CHAPTER Arkansas Soybean Production Handbook

DIVISION OF AGRICULTURE

2 RESEARCH & EXTENSION

University of Arkansas System

Soybean Growth and

Development

Larry C. Purcell, Montserrat Salmeron and Lanny Ashlock

The progenitor of soybean grows wild the response of soybean to day length are discussed
throughout eastern China, Korea, Japan and the with regard to their importance on crop management
far eastern portion of Russia. Domestication of in more detail later in this chapter.
soybean is believed to have occurred in the Yellow
or the Yangtze River valleys of central or southern Seed
China somewhere between 3,000 and 5,000 years
ago. There are numerous references to soybean in Soybean seeds are living organisms and should
some of the earliest Chinese literature. be treated with care. Avoid unnecessary dropping of
seed through augers and conveyers as this can
An important characteristic of soybean is that it damage the seed coat. Also, seed should be kept in a
is a legume and forms a symbiotic relationship with cool, dry environment to preserve seed quality and
Bradyrhizobium japonicum (commonly referred to vigor. Hot and humid conditions can result in rapid
as rhizobia) bacteria that results in nodules forming deterioration of seed quality, which can decrease seed
on the roots (Figure 2-1). These nodules reduce germination especially under stressful conditions.
atmospheric nitrogen gas to a form that the plant can
utilize. A major advantage of soybean is that because Soybean seeds vary in shape but are generally
of nitrogen fixation, it does not require any nitrogen oval. A soybean seed consists of a large embryo
fertilizer. enclosed by the seed coat. There are large variations
in seed coat color (light yellow, green, brown, black,
mottled), but commercial soybean is nearly always
yellow. The embryo is comprised of two cotyledons
(Figure 2-2), which upon germination produce a
plumule with two simple leaves (unifoliate leaves)
and a hypocotyl (which is green or purple depending
upon whether the variety produces white or purple

hilum
Figure 2-1. A well-nodulated root system of a young
soybean plant.

A second important characteristic of soybean that micropyle

has important implications for crop management is
that it is a short-day plant. That is, soybean is hypocotyl
triggered to flower as the day length decreases below
some critical value. These critical values differ among
maturity groups (MGs). Both nitrogen fixation and Figure 2-2. A mature soybean seed.

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flowers). The embryo also consists of the radicle Figure 2-3. The date when 50% of Arkansas soybeans
(root). The hilum (seed scar) is easily visible on the were (a) planted and (b) harvested from 1985 to 2008.
Data compiled from statewide averages of the Crop
surface of the seed coat and is classified by color (i.e., Reporting Service.
black, imperfect black, brown, buff and clear). The
micropyle is a very small hole located near the hilum
that is formed during seed development. The
micropyle accounts for nearly all of the gaseous
exchange between the seed and its environment,
whereas water can be absorbed through the entire
seed coat surface.

Germination and Seedling

Establishment
The soybean crop has a wide planting window,
typically from early spring, after the danger of frost
has passed, until mid- to late-June. The trends for
soybean production in Arkansas in recent years are
for planting to occur earlier each year. On average,
since 1985, planting has occurred one day earlier
each year (Figure 2-3). In conjunction with earlier
planting, the average soybean harvest date has also
been about one day earlier each year. The combina
tion of early planting and early maturity has advan
tages for avoiding late-season drought and disease
and insect pressure. The predominant MGs grown in
Arkansas today are 4 and 5, whereas MGs 5, 6 and 7
were predominant prior to 1985.

Soybean seed is typically planted at a depth of

about 1 inch. Many seed companies provide fungi
cide and insecticide treatments on their seed, which
can be especially beneficial to soybeans emerging and
growing slowly in early spring due to cool tempera
tures. In fields where soybeans have not been grown
in 3 or more years, a rhizobium inoculant (specific
for soybeans, Bradyrhizobium japonicum) should be The radical (root) is the first structure to emerge
applied as a seed treatment or in furrow at planting. from the germinating seed, usually within 48 hours
There is little supporting research to show that of planting under optimum conditions. The radical
inoculation improves yield in fields where soybeans grows downward rapidly and can provide moisture
have been grown in recent years. to the germinating seed if the soil surrounding the
seed becomes dry. If a seed begins to germinate and
High-quality seed requires three appropriate the soil dries around the radical before it reaches soil
conditions for germination soil moisture, tempera moisture, the seed will most likely die.
ture and oxygen. Provided that the soil is not
saturated, oxygen concentration is not a limitation, The hypocotyl is the seedling structure that
but germination will not occur in flooded soil due emerges from the soil surface (Figure 2-4a). The
to lack of oxygen. Within 24 hours of planting, hypocotyl is either greenish or purplish in color
assuming that soil moisture is adequate, seed size reflecting differences in white or purple flowers that
doubles and seed moisture content increases up to will be evident later in the season. As the hypocotyl
50%. During this time period, proteins become active emerges from the soil, it forms a crook as it pulls the
and respiration increases. Respiration is temperature cotyledons from the soil (Figure 2-4b). This is a criti
sensitive and requires oxygen, and germination rates cal stage in seedling emergence. In a crusted soil, the
range from 2 weeks or more in cold soil (50F or hypocotyl may be unable to push through the soil
less) to about 4 days under optimum soil tempera surface, resulting in a swollen hypocotyl, or the
tures (82 to 85F). cotyledons may break from the hypocotyls, leaving

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Figure 2-4a. Stages of soybean germination, emergence
and seedling establishment. (Drawing by Chris Meux)
Germination Management Tips
An example of the effect upon yield due to
the loss of various plant parts during seedling
development is as follows: the loss of cotyledons
at VC is around 8% to 9%; the loss of the unifoli
ate leaves plus cotyledons at V1 is around 7%.

Consider using fungicide and insecticide seed

treatments, especially when planting in cool soils.

Vegetative and
Root Development
Once the unifoliate leaves are fully expanded,
Figure 2-4b. Different stages of seedling emergence in the plant is described as being at the V1 stage of
the same field at the same time. The two soybean development. To determine when a leaf is fully
seedlings on the left have swollen hypocotyls due to a expanded, examine the young leaf at a node above.
crusted soil surface. The two seedlings in the middle
emerged normally in an area without crusting. The If the edges of the leaf at the node above the leaf in
hypocotyl of the two seedlings on the far right has question are not touching, the leaf at the node below
straightened and the cotyledons are beginning to can be considered fully expanded and that node is
unfold. (Photo by Ryan J. Van Roekel) counted. For example, Figure 2-5 illustrates a soy
bean plant with unifoliate leaves and with two trifoli
ate leaves. In this figure, the edges of the young
developing trifoliate are not touching. Therefore,
the unifoliate node is counted and the first trifoliate
node is counted, and the plant is at V2.

Figure 2-5. Vegetative structures of a young soybean

plant. (Drawing by Chris Meux)

the cotyledons and terminal beneath the soil. Under

these conditions, crust-busting equipment such as a
rotary hoe may be able to fracture the soil crust and
improve seedling emergence.

Once through the soil surface, the cotyledons

unfold, synthesize chlorophyll and begin to photo
synthesize. The cotyledons are rich in protein and oil
and are the primary source of nutrients for the devel
oping seedling for the first 7 to 10 days. Once the
cotyledons are through the soil surface, the plant is
said to be at the VE stage of development.

Two unifoliate leaves emerge opposite from

one another on the main stem as the cotyledons
unfold and expand. Once cotyledons and the edges
of the unifoliate leaves are not touching (leaf
unrolled), the plant is considered to be in the
VC development stage.

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 The nodes above the unifoliate leaves have
trifoliate leaves, and vegetative development is identi Management Tips

fied from V2 (the node with the first trifoliate leaf) to

the topmost node of the plant (Vn, Table 2-1). Trifo At full growth, more than 80% of the roots are in
the upper 4 inches of soil with a restrictive pan.
liate leaves are arranged in an alternate pattern up This creates a situation in which deep cultivation
the stem. When temperatures are warm and soil (root pruning) or drought may reduce yields and
moisture is adequate, new nodes will appear about increase the need to maintain adequate soil
every 4 days. Cool temperatures and drought can moisture (irrigation).
slow and even halt node and leaf development. The Decreasing row spacing to 20 inches or less
time required to reach full canopy closure decreases results in faster canopy coverage and lessens the
as rows are narrowed and populations increased, dependence on post-emergence herbicides.
resulting in a crop more competitive against weeds.

Under good environmental conditions, the root

depth increases faster than shoot height during the Growth Habit
vegetative development phase, but the dry weight of
the aboveground parts exceed the root dry weight. By The growth habit of soybeans is described as
V3, small root nodules are usually visible on the being either determinate or indeterminate. Tradition
main root, but if there is substantial carryover of N in ally, determinate varieties have been from MGs 5
the soil, nodule development may be delayed. Three to 10 and indeterminate varieties have been from
to four weeks after emergence, nodules begin provid MGs 000 to 4. In more recent years, however, this
division has become less distinct, and there are
ing nitrogen to the plant. A nodule continues to
numerous MG 4 varieties that are determinate and
increase in size until it is about 8 weeks old, and
MG 5 varieties that are indeterminate.
after this, it loses activity. As nodules die and lose
activity, new nodules are formed, often on branch Determinate soybean varieties stop vegetative
roots, and provide a continued N supply throughout growth and producing nodes on the main stem soon
seed fill. Active nodules have a dark reddish color on after flowering starts, whereas indeterminate varieties
the inside, whereas inactive nodules are black, gray continue producing nodes on the main stem until the
or greenish on the inside (Figure 2-6). beginning of seed fill (growth stage R5). Determinate
varieties, however, will continue producing nodes on
Both root and vegetative development depend on branches until the beginning of seed fill. While deter
a good environment such as adequate soil moisture minate varieties have a relatively short flowering
and nutrition (including adequate nodulation for period of nodes on the main stem (~3 weeks), the
nitrogen fixation) and the absence of high levels of entire flowering period when including branches is
disease and nematode infection. During vegetative similar to indeterminate varieties of the same matu
development, roots can grow 0.5 to 0.75 inch per rity. The total length of the flowering period will
day. Figure 2-5 depicts above- and below-ground depend upon planting date and maturity but may
vegetative development of a young soybean plant. range from 3 to 6 weeks.

Figure 2-6. Cross section of soybean nodules (a) actively fixing nitrogen, (b) beginning to senesce and lose nitrogen
fixation activity and (c) senescent. Note that the active nodule has a deep red color compared to the gray and dark
green color of the senescent nodule.

a b c

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 Determinate varieties are characterized as having Table 2-2. Description of reproductive stages.
a terminal raceme that results in a cluster of pods
under good growing conditions at the uppermost Stage Abbreviated
No. Stage Title Description
main stem node. Under stressful conditions, some or
all of the pods may abort and the terminal raceme R1 Beginning One open flower at any node
appears as a notched spine at the top of the plant. bloom on the main stem
Determinate varieties also typically have leaves at the R2 Full bloom Open flower at one of the
topmost three or four nodes that are similar in size. two uppermost nodes on
In contrast, indeterminate varieties lack a terminal the main stem with a fully
developed leaf
raceme, and the nodes at the top of the plant tend to
form a zigzag pattern. Leaves of indeterminate vari R3 Beginning Pod 3/16 inch long at one of the
eties progressively decrease in size beginning at about pod four uppermost nodes on the
main stem with a fully devel
the fifth node from the top to the plants terminal.
oped leaf
Table 2-1. Description of vegetative stages. R4 Full pod Pod 3/4 inch long at one of
the four uppermost nodes on
Stage Abbreviated the main stem with a fully
No. Stage Title Description developed leaf
VE Emergence Cotyledons above the soil R5 Beginning Seed 1/8 inch long in a pod at
surface seed one of the four uppermost
VC Cotyledon Unifoliate leaves unrolled nodes in the main stem with a
sufficiently so that the leaf fully developed leaf
edges are not touching R6 Full seed Pod containing a green seed
V1 First node Fully developed leaves at that fills the pod cavity at one of
unifoliate node the four uppermost nodes on
V2 Second node Fully developed trifoliate leaf the main stem with a fully
at node above the unifoliate developed leaf.
node R7 Beginning One normal pod on the main
V3 Third node Three nodes on the main maturity stem that has reached its
stem with fully developed mature pod color
leaves beginning with the R8 Full maturity 95% of the pods have reached
unifoliate node their mature pod color; 5-10
Vn nth node n number of nodes on the main days of drying weather are
stem with fully developed required after R8 before the
leaves beginning with the soybeans have less than 15%
unifoliate node moisture

Reproductive Development The main stem is used for determining reproduc

tive stages. When the main stem of a plant is broken
When a soybean plant begins to flower, it is or cut off, reproductive development on the new
classified as being in a reproductive (R) growth stage. branches may be retarded. Plants that have intact
Each reproductive stage from flowering until matu main stems are used to determine stage development.
rity is designated in the reproductive growth stage
scheme as indicated in Table 2-2. The length of time The R1 stage begins when there is one flower at
for plant development (both vegetative and reproduc any node on the plant. Usually the first flower is
tive) varies depending on several factors including located between the fourth and sixth nodes. The R2
temperature, MG and day length. The main effect of stage occurs when there are open flowers at one of
day length on soybean development is that of floral the two uppermost nodes. Between R1 and R2, open
induction. Soybeans are referred to as short-day flowers appear at nodes from the bottom of the plant
plants because short days (i.e., long nights or dark toward the top. In a late-planted soybean crop (e.g.,
periods) initiate flowering (floral induction).
double-crop soybeans), R1 and R2 may occur almost
Reproductive stages are based on flowering, simultaneously or be separated by as much as 6 days.
pod development, seed development and plant For full-season soybeans (e.g., MG 5 variety planted
maturation. Each stage description is given a repro in early May), the time period between R1 and R2
ductive stage (R) number and an abbreviated title ranges between 3 and 10 days.
(Table 2-2).

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 After flowering begins, plants continue to grow
vegetatively, producing new nodes on both the main Management Tips

stem and branches. The time between R2 and begin

Having a full canopy by R2 is important in setting
ning pod stage (R3) depends upon planting date, a maximum number of pods.
growth habit (determinate or indeterminate), MG,
temperature and other factors that affect plant growth Begin scouting soybeans regularly for diseases
and pod-feeding insects (especially bollworms
(e.g., drought). Indeterminate varieties may spend and stinkbugs) beginning at R3 and continuing
2 or more weeks at R2 because new nodes are through seed fill.
produced on the main stem after first reaching R2.
Irrigation should be continued at least until the
crop is in the R6 development stage. If soil mois
The beginning of the seed fill period is desig ture is high when leaves first begin to turn yellow,
nated as R5. At this time, seed are 1/8-inch long in irrigation can be terminated.
full-size pods at one of the uppermost four nodes.
Flowering ends and plants reach their maximum
vegetative weight at the beginning of R5. During the
flowering and early pod formation stages (R1 through
R4), plants adjust the number of flowers and young
Days Between Growth Stages
pods in response to the environment. Flowers and
Soybean development is influenced by tempera
young pods may abort when conditions are stressful
ture and day length and will also be affected by soil
for a few days, but then fewer flowers and young
moisture conditions, plant nutrition and other
pods may abort later if environmental conditions
factors. The number of days between stages will also
improve. Soon after plants reach R5, however, there
vary greatly depending on the MG and variety used.
is little pod abortion, and stress (e.g., drought, pro
Therefore, the timing of development stages will be
longed flooding, etc.) results in a short seed-filling
different depending on the variety, climate and plant
period, early maturity and small seed.
ing dates.

The R5 development stage ends and the R6 Emergence will depend primarily on temperature
development stage begins when the seed fills the pod under correct moisture conditions. Plants will emerge
cavity from a pod at one of the uppermost four nodes when soil temperatures are higher than 41F, and
on the main stem. Because much of seed dry weight higher temperatures will enhance a faster emergence.
accumulates during R6, it is critical to ensure that the For instance, emergence can take about 14 days at
crop continues to be managed for irrigation and 46F but only 4 days at 59F. During the vegetative
pests. Toward the end of R6, the canopy begins to period, the plant will produce nodes at an average
yellow and leaves begin to senesce. rate of 1 node every 4 days. The total number of
nodes will vary between approximately 16 and 23,
The R7 development stage begins when there is depending on the cultivar and length of the vegeta
one mature-color pod at any node on the plant. tive period.
Mature pod color in soybeans differs among varieties.
The pod wall color in commercial varieties is usually The beginning of flowering and subsequent
tan but may be brown. Additionally, the pubescence reproductive development is greatly affected by
(small hair-like structures on the plant) of varieties photoperiod. The soybean is a short-day plant, mean
may be either gray or tawny (brown). The mature ing that days shorter than a critical value will induce
pod color of soybeans, therefore, varies from light the plant to flower. Figure 2-7 shows the day length
gray to dark brown. At R7, seed moisture is still high, across the growing season for Dumas (33.8 latitude),
but the seed has reached its maximum dry weight. Stuttgart (34.5 latitude) and Blytheville (35.9
Seeds can still be damaged by stinkbugs at this stage. latitude), Arkansas. The yellow-shaded region near
The R7 stage is also referred to as physiological the bottom of the figures show the critical day length
maturity. Once 95% of the pods turn a mature pod at which the development rate toward flowering is
color, the plant has reached R8 and can be harvested at a maximum. For MG 3 varieties, the critical day
once seed moisture is less than 15%. length that induces soybeans to flower at a maximum

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Figure 2-7. Day length versus date for three locations in Arkansas. The yellow-shaded regions indicate the day
lengths that result in MG 3 (left panel) and MG 5 (right panel) varieties rapid progression toward flowering.
As the day length becomes longer (green region), the rate of progression toward flowering decreases.

rate is about 13.4 hours, whereas for a MG 5 variety, In addition to photoperiod, higher temperatures
the critical day length is about 12.8 hours. When day will accelerate reproductive development. Using
length is longer than these critical values, plants will long-term weather data for Blytheville and Dumas,
continue to be induced to flower but at a progres Arkansas, we have estimated dates when flowering
sively slower rate (green region in Figure 2-7). (R1), beginning seed fill (R5) and physiological
maturity (R7) are expected to occur over a range of
For example, a MG 3 variety will be induced to planting dates using a computer simulation model
flower at a maximum rate whenever the day length is (CropGro, Figure 2-8). Assuming average tempera
less than 13.4 hours, which corresponds to dates ear tures for early plantings on March 15, the time from
lier than April 1 (Figure 2-7). For a MG 5 variety, planting to flowering (averaged across the two loca
plants will be induced to flower at a maximum rate tions) will be about 60 (May 14) and 70 (May 24)
whenever the day length is less than 12.8 hours, days for MG 4 and 5 varieties, respectively. As plant
which corresponds to dates earlier than March 15
ing date is delayed, the number of days to flowering
(Figure 2-7). Note that if planting is delayed until
is shortened. For instance, with late planting on
mid-May, the photoperiod is longer than the critical
June 15, the number of days from planting to flower
day length for both MG 3 and 5 varieties (green
ing will be about 42 (July 27) and 50 (August 14)
region of Figure 2-7), which means that the progress
toward flowering will take considerably longer. For days for MG 4 and 5 varieties, respectively. The
double-crop or late-planted soybeans (late June or development rate toward flowering and the subse
early July), the day length is shorter each day, which quent reproductive stages will be faster in years
means that the development rate toward flowering with higher temperatures and/or locations in
is increasing. From a practical standpoint, these south Arkansas.
scenarios mean that early-planted or late-planted
soybeans should be planted in more narrow rows The beginning of seed filling, or the R5 stage,
(< 20 inches or twin rows) and at a higher popula will occur about 27 (June 10) and 39 (July 2) days
tion density than soybeans planted in mid-May. after beginning of flowering for MG 4 and 5 varieties
Likewise, MG 3 and early MG 4 soybean varieties planted on March 15, but it will shorten to 21
will have less time for vegetative growth compared to (August 17) and 23 (August 27) days for MG 4
MG 5 varieties, which may require more narrow rows and 5 varieties when planted on June 15.
and a higher population density.

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Figure 2-8. Estimated dates of beginning flowering (R1), beginning seed filling (R5) and physiological maturity (R7)
based upon planting date for MG IV and V soybeans. The lower edge of each colored region represents dates for
Dumas, AR (latitude 33.8N), and the upper edge represents estimates for Blytheville, AR (latitude 35.9N). Estimates
were made using a crop simulation model (CropGro) and using long-term weather data for each location.

Finally, the expected number of days from begin and weather conditions. Under non-irrigated
ning of seed filling to physiological maturity (R7) will conditions or dry years, the soybean crop can reach
be about 67 (August 16) and 61 (September 1) days maturity about 7 to 10 days earlier than the reported
for MG 4 and 5 varieties with an early planting on expected dates. Conditions of prolonged drought
March 15, and the R7 date is expected to be about during flowering through early seed fill (R5) followed
34 (September 20) and 32 (September 28) days after by favorable growing conditions may result in few
R5 for MG 4 and 5 varieties with a planting date in pods on the plant, a delay in maturity, and green
mid-June. leaves and stems when pods are mature. Likewise,
insect infestations resulting in a large loss of pods can
The number of days from R7 until all the pods also delay maturity and result in green leaves and
are mature and the crop is ready to harvest (R8) can stems when pods are mature.
range from 6 to 13 days depending on the cultivar

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