the higher eutrophication score to nitrate leaching from N fertilizer
application, whereas, human and eco-toxicity increased due to the use of agro-
chemicals. Increased acidification resulted from ammonia emissions from agri-
culture. Cherubini and Jungmeier [43] also concluded that biofuel production had
greater impacts on acidification and eutrophication.
These studies suggest that additional environmental impacts should not be
disregarded when evaluating the impact of bioenergy production systems on GHG
emissions, but no recommendations were made concerning how to rank the
importance of competing environmental impacts, or on how to compute an overall
‘‘environmental score’’ for bioenergy production. Such information will be crucial
for evaluating the advisability of using bioenergy sources to replace fossil fuels.
6.2.5 Indirect Land Use Change
The production of switchgrass for energy purposes at an industrial level requires,

as with any other crop, large expanses of agricultural croplands. Since projections
indicate that the global population will continue to increase, as will their food,
feed, and energy demands [59, 60], it is foreseen that the croplands dedicated to
produce energy feedstocks such as switchgrass will most probably come from the
displacement of existing crops or from the conversion of grasslands and forests. In
either case, the new uses of the land will lead to direct and indirect changes in the
carbon balance within and outside the boundaries of the newly introduced system
with the consequent effects on global climate, food and feed supplies, and eco-
system services [61].
In general, when a natural forest or pasture is replaced by an annual crop the

soil and biomass carbon emissions increase significantly [59]. On the other hand,
when a cropland cultivated with annual crops or an abandoned cropland is con-
verted to a perennial grass such as switchgrass, large amounts of carbon could be
sequestered in the soil and therefore a net reduction in the atmosphere could be
expected. However, this process is not always straightforward because many
variables are involved [30, 59, 62, 63]. The degree of impact will be a function of
the type of crop replaced, root mass, soil depth, soil bulk density, climatic con-
ditions, and crop management and intensity, among others.
In general, it has been estimated that when perennial grasses are introduced into
croplands the carbon stocks in the soil increase at a rate between 1.1 and 1.2 Mg
C ha-1 yr-1 [64–66]. In agreement with such general estimations, Garten and
Wullscheleger [67] predicted that during a 10–30 year conversion period of a
cropland to switchgrass the SOC sequestration rate would be 0.78 and 0.53 Mg
C ha-1 yr-1, respectively, probably because the large amount of assimilates
accumulated in its extended root system. On the other hand converting native
grasslands, such as those in U.S., to maize resulted in 5.6 Mg C ha-1 yr-1
emissions [24].
Land use change (LUC) is not a new topic as the expansion of the agricultural
frontier led to, and will continue to lead to significant CO2 emissions, especially
when tropical forest (rich in above-and below-ground carbon) are converted into
agricultural lands [68, 69]. Beyond the environmental effects, LUC could have
short-term effects on crop prices, stocks, farm incomes, and demand of agricultural
products. However, only recently have the direct and iLUC effects been taken into
account in the estimations of GHG emission and LCA studies of biofuel and
bioenergy production systems.
However, the estimation process has a number of limitations and there is not a
conventionally accepted estimation procedure. Since there are not reliable data on
LUC effects, the estimations are highly variable and susceptible to economical,
political, social, and environmental influences. For example, Fargione et al. [24]
and Searchinger et al. [70] indicated that including the effect of LUC and iLUC in
the analysis could result in GHG emissions being even higher for bioenergy crops
than that of current fossil fuels. On the other hand, other estimates indicate that the
positive benefits of biofuels could be seen under certain circumstances and type of
crops such as sugarcane or other perennial grasses [60]. Switchgrass, being a
perennial grass with low input requirements and high carbon sequestration
capacity, may be part of the group of crops with positive environmental effects.
But the available information is not sufficient to determine, with an acceptable
degree of approximation, its indirect and direct effects.
Changing the preexisting vegetation to bioenergy crops causes removal or
sequestration of CO2, but such changes could be negated or enhanced elsewhere
because of the spatial and temporal nature of the replacement effects [71].
Therefore, the iLUC effects are non-local and not specific to a feedstock, so they
cannot be quantified directly but only through modeling [71]. Since the dynamics
of iLUC are dominated by international trading trends, food and feed prices,
agricultural policies, climatic conditions, among others, its global nature makes it
very difficult to model. In fact, the validity of the current available methods is
hotly debated. But in general two approaches are widely used. In the economic
approach, linkages between complex macro- and micro-economic models with
biophysical models are used to estimate GHG emissions associated with iLUCs.
While in the deterministic approach, the iLUC analysis is based on the export/
import trends of agricultural commodities in the most relevant countries.
Examples of the most common economic and deterministic models used to
estimate iLUC effects can be found in Searchinger et al. [70] and Fritsche et al.
[61]. In both cases, however, the results and predictions remain vague and vari-
able, mainly because of insufficient analysis of market distortions, complexity of
the factors considered, and insufficient analysis of trading levels. A recent study
[60], in which seven agro-economic models were compared, indicated a wide
range (from 10 to 80 g CO2 MJ-1 of biofuel produced) of overall emissions from
iLUC. The large variability mainly depended on the assumptions used in each
model. However, in the case of switchgrass none of these models may be appli-
cable as none of them considered the iLUC effects of second generation feed-
stocks, showing the urgent need to develop estimation procedures that take into
account perennial grasses. In the case of the deterministic model, it was shown that
adding iLUC plus LUC emissions in LCAs could almost double GHG emissions
per unit energy [71].
Some authors consider model simulation approaches to not be sufficiently
accurate, therefore they use the risk-adder method, which estimates the average
LUC area per additional hectare of bioenergy production [60, 70, 72], to determine
the maximum possible effects of iLUC. Based on that approach, Searchinger et al.
[70] indicated that even when U.S. maize fields are converted to switchgrass, GHG
emissions still increase by 50% over a period of 30 years. Such results raise great
concerns about the potential of switchgrass to reduced GHG emissions associated
with iLUC. However, this seems to be an overestimation mainly because of the
arbitrary assumptions and non-replicable parameters used in the study. But it is
clear from this and other studies that the approach of eliminating any iLUC risk
provides very rough estimates, which in turn seem insufficient for generalization
and rulemaking. Therefore, further studies are needed to define more precise
evaluation methods and specific criteria to quantify consistent iLUC values in
order to opportunely include them in GHG emission balances.
In any case, it is clear that the production of biofuel and bioenergy leads to
GHG emissions associated with iLUC, and controlling them could be an important
factor for mitigating the global warming process. Several authors suggest that
optimizing the use of byproducts as biofuels feedstocks, maximizing the use of
crop residues as biofuels feedstocks, and cultivation of feedstocks on abandoned
croplands are measures that to some extent could reduce the iLUC effects on GHG
emissions [45, 60]. In addition, technological developments along the supply
chain, improved feedstocks, crop management, and improved conversion effi-
ciencies (e.g. bioelectricity instead of biofuels from lignocellulosic crops such as
switchgrass) will reduce the impact of the bioenergy feedstock on the GHG
balance [60, 73]. Global climate policies with emissions caps would also help to
control iLUC effects. In fact, any measure that reduces the land requirements for
feedstocks will contribute to mitigate the effects of direct and iLUCs. As for
switchgrass, the research window remains completely open as information on the
aforementioned aspects is almost nonexistent.
6.3 Impact on Water Quality
6.3.1 Runoff, Nutrient, and Sediment Losses
Before switchgrass became the focus of research as a bioenergy crop, its envi-
ronmental impact on surface water runoff and quality were considered from the
perspective of using this grass species in vegetative buffer strips. Switchgrass has
upright and stiff stems and a rhizomatous growth habit, characteristics that make it
desirable for intercepting sediment from surface runoff and allowing it to regrow
through sediment that has been deposited on the soil surface. The USDA Natural
Resource Conservation Service identifies these criteria for selecting desirable
species in vegetative buffers (Code 601; [74]). In one study in which several grass
species were evaluated as a narrow hedge (0.75–1.0 m) grown along the contour to
impede surface water runoff, Dabney et al. [75] determined that the primary
explanation for the sediment trapping efficacy of the hedge was the result of flow
constriction and water backing up above the hedge. The backed up water slowed
runoff and the sediment load was deposited, thus the filtering efficacy of the hedge.
In a field study in which the effectiveness of a switchgrass hedge was determined
for runoff and sediment loss from maize plots, the hedge reduced runoff (41%) and
sediment (63%) losses [76]. While species of grass in the hedge seemed unim-
portant in these studies as long as there was the physical constraint of backing up
the surface runoff, cool-season grasses may not withstand the sediment deposition
as well as non-bunch, rhizomatous warm-season grasses like switchgrass.
Rainfall simulations were used in a plot (1.5 9 16 m) study to contrast the
effectiveness of filter strips of different grass species, including: tall fescue [Lolium
arundinaceum (Schreb.) S.J. Darbyshire], tall fescue with a switchgrass barrier
(0.7 m wide), and a mixture of grasses and forbs native to the Midwestern United
States with a switchgrass barrier [77]. Compared to a tilled plot, tall fescue
improved surface water quality on this Mexico silt loam soil by reducing organic N
(55%), NO3–N (27%), NH4–N (19%), particulate P (36%), and PO4–P (376%).
The addition of the switchgrass barrier (similar to a narrow hedge) provided
greater effectiveness, reducing organic N by 67% compared to the tilled plot, and
also reducing NO3–N (68%), NH4–N (50%), particulate P (53%), and PO4–P
(54%). Expanding this experiment to consider concentrated flow paths for the
same fescue strips with switchgrass barriers (Fig. 6.1), Blanco-Canqui et al. [78]
determined that dormant or actively growing switchgrass were similarly effective
Fig. 6.1 Diagram of fescue

filter strips combined with a
narrow switchgrass barrier
that was effective in reducing
sediment and nutrient losses
in runoff (redrawn from
Blanco-Canqui et al. [78];
reprinted with permission
from the Soil Science Society
of America)
in reducing runoff and sediment losses. As with the previous study, the switchgrass
barrier was more effective than only tall fescue in reducing sediment and nutrient
losses.
In a direct comparison between cool-season grasses [smooth bromegrass, tim-
othy (Phleum pretense L.), and tall fescue] and switchgrass, switchgrass was more
effective than the cool-season grasses in removing sediment, N, and P from surface
runoff [79]. This comparison was based on 3 or 6 m wide filter strips. The 6 m
wide strips were generally 50% more effective than the 3 m wide strips in
removing sediment and nutrients. While the switchgrass was usually significantly
better than the cool-season grasses at filtering runoff sediment and nutrients, dif-
ferences were generally less than 10% (Table 6.2).
Switchgrass hedges have also been effective in reducing nutrient runoff losses
from plots receiving manure or fertilizer [80]. In no-till plots receiving manure or
fertilizer, the adjacent (and downslope) hedge reduced runoff concentrations of
dissolved P (47%), bio-available P (48%), particulate P (38%), total P (40%), and
NH4–N (60%). In the disked plots, the reduction in runoff concentration was not as
great for dissolved P (21%), but was generally comparable for the other nutrients.
Table 6.2 Efficacy of switchgrass and cool-season grasses in reducing sediment and nutrient
runoff (redrawn from [79])
Strip
Width Area Grass Sediment Total N NO3–N Total P PO4–P
(m) ratio (%) (%) (%) (%) (%)
6 20:1 Switchgrass 78.2 a* 51.2 a 46.9 a 55.2 a 46.0 a
6 20:1 Cool-season 74.8 b 41.1 b 37.5 b 49.4 b 39.4 b
Overall average 76.5 46.2 42.2 52.3 42.7
3 40:1 Switchgrass 69.0 c 31.7 c 28.1 c 39.5 c 38.1 b
3 40:1 Cool-season 62.0 d 23.5 d 22.3 d 35.2 c 29.8 c
Overall average 65.5 27.6 25.2 37.4 34.0
*
Percent within a column for reduction followed by a different letter are significantly different
(P \ 0.05)
Water quality research related to switchgrass hedges has mostly focused on the
reduction of runoff losses and improvement in associated water quality charac-
teristics, attributing these reductions to water backing up above the hedge.
Improving infiltration and/or hydraulic conductivity of the soil surface within a
switchgrass stand would contribute to additional water quality improvements by
filtering fine sediment particles and other soluble nutrients. Measuring field-satu-
rated hydraulic conductivity (Kfs) was the focus of a study in Iowa on a Monona
silt loam soil [81]. In this study Kfs was measured at three different locations on the
landscape: (1) 7 m upslope from the switchgrass hedge in a maize or soybean field,
(2) 0.5 m upslope from the hedge in the sediment depositional area, and (3) within
the grass hedge. These hedges had been in place for 10 years. The Kfs within the
grass hedge (107 and 154 mm h-1) was more than seven times greater than the Kfs
in the row crop field (13.5 and 22.5 mm h-1) and more than 24 times greater than
the depositional area (1.4 and 9.4 mm h-1). Infiltration was measured under
conditions of increasing soil water tension. As tension increased to 50 and
100 mm, infiltration within the hedge was still greater than in the row crop field;
but as tension was increased to 150 mm, the infiltration within the hedge and row
crop field became similar. A reduction in sediment losses due to a switchgrass
hedge can probably be attributed mostly to water backing up above the hedge, but
the reduction in nutrient loss is likely attributable to the greater infiltration within
the hedge.
The efficacy of a 7.1 m buffer of switchgrass was compared to the switchgrass
buffer with an additional 9.2 m length of switchgrass and woody species mix [82].
In a 2 h rainfall simulation (22 mm h-1), the switchgrass buffer trapped 70% of
the incoming sediment and 64, 61, 72, and 44% of incoming total N, NO3–N, total
P, and PO4–P, respectively. The additional length of switchgrass-woody species
buffer improved these numbers to 92% of the sediment and 80, 92, 93, and 85% of
the respective nutrients. The woody species in the additional buffer provided
greater infiltration, plus the additional length of buffer contributed to the overall
greater effectiveness of the buffer in the latter scenario.
When compared to row crops in small-plot studies, switchgrass is very effective

at reducing sediment and nutrient loads in surface runoff. The examples provided
here were often side-by-side comparisons for standing crops. Because switchgrass
is a perennial crop that will not require additional tillage or re-establishment of the
crop, an annual (and long-term) comparison of water quality between a row crops
and switchgrass should be even more favorable for switchgrass. When switchgrass
was compared to cool-season grasses, such as tall fescue, improvements in the
sediment, N, and P from surface runoff were slightly better with switchgrass, but
the differences were generally less than 10%. Switchgrass makes an effective
ground cover for improving water quality.
6.3.2 Expanding the Spatial and Temporal Scale

of Switchgrass Impacts
Numerous land use studies considering changes to switchgrass have evaluated the
environmental impact at larger scales, from the small watershed to the Mississippi
River basin. Using the Soil and Water Assessment Tool (SWAT), hillslope pro-
cesses were modeled and the environmental impacts were considered if planting
switchgrass to 10, 20, 30, and 50% of the Walnut Creek watershed (51.3 km2) near
Ames, IA [83]. Filter strips of switchgrass representing 10–50% of the sub-basin
could lead to a 55–90% reduction in NO3–N load during an average rainfall year.
In the larger Delaware River basin of NE Kansas, SWAT was used to consider
sediment yield, surface runoff, NO3 in surface runoff, and edge-of-field erosion
[84]. If the cultivated cropland (119,400 of 300,000 total ha) were converted to
switchgrass, sediment loss would be reduced by 99%, surface runoff by 55%, NO3
loss by 34%, and edge-of-field erosion by 98%. Evaluating a shift to switchgrass in
the large, agriculturally dominated Raccoon River watershed of central Iowa
(9,364 km2), results from SWAT indicated that lower water yield will correspond
with less NO3, less P, and less sediment loss [85]. These scientists suggested that
even though a shift in land use (i.e. toward more switchgrass) might resemble a
pre-1940s land use and land cover, the extensive tile drainage network would
prevent the hydrology from ever resembling pre-1940s condition. Tile drainage is
meant to move water quickly away from agricultural fields with the inadvertent
consequence of carrying its nutrient load with it. Nevertheless, growing maize
results in lower annual evapotranspiration and therefore greater runoff and
drainage than a perennial cropping system, such as switchgrass; resulting in a shift
toward fewer environmental impacts when more switchgrass is grown.
Expanding the spatial scale even further to an area encompassing much of
Missouri, Iowa, Nebraska, and Kansas in the Midwestern U.S. (15,100 km2),
Brown et al. [86] used the Erosion Productivity Impact Calculator (EPIC) to
consider future environmental impacts of growing more switchgrass in this region.
Their model scenarios also extended the temporal scale as well by considering
crop yields and sediment loss under increased atmospheric CO2 (560 lg g-1).
Alternative climate conditions were considered using the general circulation model
(GCM) from CSIRO. With increased temperature, switchgrass yield increased by
5 Mg ha yr-1, whereas other crop yields decreased (Mg ha-1 yr-1): maize, 1.5;
sorghum, 1.0; soybean, 0.8; and wheat, 0.5. With additional CO2 under this
otherwise similar future scenario, all crops responded with greater yield compared
to the future scenario with only increased temperature. Greater rainfall predicted
with climate change corresponded with greater runoff and generally increased
sediment loss, except with switchgrass for which sediment loss generally
decreased. A stochastic model was used in another study to evaluate the impact of
producing cellulosic ethanol (i.e. growing switchgrass) compared to maize-derived
ethanol in the Mississippi River basin [87]. They concluded that cellulosic ethanol
production would result in a 20% decrease in NO3 delivered to the Gulf of Mexico.
Growing switchgrass compared to growing a row crop will increase water use
and increase infiltration, both of which will have the net effect of reducing runoff.
These impacts described and measured at the small plot scale translate to reduced
nutrient losses at the larger watershed and basin scales. Including switchgrass on
the landscape will have a favorable impact on improving water quality.
6.3.3 Seasonal Nitrogen Dynamics: Implications

for Management and Environmental Impacts
Perhaps some of the more interesting questions about growing switchgrass as a

bioenergy crop are intertwined in the N dynamics of physiological characteristics,
production management, and their corresponding impacts on N in the environ-
ment. For example, when is N taken up by switchgrass? When will switchgrass be
harvested? What is the biomass N content at harvest? How much N fertilizer will
be applied? How do these physiological characteristics impact NO3 water quality
and N2O emissions? Some of these questions have already been addressed with
recent research, though some remain unanswered.
Cropland in the Midwestern U.S. is extensively tile-drained, designed to move
water quickly from the field to improve soil conditions for maize and soybean
production. This conduit for water also effectively moves NO3 from agricultural
soils to streams and rivers [88, 89], significantly contributing to the increase in
NO3 flux in the lower Mississippi River [89]. Compared to continuous maize or
maize–soybean crop rotations, perennial crops will reduce NO3 leaching in this
tile-drained landscape from more than 60 kg N ha-1 yr-1 to less than
5 kg N ha-1 yr-1 [90]. This reduction in NO3 loss from perennial crops can be
attributed to both lower NO3 concentration in the tile water and reduced drainage
as a result of greater evapotranspiration.
In a recent study [91], the impact of miscanthus and switchgrass managed as
bioenergy crops on hydrology and NO3 leaching was evaluated in the tile-drained
landscape of the Midwestern U.S. The soil profile water content was consistently
less with miscanthus than with either switchgrass or a maize-soybean rotation,

especially later in the growing season. Nitrate leaching was much greater in the
maize-soybean rotation (34–45 kg N ha-1 yr-1) than either switchgrass
(0.3–3.9 kg N ha-1 yr-1) or miscanthus (1.6–6.6 kg N ha-1 yr-1). Although N
fertilizer was not applied to switchgrass in this study, N applications will likely be
a component of switchgrass management as a bioenergy crop; but this study
illustrates that a perennial switchgrass crop will contribute much less N to ground
water than a crop rotation including maize and soybean.
In a 2 year study in Illinois [92], the biomass and N content of miscanthus and
switchgrass were evaluated in a side-by-side trial at three locations spanning a 5°
latitudinal range (37–42° N). Biomass was harvested on five dates between June
and February. As much as 60 Mg ha-1 of biomass was harvested for miscanthus
and more than 20 Mg ha-1 for switchgrass. Nitrogen concentrations of the bio-
mass for both species decreased from 1.5 to 2.5% in June to \0.5% in December.
Nitrogen concentrations in February were similar to concentrations in December,
so there was not an incentive to postpone harvest until February. Harvest in
December corresponded with greater biomass than in February and similar N
concentrations. When harvested in June, switchgrass removed as much as
187 kg N ha-1 and miscanthus as much as 379 kg N ha-1, whereas N removal in
February corresponded to as little as 5 and \17 kg N ha-1, respectively, because
N within the plant was translocated to the rhizomes during winter dormancy. The
December harvest, corresponding to low N concentrations and high biomass yield,
was the most suitable harvest date for providing a large amount of desirable
biofuel feedstock and represents an efficient N recycling cropping system—
meeting economic and environmental objectives.
A 5 year study in Knoxville, TN contrasted a two-cut (summer and fall) harvest
plan to a one-cut (fall) plan for switchgrass [93]. Biomass yield was similar for the
two approaches (17.4–18.7 Mg ha-1), but much less N was removed with the one-
cut plan (48 kg N ha-1) than with the two-cut plan (116 kg N ha-1). One other
study provided similar results, concluding that less N can be applied and less N
removed in the biomass when switchgrass is harvested late in the fall [94]. If less
N is removed (but remains in the rhizomes), less N fertilizer will be required; thus
reducing loss risks associated with NO3 leaching and N2O emissions after N
fertilizer applications.
Root distributions and dynamics and total soil respiration were evaluated for an
edge-of-field and riparian area that included zones of poplar (Populus x euro-
americana’ Eugenei), switchgrass, cool-season grasses (smooth brome, timothy,
and tall fescue), and soybean or maize [95]. The fine root biomass for switchgrass
increased from 7 to 10 Mg ha-1 between May and November, remained relatively
constant for poplar and cool-season grasses (6–8 Mg ha-1) during this same
period. Fine root biomass was always less than 2 Mg ha-1 for maize and soybean.
Small root biomass (2–5 mm) was significantly greater in switchgrass
(2 Mg ha-1) than for any of the other species (\0.65 Mg ha-1), with no differ-
ences across sampling dates. Coarse root biomass ([5 mm) was only observed
under poplar (3.8 Mg ha-1), soybean (1.1 Mg ha-1), and maize (0.3 Mg ha-1).
Root density was greater in the poplar, switchgrass, and cool-season grasses than
the maize and soybean, for the 0–50, 50–100, and 100–125 cm depths. Soil res-
piration was greatest in the poplar and cool-season grasses. Soil respiration under
switchgrass was less than for poplar or cool-season grasses; but greater than with
maize or soybean. The implications from this research is that the additional roots
provided by poplar, switchgrass, or cool-season grasses provide a carbon source
that is greater and to greater depths than provided by maize or soybean. The
additional C contributes to the riparian zone denitrification potential and the
presence of growing roots has implication for additional NO3 removal; conse-
quently, roots deeper in the soil profile represents two effective means of NO3
removal from ground water.
Switchgrass is a perennial warm-season grass that is native to North America.
It grows to a height of about 2 m, has a deep and fibrous root system, and will
produce between 5 and 20 Mg ha-1 yr-1 of above-ground biomass [96]. A stand
of switchgrass may maintain this productivity for 15–20 year with much less
fertilizer and chemical inputs than usually applied to crops such as maize and
soybean. Some of the physiological characteristics that suggest that switchgrass
should have a favorable impact on the environment compared to most other
agriculture cropping systems include: (1) after crop establishment the soil will
remain undisturbed for many years, reducing soil erosion and energy inputs (as
fuel); (2) low N fertilizer inputs translates into low energy demand for growing
the crop and reduced risk of N2O emissions and NO3 leaching that might occur
with almost any N fertilizer application; (3) low energy inputs relative to har-
vested biomass; and (4) a perennial crop with a fibrous root system translates to
reduced water and nutrient losses through leaching as well as an effective surface
runoff filter. If and when land use currently in traditional cropping systems is
converted to switchgrass, environmental impacts should be favorable.
6.4 Conclusions
The number of studies looking at the environmental impacts of switchgrass cul-

tivation is extremely limited, especially when switchgrass was managed for bio-
energy production. In particular, additional studies at multiple locations are needed
to identify the climatic and edaphic drivers of soil C sequestration. Similar
research is needed for N2O emissions, but in addition, continuous flux measure-
ments throughout the year are needed to indentify the contribution of periodic
high-emission events to total annual emission rates. Additional research on how N
fertilization rates affect N2O emissions and NO3 leaching and on interactions
between environmental effects of N fertilization and biomass production is also
warranted. What limited information there is suggests that switchgrass provides
multiple environmental benefits compared to annual crop cultivation. However,
benefits generally appear to be similar to other perennial crops.
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Chapter 7
Biochemical and Thermochemical
Conversion of Switchgrass to Biofuels
Venkatesh Balan, Sandeep Kumar, Bryan Bals, Shishir Chundawat,

Mingjie Jin and Bruce Dale
Abstract With dwindling oil reserves and growing environmental concerns,

researchers are looking at producing sustainable biofuels and chemicals from
renewable resources like switchgrass. Biofuels and biochemicals will be produced
in the near future from switchgrass in biorefineries using both biochemical and
thermochemical platforms. We have summarized recent literature pertaining to
different processing steps within the biochemical platform (pretreatment, enzyme
hydrolysis, microbial fermentation, protein extraction) and thermochemical plat-
form (pyrolysis, bio-oil, gasification, combustion, hydrothermal process) in this
chapter. Though we have improved our fundamental understanding on the different
processing steps to produce biofuels, several challenges still have to be overcome
to create a bioeconomy and produce fuels and chemicals from biomass in an
economic and sustainable manner.
V. Balan (&) B. Bals S. Chundawat M. Jin B. Dale

Biomass Conversion Research Lab (BCRL), Chemical Engineering and Materials Science,
Michigan State University, 3900 Collins Road, Lansing 48910, USA
e-mail: balan@msu.edu
V. Balan B. Bals S. Chundawat M. Jin B. Dale
DOE-Great Lakes Bioenergy Research Center (GLBRC),
Michigan State University,
East Lansing, MI, USA
S. Kumar
Department of Civil and Environmental Engineering,
Old Dominion University,
Norfolk, VA 23529, USA

DOI: 10.1007/978-1-4471-2903-5_7, Springer-Verlag London 2012
154 V. Balan et al.
7.1 Introduction
Like a petroleum refinery which uses crude oil as a feedstock to produce different
products, the biorefinery combines different process technologies to convert bio-
mass residues generated from agriculture and forest industries to fuels and
chemicals. Lignocellulosic feedstocks offer several advantages, such as avail-
ability in great abundance, being a renewable resource and non-edible and does not
interfere with food industries. It is comprised of a complex network of 60–70%
carbohydrates (cellulose, hemicellulose) and 15–20% lignin as major constituents.
The remaining 10–25% constitutes minor components such as protein, ash, oil/
waxes and others. There are two distinct platforms for producing fuels and
chemicals (Fig. 7.1).
The first platform is a biochemical route where the biomass residues are
chemically pretreated using different process technologies followed by enzymat-
ically hydrolyzing the pretreated biomass to fermentable sugars. Since sugars are
the building blocks for most microorganisms; they are fermented to produce dif-
ferent fuels and chemicals.
The second platform is using a thermochemical route where the biomass resi-
dues are subjected to hydro-thermolysis, pyrolysis, thermolysis or burning to
produce bio-oil/bio-char/syngas followed by catalytically converting them into
fuels and chemicals. Both platforms cogenerate heat and power to meet their
internal energy demand. There are some hybrid biorefinery concepts where the
hydrolysed sugars or the fermented products are catalytically converted to produce
intermediates and final products.
Green biorefineries is another concept in which green plants are processed to
produce a fiber-rich press cake and nutrient-rich green juice. The fiber is used for
making biofuels via either the biochemical or thermochemical route, while the
green juice which contains proteins, minerals and nutrients can be used as an
animal feed supplement [1, 2].
7.2 Biochemical Platform
The processing steps involved in this platform include: biomass harvest and
transport [3], milling, pretreatment [4], solid liquid separation (optional), high
solid loading enzymatic hydrolysis [5], microbial fermentation [6], product
recovery and burning un-hydrolyzed biomass to generate heat/electricity [7]
(Fig. 7.2). The number of processing steps can increase or decrease depending on
the choice of pretreatment. For example, lignin (e.g., with organosolv pretreat-
ment) or hemicellulose (e.g., with acid pretreatment) can be removed which can be
used as a starting material for making other products [8]. In some pretreatments the
biomass composition is unaltered [e.g., Ammonia Fiber Extraction (AFEXTM)]
and hence there is no need for solid–liquid separation. There are different methods
7 Biochemical and Thermochemical Conversion of Switchgrass to Biofuels 155
Fig. 7.1 Different biorefinery platform for making fuels and chemicals
Fig. 7.2 Different processing steps involved in biochemical platform
of converting pretreated biomass to biofuels, including separate hydrolysis and

fermentation (SHF) or simultaneous saccharification and co-fermentation (SSCF).
In both cases, commercial enzymes and microbial seed cultures are needed for the
process. Alternatively consolidated bioprocessing (CBP) can be performed using a
microbe which produces enzymes and also ferments the sugars to fuels and
chemicals [6]. The fermentation broth is further processed to recover products
from un-hydrolysed solids and insoluble lignin is burned to generate enough heat/
electricity to carry out the other processing steps. In order to make the biorefinery
process environmentally sustainable and economically feasible compared to a
petroleum refinery, the amount of water and energy used in different processing
steps should be minimized, the catalyst used in the process should be recovered
and recycled, enzyme cost should be minimized [9] and the microbe used should
156 V. Balan et al.
be highly efficient. In addition, several co-products (e.g., lignin, proteins) can be

generated which will further reduce the processing cost.
7.2.1 Thermochemical Pretreatments
Pretreatments are necessary to reduce the native recalcitrance of lignocellulosic

biomass to biological conversion. The choice of pretreatment technology (thermal,
thermo mechanical, thermo chemical or biological) would also have pervasive
impacts on all aspects of the biorefinery operation ranging from choice of feed-
stock selection to product recovery and waste-water treatment [4]. However, in
order to correctly estimate the impact of different pretreatments on biomass con-
version it is necessary to carry out these studies using a standard basis. The
Biomass Refining Consortium for Applied Fundamentals and Innovation (CAFI)
was established as a partnership among several university and federal labs that
were leaders in biomass pretreatment and hydrolysis in late 1999 in Dallas and
early 2000 in Chicago to establish a common standard to assess pretreatments [10].
Three CAFI projects have been carried out over the last decade on three different
feedstocks: corn stover, poplar and switchgrass [10–12]. Three different varieties
of switchgrass, supplied by Ceres Corporation (Thousand Oaks, CA), were used in
the last CAFI project: Alamo (lowland), Dacotah (upland), and Shawnee (upland)
with variable planting and harvesting time periods (spring vs. late-fall). The var-
iable composition (glucan and lignin, in particular) seemed to depend strongly on
harvest time than on variety or ecotype.
Six different pretreatments were investigated in this project; AFEXTM, dilute
acid (DA), liquid hot water (LHW), lime, sulfur-dioxide impregnated steam
explosion (SO2), and soaking in aqueous ammonia (SAA). The optimal pretreat-
ment conditions that were determined for Dacotah switchgrass variety are high-
lighted in Table 7.1. All chemical pretreatments are known to modify plant cell
walls through various physicochemical modifications depending on pretreatment
chemistry (acid vs. base) and type of biomass that eventually helps enhance
enzymatic digestibility via improving enzyme accessibility to embedded
polysaccharides.
The CAFI pretreatments can be split into two main categories; acidic (DA,
LHW, SO2) or basic (AFEXTM, Lime, SAA), that vary in thermochemical severity.
In recent years there have been several advances highlighting the physicochemical
mechanism of acidic and ammonia based pretreatments for monocots and dicots
[13–18]. Cleavage of lignin-carbohydrate complex (LCC) linkages between lignin
and hemicellulose helps improve accessibility of cellulose. While in acidic pre-
treatment the glycosidic bonds are cleaved to result in the formation of lower
molecular gluco-/xylo-oligomers and monomers; in the case of alkaline pretreat-
ments only oligomers are released. Lignin-based ether linkages are prone to
cleavage only during acidic (or oxidative) pretreatments further coupled with the
re-condensation and deposition of lignin-based globular structures (10–10,000 nm
Table 7.1 Optimal pretreatment conditions for Dacotah switchgrass (adapted from [11])
Pretreatment Temperature Reaction Pretreatment Catalyst loading Water loading
category (C) (min) chemical (g/g BM) (g/g BM)
AFEXTM 140 30 NH3 1.5 2
DA 140 40–45 H2SO4 0.01 10
LHW 200 10–20 None – 6.7
Lime 120 240 Ca(OH)2, O2 1 15
Steam expl. 180 10–15 None or SO2 0.05 10
SAA 160 60–80 NH4OH 1.4 7.7
in cross-section diameter) on outer wall surfaces. On the contrary, the lignin ether
linkages are intact during AFEXTM with deposition of heterogeneously-shaped
lignin rich extractives (10–500 nm) on outer wall surfaces only under certain
pretreatment conditions [19]. High resolution analytical microscopy imaging
techniques on untreated and variously pretreated switchgrass samples have shown
that the relative extent of cell wall delamination, lignin re-localization, porosity
and cell wall morphological disruption is closely dependent on pretreatment type
[20] and [19]. Both DA and AFEXTM pretreatment were found to significantly
alter the ultrastructure of the compound middle lamella and the outer secondary
cell walls for corn stover and switchgrass suggesting mass transfer is a major
limitation for effective cell wall pretreatment. Cellulose degree of polymerization
has been shown to decrease for acidic pretreatments with no major decrease in
crystallinity index [21]. Though pretreatment of cellulose with anhydrous liquid
ammonia has been shown to produce a novel allomorph named cellulose IIII
(an allomorph that has up to 5 fold higher rate of saccharification than native
cellulose Ib), without producing significant amounts of amorphous cellulose, no
major alteration in cellulose crystal structure (to a more readily digestible form) is
seen during conventional AFEXTM or other aqueous pretreatments [15, 19].
Acidic pretreatments have similar chemistries but vary in thermochemical
severity. The acid is either added externally (H2SO4, SO2) or formed during
pretreatment (e.g., degradation of polysaccharides and lignin to short-chain
aliphatic acids and phenolic acids, respectively). Water is a strong acid at high
temperatures above 200C as well [22]. The extent of hemicellulose (88–93%
removal) and lignin (13–19% removal) removal depends on several factors (pH,
temperature, residence time and liquid to solid loading) as highlighted in
Table 7.2. In alkaline pretreatments, the pretreatment catalysts varies considerably
such as in AFEXTM (43% anhydrous ammonia in water solution), SAA (15%
anhydrous ammonia in water solution), and oxidative lime (7% calcium hydroxide
solution with dissolved oxygen).
The low liquid-to-solid loading employed during AFEXTM is responsible for
the low mass loss seen after pretreatment in contrast to SAA and lime pretreatment
where a significant fraction of lignin (55–60%) and hemicellulose (38–40%) is
solubilized. One critique of the acetyl mass balance for AFEXTM treated switch-
grass is that conventional chromatography methods used are unable to detect
acetamide that is only formed during AFEXTM [16], hence underestimating the
158 V. Balan et al.
Table 7.2 Relative extent of solids recovery and individual component mass balance for various
CAFI pretreatments conducted on Dacotah switchgrass (adapted from [97])
Pretreatment Recovery Cellulose Xylan Arabinan Acetyl Lignin Others
category (% solids) (% residual)
Untreated – 35.6 22.6 3.1 3.6 21.1 13.9
AFEXTM 100 35.9 22.5 3.4 2.4 24.4 11.4
DA 60 50.3 4.5 0.5 0.3 29.4 15.0
LHW 60 50.1 2.5 0 0.3 30.6 16.6
Lime 65 53.0 21.5 1.7 0 14.6 9.2
Steam expl. 62 53.9 2.7 0.7 0.5 27.6 14.6
SAA 62 55.6 21.9 2.4 1.5 13.9 4.7
true extent of acetyl removal. Previous work on AFEXTM and DA pretreatments

for corn stover has shown that more than 75 cell wall decomposition products are
formed (e.g., amides, acids, furans, imidazoles, phenolics, aldehydes) due to
degradation of carbohydrates and lignin [16]. A similar study needs to be carried
out for switchgrass since varying thermochemical severity would have significant
impact on formation of degradation products that can differentially impact enzyme
and microbial action.
7.2.2 Effect of Enzyme Combination on Hydrolysis

of Pretreated Switchgrass
7.2.2.1 Effects of Cellulases
Cellulase enzymes are needed to break down the sugar polymers to monomeric
sugars [23]. The effect of CAFI pretreatment followed by enzymatic hydrolysis on
total monomeric and oligomeric sugar release (glucose and xylose only) is high-
lighted in Table 7.3. Hydrolysis experiments were done using commercial
enzymes. Spezyme CP (Genencor Division of Danisco US, Inc, NY, USA), with a
protein content of 82 mg/mL and specific activity of 50 FPU mL-1 was loaded at
15 FPU g-1 glucan in untreated biomass. b-Glucosidase (Novozyme 188, Novo-
zymes Corp.) with a protein content of 67 mg mL-1 and specific activity of
600 CBU mL-1 was loaded at 30 CBU g-1 glucan in untreated biomass. The
sugar yields are reported based on maximum possible total glucose contribution of
60.6% and xylose contribution of 39.4% from the Dacotah switchgrass at 1%
glucan loading during hydrolysis. Acidic pretreatments solubilized higher levels of
glucose in stage 1 (during pretreatment) than alkaline pretreatments. All pre-
treatments except lime and SO2 were effective in releasing near-theoretical yields
of glucose and xylose after enzymatic hydrolysis (Stage 2). Most xylose was
released during acidic pretreatments in stage 1; however, the relative extent of
monomeric and oligomeric sugar yield depended on severity of pretreatment.
Nearly one third of total available xylose sugars, as oligomers, are released during
Table 7.3 Sugar yields from pretreatment (Stage 1) and enzymatic hydrolysis at a fixed cellulase loading of 30 mg/g glucan (Stage 2) for prewashed and
pretreated Dacotah switchgrass.
Pretreatment category % Glucan conv. Total Gluc. % Xylan conv. Total Xyl. Combined Sugars MESP ($/gal EtOH)
Stage 1 Stage 2 Stage 1 Stage 2
Untreated – 8.4 8.4 – 1.9 1.9 10.3 –
Theor. max 60.6 39.4 100 –
AFEXTM 0.8/0.8 47.1 47.9/0.8 11.1/11.1 25.6/3.0 36.7/14.1 84.6/14.9 2.50
DA 4.3/0.5 42.2 46.5/0.5 29.3/1.7 3.4 32.6/1.7 79.2/2.2 2.59
LHW 4.1/3.8 47.3 51.4/3.8 25.9/17.2 5.30/1.1 31.3/18.3 82.6/22.1 2.32
Lime 0.9/0.8 54.0/3.0 54.9/3.8 13.6/13.6 22.4/0.8 36.0/14.3 90.9/18.2 2.63
Steam expl. 3.0/1.5 48.3 51.4/1.5 28.7/1.5 3.2 31.9/1.5 83.2/3.0 2.73
SAA 0.2/0.2 39.8/1.2 40.0/1.4 9.5/8.7 17.8/6.9 27.3/15.5 67.3/17.0 2.93
Stage 1 refers to pretreatment and Stage 2 refers to the enzymatic digestion of the solids produced during pretreatment. First and second value reported in
each column corresponds to total monomeric/oligomeric sugars and oligomers only released, respectively. A single value indicates release of only monomers
(adapted from [11])
7 Biochemical and Thermochemical Conversion of Switchgrass to Biofuels
159
160 V. Balan et al.
a post-wash of AFEXTM pretreated switchgrass (only to allow fair comparison

with other pretreatments since during conventional AFEXTM no water washing of
pretreated biomass is necessary following AFEXTM treatment).
This suggests that a significant proportion of hemicelluloses are solubilized and
re-deposited on outer cell walls during AFEXTM pretreatment, however, these
oligomers can be easily removed as shown elsewhere as well [15, 19]. Unlike the
previous work with corn stover, CAFI pretreatments show distinct effectiveness on
late-spring harvest Dacotah switchgrass. A detailed techno-economic analysis was
carried out for all CAFI pretreatments based on the results reported for Dacotah
switchgrass [24]. The minimum ethanol selling price (MESP) per gallon fuel for
an integrated, commercial-scale lignocellulosic ethanol process based on each
pretreatment has been provided in Table 7.3 (inclusive of oligomers credits) that
highlights LHW and AFEXTM as the less expensive options.
For industrial processing, a high solid loading ([18%) is preferred since it gives
higher product concentrations resulting thereby lowering energy and water input.
It is important to note that as the solid loading increases from 3 to 18% or higher,
the sugar conversion drops significantly [5]. The possible reason for such inhibi-
tion is due to enzyme inhibition at higher sugar concentration (particularly
cellobiose and oligomeric sugars), higher concentration of pretreatment degrada-
tion products and lesser adsorption of enzymes to substrates. Several studies are
under way to overcome this problem for economically producing biofuels through
biorefinery process.
7.2.2.2 Effect of Hemicellulases
By looking at the composition of the pretreated switchgrass residues (Table 7.2), it

is clear that hemicellulose is either partially or completely retained. Therefore,
supplementing cellulases with accessory enzymes like hemicellulases may be
important to further improve both glucose and xylose yields for some pretreat-
ments. About 5–10% increase in glucan conversion and about 5–20% improve-
ment in xylan conversion is usually observed depending on the enzyme loading for
different alkaline pretreated biomass [25]. The dilute acidic, sulfur dioxide both
produced solids with very low xylan contents of 4.5 and 7.3%, respectively.
Supplementing at low levels, xylanase had negligible effect on xylan hydrolysis
yields.
7.2.3 Co-Production of Protein and Biofuel

from Switchgrass
With the increasing cost of food and feed products, alternative feeds such as leaf
protein concentrate (LPC) are gaining more attention. LPC is protein from
herbaceous biomass such as grasses or alfalfa that has been removed from the cell
wall carbohydrates, and is generally 50–80% protein by weight [26]. This concept
Fig. 7.3 Schematic representation of leaf protein processing of switchgrass integrated with a
cellulosic biofuel production facility (‘biorefinery’) using an ammonia based pretreatment [34]
of a green biorefinery has been studied for decades and has been scaled up and
commercialized using alfalfa as a feedstock [26, 27]. In the basic process, the
fresh, wet feedstock is first macerated and then mechanically pressed to produce a
protein-rich juice. This juice is rapidly heated via steam injection to precipitate the
protein, which is then dried and sold. The de-proteinated juice can be evaporated
and the stillage added to the fiber to be sold as an animal feed or, alternatively, as a
feedstock for biofuel production [27, 28] Alternatively, dried biomass can be
added to a dilute alkaline solvent to extract the protein, and ultrafiltration can be
used as an alternative to heat for coagulation for protein concentration [29, 30].
Yields tend to be fairly low, with 40–80% of the protein recovered during the
pressing/extraction step and 50–60% of those proteins recovered during the coa-
gulation/filtration step.
Most of the research on LPC has focused on using alfalfa as a feedstock, but
switchgrass has also been considered as a protein source. Prior to biofuel
research, switchgrass was considered forage for animals, noting that protein in
excess of 12% of the total weight of the plant was available if harvested in
early summer [31]. Protein extraction of a late May harvest of Alamo
switchgrass were reported [32]. Only 35% of the protein was extracted in most
cases. However, the remaining protein could be solubilized during cellulose
hydrolysis and is also potentially recoverable. A later study showed that the
protein was only partially recoverable, but that the process when integrated with
biofuel production (Fig. 7.3) would produce a net profit of $34 Mg-1 biomass if
moderate improvements in extractability and recovery were possible [33].
Proteins have been extracted from fresh and stored switchgrass harvested in the
summer and autumn [35]. Protein content within the biomass, extraction yields,
and concentration yields were lower than obtained from orchardgrass,
162 V. Balan et al.
suggesting that this may not be a viable co-product for biofuel production from
switchgrass.
7.2.4 Impact of Harvest Dates and Phenotype Variety

on Biofuel Yields
While numerous studies exist on pretreatment and enzymatic hydrolysis of

switchgrass, the massive variation in switchgrass harvest locations, harvest dates,
and phenotype variety make it difficult to directly compare these studies. There are
many varieties of switchgrass broadly categorized in two types; lowland switch-
grass such as Alamo tends to have greater biomass yields while upland switchgrass
such as Cave in Rock are more suitable to cooler climates [35]. As a perennial
grass, it can be harvested multiple times throughout the growing season. It is
known that forages have significant structural changes over the course of the
growing season, with cellulose and lignin content increasing over time and protein
and soluble sugars decreasing. The stem to leaf ratio also increases, and leaves will
drop off as senescence occurs if the plant is left standing over winter.
Little research has been performed on differences in biofuel yields of different
harvest dates. Sugar yields from three different stages of switchgrass growth
pretreated with DA were reported [36]. While total sugar yields released were
similar among the three stages, the later stages had greater cellulose content. Thus,
the percentage yield was lower for the later stages, with a steeper decline for
glucose than non-glucose sugars. The CAFI project also compared multiple
varieties harvested in early winter with a Dacotah switchgrass harvested after
allowing it to stand all winter [37]. The Dacotah harvest had significantly poorer
glucose yields among all pretreatments tested, with the largest decreases in
AFEXTM, DA, and SAA (Fig. 7.4).
The xylose trend was less clear, with significant decrease in yields seen only for
the two ammonia pretreatments. Two different switchgrass varieties harvested in
July and October: one planted in Michigan and one in Alabama were investi-
gated [38]. Differences in fiber digestibility were seen in the Michigan harvest but
not in Alabama. This is likely due to a shorter growing season in Michigan,
causing the July harvest to be more immature than an equivalent July harvest in
Alabama as well as the fact that the October harvest in Alabama was not yet at full
senescence. In general, however, a clear trend of decreasing cellulose digestibility
with a slight decrease in hemicellulose digestibility as the growing season con-
tinues is present in all studies.
In contrast, different strains of switchgrass can also have different digestibility,
although there is currently very little research in this field. Shawnee and Alamo
switchgrass, both planted in Oklahoma and harvested in December did not show
much difference for any pretreatment between sugar yields [38]. A separate study
considered ethanol yields from pretreated switchgrass varieties specifically bred
SAA
(a)
LIME
LHW
DA
AFEX
Untreated
0 20 40 60 80 100
% Glucan Yield
SAA (b)
LIME
LHW
Dacotah
DA Shawnee
Alamo
AFEX
Untreated
0 20 40 60 80 100
%Xylan Yield
Fig. 7.4 Switchgrass pretreated by various methods and hydrolysed using commercial enzymes.
a % glucan yield, b % xylan yield. The experiments were done at 3% solid loadings using
commercial enzymes (15 FPU cellulase in Spezyme CP plus 30 CBU b-glucosidase in Novozyme
188) per g glucan of untreated raw switchgrass (equivalent to total 27 mg protein/g glucan in
untreated biomass). The hydrolysis was carried out at 50C and an agitation rate of 150 rpm.
Pretreatment conditions are given in Table 7.1. Sugar yield is based on glucan or xylan in
pretreated/hot washed solids for all pretreatments except for AFEXTM (adapted from [37])
for either high digestibility or high biomass yield. The highly digestible sample did
not significantly increase ethanol yields over one of the lower digestibility breed,
but did increase yields over the other breed [39]. In contrast, Sarath et al. [40]
found significant differences in varieties specifically bred for changes in dry matter
digestibility, and noted that most of the change in ethanol yield after DA pre-
treatment came from variations in stem lignin content [40]. Other important
factors were present, however, including the amount of hemicellulose that
could be solubilized during pretreatment. Besides the total lignin produced, the
164 V. Balan et al.
syringyl/guaiacyl ratio within lignin can be important, and genetically engineered

strains that improved these ratios have been developed and shown to greatly
improve biofuel potential [41].
Several studies have also considered different strains for fiber digestibility in
sheep and cattle, which has been shown to correlate strongly with biofuel yields [42].
Pathfinder and a variety bred specifically for feed were tested for high fiber
digestibility, and noted a slight but not significant difference in fiber digestibility
when fed to lambs [43]. Likewise, Trailblazer and Pathfinder switchgrass were
compared and also no differences were found in fiber digestibility [44]. More
recently, three varieties across three separate years were compared, and found that
the differences among years were greater than the differences between strains [45].
A later study did show that Trailblazer had significantly higher cellulose digest-
ibility than Cave in Rock and Shawnee, but not hemicellulose digestibility [46].
The authors note that the difference is likely due to a different leaf/stem ratio as
opposed to differences in cell wall structure.
7.2.5 Biofuel Production
Biochemical conversion of lignocellulosic biomass to biofuel consists of four

biological events: saccharolytic enzymes production, enzymatic hydrolysis,
hexose fermentation and pentose fermentation [6]. Depending on how the four
biological steps are integrated, the process configuration can be separate hydrolysis
and fermentation/co-fermentation (SHF/SHcF), simultaneous saccharification and
(co)fermentation (SSF/SSCF), or CBP [6]. SHF/SHcF performs the four biological
steps separately (co-fermentation performs hexose and pentose fermentation
together). SSF combines enzymatic hydrolysis and hexose fermentation together.
SSCF further integrates hexose fermentation and pentose fermentation. CBP
performs the four biological steps in a single bioreactor. The major advantage of
SHF/SHcF is that enzymatic hydrolysis and fermentation can be carried out at
optimal conditions [47]. Nevertheless, the enzymes are easily inhibited by the end-
products (sugars) and thus affecting final sugar conversions. SSF/SSCF removes
sugars by fermentation and thereby reduces the sugar inhibition [47]. SSF/SSCF
also has other advantages such as lower cost, shorter process time, and lower
contamination risk. CBP is the ultimate low-cost industrial configuration to pro-
duce biofuel [48, 49]. However, currently there’s no perfect microbe or microbial
consortium capable of degrading lignocelluloses, utilizing all the carbohydrates
and at the same time producing ethanol at a high yield as CBP requires [48].
Biofuel (such as ethanol) production from switchgrass is affected by many
factors such as switchgrass type, harvest time, pretreatment, glucan loading,
enzyme loading, fermentation strain, and process configuration (Table 7.4).
Switchgrass type and harvest time affect the sugar compositions in the bio-
mass and hence affect the biofuel yield [33]. The two factors might also affect
other chemicals or nutrients in the switchgrass, which in turn affect fermentation
Table 7.4 Ethanol production from switchgrass
SG type Pretreatment Process Strain(s) Glucan Cellulase Nutrients EH + ferm. Ethanol Ethanol Ref.
(harvested configuration loading loading suppl. Time (h) titer yield
time) (%) (FPU/g (g/L) (g/g
glucan) biomass)
Unspecified Lime SSF S. cerevisiae D5A 3 25 YEPb 168 14 0.16a [98]
Unspecified Dilute acid SSF S. cerevisiae D5A 3 25 YEP 168 14 0.15a [99]
Kanlow Hydrotherm. SSF K. marxianus IMB3 4.2 15 YE & amm. 96 19 0.15a [100]
Sulf.
Kanlow Hydrotherm. SSF S. cerevisiae D5A 4.2 15 YE & amm. 168 22 0.18a [100]
Sulf.
Kanlow Hydrotherm. SSF K. marxianus IMB4 4.1 15 YE & amm. 72 17 0.15a [62, 101]
Sulf.
CIR Aq. ammonia SSF S. cerevisiae D5A 6 38.5 YEP 96 22 0.15a [102]
CIR Aq. ammonia SSFc S. cerevisiae D5A 2 77 YEP 72 10 0.16a [103]
CIR (Oct) AFEXTM SHcF S. cerevisiae 424A 6 15 None 96 ? 96 35 0.18 [63]
(LNH-ST)
CIR (Oct) AFEXTM SSCF S. cerevisiae 424A 6 15 None 8 ? 192 32 0.17 [63]
(LNH-ST)
CIR (Oct) AFEXTM SSCF S. cerevisiae 424A 6 15 YEP 8 ? 192 36 0.19 [63]
(LNH-ST)
CIR (Jul) AFEXTM SHcF S. cerevisiae 424A 6.1 10 None 72 ? 96 34 0.16 [33, 42]
(LNH-ST)
CIR (Oct) AFEXTM SHcF S. cerevisiae 424A 6.7 12.2 None 72 ? 96 30 0.14 [33, 42]
(LNH-ST)
7 Biochemical and Thermochemical Conversion of Switchgrass to Biofuels
Alamo (Jul) AFEXTM SHcF S. cerevisiae 424A 6.5 9.8 None 72 ? 96 30 0.14 [33, 42]
(LNH-ST)
(continued)
165
Table 7.4 (continued)
166
SG type Pretreatment Process Strain(s) Glucan Cellulase Nutrients EH + ferm. Ethanol Ethanol Ref.
(harvested configuration loading loading suppl. Time (h) titer yield
time) (%) (FPU/g (g/L) (g/g
glucan) biomass)
Alamo AFEXTM SHcF S. cerevisiae 424A 3.3 9.7 None 72 ? 96 21 0.2 [33, 42]
(Oct) (LNH-ST)
Unspecified Dilute acid SSF B. clausenii Y1414 & 4.4a 26 YEP 168 * 192 unspecified 0.16a [61]
S. cerevisiae D5A
St6-3F Dilute acid SHF S. cerevisiae 1.1a 127a None 72 ? 48 unspecified 0.08 [39,
ATCC 24859 104]
CIR Cave in rock
a
Calculated or estimated
b
YEP medium yeast extract and peptone medium; YE yeast extract
c
Pilot scale
V. Balan et al.
performances [33]. For instance, more inhibitory effects were observed during
the fermentation of the biomass harvested in July when compared to October
[33]. Pretreatment largely determines the substrate properties for biofuel pro-
duction. Degradation products generated during pretreatment reactions also affect
fermentation performance [50, 51]. Typically, DA pretreatment destroys the
nutrients present in the biomass and hence nutrient supplementation is required
during fermentation. However, AFEXTM pretreatment conserves nutrients, which
renders nutrients supplementation unnecessary and reduces cost [51, 52]. Glucan
loading determines the final biofuel concentration. Higher glucan loading typi-
cally leads to higher biofuel concentration and reduces the cost of biofuel
recovery/distillation as well as water use. However, higher glucan loading nor-
mally results in lower sugar conversions and hence lower biofuel yield [5].
Commercial enzyme supplementation is a large cost in the biofuel production
process. High enzyme loading can enhance the biofuel yield while also increase
the production cost.
The oldest ethanol fermentation strain, Saccharomyces cerevisiae, cannot
natively ferment pentose. During the past decades, a great deal of effort has
been made to genetically engineer microorganisms, such as Escherichia coli
[53], Zymomonas mobilis [54], and Schefferosomyces stipitis [55] in order to
efficiently ferment xylose into ethanol. However, the robustness of E. coli and
Z. mobilis during fermentations of cellulosic hydrolysates might not be ideal
[56]. Dissolved oxygen control is the key for ethanol production using P. stipitis
[57]. Genetically modified S. cerevisiae strains were also widely utilized for
glucose and xylose co-fermentation [58–60]. Depending on the biofuel pro-
duction strains capacity, the process can convert glucose or both glucose and
xylose to biofuel. Typically, higher biofuel yield can be obtained when xylose
was also converted. For instance, Yang et al. [39] reached a yield of 0.08 g
ethanol per gram switchgrass using a SHF process configuration, while Bals
et al. [33] achieved higher than 0.14 g ethanol per gram switchgrass even at a
higher solids loading and lower enzyme loading. Comparing to SHF/SHcF,
SSF/SSCF has a potential to reach higher yields [61]. The ethanol yields from
switchgrass using SSF were around 0.14–0.18 (Table 7.4). One large concern of
SSF is the discrepancy of the optimal temperature for enzymes, which is around
50C, and for fermentation strains, which is around 30–37C. Typically, SSF is
performed at fermentation temperatures as a compromise resulting in lower
enzyme activities. High temperature tolerance strain such as Kluyveromyces
marxianus was also investigated for SSF of switchgrass [62]. However, lower
ethanol yield was obtained probably due to the ethanol fermentation was not as
good as S. cerevisiae.
Those SSF studies mentioned above were all performed at a relatively low
glucan loading, which resulted in low ethanol concentrations that did not reach the
industrial titer threshold of 40 g L-1. Jin et al. [63] tried 6% glucan loading using
AFEXTM treated switchgrass aiming to produce an industrial relevant ethanol titer
in both SHcF and SSCF configurations; 35 and 32 g L-1 ethanol was produced,
respectively. It turned out that the solid switchgrass biomass showed severe
168 V. Balan et al.
(a) SHF
- Spezyme CP: 89.3ml (7.9 g Protein)
- Novozyme 188: 89.9ml (13.5 g Protein) - Yeast innoculum
- Multifect Xylanase: 31.5ml (1.1 g Protein) (Dry Weight):
- Multifect Pectinase: 22.2ml (2.0 g Protein) 5.0 g
1kg AFEXTM
pretreated Enzymatic
Switchgrass Fermentation -178.4g Ethanol (34.6g/L)
hydrolysis
Time: 96h Time: 96h - Xylose: 9.9g (1.9 g/L)
Hydrolysate
- Glucose: 388.9g Temp.: 50°C Temp.: 30°C - Gluco-oligomers: 23.6g (4.7 g/L)
- Glucose: 235.0g (45.5 g/L)
- Xylose: 287.5g - Xylo-Oligomers: 50.1g (9.8 g/L)
- Xylose: 165.0g (31.8 g/L)
- Gluco-oligomers: 23.6g (4.7 g/L) Sugar conversions:
- Xylo-Oligomers: 50.1g (9.8 g/L) Glucan: 66.5%
Xylan: 74.7%
Residual solids
Ethanol Metabolic yield: 89.7%
- Glucose: 130.4g
- Xylose: 72.6g
(b) Two-step SSCF

- Spezyme CP: 89.3 ml (7.9 g Protein)
- Novozyme 188: 89.9 ml (13.5 g Protein)
- Multifect Xylanase: 31.5 ml (1.1 g Protein)
- Multifect Pectinase: 22.2 ml (2.0 g Protein)
- Yeast innoculum
(Dry Weight):
11.2 g
1kg AFEXTM
pretreated 165.3g Ethanol
Switchgrass SSCF (32.1g/L)
192 h
- Glucose: 2.4 g (0.5 g/L)
- Glucose: 388.9 g
- Xylose: 55.0 g (11.2 g/L)
- Xylose: 287.5 g
- Glc-oligomers: 11.2 g (2.2 g/L)
- Xyl-Oligomers: 40.5 g (8.2 g/L)
Sugar conversions:
Residual solids: Glucan: 80.3%
- Glucose: 76.5 g Xylan: 84.3%
- Xylose: 45.2 g Ethanol Metabolic yield: 72.7%
Fig. 7.5 a Mass balance for SHcF, b SSCF (b) of AFEXTM pretreated switchgrass (Cave-in-
rock, harvested in October). Data were collected from 6% (w/w) glucan loading experiments
(adapted from [63])
inhibition on the yeast fermentation, which rendered the SSCF results were not as
good as the SHcF ones. However, with washing and supplementation of YEP
medium, actions typically performed with other pretreatments, SSCF yielded
36 g L-1 ethanol (Table 7.4). The mass balances for SHcF and SSCF of AFEXTM
treated switchgrass were shown in Fig. 7.5. Low sugar conversions were observed
for SHcF process. SSCF enhanced the sugar conversions while the ethanol met-
abolic yield was reduced by solids. It should be noted that there was a considerable
amount of sugar loss in the oligomeric forms. Improved enzymes or engineered
yeasts able to consume oligomers would help resolve this problem.
CBP of dilute-acid pretreated switchgrass using Clostridium thermocellum was
also tried with a 0.32 g L-1 ethanol yield [64]. ABE (Acetone butanol ethanol)
fermentation was also tested on dilute-acid pretreated switchgrass and yielded
14.6 g L-1 [65]. It is promising to produce biofuels from switchgrass, while some
improvements are still required such as fermentation strains and saccharolytic
enzymes.
7.3 Thermochemical Conversion Processes

for Switchgrass to Biofuels
Switchgrass is an oxygenated, low energy density feedstock when compared to

conventional fossil fuels including crude oil and coals. About 38–42% of the dry
matter of switchgrass is oxygen [66]. A useful means of comparing biomass and
fossil fuels is in terms of their oxygen to carbon (O:C) and hydrogen to carbon
(H:C) ratios. The lower the respective ratios the greater the energy content of the
material [67]. Therefore, removal of oxygen from switchgrass is the major
objective for upgrading its energy density. The deoxygenation can be achieved
most readily by dehydration, which removes oxygen in the form of water, and by
decarboxylation, which removes oxygen in the form of carbon dioxide [68, 105].
For an efficient carbon conversion to fuels, it is desirable to keep decarboxylation
as low as possible.
Thermochemical conversion processes involve the use of temperature and/or
catalysts to convert biomass to liquid or gaseous intermediate products which is
further upgraded to transportation fuels or chemicals. The choice of an appropriate
conversion technology depends on several factors such as biomass composition,
availability, logistics, processing characteristics and types of biofuels desired. The
robustness of the technology, production of fungible fuels, ability to process
diverse feedstocks, and almost complete utilization of biomass components are
some of the major advantages of the thermochemical pathways. Pyrolysis, gasi-
fication, and combustion that are conducted at a temperature of several hundred
degrees Celsius are the most promising technologies under thermochemical
pathways to convert switchgrass to liquid fuels and/electricity (Fig. 7.6). These
three major technologies can be compared based on the basis of oxygen supply
during the respective conversion processes (Fig. 7.6b) using equivalence ratio (U)
which is defined as the actual air fuel ratio/the air fuel ratio for complete
combustion.
The proximate and ultimate analysis, ash/inorganic contents, moisture content,
heating value, and bulk density are some of the important properties which play a
very important role in thermochemical conversion processes. The elemental
composition of switchgrass is a basic property that is required to determine the
carbon conversion efficiency during different thermochemical processes and
potential of switchgrass for biofuels applications. Literature average value of
proximate analysis of switchgrass is 81.4% volatile matters, 3–6% ash, and 15%
fixed carbon with a higher heating value in the range of 18.5–18.7 MJ kg-1. High
volatile matter of biomass makes it more readily devolatilized than coal leaving
less fixed carbon. The elemental compositions of switchgrass are comparable with
other potential biofuels feedstock such as hybrid poplar and corn stover. Recently
the chemical properties of lowland and upland varieties of switchgrass (Table 7.5)
which are relevant to biofuel production were reviewed [69]. Ash is an integral
part of the plant structure which consists of a wide range of mineral matter such as
salts of calcium, potassium, sodium, silica, and magnesium. Ash content depends
170 V. Balan et al.
Switchgrass
(a) (b)
Thermochemical
Combustion 100% or excess
oxygen
( 1)
Pyrolysis Gasification Combustion
Catalytic Fischer-Tropsch Partial oxygen

Upgradation Synthesis Gasification
Oxygen Supply
( = 0.2 – 0.4)
Pyrolysis No oxygen
Liquid Liquid Heat / ( = 0)
Fuels Fuels Electricity
Fig. 7.6 a A simplified schematics of thermochemical routes for switchgrass, b comparison of

pyrolysis, gasification, and combustion with respect to equivalence ratio
Table 7.5 The ultimate analysis and inorganic constituents of different switchgrass species [69]
Variety Ultimate analysis (wt% dry Inorganic constituents (mg/kg, dry basis)
basis)
C H N O Al Cl K Si P S Ca Ash%
CIR 47.5 6.8 0.51 42.5 74 1,624 9,148 8,623 3,577 820 3,572 6.0
Alamo 47.3 5.3 0.51 41.6 – – – – – – – 5.2
Trailblazer 45.9 6.0 0.96 – 75 1,500 8,674 9,420 4,176 920 3,712 6.4
Blackwell 46.3 6.0 1.10 – 82 1,514 9,323 9,904 3,662 881 3,792 6.2
Kanlow 48.0 5.4 0.41 41.4 76 1,596 10,894 8,767 3,844 865 3,512 5.4
on the type of the plant and the soil contamination in which the plant grows. The
time and frequency of harvesting of switchgrass is also important and these
conditions affect the ash content along with the total dry matter yield. As an
example, Monti et al. [70] reported that harvesting twice per year may have
advantages from time-management point of view, but biomass quality was sig-
nificantly affected leading to higher ash content in biomass. The biomass pro-
ductivity and plant vigour generally decreased after initial two years in twice-cut
system. Therefore, the additional harvesting costs due to low re-growth biomass
yield potentially offsets the benefits [71].
The chemical composition, ash content, and mineral composition (C, N, Al, Ca,
Cl, Fe, K, Mg, Na, P, S, Si) of switchgrass varies along the stems, leaves, and flower
head. Generally, leaves have higher mineral concentration when compared with
other parts of the plant. Monti et al. [70] compared mineral compositions and ash
contents of six major energy crops (four perennial and two annual crops).
Switchgrass and miscanthus showed the overall better biomass quality with respect
Table 7.6 Products of biomass pyrolysis

Process Time Bio-oil Gas Biochar
(s) (%)
Fast pyrolysis 1–2 75 13 12
Slow pyrolysis 30–1,800 30 35 35
to ash and mineral contents. It has been demonstrated that ash contents also neg-
atively affects the heating value of biomass. The ash content of perennial grasses
(e.g. switchgrass, giant reed, miscanthus) are generally higher than wood probably
due to the soil contamination. The time of harvesting also affects the biomass yield
and quality. Switchgrass yield is generally decreased due to delayed harvesting, but
delayed harvesting has been reported to have less ash and moisture content in
switchgrass thereby increasing the overall heating value of the biomass [72].
Although the ash content of switchgrass is lower than coal, the ash chemistry is
different when subjected to the thermochemical conversion processes. In general,
the ash present in biomass has low ash fusion temperature (750–1,000C) which
becomes a concern due to the propensity of agglomeration, fouling, deposits,
slagging and corrosion in the reactor at high temperature. Further, ash needs to be
properly disposed after the conversion process to minimize its impact on process
equipment. The melting and slagging of ash in biomass is dependent on the
concentrations of K, Cl, S, Al and Si in ash [73]. The problems arising from the
high ash content of energy crops during thermochemical conversion may
be addressed by separating the problematic elements through leaching or by
adding additives such as lime to increase the melting point of slag. Another
possibility is to blend the perennial crops with wood which has comparatively
lower ash content and also higher ash fusion temperature [74].
7.3.1 Pyrolysis
Pyrolysis, in the form of wood distillation, has been used since the historical
times. Pyrolysis is the thermal degradation of biomass in the temperature range
of 400–550C in the absence of oxygen. Overall, biomass pyrolysis is an
endothermic process, which means that energy needs to be supplied to drive the
process. The process yields gaseous (non-condensable gases e.g., CO, CO2, CH4,
and H2), liquid (bio-oil and water) and solid (bio-char) products with varying
yields (Table 7.6) depending upon the pyrolysis conditions such as feedstock
composition (lignin, ash, and carbohydrates), pyrolysis temperature, vapor resi-
dence time, heating rate, and particle size. Primarily following three basic
approaches (fast, medium, and slow pyrolysis) are used based on the product
requirement:
• Fast pyrolysis where technologies are dedicated to bio-oil production and so bio-
char is an undesirable by-product;
172 V. Balan et al.
• Medium pyrolysis where technologies are dedicated for co-production of bio-

char and useful by-products (liquid fuels, syngas, chemicals, heat, and
electricity);
• Slow pyrolysis where technologies are dedicated to bio-char production, without
or with minimum production of useful by-products (gases and liquids).
To produce transportation fuels, the process is generally optimized for the
production of bio-oil (fast pyrolysis) that can be catalytically upgraded to trans-
portation fuels, chemicals, adhesives, and many other applications. Fast pyrolysis
typically requires dry (\10% moisture) and finely ground biomass (2–6 mm). The
thermal degradation of lignocellulosic feedstock follows a complex mechanism,
which can be influenced by heating and cooling rates. The rapid heating and rapid
quenching can produce intermediate bio-oil, which condenses before further
decomposing into gaseous products.
Bio-char which is the non-liquefied carbonaceous solid product resulting from
the thermal decomposition of biomass has historically been used in soil to enhance
plant growth [75]. Switchgrass bio-char has also been reported to be a potential
adsorbant for both organic pollutants and heavy metals [76, 77].
7.3.2 Bio-oil
Bio-oil is a dark brown, free-flowing organic liquid that is comprised of highly

oxygenated compounds with a density of about 1.2 g mL-1. The synonyms for
bio-oil include pyrolysis oil, pyrolysis liquids, bio-crude, wood liquids, wood oil,
liquid smoke, wood distillates, pyroligneous acid, and liquid wood. Bio-oil yield
from fast pyrolysis can be as high as 70–80% based on the starting dry biomass
weight [78]. It is important to note that bio-oil produced via fast pyrolysis is not
stable and its composition tends to change toward thermodynamic equilibrium
during storage [79]. The oxygen content of bio-oils is usually 35–40 wt% and the
single most abundant component of bio-oil is water. It contains major groups of
compounds, including acids, alcohols, aldehydes, esters, ketones, sugars, phenols,
guaiacols, syringols, furans, lignin derived phenols and extractible terpene with
multi-functional groups. Bio-oil has water content of typically 15–30 wt%, which
cannot be easily removed by conventional methods. The presence of aldehydes
and ketones make bio-oils especially hydrophilic and highly hydrated, which leads
to the water being difficult to eliminate [79]. Oxygen containing components
present in the oil, especially the carbonyl compounds and low pH value are
responsible for bio-oil’s instability [80]. In fact, bio-oil is usually not miscible and
chemically very different than the petroleum due to the presence of water, high
oxygen content, higher viscosity, and low pH. The higher heating value of bio-oil
is about 15–22 MJ kg-1, which is much lower than that of liquid fuels due to
presence of oxygenated compounds.
Table 7.7 Physical Property 450 500 550

properties of switchgrass bio- (C) (C) (C)
oil produced at three
pyrolysis temperatures [82] Density at 15C (103 kg m-3) 1.17 1.13 1.11
pH value 2.74 3.29 2.96
Water content (wt%) 20.1 24.7 27.5
HHV (MJ kg-1) 18.7 17.4 17.1
Viscosity at 40C (centistokes) 28.2 20.9 17.0
Solid content in bio-oil (wt%) 1.62 1.44 1.50
Ash content (wt%) 0.42 0.40 0.43
7.3.2.1 Switchgrass Bio-Oil
There have been several studies which reports the fast pyrolysis of switchgrass
to produce bio-oils and discusses the bio-oil properties [69, 81–83]. The bio-oil
yield varied from 43 to 77% depending upon the pyrolysis conditions and types
of switchgrass [69]. It has been reported that the switchgrass bio-oils had much
higher levoglucosan levels, less aromatic compounds, and also less nitrogen
containing compounds than alfalfa stems bio-oils [69]. Alkali metal, especially
potassium present is switchgrass has been reported to have a strong catalytic
effect on pyrolysis [84]. Table 7.7 shows the important properties of bio-oil
produced from the fast pyrolysis of switchgrass. He et al. [82] studied fast
pyrolysis of switchgrass in a pilot fluidized-bed pyrolysis reactor at three dif-
ferent temperatures (450, 500, and 550C) and moisture (5, 10, and 15%).
Switchgrass was collected in Iowa (USA) from mature stands of the Cave-in-
Rock cultivar while dormant (early spring). The study concluded that moisture
content and pyrolysis temperature caused large variations in product yield,
chemical composition, and most of the measured physicochemical properties of
bio-oil. Solid content in bio-oil are generally fine char particle entrained with
vapors.
The inconsistent physicochemical properties of bio-oil possess a significant
barrier to the commercialization. However, the volumetric energy density of bio-
oil is 5–20 times higher than the original biomass [80]. This makes the concept of
developing small scale fast pyrolyzer available near to the feedstock where bio-
mass can be converted to bio-oil and subsequently, the liquid product can be
transported to a central refining location for upgrading it to transportation fuels.
The deleterious properties of bio-oil such as high viscosity, thermal instability and
corrosiveness can be addressed by different upgrading processes. Some of the
recent bio-oil up gradation techniques include hydro-de-oxygenation, catalytic
cracking of pyrolysis vapors, emulsification, steam reforming, and chemicals
extraction techniques [79].
174 V. Balan et al.
7.3.3 Gasification
Gasification is another transformation pathway for production of liquid fuels from

biomass. Biomass gasification is an endothermic process where biomass is con-
verted into combustible gas under high temperature and limited supply of oxygen
(partial oxidation). Biomass gasification processes vary considerably, and typical
operation is conducted at C700C and 1–5 atmosphere pressure under limited
supply of air/oxygen/steam using relatively dry feedstock (moisture \10 wt%)
[85, 86]. Biomass inside a gasifier mainly goes through four conversion stages:
(1) drying ([150C); (2) pyrolysis or devolatilization (150–700C); (3) combus-
tion (700–1,500C); and (4) reduction (800–1,100C). The stages 1, 2, and 4
absorb heat (endothermic) whereas step 3 releases (exothermic) heat. The gasifiers
are generally classified according to the relative movement of the fuel and the
gasifying medium as either fixed beds (up-draft, down-draft and cross-draft) or
fluidized beds (bubbling, circulating, spouted and swirling). The key step in the
gasification process includes feedstock conditioning, gasification, heat recovery,
gas cleaning and conditioning, and gas utilization. The product gas from a gasifier
is mainly composed of CO; CO2 ; H2 O; H2 ; CH4 ; other gaseous hydrocarbons
ðCHs Þ; tars, char, inorganic components, and ash. The gas composition can greatly
vary depending upon the gasification process, the gasifying agent, and the biomass
composition. A generalized reaction scheme can be written as:
CHx Oy þ O2 ! CO þ CO2 þ H2 O þ H2 þ CH4 þ other ðCHs Þ þ tar

þ char þ ash
The product gas is either a medium-energy content gas referred synthesis gas
(syngas) or a low-energy content producer gas. Syngas consists primarily of
carbon monoxide and hydrogen. Higher quality syngas can be produced by using
pure oxygen as the oxidizing agent. Producer gas results if air is used as the
oxidizing agent, which dilutes the combustible components of the gas with
nitrogen. Generally, producer gas is adequate for power generation and avoids
the energy use associated with oxygen production. Syngas is required for
chemicals and liquid fuels production. Gasification technology has been widely
used to produce commercial fuels and chemicals. Technological advances par-
ticular to biomass gasification have been successfully demonstrated and being
developed at commercial scales. The main advantage of technology is its ability
to produce a reliable, high-quality syngas product from organic matters that can
be used for energy production or as a building block for chemical manufacturing
processes (Fig. 7.7).
In fact, the easiest way to convert biomass into liquid transportation fuels is
via routes involving C1-building blocks (i.e. synthesis gas) [68]. In 1923,
German scientists Franz Fischer and Hans Tropsch first studied conversion of
coal-derived syngas to liquid hydrocarbons over metal catalysts through a series
of chemical reactions. The process is known as Fischer-Tropsch (FT) synthesis.
Fig. 7.7 Major pathways Diesel /

for syngas conversion to F-T Synthesis
Gasoline /
fuels and chemicals. WGS Waxes
Water–Gas Shift reaction:
CO(g) ? H2O(v) $
WGS* Reaction Hydrogen
CO2(g) ? H2(g)
Syngas
Methanol Dimethyl Ether
(CO & H2)
Gasoline /
Olifins
Ethanol
Fermentation
Power
* WGS: Water-Gas Shift Reaction: CO(g) + H2O(v) CO2(g) + H2(g)
FT synthesis is typically carried out in the temperature range of 150–300C and

at high pressures (15–40 bars) described by the following equation [87]:
nCO þ ð2n þ 1Þ H2 ! Cn H2nþ2 þ nH2 O
where n is the number of carbon atoms in the hydrocarbon produced.
All reactions (for varying value of n) are exothermic (DH = -165 kJ mol-
CO-1) and the product is a mixture of different hydrocarbons mainly consisting of
paraffins and olefins [87]. The relative distribution of the products depends on the
catalyst and the process conditions (temperature, pressure, and residence time).
The ratio of H2 and CO in syngas is an important parameter and ideally, a good mole
ratio is 2:1 to form alkanes (–CH2-) [88]. The synthesis gas from carbohydrates and
other biomass normally has 1:1 molar ratio (C6H12O6 ? 6CO ? 6H2). However,
H2 to CO ratio can be adjusted by the use of water–gas shift (WGS) reaction
(CO ? H2O ? CO2 ? H2).
The gasification process has several benefits for near-term commercial appli-
cation due to the inherent advantages over combustion including more flexibility
in terms of energy applications, higher economical and thermodynamic efficiency
at smaller scales, and potentially lower environmental impact when combined with
gas cleaning and refining technologies [88]. The directly evolved synthesis gas
from a gasifier contains undesired impurities derived from biomass feedstock
including tar, hydrogen sulfide, carbonyl sulfide, ammonia, hydrogen cyanide,
alkali, and dust particles. It is important to clean synthesis gas to avoid FT catalyst
poisoning. Impurities from synthesis gas can be removed through several routes
176 V. Balan et al.
Table 7.8 Average gas Compound Switchgrass Corn Wheat straw

composition [90] stover
H2 23.5 26.9 24.4
CO 33.2 24.7 27.5
CO2 19.4 23.7 22.0
CH4 17.0 15.3 16.3
C2H4 5.1 4.2 4.3
C2H2 0.34 0.45 0.31
C3H8 0.82 0.40 0.81
C3H6 0.10 0.12 0.10
%Mass closure 101.1 97.5 0.08
H2/CO 0.71 1.09 0.92
Gas yield (kg/kg feed) 0.62 0.54 0.54
including physical separation, thermal cracking, and catalytic hot gas cleanup [89].
The significant problem with FT synthesis is the cost of clean-up and tar
reforming. The presence of tars in gas causes coking on catalyst surfaces which
must be removed to sustain an effective catalyst.
7.3.3.1 Switchgrass Gasification
There have been few studies on gasification of switchgrass. One of the earlier
studies in fluidized bed gasifier used air (10 Nm3 h-1) as oxidizing medium. The
gas production over the experiment was reasonably constant at 650C, with 5%
H2, 3% CH4 and 11% CO as main combustible components. Lower heating value
of the gas was 4 MJ/Nm3. Carpenter et al. [90] performed a detailed parametric
study of the gasification of switchgrass, corn stover, and wheat straw on an
experimental, fluidized bed pilot-scale (0.5 Mg d-1) gasification facility. The
study compared the performance of the gasifier as a function of feedstock, in terms
of the syngas production and composition. Biomass was continuously fed into
gasifier and the product gases were further cracked in a tubular flow reactor. The
resulting synthesis gas was scrubbed and analyzed. Biomass of particle size of less
than 2.3 mm was fed continuously in fluidized-bed reactor. Table 7.8 provides the
comparison of average gas compositions (vol% on a dry nitrogen-free basis)
during a gasification experiments conducted at steam-to-biomass ratio of 1.0,
a fluidized-bed reactor temperature of 650C, a thermal-cracker temperature of
875C and biomass feed rate of 20 kg h-1.
The residence time in the fluidized-bed reactor was 6.7 s and the residence time
in the thermal cracker was 0.63 s. The study concluded that there was no signif-
icant variation (\10%) in the total tar formation between the feedstock and the
gasification results were in agreement with the literature.
7.3.4 Combustion
Biomass is one of the earliest sources of energy produced by combustion espe-

cially in rural areas where it is often the only accessible and affordable source of
energy [91]. Direct combustion and co-firing with coal for electricity production
from biomass has been found to be a promising method in the near future.
Combustion is the burning of biomass in the presence of air to convert the
chemical energy stored in biomass into heat, mechanical power, or electricity
using stoves, furnaces, boilers, steam turbines, turbo-generators, etc. It is an
exothermic process and can be used for any types of biomass. Fluidized bed
combustion is the best technology used to burn a fuel such as biomass with low
quality, high ash content and low calorific value. Co-firing (direct co-firing,
indirect co-firing and parallel co-firing) of biomass in coal-fired power plants is an
especially attractive option because of the high conversion efficiency of these
plants.
Biomass is very different from coal with respect to physical properties and
organic, inorganic, and energy contents. In comparison to coal, biomass has less
carbon, more oxygen, higher hydrogen content, higher volatile matters, lower ash
fusion temperature, more silica and potassium, and lower heating value [91].
Biomass offers important advantages as a combustion feedstock because of the
high volatile matters and the high reactivity of both the fuel and the resulting
char. The relative volatility which is defined as ratio of volatile matter (V) to
fixed carbon (FC) is generally around five (V/FC) which is significantly higher
than that of coal (V/FC \ 1). Besides the advantages of biomass being a
renewable fuel and important feedstock for the energy security, it is useful in the
mitigation of hazardous emissions from combustion such as CO2, NOx, CH4, SOx
and CO from power plants. It can be used for electricity generation in power
plants which can provide a viable alternative for reducing greenhouse gases
(GHG) and also for the combined heat and power (CHP) in the process
industries.
7.3.4.1 Combustion Properties of Switchgrass
The suitability of switchgrass for bioenergy through combustion is measured by

several indices that reflect energy content, density, and ease of recovery. The study
conducted by McLaughlin et al. [92] indicated that switchgrass should be a ver-
satile bioenergy feedstock for combustion and have some important advantages
over wood with respect to combustion properties. The heating value of switchgrass
(17.4 MJ kg-1) is comparable to that of poplar (18.6 MJ kg-1) with significantly
lower initial moisture content. Switchgrass is baled at moisture contents of
13–15%, while poplar typically contains 45–50% moisture at harvest. Moisture
content at harvest influences the cost of drying, transportation and handling. Ash
content and chemistry is important in the combustion process because it can
178 V. Balan et al.
Table 7.9 Emission factor (g/kg fuel) from switchgrass and coal [91]
Emission factor CO2 N2O CH4 SO2 CO
Switchgrass 1,525 0.090 0.140 0.10 4.12
Coal 2,085 0.031 0.022 17.16 0.25
contribute to slagging problem. McLaughlin et al. [92] reported the average ash
content of switchgrass across a wide variety of locations, varieties and treatments
as 4.5% (range 2.8–7.6%). The ash fusion temperature of switchgrass is 1,016C
which is lower than poplar (1,350C) and coal (1,287C) [92]. Coulson et al. [93]
measured the initial deformation temperature (IDT) of switchgrass ash as
1,035–1,160C in oxidizing atmosphere (air) and 1,055C in nitrogen atmosphere.
The IDT is one of the most important indicators to anticipate the problems from
slag melting during combustion. It is recommended to run thermochemical
operations well below the temperature at which ash starts to soften. The GHG
emissions (CO2, NOx and SOx) are considerably lower than that of coal
(Table 7.9) which makes switchgrass a potential fuel to mix with coal to reduce
the environmental impacts.
The lower heating value, low bulk density, issues with storage, collections, and
handling of switchgrass possess a significant challenge in commercialization of co-
firing with coal. The direct injections of switchgrass with coal in existing coal-fired
power plants possess significant challenges due to its difference in physical
properties with coal. However, the environmental benefits from reduced emission
of NOx, SO2, fossil CO2, and trace metal emissions such as mercury makes a
favorable case to employ switchgrass or other biomass as a renewable energy
source in electricity generating stations.
7.3.5 Hydrothermal Processes
Hydrothermal processes refers to processing of biomass in sub- and super-critical

water (critical point: 374C, 221 bar) medium. One of the major challenges in
utilization of biomass is its high moisture content and the variable feedstock
composition. The conventional thermochemical processes require dry biomass for
the efficient production of biofuels. Hydrothermal processes, which can utilize wet
biomass, capitalizes on the extraordinary solvent properties of water at elevated
temperature for converting biomass to high energy density fuels and functional
carbonaceous materials. Here, water acts as reactant as well as reaction medium
which help in performing hydrolysis, de-polymerization, dehydration, decarbox-
ylation, and many other chemical reactions. The technology has attracted much
attention as a non-toxic, environmentally benign, inexpensive and tunable reaction
medium for conducting ionic/free radical reactions. The process can be applied to
produce solid (bio-char), liquid (bioethanol, bio-crude/bio-oil), and gaseous
(methane, hydrogen) fuels from switchgrass depending on the processing tem-
perature and pressure.
7.3.5.1 Conversion of Switchgrass in Hydrothermal Medium
Kumar et al. [76, 77] used hydrothermal pretreatment of switchgrass to enhance

and optimize its enzymatic digestibility during bioethanol production. More than
80% of glucan digestibility was achieved by pretreatment at 190C. The near-
critical water (250–380C) is used for the liquefaction of biomass to produce bio-
crude. Bio-crude can be upgraded to liquid fuel, hydrogen gas or chemicals by
aqueous phase reforming process in hydrothermal medium. Switchgrass was
effectively liquefied to produce bio-crude in subcritical water at a comparatively
low temperature (235C) in a flow-through reactor. More than 50 wt% of the
organic carbon available in switchgrass was converted to bio-crude using sub-
critical water in the presence of 0.15 wt% of K2CO3 [106].
Supercritical water has liquid-like density and gas-like transport properties. The
lower density favors free-radical reactions, which is favorable for supercritical
water gasification of biomass. Methane gas is a main product from biomass gas-
ification in near-critical or supercritical water (350–400C). At higher temperature
in supercritical water, biomass is converted to hydrogen rich gas without catalyst
or with non-metal catalysts. Adam et al. studied the hydrogen production by
gasification of switchgrass bio-crude in supercritical water at 600C. Nickel,
cobalt, and ruthenium catalysts were tested on titania, zirconia, and magnesium
aluminum spinel supports. Ni/ZrO2 gave 0.98 mol H2/mol C, the highest hydrogen
yield of all tested catalysts; however, over time, increase in pressure drop lead to
reactor plugging with all zirconia supported catalysts [94]. Ramsurn et al. [95]
gasified switchgrass bio-char in hydrothermal medium at 400–650C to produce
syngas. The carbon gasification efficiency in hydrothermal medium was reported
to be much better than that in the thermal medium [95]. Kumar [96] studied the
hydrothermal carbonization (HTC) of switchgrass to produce the high energy
density bio-char at 280C. The results demonstrated that the HTC process can
provide a promising route for high energy density (28 MJ kg-1) solid fuel and
carbonaceous functional materials for use in a variety of applications.
7.4 Conclusions
We have given an outline for two different routes to processing switchgrass. For
the biochemical platform, sugar conversion and products yield are summarized
using the reported literature data. A different scenario of producing fuels and
chemicals using the thermochemical route has also been presented. With the help
of this review one can understand what has been accomplished so far in this field
and identify other barriers which can be overcome in the near future for com-
mercially producing biofuels from switchgrass in a cellulosic biorefinery.
Acknowledgments This work was supported by U.S. Department of Energy through the DOE
Great Lakes Bioenergy Research Center (GLBRC) Grant DE-FC02-07ER64494. AFEX is a
180 V. Balan et al.
trademark of MBI International. We would like to thank CAFI3 members for allowing us to use
some of their data in this chapter. We would also like to thank Nirmal Uppugundla from the
Biomass Conversion Research Laboratory (BCRL) at Michigan State University for helping draft
some of the figures.
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Chapter 8
Estimating Region Specific Costs
to Produce and Deliver Switchgrass
Anthony Turhollow and Francis Epplin
Abstract Feedstock costs are expected to be a major component of the cost

of producing bio-based products from lignocellulosic biomass. The economic
viability of using switchgrass to produce feedstock will depend on its cost relative
to alternatives. The purpose of this chapter is to present estimates of the cost to
produce, harvest, store, and transport switchgrass biomass. Enterprise budgets and
sensitivity analysis are used to produce cost estimates. Delivered switchgrass costs
can vary widely, depending on yields, input prices (seed, fertilizer and lime, and
diesel fuel), input quantities (fertilizer and lime, herbicides), and land costs.
Establishment costs amortized over 10 years range from $38 to $112 ha-1 and
reseeding costs (25% of land in second year) (amortized over 9 years) range from
$10 to $18 ha-1. Baler productivity is important, and can impact costs up to a $16
dry Mg-1. Costs of switchgrass delivered range from as low as $42 to well over
$100 dry Mg-1 if yields are low. The ultimate challenge is to formulate a prof-
itable switchgrass production, storage, and delivery system simultaneously with
profitable conversion to bio-based products.
A. Turhollow (&)
Environmental Sciences Division, Oak Ridge National Laboratory, Bioenergy Resource
and Engineering Systems Group, 1511 East 2050 North, North Logan, UT 84341, USA
e-mail: turhollowaf@ornl.gov
F. Epplin
Agricultural Economics, Oklahoma State University, 416 Ag Hall Stillwater,
Stillwater, USA
e-mail: f.epplin@okstate.edu

188 A. Turhollow and F. Epplin
8.1 Introduction
Numerous cost estimates have been made for switchgrass. These estimates vary
widely and depend greatly on the assumptions made about yield, land rents, fer-
tilizer application rates, and machine costs (Table 8.1). In this chapter we provide
a set of crop budgets for switchgrass that cover a range of possible scenarios. We
use a discount rate of 6.5%, a labor cost of $17.00 implement hr-1, and a diesel
fuel price of $0.93 L-1.
8.2 Land Procurement
If switchgrass (or other purpose-grown crops) are to contribute significant amounts

of biomass, many hectares of land are required. The vast majority of land in
the United States capable of producing switchgrass and other biomass crops is
privately owned. There is precedence in the United States for removing large
quantities of cropland from production, most recently via long-term Conservation
Reserve Program (CRP) leases. Historically, in the United States, the challenge
facing the farm sector has been excess production capacity. Various federal
government programs have attempted to reduce this by idling cropland from
production of commodity crops [1].
Perlack and Stokes [2] estimate how much biomass crops (including perennial
grasses such as switchgrass) could be produced at differing farm-gate prices
(Table 8.2). In 2007 U.S. farmers planted 38, 24, and 26 9 106 ha of corn
(Zea mays), wheat (Triticum aestivum), and soybeans (Glycine max), respectively.
To obtain between 9 and 21 million ha of land to grow switchgrass, switchgrass
must compete economically with these and other crops and would impact cropland
rental rates and land values.
An economically efficient switchgrass processing facility would be expected to
be located near a concentration of feedstock production. Wright and Brown [3]
estimated optimal biorefinery sizes and to achieve economies of scale annual
feedstock requirements ranged from 1 9 106 Mg for a fast pyrolysis-to-bio-oil
facility to 7 9 106 Mg for a gasification to mixed alcohol facility. Depending on
yield, area required could range from 50,000 ha at 20 dry Mg ha-1 to 644,000 ha
at 11 dry Mg ha-1.
In the absence of spot markets (as in the case of corn for corn-to-ethanol
facilities), obtaining a reliable flow of feedstock could involve: (1) contracts with
individual growers, (2) contracts with a group of owners through a cooperative, (3)
long-term leases similar to the CRP, and/or (4) land purchases. U.S. landowners
have experience with long-term (10–15 year) CRP contracts, with more than
13 9 106 ha having been under contract. CRP annual payments have been in the
range of cropland rental rates and vary widely by state. Such contracts may pro-
vide a blueprint for biorefineries that need to ensure a reliable flow of feedstock
8 Estimating Region Specific Costs to Produce and Deliver Switchgrass 189
Table 8.1 Estimates of switchgrass farm gate costs

Year Location Nitrogen Land rent Mature yield Farm gate cost Source
(kg ha-1) ($ ha-1) (dry Mg ha-1) ($ Mg-1)
2008 Indiana 90 173 11.2 50 Brechbill and
Tyner [15]
2007 Iowa 112 198 9.0 90 Duffy [16]
1996 Oklahoma 56 74 9.0 25 Epplin [17]
2007 Oklahoma 90 148 8.4–14.6 41–58 Epplin et al. [18]
2008 Illinois 112 193 6.4 90 Khanna et al. [19]
2008 N. Dakota, 75 148 5.0 60 Perrin et al. [20]
S. Dakota,
Nebraska
2009 N/A 153 13.8 49 USEPA [21, 22]
2008 Tennessee 67 N/A 14.4 68 ? land UT Extension [23]
2008 Wisconsin 140 198 10.8 58 Vadas et al. [24]
Table 8.2 Estimates of U.S. land converted to perennial grasses at three farm gate prices with a
4% annual growth rate in crop yield
Year $44 Mg-1 $55 Mg-1 $66 Mg-1
ha (106)
2017 2.9 8.5 12.1
2022 5.3 14.2 17.8
2030 9.3 18.6 21.4
Percent of land basea
2017 1.6 4.7 6.7
2022 2.9 7.8 9.8
2030 5.1 10.2 11.8
Biomass produced (106 dry Mg)
2017 32 96 140
2022 91 245 307
2030 183 368 419
a
Land base available in the POLYSYS model used in Perlack and Stokes [2] includes
183 million ha for the eight major commodity crops (corn, wheat, soybeans, cotton, barley, oats,
sorghum, and rice), hay, cropland pasture, and non-irrigated permanent pasture
and for landowners that desire a reliable rent and reduced risk. However, the
risk of default would be higher with a private biorefinery than with the U.S.
government.
Given the required investment in harvest equipment and the need to provide a
continuous flow of a consistent quality of biomass throughout the year and size
economies in a biorefinery, due diligence would dictate a business plan that
includes a well-designed and coordinated feedstock production, harvest, storage,
and delivery system. Production characteristics and harvest cost economies could
result in a structure for switchgrass production that more closely resembles
integrated timber production and processing. For a low-cost perennial feedstock

with a long stand life and wide harvest window, such as switchgrass, market forces
may drive the industry toward vertical integration. A mature switchgrass to
bio-based products industry may more resemble a vertically integrated timber
harvest, production, and processing industry than the atomistic corn grain system.
For a price, the history with the CRP indicates that a substantial quantity of U.S.
farmland could be bid away from current uses and put into switchgrass for bio-
mass production.
8.3 Machinery Costs
We cost machinery used in the establishment, maintenance, and harvest operations

following the methodology set forth in AAEA [4], ASABE [5, 6], and Turhollow
et al. [7]. Included are costs for depreciation and interest; repairs; insurance,
housing, and taxes; fuel; and labor. Turhollow et al. [7] included operating interest
for machinery in their methodology, but in this chapter operating interest is
accounted for in the crop budgets. Table 8.3 summarizes the costs for machinery
on a per hr and per ha basis. Hours of use for the costs in Table 8.3 are based on a
10 year machine life. In Table 8.4 we show how hours of annual operation affect
costs. Note that there are two baler productivities, one from Larson and English [8]
and the other from Shinners et al. [9], which are significantly different and affect
baler costs.
The conclusion one can make from Table 8.4 is that if not enough hours of
annual machine use occur, then machine costs are very expensive. A producer
would be better off hiring custom operators or renting the equipment than owning
if annual hours of use are limited. Harvest companies could develop in a manner
similar to custom grain harvesting firms that harvest a substantial quantity of the
grain produced in the U.S. Great Plains. Custom grain harvest firms take advantage
of the economics of size associated with ownership and operation of machines
used to harvest grain.
8.4 Establishment
Costs are incurred in the first year, and possibly second year for reseeding, to
establish a stand of switchgrass. These include preparation of the soil, seeding,
weed control, and possibly fertilization. Griffith et al. [10] have two conventional
and two no-till establishment scenarios for (1) cropland harvested in fall or pas-
tureland and (2) winter wheat grazed out or harvested for hay in April (Table 8.5).
These were done for Oklahoma and are based on custom rates. Griffith et al. use a
lower cost for seed and fertilizer (diammonium phosphate). We have updated the
Table 8.3 Machinery costs

Implement Size (m) Tractor Use Implement ($ hr-1) Tractor Total Total
(kW) (hr yr-1) (hr ha-1) ($ hr-1) ($ hr-1) ($ ha-1)
Chisel plow 5.49 97 200 0.266 19.84 61.63 81.46 21.70
Offset disk 3.66 97 200 0.354 15.28 61.63 76.90 27.21
Fertilizer & 12.2 45 120 0.104 23.28 34.14 57.43 5.97
lime
spreader
Boom sprayer 15.2 45 150 0.096 26.94 34.14 61.08 5.89
Press wheel 6.10 97 150 0.291 31.67 61.63 93.30 27.16
drill
Mower-condit. 4.57 97 250 0.138 29.56 61.63 91.18 30.97
Side-delivery 2.44–3.66 45 250 0.191 5.90 34.14 40.05 18.89
rake
(kW) (hr yr-1) (dry ($ hr-1) ($ hr-1) ($ hr-1) ($ dry
Mg hr-1) Mg-1)
Round bale mover 14 bales 97 300 23.10 23.80 61.63 85.42 4.08
Rectangular bale mover 16 bales 97 300 18.33 34.83 61.97 96.45 5.80
Baler productivity from Larson and English [8]
Round balera 97 150 5.44 59.72 61.63 121.34 22.29
Rect. balerb 119 300 10.89 82.48 75.60 158.08 14.52
Baler productivity from Shinners et al. [9]
Round balera 97 150 19.00 59.72 61.63 121.34 6.39
Rect. balerb 119 300 25.20 82.48 75.60 158.08 6.27
a b
1.83 m dia, 1.52 m wide; 0.91 9 1.22 9 2.44 m
seed cost to a current quote of $33.07 kg-1 and for all fertilizers use a 4 year
average (2008–2011) because fertilizer prices have been so volatile in recent years.
For conventional tillage, total costs are $589 and $508 for land that was pre-
viously in a fall harvested crop and winter wheat that was either grazed or hayed,
respectively (Table 8.5). Amortized over 10 years, costs are $82 and $71 ha-1.
If lime (assume 2.24 Mg ha-1 at $34.94 Mg-1 and an additional application) and
potassium (assume 90 kg ha-1 as K2O at $0.697 kg-1 as K2O) are needed, costs
increase by $147 ha-1 and amortized costs increase by $20 ha-1.
If no-till is used, overall costs are lower, with no plowing, disking, or culti-
packing, but herbicide costs are higher ($42 and $32 ha-1 for land that was pre-
viously in a fall harvested crop and winter wheat that was either grazed or hayed,
respectively). Total costs for no-till are $506 and $482 ha-1 for land that was
previously in a fall harvested crop and winter wheat that was either grazed or
hayed, respectively (Table 8.5). If seed costs are only $13.23 kg-1 (as they were at
the time Griffith et al. did their calculation), then with minimal herbicides and no
mowing, no-till establishment costs can be as low as $276 ha-1 and amortized
costs $38 ha-1.
Table 8.4 The effect of annual hours of use on machine costs

Equipment Baseline ha of annual use
hours
50 100 200 400 800 1,600
Chisel plow-hr 200 13 27 53 107 213
Chisel plow-$ hr-1 32.68 163.96 89.91 55.33 39.94 32.13
Offset disk-hr 200 18 35 71 142 283
Offset disk -$ hr-1 15.28 68.64 37.61 23.65 17.29 13.64
Fertilizer and lime spreader-hr 120 5 10 21 42 83 166
Fertilizer and lime spreader-$ hr-1 23.28 189.40 99.38 55.35 35.48 26.37 21.02
Press wheel drill-hr 150 15 29 58 116 233
Press wheel drill-$ hr-1 31.67 125.73 70.89 46.33 34.60 27.53
Boom sprayer-hr 150 5 10 19 39 77 154
Boom sprayer-$ hr-1 26.94 305.31 157.59 84.20 49.80 34.58 26.68
Mower-conditioner-hr 250 17 34 68 136 272
Mower-conditioner-$ hr-1 29.56 156.06 84.45 51.11 36.35 28.86
Side delivery rake-hr 250 24 47 94 189 377
Side delivery rake-$ hr-1 5.90 25.25 13.96 8.91 6.55 5.11
Hours of annual use not based on area, but on crop
production
Round baler-hr 150 100 200 400 800 1,500
Round baler-$ hr-1 59.72 67.71 55.19 46.80 41.04 37.51
Rectangular baler-hr 300 100 200 400 800 1,500 3,000
Rectangular baler-$ hr-1 82.48 127.91 94.35 75.76 63.29 55.40 49.49
Round bale mover-hr 300 100 200 400 800 1,500
Round bale mover-$ hr-1 23.80 37.03 27.34 21.74 17.80 15.23
Rectangular bale mover-hr 300 100 200 400 800 1,500
Rectangular bale mover-$ hr-1 34.83 54.18 40.01 31.82 26.05 22.29
For herbicides, Mitchell et al. [11] suggest using a combination of quinclorac

(0.56 kg ha-1) and atrazine (1.12 kg ha-1) to control grassy and broadleaf weeds
for establishment and 2,4-D (1.06–2.13 kg ha-1) to control broadleaf weeds
during the establishment year. This combination of herbicides is more expensive
($65–$76 ha-1) than those used by Griffith et al. [10] ($21–$42 ha-1).
We estimated costs for machinery using the values in Table 8.3 and used the
herbicide costs based on Mitchell et al. [11] and include a potassium application
of 90 kg ha-1 (as K2O) at a cost of $0.697 kg-1 (as K2O) and a lime application of
2.24 Mg ha-1 at a cost of $34.94 Mg-1. Total establishment costs are $721 and
$692 ha-1 and amortized over 10 years are $100 and $96 ha-1 for conventional
and no-till, respectively.
Establishment costs will vary depending on land prices and the need for
fertilizer and lime. Depending on the soil pH, lime could be applied at from
0 to 2.24 Mg ha-1. Depending on soil levels of phosphorus and potassium,
application may or may not be needed. There are large areas in the United States
Table 8.5 Establishment costs for conventional and no-till scenarios based on Griffith et al. [10],
with seed at $33.07 kg-1
Item Units $ unit-1 Previous crop
Cropland-fall Winter wheat
harvest or grazed or
pasture harvested for
hay in Spring
Conventional tillage
Units $ ha-1 Units $ ha-1
ha-1 ha-1
Chisel plow ha 27.17 1 27.17 0 0
Fertilizer (diammonium phosphate) kg 0.744 48.2 35.83 48.2 35.83
Apply fertilizer ha 10.23 1 10.23 1 10.23
Disk ha 24.70 3 74.10 1 24.7
Cultipack ha 22.23 1 22.23 1 22.23
Seed kg 33.07 5.6 185.19 5.6 185.19
Seeding ha 33.10 1 33.10 1 33.10
Rotary mow ha 8.65 1 8.65 1 8.65
Herbicide
Glyphosate kg 8.11 1.26 10.23 1.26 8.11
Broadleaf, post emergent ha 11.12 1 11.12 1 11.12
Apply herbicide ha 12.20 2 24.40 2 24.40
Land ha 111.15 1 111.15 1 111.15
Interest on operating costs $ 0.065 553.38 35.97 476.81 30.99
Total ha 589.35 507.81
Amortize over 10 years ha 0.1391 81.98 70.64
Total without fertilizer, rotary mower, ha 507.21 425.66
and glyphosate
No-till
Total for no-till ha 505.60 481.71
Total for no-till without fertilizer, rotary ha 418.11 394.23
mower, and Glyphosate
where limited or no potassium and lime are applied, especially in the western half
of the United States. Land prices have a wide variance. For example in 2010, Iowa
cropland and pastureland rented for $435 and $99 ha-1, respectively, and Texas
non-irrigated cropland rented for $64 ha-1 [12]. With land cost at $111 ha-1,
based on Griffith et al. (Table 8.5) costs can range from $394 ha-1 ($31.57 ha-1
amortized over 10 years) for no-till with no potassium fertilization, no lime and
minimal herbicides ($276 and $38 ha-1 amortized if seed costs are $13.23 kg-1)
to, based on our calculation, $721 ha-1 ($100.26 ha-1 amortized over 10 years)
for conventional tillage with fertilization/lime and a full spectrum of herbicides.
Table 8.6 Reseeding costs (25% of land)

Based on Griffith et al. [10] Our calculations
Item Unit $ unit-1 Quantity $ ha-1 $ unit-1 Quantity $ ha-1
Seed kg 33.07 1.4 46.30 33.07 1.4 46.30
Seeding ha 33.10 0.5 16.55 13.23 0.5 13.58
Herbicide
Glyphosate kg 8.11 0.32 2.56
Broadleaf, post emergent ha 11.12 0.25 2.78
Atrazine 9.53 0.14 1.33
Quinclorac 77.16 0.40 30.86
2,4-D 10.44 0.75 7.83
Herbicide application ha 12.20 2 24.40 5.89 3 17.68
Interest on operating costs $ 0.0325 92.58 3.01 0.0325 117.58 3.82
Total ha 95.59 121.40
Amortize over 9 years ha 0.1502 14.36 0.1502 18.24
Total with seed at $13.23 kg-1 66.91 84.64
Amortize over 9 years ha 0.1502 10.05 0.1502 12.72
If land cost is $198 ha-1, then establishment cost is $813 ha-1 ($113.12 ha-1
amortized over 10 years).
Some land planted to switchgrass may not successfully establish in the first
year, so reseeding may be necessary. Based on Griffith et al. [10], if 25% of
switchgrass needs reseeding, we estimate a cost of $96 ha-1 and amortized over
9 years’ costs are $14.36 ha-1 (Table 8.6). Based on our calculations, including
the herbicides suggested by Mitchell et al. [11], reseeding costs are $121 ha-1 and
amortized over 9 years’ costs are $18.24 ha-1. If seed costs are $13.23 kg-1, then
reseeding costs are $67 and $85 ha-1 and amortized over 9 years are $10.05 and
$12.72 ha-1, based on Griffith et al. and based on our calculations, respectively.
Depending on the assumed values for inputs, amortized reseeding costs can range
between $10.05 and $18.24 ha-1.
The longevity of the stand and the interest (discount) rate affect the amortized
cost of establishing switchgrass (Table 8.7). In Table 8.7 the establishment costs
for no-till establishment on fall-harvested cropland or pasture from Table 8.5
($505.60 ha-1) amortized over the number of years in the first column plus the
re-seeding cost from Table 8.6 ($92.58 ha-1) amortized over the number or years
in the first column minus one, are shown as affected by discount rate and years of
stand life.
8.5 Maintenance
Once established, switchgrass should require limited attention until harvest.

Fertilizer is assumed to be applied each year. Based on Griffith et al. [10], using
up-dated fertilizer prices, maintenance costs are $119 ha-1, including amortized
Table 8.7 Effect of interest (discount) rate and years of stand life on amortized establishment
and reseeding cost ($ ha-1 yr-1), based on no-till establishment on fall-harvested cropland or
pasture
Years of stand life Interest (discount) rate
0.04 0.065 0.10
5 140 150 164
6 118 127 141
7 102 112 126
8 91 100 114
9 82 92 106
10 75 85 99
11 70 79 93
12 65 74 89
13 61 71 85
14 57 67 82
15 55 64 79
20 44 55 71
Table 8.8 Maintenance costs for years after establishment, excluding harvest, based on Griffith
et al. [10]
Item Unit $ unit-1 Quantity $ ha-1
Urea kg 0.554 103 57.13
Diammonium phosphate kg 0.744 48 35.83
Fertilizer application ha 10.23 1 10.23
Land ha 111.15 1 111.15
Establishment+ reseeding cost amortized ha 81.98
Interest on operating capital 0.0325 296.32 9.63
Total 305.95
establishment, reseeding and land (Table 8.8). In the Great Plains and Midwest,
for each dry Mg of anticipated harvest, Mitchell et al. [11] recommend 10 kg N
dry Mg-1 if harvested before a killing frost and 6–7 kg N if the previous year is
harvested after a killing frost. We assume harvest takes place after a killing frost,
and N is applied at a rate 6.5 kg N dry Mg-1. Phosphorus and potassium may need
to be applied, depending on the soil levels. In this analysis, we cost the application
of 44 and 88 kg ha-1 of phosphorus (as P2O5) and potassium (as K2O), respec-
tively, as well as no potassium application (Table 8.9).
A well-established stand should typically require only herbicides to control
broadleaf weeds only once or twice every 10 years, using 2-4,D at
1.06–2.13 kg ha-1. One application of 2-4,D costs $11–$22 ha-1 and application
cost is $11.50 ha-1.
Table 8.9 Fertilizer maintenance costs with and without potassium application as a function
of yield
Yield (dry Mg ha-1)
4 8 12 16 20
Fertilizer $ kg-1 kg ha-1
Nitrogen 1.23 26.0 52.0 78.0 104.0 130.0
Phosphorous (as P2O5) 1.18 44.8 44.8 44.8 44.8 44.8
Potassium (as K2O) 1.14 89.6 89.6 80.6 89.6 89.6
$ ha-1
Nitrogen 32.04 64.08 96.12 128.17 160.21
Phosphorous (as P2O5) 53.04 53.04 53.04 53.04 53.04
Potassium (as K2O) 102.32 102.32 102.32 102.32 102.32
Application cost 5.97 5.97 5.97 5.97 5.97
Total ($ ha-1) 193.37 225.41 257.45 289.50 321.54
Total ($ dry Mg-1) 48.34 28.18 21.45 18.09 16.08
Total without potassium ($ ha-1) 91.05 123.09 155.13 187.17 219.22
Total without potassium ($ dry Mg-1) 22.76 15.39 12.93 11.70 10.96
8.6 Harvest
Harvest is the single most expensive operation. The standard for herbaceous
biomass to be harvested is as bales of hay. Costs for both large round
(1.83 m diameter 9 1.52 m wide) and large rectangular (square) bales
(0.91 9 1.22 9 2.44 m) are estimated. The operations assumed for the harvest
stage are: mow-condition, rake, bale, and move bales to field edge. Mowing-
conditioning and raking are assumed to be independent of yield and are a function
of area. Baling and bale moving are a function of how much biomass is handled.
Based on work by Larson and English [8] on switchgrass, the throughput capacity
for large round and large rectangular balers are 5.44 and 10.89 dry Mg hr-1,
respectively. The throughput for the large rectangular baler is consistent with
Kemmerer and Liu [13] with a throughput of 11.37 dry Mg hr-1, based on a 0.875
dry matter fraction. Large round bales are 156 kg m-3 and 624 kg, while large
rectangular bales are 175 kg m-3 and 476 kg. Round bales use mesh wrap and
rectangular bales use twine.
Shinners et al. [9] found similar bale densities, averaging 163 and 175 kg m-3
for reed canarygrass and switchgrass, but much higher baler productivities, aver-
aging 19.0 and 25.2 dry Mg hr-1 for switchgrass for a round baler with net wrap
and a large rectangular baler, respectively.
A mower-conditioner is used to cut the switchgrass and condition it so it will
dry faster, and a rake puts it into a windrow for the baler. We assume that the costs
of these two operations are independent of the yield and are based on the area
covered. Including the tractor, the mower-conditioner costs $30.97 ha-1 and the
rake costs $18.89 ha-1 (Table 8.10). So the cost of these two operations decreases
as yield increases.
Table 8.10 Harvest (including tractor) and transport costs

Equipment $ ha-1
Mower-conditioner 30.97
Rake 18.89
Round bales Rectangular bales
$ dry Mg-1
Baler (Larson and English [8] productivity) 22.29 14.52
Baler (Shinners et al. [9] productivity) 6.39 6.27
Twine/wrap 3.31 2.30
Bale mover (to field edge) 4.08 5.80
Transport 8.02 7.50
Based on the baler productivities in Larson and English [8], baling costs
(including wrap/twine) are $26 and $17 Mg-1 for round and rectangular bales,
respectively (Table 8.10). Based on the baler productivities in Shinners et al. [9],
baling costs (including wrap/twine) are $9.70 and $8.60 Mg-1 for round and
rectangular bales, respectively, which are significantly lower (Table 8.10). Griffith
et al. [10] use a cost (based on custom harvest) of $24.50 Mg-1 for rectangular
bales.
Mooney and English [14] found that when harvest and transport costs and
storage losses are considered, the use of a mixture of large round and large
rectangular bales are optimal. The rectangular bales would be harvested and
immediately taken to the biorefinery, whereas the round bales would be stored for
later use. Using the baler productivities from Larson and English [8], harvest and
trans-port costs for rectangular bales are slightly lower than round bales. Using the
baler productivities from Shinners et al. [9], harvest and transport costs are lower
for round bales.
8.7 Storage
Storage costs consist of out-of-pocket expenses as well as losses of dry matter.

Dry matter losses from three studies were summarized by Turhollow et al. [7].
Shinners et al. [9] studied switchgrass losses (Table 8.11). If storage structures are
only going to be used one time per year, then storage tends to be expensive.
Turhollow et al. [7] present costs for a number of storage structures, plastic wrap
on large rectangular bales, and a tarp (Table 8.12).
Turhollow et al. [7] estimated that the cost (out-of-pocket expenses for land rent
at $210 ha-1, structure or tarp, labor, and insurance and repair) for storing large
rectangular bales four high on (1) the ground, (2) a gravel pad with a tarp, and (3)
in a pole frame structure with one side open is $1.12, $ 10.87, and $ 25.20 dry
Mg-1, respectively. If losses are for (1), (2), and (3) are 25, 6, and 2%, respec-
tively, and switchgrass is valued at $55.12 dry Mg-1, then total costs of storage
Table 8.11 Storage losses

Turhollow et al. [7] % dry matter loss
Enclosed shed 2–5
Open-sided pole structure 3–10
Reusable tarp on ground 5–10
Plastic wrap on ground 4–7
Uncovered on gravel pad 13–17
Shinners et al. [9], switchgrass stored 293 days, large round bales
Indoor 4.9
Plastic film, outdoor on grass 5.7
Breathable film, outdoor on grass 5.4
Net wrap, outdoor on grass 9.0
Plastic twine, outdoor on grass 9.3
Sisal twine, outdoor on grass 15.4
Table 8.12 Initial costs for storage structures, a tarp, plastic wrap on large rectangular bales [7]
Initial cost ($ Useful life
m-2) (years)
Pole frame structure, all sides open 71.37 20
Pole frame structure, one side open 99.35 20
Pole frame structure, enclosed 111.62 20
Enclosed shed with concrete floor and foundation 169.43 20
Gravel storage pad 11.75 10
Asphalt storage pad 30.68 10
Hay tarp (including labor to place and remove) 2.91 5
Plastic wrap (2 large rectangular bales; equipment, labor, 6.71 dry Mg-1 1
materials)
(including losses) are $14.90, 14.18, and 26.30 dry Mg-1, respectively. If losses
for the plastic wrap option are 6%, then total storage costs for this option are
$11.34 dry Mg-1 ($6.71 dry Mg-1 for the plastic wrap and $0.084 dry Mg-1 for
the land then increased by 6% to account for the losses, and $3.31 for the lost dry
matter). For outdoor storage if losses are kept to 9% [based on Shinners et al. [9]
(Table 8.11)], then storage cost for net-wrapped large round bales is $5.88 dry
Mg-1 (including $0.92 dry Mg-1 for land and 4.96 dry Mg-1 for lost biomass).
If biomass can be directly taken from the field to a conversion facility then the
storage costs (including losses) can be avoided. Land was valued at $210 ha-1 in
Turhollow et al. and bales were assumed to be stacked four high. For land valued
at $111 ha-1 and bales stacked one (two) high, the land cost is $2.37 ($1.19) dry
Mg-1 (in Turhollow et al. the large rectangular bales are 463 kg, which is
approximately the same as the large rectangular bales which are assumed to be
476 kg in this chapter).
For estimating storage costs in this chapter, it is assumed that round bales are
not stacked and rectangular bales are plastic wrapped at a cost of $6.71 dry Mg-1.
Land area for storage allows space for equipment to maneuver around the piles and
is assumed to be 0.21 and 0.13 ha for 100 dry Mg of bales (before storage losses),
for round and rectangular bales, respectively. Using a range of land costs of
$111–$198 ha-1, land costs for storage are $0.23–$0.41 and $0.14–0.26 dry Mg-1 for
round and rectangular bales, respectively. For simplicity, a cost of $0.30 dry Mg-1 is
assumed for the land cost of storage. For storage, losses are assumed to be 9% for
net-wrapped round bales and 6% for plastic-wrapped rectangular bales. The cost
estimated for the plastic wrap in Turhollow et al. [7] was for 1.22 9 1.22 9 2.44 m
bales, while in this chapter the bales are 0.91 9 1.22 9 2.44 m. The amount of wrap
is reduced by 25%, so the cost is 75% of $6.71 dry Mg-1, or 5.03 dry Mg-1.
8.8 Transport
We use a very simple model for transport costs, assuming a truck with a flatbed
trailer costs $75 h-1. Also assumed is that a round trip takes 2 h, so each load
costs $150. A load consists of 30 round bales (18.7 dry Mg) or 42 rectangular bales
(20.0 dry Mg). Transport costs are $8.02 and $7.50 dry Mg-1. Griffith et al. [10]
assume a transport cost for rectangular bales of $4.50 per 680 kg bale (wet), or
$7.56 dry Mg-1 assuming 12.5% moisture. In the low-cost scenario in the sum-
mary, we assume only 1 h is needed for a round trip, which cuts the transport cost
into half.
8.9 Summary
Costs for the entire operation from growing to delivering the switchgrass are
summarized in Table 8.13.1 Low-cost and high-cost scenarios are presented to
give an idea of the possible range of costs, which is significant. In the low-cost
scenario, establishment costs are low because seed costs are low, no potassium and
lime are applied, weed control (herbicide) requirements are low, and a lower land
price is used; reseeding also uses a low seed cost; baling uses the lower cost
harvest choice (round versus rectangular bales),2 there is no storage (costs and
losses), and only one hour (instead of two) is required for a round trip to transport
bales to the biorefinery. Baler productivity makes a huge difference, as evidenced
by the difference in harvest costs between those based on the productivities of
Larson and English [8] and those of Shinners et al. [9], particularly the round baler
1
Note that storage costs in Table 8.13 consist of land rent plus the cost of plastic wrap for the
rectangular bales. Storage losses, in the high-cost scenario, are accounted for as the difference
between Total and Total after losses, and Total after loss is calculated as: Total/(1-loss), where
loss is 9% for round bales in net wrap and 6% for rectangular bales in plastic wrap.
2
Rectangular bales for Larson and English [8] and round bales for Shinners et al. [9].
Table 8.13 Delivered cost of switchgrass to a biorefinery for round bales in net wrap and rectangular bales in twine followed by plastic wrap
200
Low cost High cost

Yield (dry Mg ha-1) 4 8 12 16 20 4 8 12 16 20
$ ha-yr-1
Establishment 41.70 113.12
Reseeding 10.05 18.24
Fertilizer 91.05 123.09 155.13 187.17 219.22 193.37 225.41 257.45 289.50 321.54
Land 111.15 197.60
Mower-conditioner 30.97 30.97
Rake 18.89 18.89
Subtotal 303.81 335.85 367.89 399.94 431.98 572.19 604.23 636.27 668.32 700.36
Interest on operating capital 9.87 10.92 11.96 13.00 14.04 18.60 19.64 20.68 21.72 22.76
Subtotal ($ ha-1) 313.68 346.77 379.85 412.93 446.02 590.79 623.87 656.95 690.04 722.12
Subtotal ($ dry Mg-1) 78.42 43.35 31.65 25.81 22.30 147.70 77.98 54.75 43.13 36.16
Baler productivity based on Larson and English [8]
$ dry Mg-1
Rectangular bales Round bales
Baler 14.52 22.29
Twine/wrap 2.30 3.31
Bale carrier 5.80 4.08
Telescopic handler 1.92 1.46
Transport 3.75 8.02
Storage 0.30
Subtotal 28.28 39.46
Interest on operating capital 0.92 1.28
Total 107.62 72.55 60.86 55.01 51.50 188.43 118.72 95.48 83.87 76.89
Total after losses 207.07 130.46 104.93 92.16 84.50
(continued)
A. Turhollow and F. Epplin
Table 8.13 (continued)
Low cost High cost
-1
Yield (dry Mg ha ) 4 8 12 16 20 4 8 12 16 20
Baler productivity from Shinners et al. [9]
Round bales Rectangular bales
Baler 6.39 6.27
Wrap/twine 3.31 2.30
Bale carrier 4.08 5.80
Telescopic handler 1.46 1.92
Transport 4.01 7.50
Storage 5.33
Interest on operating capital 0.63 0.95
Total 98.29 63.21 51.52 45.68 42.17 177.48 107.91 84.72 73.12 66.16
Total after losses 186.29 112.27 87.60 75.27 67.87
8 Estimating Region Specific Costs to Produce and Deliver Switchgrass
201
productivities of 5.44 versus 19.0 dry Mg hr-1, respectively. In recent years the
prices of fertilizers and lime have increased, giving regions that do not require
potassium and/or lime a competitive advantage. Not having to apply potassium in
the maintenance years lowers costs by $102 ha-1. The balers in the Shinners
et al. study were limited by their capacity to process the volume of switchgrass.
Shinners et al. indicate that modifications to the baler pickup and throat might
be needed to handle both the high tonnage and physical volume of biomass.
Not having to store switchgrass lowers costs (from both out-of-pocket storage
expense and avoiding loss of dry matter) by $3–$19 and $8–$17 for round and
rectangular bales, respectively.
Yield has a large impact on the cost of producing switchgrass. In the high-cost
scenario, the cost of switchgrass is above $100 dry Mg-1 at either 4 or 8 dry
Mg ha-1. In the low-cost scenario, at a yield of 8 dry Mg ha-1 or greater with the
baler productivity from Shinners et al. and a yield of 12 dry Mg ha-1 or greater
with the baler productivity from Larson and English, the cost of switchgrass is
below $61 dry Mg-1.
In the right circumstances, switchgrass using baling can provide biomass
delivered to a biorefinery at a cost of less than $60 dry Mg-1. These circumstances
include a combination of some, but not necessarily all of: a reasonable land cost,
limited need for fertilizers and herbicides, baling technology with the throughput
from Shinners et al. [9] (19–25 dry Mg hr-1). In the high-cost scenario, using
round baling and the throughput based on Larson and English [8] (5.44 dry Mg
hr-1) the cost of switchgrass delivered to a biorefinery after accounting for a 9%
loss of dry matter is $84 dry Mg-1 at a yield of 20 dry Mg ha-1 (before losses). If
the round baler can achieve a throughput of 19.0 dry Mg hr-1 (as in Shinners et al.)
and transport cost is $4.01 dry Mg-1, then delivered cost is $62 dry Mg-1. If in
addition the land price is at $111 ha-1 instead of $198 ha-1 and no potassium is
needed, then at 12 dry Mg ha-1 (before losses) delivered cost (after accounting for
losses of 9%) is $64 dry Mg-1. If land prices are high, then a high yield is needed
to offset this cost. In the low-cost scenario, which represents geographic locations
such as Oklahoma and Kansas, costs are below $61 dry Mg-1 for yields above 12
dry Mg ha-1. In the right circumstances it is possible to supply switchgrass at
prices below $61 dry Mg-1.
References
1. Tweeten LG (1979) Foundations of farm policy. University of Nebraska Press, Lincoln

2. Perlack R, Stokes B (2011) U.S. billion ton update: Biomass supply for a bioenergy and
bioproducts industry. ORNL/TM-2010/224, Oak Ridge National Laboratory
3. Wright M, Brown RC (2007) Establishing optimal sizes of different kinds of biorefineries.
Biofuel Bioprod Bior 1:191–200
4. AAEA (2000) Commodity costs and returns handbook. A report of the AAEA task force on
commodity costs and returns. ftp://ftp-fc.sc.egov.usda.gov/Economics/care/AAEA/PDF/AAEA
%20Handbook.pdf. Accessed 15 Nov 2011
5. ASABE (2010a) D497.6 JUN2009: Agricultural machinery management data. In: ASABE
standards, American society of agricultural and biological engineers, St Joseph MI
6. ASABE (2010b) E496.3 FEB2006: Agricultural machinery management. In: ASABE
standards, American society of agricultural and biological engineers, St Joseph MI
7. Turhollow A, Wilkerson E, Sokhansanj S (2009) Cost methodology for biomass feedstocks:
herbaceous crops and agricultural residues. ORNL/TM-2008-105, Oak Ridge National
Laboratory
8. Larson JA, English BC (2009) Risk management for energy investments: agricultural policy
and extension recommendations. In: English BC, Menard J, Jensen K (eds) Proceedings of
the role of extension in energy, Little Rock, AR
9. Shinners KJ, Boettcher GC, Muck RE, Weimar PJ, Casler MD (2010) Harvest and storage of
two perennial grasses as biomass feedstocks. Trans ASABE 53(2):359–370
10. Griffith A, Epplin F, Redfearn D (2010) Cost of producing switchgrass for biomass feedstock.
Oklahoma State University, Cooperative Extension Service
11. Mitchell R, Vogel KP, Schmer M, Pennington D (2010) Switchgrass for biofuel production.
http://extension.org/pages/Switchgrass_for_biofuel_production. Accessed 5 Dec 2011
12. USDA/NASS (2010) Land values and cash rents 2010 summary. National Agricultural
Statistics Service, U.S. Department of Agriculture
13. Kemmerer BJ, Liu JL (2010) Spring switchgrass harvest with a New Holland large square
baler. ASABE Paper number 1009029, presented at 2010 ASABE annual international
meeting, Pittsburgh, PA 20–23 June 2010
14. Mooney DF, English BC (2009) Economics of the switchgrass supply chain: enterprise
budget and production cost analyses. In: English BC, Menard J, Jensen K (eds) Proceedings
of the role of extension in energy, Little Rock, AR 30 June-1 July 2009
15. Brechbill S, Tyner W (2008) The economics of biomass collection, transportation, and supply
to Indiana cellulosic and electric utility facilities. Working paper #08-03, Department of
Agricultural Economics, Purdue University
16. Duffy M (2008) Estimated costs for production, storage and transportation of switchgrass.
File A1-22, Ag decision maker, University Extension, Iowa State University
17. Epplin F (1996) Cost to produce and deliver switchgrass biomass to an ethanol-conversion
facility in the Southern Plains of the United States. Biomass Bioenerg 11:459–467
18. Epplin F, Clark C, Roberts R, Hwang S (2007) Challenges to the development of a dedicated
energy crop. Am J Agr Econ 89:1296–1302
19. Khanna M, Dhungana B, Clifton-Brown J (2008) Costs of producing miscanthus and
switchgrass for bioenergy in Illinois. Biomass Bioenerg 32:482–493
20. Perin R, Vogel K, Schmer M, Mitchell R (2008) Farm-scale production cost of switchgrass
for biomass. Bioenerg Res 1:91–97
21. USEPA (2009a) Draft regulatory impact analysis: changes to renewable fuel standard
program. EPA-D-09-001. US Environmental Protection Agency, Washington, DC
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program. US Environmental Protection Agency, Washington, DC. Federal Register 26 May
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Accessed 18 Nov 2011
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Index
A C
Acidification, 138 C4 pathways, 5, 6
Adaptation, 30, 36, 41, 92 Canadian, 20
Adventitious roots, 61, 98 Canopy architecture, 114, 125
Agricultural policies, 140 Carbohydrates, 47, 61, 62, 66,
Allopolyploid, 35 154, 175
American, 7, 9, 10, 21, 22, 24 Carbon, 118
Ammonia, 137, 154, 157, 175 Carbon debt, 133
Ammonium nitrate, 104, 132, 133 Carbon sequestration, 129, 130, 139
Annual, 12, 67, 75, 100, 129, 190 Cellulosic, 22, 119, 121, 145
Ash, 169, 177 Chromosome, 3, 35, 44
Association panel, 46 Chromosome number, 35
Aneuploid, 35 Co-firing, 177
Coleoptile, 58, 61
Combustion, 177
B Companion crop, 99
Bacterial artificial chromosomes, 46 Conservation Reserve
Bale(s), 197, 199 Program (CRP), 188
Baling, 196 Conversion (Biochemical, Thermochemical),
Best management practices, 88–90, 135 117, 123
Bio-char, 172 Conversion efficiency, 39, 169, 177
Bioconversion, 17 Cool-season grass, 100, 103, 142
Bio-crude, 179 Cost, 101, 120, 121
Bioenergy, 17, 19, 69, 85 Costs, 121, 130, 137
Biofuel, 47, 131, 140, 146, 160 Cropping systems, 97, 129, 147
Biomass, 17, 24, 51, 146 Cytotypes, 8
Biomass yield, 40, 50, 52
Bio-oil, 172, 173
Biorefinery, 113, 115, 119, 137, 179 D
Breeding, 29, 38, 40, 41, 44, 51 DAYCENT, 131
Bromegrass, 142 Decarboxylation, 169
Budget, 18, 187, 188, 190 Degradation, 120, 122, 158, 167
Buffer, 141, 144 Denitrification, 133, 147

DOI: 10.1007/978-1-4471-2903-5, Ó Springer-Verlag London 2012
206 Index
D (cont.) Frequency grid, 102

Department of Energy, 1, 15 Fuel, 25, 107, 137, 154, 169
Digestibility, 38, 156, 162
DNA, 4, 5, 35
DNA markers, 33, 43 G
DOE/US, 40 Gas exchange, 81
Dormancy, 95 Gasification, 174, 176
Drought, 81, 85 General circulation model, 145
Genes, 4, 6, 44, 47, 58
Genetic variability, 9, 10, 25, 43, 49
E Genetically modified organisms, 48
Economics, 190 Genome, 46
Ecosystems, 6, 12, 13, 34, 130 Genomic selection, 46
Eco-toxicity, 138 Geographic information system, 101
Ecotype, 8, 9, 29, 43, 96 Germplasm, 29, 36, 40, 41
Ecotypic variability, 36 Global warming potential, 133, 135, 138
Electricity generation, 137, 177 Gramineae, 2
Elemental composition, 169 Grazing, 52
Elongation rate, 65 Greenhouse gas emissions, 2, 129
Emission factors, 135 Growing degree days, 114
Enterprise, 187 Growth rate, 55, 61, 66, 79
Environmental impacts, 129, 138, 144
EPIC, 145
Equivalence ratio, 169 H
Erosion, 145 Hardiness zone, 34, 36
Establishment, 51, 90, 100–102, 110, Harvest, 146, 190, 196
187, 190, 192 Harvest system, 68, 103, 116
Ethanol, 21, 124, 138, 167 Hedge, 141, 143, 144
European, 7, 21, 22, 24 Herbicides, 21, 91, 99, 100, 106
Eutrophication, 138 Heterosis, 52
Evapotranspiration, 73, 145, 146 Hexaploid, 35, 55, 62
Evolution, 1, 2, 5 High-diversity, 99
Exome sequences, 46 Hormones, 58, 65
Expressed sequence tags, 46 Human toxicity, 138
Hybrid, 43
Hydraulic conductivity, 68, 143
F Hydrology, 145, 146
Fast pyrolysis, 172 Hydrolysis, 158
Feedstock, 187 Hydrothermal, 178, 179
Fermentation, 39, 47, 155, 167
Fertilization, 97, 102
Fertilizer, 194 I
Filter, 144 Ice Ages, 9, 10, 24
Fischer–Tropsch, 174 Incompatibility, 50
Fitness, 6, 42, 48 Infiltration, 143
Flowering, 44 Inheritance, 35, 44
Forage quality, 52 Inorganic N, 134
Forage yield, 38, 88 Insects, 40, 94, 105
Forests, 131, 132, 135, 138 Intercropping, 135
Fossil fuels, 25, 135, 137–139, 169 International trade, 139
Index 207
IPCC, 135 O
Irrigated, 71, 118 Oak Ridge National Laboratory, 15
Octoploid, 8–10, 35, 55, 62, 114
One-cut, 116, 146
K Organic N, 134, 141
Killing frost, 68, 102, 114, 116, ORNL, 15–19
117, 145 Osmotic adjustment, 78
Ozone layer depletion, 138
L
Land rent, 188, 189, 197, 199 P
Leaching, 69, 129, 134, 146, 148, 171 Panicum, 2, 3, 6–8, 11, 56, 100, 129
Leaf area index, 56, 70, 71, 115 Particle bombardment, 47
Leaf water potential, 77 Pathogens, 40
Legumes, 130, 135 Perennial, 16, 22–24, 44, 56, 63, 65, 73, 75,
Life cycle, 63, 118, 129, 130, 135–137 77–79, 89, 90, 91, 97, 99, 100, 106,
Lignin, 68, 94, 154, 156, 158, 164 118, 129, 130
Lignocellulosic biomass, 16, 164, 187 Pests, 40, 90, 105
Lime, 103, 104, 156, 165, 192 Phenological stages, 62–64
Linkage mapping, 44 Phenotype, 8, 33, 37, 46, 47, 162
Liquid fuels, 47, 169, 172, 174 Phosphorus, 68, 69, 79, 80, 97, 103, 104,
Losses, 6, 77, 90, 115, 118, 122, 142, 147, 197, 192, 195
198, 202 Photochemical oxidation, 138
Low-input, 99, 119 Photoperiod, 43, 30, 33, 55, 56, 62, 63, 65, 66,
Lowland, 7–10, 31, 35, 40, 62, 63, 69, 74, 77, 92, 114
96, 106, 116, 162 Photoperiodism, 30
Lowland ecotype, 8, 31, 41–44, 56, 62, Photosynthesis, 5, 6, 55, 71, 72, 77, 78, 80, 132
70, 91, 95 Phylogenetic, 3–6, 45
Phylogeny, 4, 6
Plantation, 67, 131
M Pleistocene Era, 5
Maize, 2, 4, 5, 11, 44, 91, 131, 134, Ploidy, 1, 10, 13, 25, 35, 37, 39, 60, 62, 71,
146, 147 78, 114
Manure, 104, 114, 132, 133, 137, 143 Poaceae, 2, 6, 9
Market, 22, 89, 140, 188, 190 Pollen, 2, 9, 33, 35, 48
Methane, 133, 135, 178, 179 Polyploid, 25
Miscanthus, 3, 4, 21, 24, 75, 130, 132, 134, Poplar, 21, 131, 137, 134, 147, 156, 169,
146, 171 177, 178
Mixtures, 12, 36, 99, 100 Post-anthesis, 64, 116, 118
Model, 17, 18, 25, 36, 44, 69, 84, 131 Potassium, 69, 96, 103, 169, 173, 177,
Model species, 18, 25, 29, 36 191–193, 195, 196, 199, 202
Molecular clock, 4–6, 8 Prairie, 7, 9–14, 24, 29, 30, 33–36, 48, 56, 63,
Multi-harvest, 116 94, 99, 114, 132
Prairie grasses, 7, 12, 13
Prescribed fire, 107
N Pretreatment, 23, 47, 124, 153, 154, 156–160,
Native plant, 130 162–165, 167, 168, 179
Nitrification, 133 Productivity, 5, 16, 21, 22, 24, 40, 62, 66, 71,
Nitrogen, 97, 102, 103, 118, 119, 145, 146 74, 75, 77–79, 90, 102, 116, 117, 134,
Nitrogen removal, 75, 118 147, 187, 191, 199, 200, 202
Nitrogen use efficiency, 6, 74, 76 Projects, 21–24, 36, 88, 156
Nitrous oxide, 133 Protein extraction, 153, 161
No-till, 97, 100, 191, 193, 194 Proximate analysis, 169
Nutrients, 14, 55, 67, 79 Pyrolysis, 21, 153, 154, 169, 171–174, 188
208 Index
R Soil pH, 104, 134, 182

Radiation use efficiency, 56, 70, 71, 84 Soil temperature, 197, 98
Recalcitrance, 47, 156 Soil texture, 60, 133
Reed canarygrass, 196 Sorghum, 3, 7, 2, 24, 35, 44, 100, 106,
Refugia, 7–10, 33 145, 189
Reseeding, 101, 102, 187, 190, 194, 195, Soybean, 91, 97, 100, 119, 130, 143, 145–147,
199, 200 188, 190
Residue, 91, 97, 118, 131, 132, 135, 154, 160 Spoilage, 123, 124
Respiration, 56, 57, 59, 67, 68, 72, 132, Stomata, 56, 72, 73, 77, 78
133, 147 Storage, 62, 66, 68, 75, 90, 94, 113, 115, 117,
Restoration, 34, 36, 38, 40 119, 120, 123–124, 172, 178, 187, 189,
Rhizomes, 62, 65, 67, 68, 75, 114, 146, 147 197–199, 202
Risk assessment, 47, 48, 89 Strategic planning, 89
Risk management, 90 Subcoleoptile internode, 58–61, 98
Root, 6, 8, 39, 59, 61, 62, 66–69, 73, 75, Subcritical water, 179
77–80, 83, 98, 101, 105, 114, 117, 118, Sugarcane, 139
131, 133, 134, 137, 139, 147 Supercritical water, 179
Roots, 105, 114 Syngas, 154, 172, 174–176
Row crop, 89, 143–145
Runoff, 141
T
Tall fescue, 141, 142, 147
S Tallgrass prairie, 29, 30, 33, 34, 99
Salinity, 77, 78 Temperature, 70, 81
Satellite, 119, 121 Testcross, 44
Savanna, 3, 9, 89, 30, 33, 34, 36, 48 Tetraploid, 35
Sediment, 33, 141–145 Thermochemical, 153, 156, 169
Seed, 40, 48 Thrifty, 75, 80
Seed dispersal, 48 Tile drainage, 145
Seed dormancy, 40, 48, 51, 56, 57, Tillage, 43, 96, 97, 135
81, 94, 95 Tillering, 56, 65, 66
Seed quality, 94, 95 Timothy, 147
Seed shattering, 63, 64 Transformation, 47, 136, 174
Seed yield, 10, 140 Transgenes, 48
Seedling, 39, 46, 47, 55, 57, 59, 61, 68, 78, 90, Translocation, 62, 68, 75, 78
94, 95–105 Transpiration, 55, 56, 73, 77, 78
Seedling vigor, 39, 95, 96 Transport, 199
Selection, 2, 3, 5, 8, 9, 13, 15, 36–44,
46–49, 61, 89–91, 94, 95, 98,
99, 130, 156 U
Senescence, 68, 78, 77, 103, 114, 115, 118, Upland, 7–10, 30, 31, 35, 65
130, 162 Upland ecotype, 91
Silage, 121 USDA/US Department
Slagging, 171, 178 of Agriculture, 56
Slow pyrolysis, 171, 172
Smooth, 101, 131, 142, 147
Soil moisture, 59, 61, 67, 69, 70, 79, 96, V
98, 133 Volatile matters, 169, 177
Soil organic carbon (SOC), 118, 130 Volatilization, 133, 134, 174
Index 209
W Y
Warm-season grass, 99, 114 Yield, 75, 202
Water quality, 143
Water uptake, 68
Water use efficiency, 55, 73, 79 Z
Wet, 121 Zea mays, 11
Wheat, 11, 100, 134, 145, 188
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Article in Green Energy and Technology · March 2012

Sujithkumar Surendran Nair Andrea Monti

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Switchgrass (Panicum virgatum L.) is a warm-season (C4) perennial grass that is

switchgrass. Information is provided about the evolution of switchgrass as an

improved through developing permanent switchgrass physiology programs. In

1 The Evolution of Switchgrass as an Energy Crop . . . . . . . . . . . . . 1

2 Switchgrass Breeding, Genetics, and Genomics . . . . . . . . . . . . . . . 29

4 Crop Management of Switchgrass. . . . . . . . . . . . . . . . . . . . . . . . . 87

5 Switchgrass Harvest and Storage . . . . . . . . . . . . . . . . . . . . . . . . . 113

6 Environmental Impacts of Switchgrass Management

7 Biochemical and Thermochemical Conversion of Switchgrass

8 Estimating Region Specific Costs to Produce

David J. Parrish, Michael D. Casler and Andrea Monti

Abstract This chapter discusses the prehistoric origins of switchgrass, its

A. Monti (ed.), Switchgrass, Green Energy and Technology, 1

1.2 The Evolution of Switchgrass

1.2.1 Taxonomic and Phylogenetic Relationships

Systematics underwent a revolution in the last quarter of the twentieth century

some fossil evidence reveals morphological differences equating to some sub-

1.2.2 Evolution of C4 Pathways

Because it is so important for both productivity and drought tolerance, the

1.2.2.1 Where do C4 Pathways Appear Phylogenetically?

C4 pathways occur in at least 7,500 angiosperm species found in 19 families—16

1.2.2.2 When did C4 Pathways Appear?

The distinctive 12C:13C isotopic fingerprint associated with C4 species begins to be

1.2.3 Center of Switchgrass’s Origin and Spread Across North

1.2.4 Upland and Lowland Origins, Distinctions, and Connections

1.3 The Agronomic History of Switchgrass

Fig. 1.1 Annual (a) and

1.3.1 ‘Prairie Grass’ Origins

in 1932 from Massachusetts, is the first CAB-indexed citation where switchgrass

1.3.2 Early Studies and Uses as a Monoculture, and the Growth

1.3.3 Current and Proposed Uses

1.4 The Origins of Switchgrass as an Energy Crop

A few published articles have discussed the brief history of switchgrass as an

1.4.1 In North America

1.4.1.1 DOE Extramural Screening Studies

that some of the subcontractors did switchgrass-specific studies looking at man-

1.4.1.2 Intramural Efforts at ORNL and Other DOE Agencies

1.4.1.3 DOE Extramural ‘Model Species’ Studies

Switchgrass, which was somewhat serendipitously chosen as the benchmark

1.4.1.5 Canadian Efforts

Switchgrass is native to southern portions of Canada, and Canadian workers

1.4.2 In Europe and Elsewhere

1.4.2.1 A Brief History of Switchgrass in Europe

If the North American history of switchgrass as an energy crop can be considered

production may not always be satisfactory; (2) production of up to 25 Mg biomass

upstream and down-stream processes, such as pretreatment steps; (5) Pellets-

1.4.2.2 Switchgrass Studies in Other Countries

The number of non-North American and non-European countries in which