HERBICIDES

1. Introduction
a. Why the Herbicide?

A herbicide is a type of pesticide or chemical that is used to kill plants, mostly

weeds, by causing one or more physiological disorders.

 As a weed control tool, herbicides are the fastest and only feasible means
available to broad-acre farming.
 Herbicides are a cheaper means of controlling weeds on large scale, especially
where labour is scarce or expensive
 In the mostly broadcast sown crops like finger millet and rice, herbicides are
more effective in reaching almost every weed while mechanical means find
more use in controlling weeds inter-row spaces in row-sown crops.
 A weed-free paddock at the time of crop emergence (early season weed
control) is usually possible only with use of herbicides (pre-plant and pre-
emergence). Bear in mind that weed competition during early stages of crop
growth is responsible for the greatest loss in crop yield.
 If properly applied, herbicides do not cause injury to crops, while mechanical
methods may cause damage to roots and other plant parts.
 Most perennial weeds are not controlled using physical means but are
susceptible to systemic herbicides.

b. Brief history

Chemicals such as salt, ash, smelter wastes and other materials were in use for
centuries as soil sterilants before the advent of modern herbicides in the late 19 th
century. Interest in chemical weed control was triggered in 1896 when discovery
that a Bordeaux spray applied to grapevines to protect them against downy mildew
also controlled some weed species. Then several independent researchers focussing
on control of fungal diseases, accidentally found that copper salts applied to broad-
leaved weeds in cereals resulted in selective control. By the end of the century into
the 20th century, other products including solutions of sulphuric acid, copper nitrate

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potassium salts etc. were added to the selective herbicidal squad. The discovery of
2, 4-dichlorophenoxy acetic acid (2, 4-D) during the period leading to WWII
heralded the advent of modern era herbicides. Research on 2, 4-D showed that
herbicides could not only be effective in very small quantities but also highly
selective and systemic in action.

Decreasing number of field workers, rising costs of labour and narrower profits
margins called for cheaper but more effective weed control tactics. Herbicides filled
this niche and as a result, this gave impetus to the development of the herbicide
industry. (Source: National Research Council (U.S.). Committee on Plant and Animal
Pests. 1968)

c. Moderators of Herbicide Activity

The effect of herbicides on a plant can be modified by:

 The environment:- wet, dry, dark etc. and so they work best in specific
environments
 Stage of weed maturity – some are more effective on young weeds while others
on older, mature ones
 Part of the plant treated or where the herbicide is applied – foliar or soil
application
 The route taken by the herbicide in the plant – apoplastic or symplastic or a
combination of these.
 Concentration of herbicides
 Time of the day when herbicides are applied – morning or afternoon

2. Herbicide Classification

Herbicides do not cause the same type of disorder in weeds. Based on their effects,
timing of application, method of application and chemistry, herbicides are classified
into groups, although some can fall into more than one group.

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 a) Timing of application

This separates herbicides into pre-emergence and post-emergence with respect to

either weeds or crops or both – hence specify- and pre-planting, which are applied
before planting the crop to kill weed seeds and vegetative plant parts.

Pre-plants: Most pre-plants are incorporated into the soil as granules (soil fumigants)
to destroy germinating weed seeds. Care should be taken so that the crop that will
be planted has tolerance to the herbicide or wait until the herbicide has lost activity.
Trifluralin, EPTC and methyl bromide are examples of pre-plants incorporated into
soil. Methyl bromide was outlawed in 2005 due to its effects on the ozone layer
(depletes it)

Pre-emergence: this is any treatment made prior to weed or crop emergence or

both. An example is application of metolachlor to control yellow nutsedge. To clearly
establish the timing of application, state ‘pre-emergence to weeds’ or ‘pre-
emergence to crops’ or ‘pre-emergence to both weeds and crops’. They are mostly
applied to the soil before the target has emerged – whether annual or perennial
weed seed but they do not kill new shoot arising from vegetative structures. The
pre-emergence herbicide does not kill the weed seed nor prevent its germination but
establishes sort of a thin chemical barrier at or right below the soil surface. As weed
seedlings begin to emerge, they come into contact with the treated zone, absorbing
the lethal herbicide, and eventually dying. The herbicide has to come into contact
with the primary growing point for the weed hence the emphasis to apply pre-
emergence herbicides before the weed emerges. Activation by water is necessary to
improve on use of these herbicide or else they photo-decompose or volatilise if left
for a long time on the surface before rains or irrigation.

Post-emergence: applied after the emergence of specified crop or weed. The post-
emergence herbicides are usually more effective if applied early i.e. onto seedlings
and during early vegetative growth stage than onto reproductive, slow-growing
stage. The herbicide 2, 4 D gives effective control of most broad leafed weeds in
maize, sorghum and grass pastures. It may be applied post-emergent to the crop
but pre-emergent to the weeds.

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Herbicides in this category combine other classifications as they can be foliar or root
absorbed, selective or nonselective, contact or systemic. Repeated applications are
necessary to combat weeds as others emerge later.

b) Target of application

For most weeds, herbicides are applied to leaves (foliar application) or to the soil. All
foliar applications enter the plant through leaves and the herbicide is translocated to
all parts of the plant through the symplastic path. In foliar applications, the active
ingredient is absorbed through leaves e.g. dicamba, glyphosate and 2, 4 D. This
means that foliar herbicides can only be applied post-emergent in relation to the
target organism. Most of the herbicides applied require that there should be no
rainfall until after a good number of hours e.g. six (6) hours for glyphosate. This
requirement is only for water-based herbicides; as for soil applied ones, they need to
enter the soil solution which the weed seeds will imbibe or else their efficacy is
compromised if the soil is dry.

If the active ingredient is absorbed through roots such as is the case with atrazine,
the herbicide is classified as root herbicide. Root herbicides are used for pre-
emergence or post-emergence weed control. Adequate moisture is required to
enhance the effectiveness of root herbicides.

c) Type of movement in the plant

Herbicides exert their effect either on contact or need to move to other plant parts.
Contact herbicides kill susceptible plant tissues they come into contact with e.g.
diquat, paraquat, propanil. Contact herbicides will therefore require uniform spray
coverage and uniform particle size to achieve the desired results – best results are
obtained if the entire plant receives the spray. Apply fine droplets by choosing the
right nozzle type and size. Contact herbicides are also foliar. Contact herbicides are
therefore not effective at controlling weed species with underground perennating
structures. But repeated applications to regrowths deplete the plants’ underground
resources and thus they can be controlled in this way. Nevertheless, use of systemic
herbicides is a better option for controlling perennial weed species with underground
reserves.

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Systemic ones are extensively translocated through a plant’s vascular system - that
is the phloem, xylem or interconnected cells - from absorption sites (leaf or root) to
sites of action. Examples include atrazine, glyphosate, dalapon. Entire coverage of
the target plants (weeds) is not necessary for systemic herbicides and therefore
spray can be applied either as small or large droplets.

d) Type of selectivity

While some herbicides are selective, others are nonselective (also known as broad
spectrum or knock-down) e.g. 2, 4-D and metolachlor are selective while glyphosate,
diquat, paraquat and sodium chlorate are nonselective. Selectivity may be based on
differences between plants species in the way they absorb and translocate
herbicides or it may be due to morphological factors. However, selectivity is also
influenced by concentration so that if a selective herbicide is applied as an overdose,
it may lose is selectivity and kill all plants – hey, including your crop!

Herbicide tolerance (HT) versus herbicide resistance (HR): Plants that are
not killed after the application of a herbicide at rates that normally kill susceptible
ones are said to be tolerant to that herbicide. HT is defined as the inherent ability of
a plant species to survive and reproduce after herbicide treatment at a normal use
rate. It is the underlying cause for selectivity. Development of HT crops (e.g.
glyphosate-tolerant soyabeans and triazine-tolerant canola) rides on the same
principle as tolerant weeds. HT is different from HR. HR is the inherited ability of a
plant to survive and reproduce following exposure to a dose of a herbicide normally
lethal to the wild type. HR is normally present at very low frequencies in weed
populations before the herbicide is first applied. Continued application of a herbicide
is continued application of selection pressure and hence the resistant individuals in a
population increase.

Glyphosate is the most widely used herbicide and over reliance on it has led to
glyphosate-resistant weed problems, especially in countries where genetically
modified (GM) crops such as glyphosate-tolerant soyabeans are grown.

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 e) Chemical structure

Chemical classification groups herbicides based on the chemical structure of the

active ingredient. Some may have similar structures but their modes of action differ
through physiological effects. Chemical groups – herbicides are either organic,
inorganic or biological (concentrates of fungi).

f) Physiological action – Mode of Action Groups

There is a number of physiological effects that herbicides have on plants and based
on those effects they are put into groups some of which are:

 Mitotic inhibitors,
 Photosynthetic inhibitors
 Nitrogen metabolism inhibitors (affect protein synthesis)
 Respiratory inhibitors
 Inhibitors of production of vital pigments such as carotenoid
 Seedling shoot and root inhibitors
 Desiccants or growth regulators

These physiological effects herbicides have on plants indicate that herbicides are
specific in their effects. The effects that herbicides have on plants can be used to
conveniently classify herbicides into mode of action (MOA) groups. A MOA is the
biochemical mechanism by which a herbicide causes growth to cease in the
target/susceptible plant leading to the plant’s eventual death. MOA groups are
assigned group letters from A to N, but there are also subgroups and other
herbicides whose groups are unknown because their physiological effects on weeds
are unknown. Each herbicide should have its MOA group indicated on the label. This
is helpful in guiding users to avoid repeated application of a herbicide or herbicides
from the same MOA group, which encourages resistance. Commonly used MOA
groups and examples of herbicides under them are given below.

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MOA GROUP CHEMICAL FAMILY EXAMPLES (Active
EXAMPLES ingredients)
A - Inhibitors of acetyl Aryloxyphenoxypropionates Clodinafop, cyhalofop ,
coA carboxylase (Fops) diclofop, haloxyfop
(Inhibitors of fat synthesis Phenylpyrazoles (Dens) Pinoxaden
/ ACC’ase inhibitors) Cyclohexanediones (Dims) Clethodim, profoxydim,
tepraloxydim
B - Inhibitors of Imidazolinones (Imis) Imazapic, imazethapyr,
acetolactate synthase imazamox
(ALS inhibitors) Sulfonylureas (SUs) Bensulfuron,
chlorsulfuron, metsulfuron
MOA GROUP CHEMICAL FAMILY EXAMPLES (Active
EXAMPLES ingredients)
C - Inhibitors of Triazines Atrazine, simazine
photosynthesis at propazine, cyanazine
photosystem II (PS II Ureas Diuron
inhibitors) [there are Triazinones Hexazinone, metribuzin
many other chemical Amides Propanil
families than indicated]
D - Inhibitors of Dinitroanilines Trifluralin, oryzalin,
microtubule assembly pendimethalin
Benzoic acids Chlorthal
E- Carbamates Carbetamide,
Inhibitors of mitosis / chlorpropham
microtubule organisation
F - Bleachers: Inhibitors Nicotinanilides Diflufenican
of carotenoid biosynthesis
at the phytoene
desaturase step (PD S
inhibitors)
G - Inhibitors of Diphenylethers Acifluorfen, oxyfluorfen
protoporphyrinogen
oxidase
H - Bleachers: Inhibitors Pyrazoles Benzofenap, pyrasulfotole
of 4-hydroxyphenyl-
pyruvate dioxygenase
(HPPD s)
I - Disruptors of plant cell Phenoxycarboxylic acids 2,4-D, MCPA, 2,4-DB,
growth (Phenoxys)
Benzoic acids Dicamba
K - Inhibitors of cell Chloroacetamides Metolachlor,
division / Inhibitors of dimethenamid, propachlor
very long chain fatty
acids (VLCFA inhibitors)
L - Inhibitors of Bipyridyls Diquat, paraquat
photosynthesis at
photosystem I (PSI

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inhibitors)
M -Inhibitors of EPSP Glycines Glyphosate
synthase
N - Inhibitors of Phosphinic acids Glufosinate
glutamine synthetase
Z - Herbicides with Arylaminopropionic acids Flamprop
unknown and probably
diverse sites of action
Organoarsenicals DSMA

Please, note that the last column names the herbicides by their active ingredients.
Most herbicides are given trade names by which they are more commonly referred
to than by active ingredient names. In fact, one herbicide (as well as other pesticide
types) may have more than one trade name and more than one active ingredient.
Examples of active ingredients and some of their other names:

 Paraquat: Gramoxone, Alliance

 Glyphosate is traded as Roundup, Trounce, Xpress, Illico, Arsenal
 Metolachlor: Boxer Gold, Dual Gold
 2, 4-D: Amicide
 Atrazine: Gesaprim, Gesapax
 Trifluralin: Treflan
 MCPA: MCPA, Buctril, MA, Banvel M, Midas, Paragon, Tigrex, Barrel, Tordon
242, Basagran M60, Chipco Spearhead, Agtryne, Precept (Most of which have
more than one active ingredient),

3. Types of Application

There is a number of ways in which herbicides can be applied:

• Broadcast or blanket application: this is where application is uniform to the

whole area

• Band treatment: Application to narrow strips or selected areas such as inter-

rows or crop rows only. This method finds greater use where you use highly toxic

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herbicides; and it reduces the chemical cost because the treated area is often one-
third of the total area.

• Directed sprays: This is where sprays are applied only to particular plant parts
– usually the low part of the stem or trunk just above the ground level. This method
is usually practised in plantations to kill trees and has less usage in arable crops.

• Spot treatment: This is the treatment of a restricted area to control an

infestation of a weed species. The weed has not yet spread to other portions of the
paddock. Treatment is often done to contain the spread of a weed species.

4. The Herbicide and the Plant

When in contact with a plant, the effects of a herbicide is moderated by the

morphology and anatomy of the plant as well as numerous physiological and
biochemical processes that occur within the plant and the environment. These
processes include (1) absorption, (2) translocation, (3) molecular fate of the
herbicide in the plant, and (4) effect of the herbicide on plant metabolism. The
interaction of all these factors with the herbicide determines the effect of a specific
herbicide on a given plant species. When one plant species is more tolerant to the
chemical than another plant species, the chemical is considered to be selective.
These topics and methods to influence selectivity. Although the effectiveness of any
herbicide ultimately depends on the concentration of active ingredient that finally
reaches the target site, as stated earlier, there are many other factors that regulate
herbicide efficacy. Considering plant aspects, many barriers prevent the herbicide
molecule passing from the outside of the leaf through the cuticle and underlying
cells, utilising transport systems, to reach the site of action.

a. Role of Plant Morphology

The effects of a herbicide in a plant are influenced by morphology and

biochemical/physiology processes in a plant such as translocation and absorption.

Morphological aspects that affect herbicides include:

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 Inclination of leaves – vertical or horizontal- by sliding movement, more herbicide
applied is lost when applied to vertically inclined leaves than horizontal ones.
 Leaf size – broad leafs retain more herbicide than grass/narrow ones.
 Nature of leaf/stem surface – pubescent (downy, covered with short soft hair) vs
glaborous (bald)
 Pubescent leaves and stems have hair-like structures and therefore are able to
retain more herbicide than glaborous ones, which are smooth
 The cuticle: The outer leaf surface is covered with a waxy cuticle that
waterproofs the leaf and provides the first line of defence between the plant and
its hostile environment. Its structure and chemical content are both varied and
complex, but the successful passage across it is a vital aspect of herbicide
efficacy. Generally, the cuticle is 0.1 – 13 μm thick and contains three
components: an insoluble cutin matrix, cuticular waxes, and epicuticular waxes.
The epicuticular waxes are the most significant barriers to absorption of water-
soluble herbicides. It is not a homogeneous layer and varies greatly from species
to species. The change from high to low humidity can trigger wax production by
more than an order of magnitude, an important factor to consider when
extrapolating data on herbicide trials from the glasshouse to the field
environment. Generally, the cuticle will thicken during conditions that are
unfavourable to plant growth, including low temperatures, photon flux density
and water availability, and so herbicide absorption is maximised when opposite
conditions prevail. Thick cuticles do not easily absorb herbicides e.g. pineapple
leaves have thick wax and cuticle layer which make most herbicides ineffective.

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Schematic representations of the cuticle

Other crops with think cuticle include onion and Brassica species like cabbage, also absorb a
minimal amount of a herbicide as a result of their thick leaf cuticles. Santier and Chamel
(1992) showed that glyphosate absorption was 94% through thin tomato fruit cuticles, but
only 1 to 6% for thicker cuticles of box-tree leaves, rubber plant leaves, and pepper fruit.
Portulaca oleracea is a good example of a weed species with think cuticle.

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 b. Absorption

Absorption and transport are more important for systemic herbicides than for
contact ones. Some plant surfaces absorb chemicals faster/more effectively than
others i.e. there is differential/selective absorption in different plants and plant parts.
Therefore, apply systemic herbicides to plant parts that are likely to absorb more of
them. Leaves and roots are the two common points of herbicide entry but some can
enter through stems, shoots and seeds. Absorption through leaves is by far the most
important. Surfactants may be added to the herbicide to improve absorption by
reducing interfacial/surface tension, polarity and also making the active ingredient
stick to the leaf surface. Absorption may also increase with temperature. In the case
of herbicides crossing the plant membranes by simple diffusion, movement is related
to the concentration difference (gradient) across the membrane. The herbicide
moves from a region of higher concentration to a region of lower concentration.
When the concentration is equal on both sides of the membrane, accumulation
stops. The herbicide concentration gradient across the membrane is the driving force
that moves the herbicide across the membrane.

Root absorption is for soil-applied herbicides. Water dissolves minerals and other
substances to form a soil solution. When herbicides have been applied to the soil,
they will dissolve and also be part of that soil solution that plant root will absorb –
this movement is by mass flow. The root may also intercept the herbicide, or the
herbicide moves by diffusion, especially in the case of fumigants. Once inside the
root hair, the herbicide has to be transported to the vascular system for long -
distance transport to the target site. Herbicides can be absorbed passively (through
the apoplast route - that movement down a concentration gradient along the cell
walls) or actively (through the symplast route) or a combination of these. The
apoplast route is the total cell wall continuum, intercellular spaces and xylem vessels
(major component). This route requires herbicides to bypass the impermeable
Casparian strip to reach the xylem or phloem tissue for long-distance translocation.
Therefore, it is not possible for herbicides to move into xylem vessels entirely by the
apoplast pathway. The Casparian strip is made up of suberin wax and serves to
protect the root from dehydration in a similar manner the cuticle is to the leaf. The

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herbicide must penetrate the cell membrane, thus entering the symplast, in order to
bypass the Casparian strip (see figure below). Having bypassed the Casparian strip,
the herbicide may enter the xylem or phloem for long-distance translocation. In the
apoplast route, herbicides follow the same route as solutes through the transpiration
stream (i.e. with water). Since xylem vessels are non-living, an overdose is usually
not a problem. In case of moisture deficit, the herbicides will not go the upward
transpiration way but move downward, so ensure plants are well watered. It is a
faster route than the symplastic one. On the other hand, the symplast is made up of
the living/protoplasm of a plant. However, it involves initial entry into cell walls and
then into the protoplasm of the epidermis, cortex, stele (conducting tissue of the
stems and roots, which is in the form of a cylinder, principally containing xylem,
phloem, and pericycle) and then the phloem vessels. This is a much longer route
through the plasmodesmata (channels of cytoplasm lined by plasma membrane that
transverse cell walls – kind of pipes that link one cell to the other). Under the
symplast route, herbicides follow the same route as sugars produced during
photosynthesis.

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Figure Showing Lateral Transport of Soil Solution (with Water, Minerals and Herbicides)

Since phloem vessels are sensitive, as living cells, they can get killed should an
application be an overdose. The fact that phloem vessels are living makes them 50
to 100 times poorer conductors of herbicides than xylem vessels – the interaction
with cell sap. Metabolism of a herbicide within the plant will also influence its
movement, since metabolism generally reduces lipophilicity. Nevertheless, the
potency of some herbicides at the grass meristem is such that even though less than
1% of applied dose is mobile, it is sufficient to control grass weeds.

5. Fate of herbicides: interaction with the environment.

After application on crops, pesticides can reach groundwater and surface waters,
including streams and lakes, because of leaching and soil runoff processes.
Fortunately, many pesticides are not recalcitrant in water environments and they
can be transformed through chemical (e.g., hydrolysis) and biological processes,
as well as photochemical reactions in sunlit surface waters

(a) Molecular fate of herbicides on and in plants

Once the herbicide has contacted the plant

surface, six things can happen to the

active ingredient:

i. Volatilize and be lost to the atmosphere

or be washed off by rain.
ii. Remain on the outer surface in a viscous
liquid or crystalline form.
iii. Penetrate the cuticle but remain absorbed
in the lipoid (wax) components of the cuticle.

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iv. Penetrate the cuticle, enter the cell walls, and then translocate prior to entering
the symplasm. This is called apoplastic translocation, which includes movement
in the xylem.
v. Penetrate the cuticle, enter the cell walls, and then move into the internal cellular
system (through the plasmalemma) for symplastic translocation, which includes
phloem movement.
vi. Its structure gets changed. The molecular structure of a herbicide once applied
changes with time. Most changes reduce the phytotoxicity of the herbicide i.e.
the herbicide becomes inactivate. But some herbicides can become more
phytotoxic through activation. Different plant species have varying abilities to
modify herbicides; and tolerance or susceptibility of some plants to some
herbicides is based on this difference. Some plants inactivate herbicides so
rapidly that they do not get affected by them e.g. maize quickly inactivates
atrazine and therefore, this herbicide can be used to control weed species that
cannot inactivate it. The inactivation process may involve molecular degradation
or formation of a conjugate with plant constituents such as sugars, amino acids
etc. the table below compares responses of different plant species to atrazine.

The amount of atrazine in shoots of 8 plant species 10 days after pre-

emergence Application (Negi et al., 1964)

Sourced from Fundamentals of Weed Science [3rd Edition, 2007] by Robert L. Zimdahl

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Some reactions are may be catalysed by enzymes while others are spontaneous.
The faster an herbicide is metabolized, the less there is available for translocation
and activity at the site of toxic action. Another example of plant metabolism is
conversion of simazine to hydroxysimazine, a derivative with no herbicidal
properties (see figure below).

Herbicides are degraded either through oxidation, reduction, hydrolysis,

deamination dehalogenation, dealkylation, hydroxylation, decarboxylation etc.
“Plant metabolic reactions have been separated into three phases (Hatzios and
Penner, 1982; Shimabukuro et al., 1981). Phase one includes nonsynthetic,
generally destructive processes such as oxidation, reduction, and hydrolysis.
Phase two reactions are conjugations that result in synthesis of a new molecule.
Phase one reactions add OH, NH2, SH, or COOH functional groups that usually
change phytotoxicity, increase polarity, and lead to a predisposition for further
metabolism. Phase one reactions can be enzymatic or nonenzymatic. An example
of the latter is photochemical reduction (detoxification) of bipyridyllium
herbicides. Phase two metabolism is conjugation that yields metabolites with
reduced or no phytotoxicity, higher water solubility, and reduced plant mobility.
Conjugations occur with glutathione, amino acids, and glucose and other sugars.
Phase three metabolism is unique to plants because plants cannot excrete
metabolites as animals can. Conjugated metabolites must be compartmentalized
in plant cells or somehow removed from further metabolic activity. Herbicides
become more water soluble as they are metabolized from phase one to two and
they remain water soluble or become insoluble in phase three. Phytotoxicity is
reduced with each phase and herbicides metabolized to phase three are no
longer toxic.”

Below is the table show possible fates of various herbicide groups.

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Plant Metabolic Reactions and the Herbicide Chemical Groups Affected

Sourced from Fundamentals of Weed Science [3rd Edition, 2007] by Robert L. Zimdahl

Cationic (positively charged) salt herbicides (e.g., paraquat) are water soluble but
rapidly adsorb to the negatively charged cuticle and thus are less subject to removal
from leaves by rain. Lipophilic herbicides (usually formulated as an EC or flowable)
have low water solubility but are readily absorbed into the lipophilic cuticle. These
characteristics make both cationic and lipophilic herbicides less subject to loss from
the leaf surface by rainfall. Environmental factors 1 to 2 weeks before and
immediately after herbicide application can influence absorption of postemergence
herbicides. High light coupled with low relative humidity and, most important, low
soil moisture tend to induce synthesis of leaf cuticles with increased lipophilic
character; thus, when herbicide —particularly, water-soluble herbicide—application
occurs, performance decreases. Stomatal penetration of spray solutions is not a
common occurrence (Schönherr and Bukovac, 1972). However, if the surface tension
of the spray solution is reduced enough, stomatal penetration can occur. One thing
to remember is that in most weeds, the stomates are on the abial surface; hence,
for most plants, stomatal penetration is not a major means of spray entry for any
foliar-applied compound.

Herbicides affect plant metabolism in a complex manner. Once one reaction starts in
one part of the plant, it triggers a chain reaction. A herbicide may interfere with only
one particular biochemical process e.g. photosynthesis, or be non-specific and affect
several processes e.g. photosynthesis and respiration. But since each biochemical
process is linked to others, affecting only one affects others too hence the entire

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metabolism of a plant (photosynthesis, respiration, DNA synthesis, lipid metabolism
etc.) is affected leading to death or injury to a plant.

“In general, for maximum effectiveness the ideal herbicide should have the
following:

1. Ability to enter plants at various sites.

2. Ability to enter plants without local damage.
3. Activity or ability to affect plant growth that is not confined to a
particular stage of plant development or plant size.
4. Ability to translocate in plants to appropriate sites of action.
5. Metabolism or degradation to inactivity in target plants should be slow
enough to permit full expression of activity.
6. Moderate soil absorption to decrease leaching.
7. Reasonable stability in soil except for foliar active, contact herbicides
where soil persistence is of no consequence to plant action but may
nevertheless have environmental consequences.
8. A wide weed control spectrum or specific activity against target
weeds.”

(b) Fate of herbicides in and on the soil

Regardless of the application target – plant or soil – portions of all herbicides end up
onto the soil. Herbicides applied onto plant surfaces can be washed off by rainfall. In
pre-emergence weed control, the herbicide is directly applied onto the soil surface,
while fumigants are injected into the soil. For the root absorbed herbicides, it is
desirable that they persist in the soil at least for the length the crop will be growing
as long as they crop is tolerant to it. Atrazine is an example of a full season
herbicide. Immediately after application, herbicides may undergo changes in the soil
or end up in various places.

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 i. Decomposition

Herbicides can be decomposed biologically or chemically or by light. Biotic and

abiotic photochemical degradations usually play a more important role in the
environmental fate of pesticides. Biological decomposition is a microbial process.
Microorganisms begin to attack herbicides as soon as they are applied to the soil.
The rate of herbicide decomposition increases with time as the population of
microorganisms thriving on the herbicide increases. Nevertheless, microbial activity
is affected by ecological factors such temperature, soil acidity, moisture, oxygen
minerals. If these are not optimum, microbial decomposition will decline and the
herbicide with persist for a longer period.

Chemical decomposition includes oxidation, reduction hydrolysis etc. This can be

aerobic on anaerobic.
Photolysis (or photodecomposition) is influenced by the chemical structure of the
herbicide, intensity of light and the duration of exposure of the herbicide to light.
Photolytic reactions can be distinguished into direct photolysis and indirect
photochemistry. Direct photolysis refers to the transformation of a molecule upon
direct light absorption. Indeed, some pollutants are able to absorb sunlight, reach
excited molecular states and then undergo chemical transformation. Direct
photolysis can be inhibited by competitive light-absorbing compounds, including
most notably the dissolved organic matter (DOM) that naturally occurs in surface
waters. The absorbed triggers the indirect photodecomposition of water pollutants
including herbicides.

ii. Absorption

Herbicides meant for root absorption are applied to the soil for plant uptake - as
stated above. Absorption is the desirable fate of applied herbicides for in-season
crops. Shoot absorption is an important means of entry for many soil-applied
herbicides that are active on germinating seeds or small seedlings (e.g.,
carbamothioates and chloroacetamides). Before emergence, a shoot has a poorly
developed cuticle and probably no wax layers, making it more easily penetrated by
herbicides. In addition, the Casparian strip barrier is not present in shoot tissues.

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Shoot absorption is a particularly important route of herbicide entry in grass species,
but less so for dicot weeds. Shoot zone entry is by diffusion, from herbicide
dissolved in the soil solution in contact with the shoot tissue or, probably more
important, diffusion from herbicide present in the vapour phase of the soil (e.g.,
EPTC). Herbicides known to have a major route of entry in the shoot zone are
primarily growth inhibitors having their site of action in the shoot meristem as it
emerges through the soil.

iii. Adsorption

The herbicide adheres to a surface or particle such as soil particle. This is a cause of
concern to environmental safety as it may cause hazards. In this way, some early
herbicides got outlawed. If adsorption is irreversible, then the herbicide will no
longer be available to the plants. Adsorption is affected by soil type being very high
in organic matter and clay soils. Where adsorption is high, more herbicide will be
required to effectively control weeds because less herbicide ends up in solution than
in sandy soils – where adsorption is low. The option in this case would be to apply
foliar absorbed herbicides.

iv. Leaching

Herbicides are carried in water and washed down the profile through percolation and
seepage. So then, water-based/water soluble herbicides in sandy soils will quickly be
leached and not help much with weed control. In this case, opt for oil-based ones in
such soils. Herbicides not easily washed down the profile but are reversibly adsorbed
will be more effective in weed control. Clay soils and organic soils are good at
retaining herbicides than sandy soils. Amount of water moving down the profile also
regulates the persistence of herbicides within the plough layer.

v. Volatilisation

Herbicides on a dry soil surface are more prone to volatilisation under high
temperature than if applied onto moist soil and in cool weather. When herbicides
vaporise, they are lost as gasses e.g. trifluralin is very volatile. To be more effective
then, volatile herbicides should be applied as pre-plants to moist soils in cool

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weather to allow more contact with weeds. If the soil is not moist, irrigation should
be done before incorporation of the herbicides. Vapours can migrate to neighbouring
paddocks and kill crops in them, so be careful mwana! Volatilisation increases as air
temperature and the temperature of the surface on which the herbicide was applied
increases. Adsorption and absorption of the herbicide into the plant or soil reduce or
eliminate volatilisation.

Adsorption, leaching and volatilisation simply relocate the herbicide without changing
the chemical structure and so do surface runoff and spray drift discussed below.

Diagrammatic Representation of Herbicide Fate

(c) Surface Runoff

Herbicides can be washed off the soil and the target surface with rain water into
run-off that will transport them to rivers and other water bodies. Any negative
effects the pesticide has downstream on living and non-living things amounts to
environmental pollution. Some of the pesticides like atrazine that are stable may end
up in drinking water.

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 (d) Spray Drift

Judicious use of pesticides calls for the minimisation of off-target effects of the
application as the impact on neighbouring situations within the same landowner or a
different farmer may be unwanted. Chemicals such as the ester form of 2 4-D, very
prone to drifting, may harm crops like cotton, soybean, okra if they are in the near
neighbourhood. Spray drift is affected by a number of factors:

i. Droplet size –fine droplets are more prone to drifting than large ones
ii. Droplet velocity- very high spray pressure increases spray velocity
iii. Droplet discharge angle (trajectory) – less drift if spray is discharge at right
angle
iv. Sprayer height at which the droplets are released
v. Formulation of spray mixture- more viscous formulations are less prone to
drifting than lighter ones
vi. Ambient temperature at the time of spraying –drift increases as temperature
increases
vii. Humidity at the time of spraying – high relative humidity minimises spray drift
viii. Air turbulence – in calm weather, there is little or no spray drift
ix. Wind speed - spray drift increasing with increasing wind speed.

Therefore, spray drift is worse in circumstances with the highest intersection of pro-
drift factors as stated above. Spray drift is related to volatilisation, and both factors
cause pesticides to get into the atmosphere.

…………………………………………………………………………………………………………………………

[Note: italicised material is “copy-and-paste” from the book Fundamentals of

Weed Science [3rd Edition, 2007] by Robert L. Zimdahl]

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