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Herbicides are chemicals that inhibit or interrupt
All herbicide interactions with a plant, from
normal plant growth and development. They are
application to final effect, are considered the mode
used widely in agriculture, industry, and urban
of action. The herbicide mode of action involves
areas for weed management. Herbicides can provide
absorption into the plant, translocation or movement
cost-effective weed control with a minimum of
in the plant, metabolism of the herbicide, and the
labor. However, improper herbicide use can injure
physiological plant response. Herbicide site of action
crops, damage the environment, and pose a threat to
refers to the specific process in the plant that the
the applicator and others exposed to the chemical.
herbicide disrupts to interfere with plant growth and
Herbicide mode of action is how herbicides work.
Understanding how herbicides work provides insight
Herbicides may be classified according to
into how to use the chemicals and helps diagnose
selectivity (nonselective, grass control, broadleaf
performance problems and related injury symptoms.
control, etc.), time of application (preplant
The best source of information for herbicide
incorporated, preemergence, or postemergence),
use is the herbicide label. Always apply herbicides
translocation in the plant (contact or systemic),
according to label directions. Publications such
persistence, or site of action.
as the K-State Research and Extension Report of
Progress, Chemical Weed Control for Field Crops,
Herbicide Selectivity
Pastures, Rangeland, and Noncropland, also
The herbicide’s ability to kill certain plants
provide information on available herbicide options
without injuring others is called selectivity.
and application guidelines. The herbicide label,
Herbicides that kill or suppress the growth of
however, is a legal document, and an applicator is
most plant species are relatively nonselective.
responsible for applying the herbicide according to
Nonselective herbicide use is limited to situations
label directions.
where control of all plant species is desired, or by
Herbicides kill plants in different ways. A
directing the herbicide on the target weed and away
herbicide must meet several requirements to be
from desirable plants. Glyphosate and paraquat
effective. It must come in contact with the target
historically have been considered nonselective
weed, be absorbed by the weed, move to the site of
herbicides. However, glyphosate is a highly selective
action in the weed, and accumulate sufficient levels
herbicide when used in conjunction with crops that
at the site of action to kill or suppress the target
have been genetically engineered with resistance to
plant. Weed control is unsatisfactory unless these
requirements are met.

Most herbicides used in crop production are
waxy leaves. Sparse leaf pubescence or hairs may
selective. Herbicide selectivity is relative and
help retain spray droplets, but dense pubescence
depends on several factors, including environment,
can hold spray droplets above the leaf surface and
herbicide application rate, application timing, and
reduce spray contact with the leaf (Figure 1).
application technique. Even tolerant plant species
may be susceptible to a herbicide if the application
Growth Habit
rate is high enough. Herbicide selectivity may be
Grass plants tend to be more difficult to wet than
based on herbicide placement, differential spray
broadleaf plants because grasses often have narrow,
retention, absorption, translocation, metabolism, or
waxy leaves with upright orientation. The grass
an altered site of action.
leaf presents a small target, leaving a good chance
the spray droplet will roll off the leaf upon contact.
Herbicide Placement
Broadleaf plants may be easier to wet because they
Herbicide placement can be critical to
present a large target with some pubescence and
effectiveness and selectivity. Most small weed
horizontal orientation (Figure 2).
seeds germinate and emerge from the top ½ inch
of soil. Herbicides applied and positioned near the
soil surface will be most available for absorption by
shallow-germinating weed seeds. However, larger
Herbicide Droplets
seeded weeds that emerge from deeper in the soil
may not be controlled very well by a preemergence
Droplets Stay
On Leaf
herbicide unless it is incorporated or moved deep
Roll Off
enough into the soil by water movement. Selectivity
may be achieved by seeding the crop below the
herbicide-treated zone, especially if the herbicide is
root absorbed and relatively immobile in the soil.
Figure 2. Spray droplet retention on grass and broadleaf
leaves due to leaf orientation.

Spray Retention
Greater spray retention by a plant is likely to
Spray Adjuvants
result in more herbicide absorption. Spray retention
Spray adjuvants or additives often improve spray
depends on the properties of the spray solution
retention and absorption by reducing the surface
and the target plant. Leaf waxiness, pubescence
tension of the spray solution, allowing the spray
(hairiness), and orientation are among the
droplet to spread more evenly over the leaf surface
characteristics that affect spray retention. Waxy leaf
(Figure 3). Herbicide absorption may be further
surfaces repel water-based spray solutions, allowing
enhanced by interacting with the waxy cuticular layer
spray droplets to run off more easily than on less
on the leaf surface.
Droplet with
Droplet with
no surfactant
Epicuticular wax
Pubescent surface
Wax lamellae and
platelets embedded
in highly polymerized
Semi-polar cutin
Waxy surface
Pectin layer
Cellulose cell wall
Plasma membrane
Nonwaxy surface
Figure 3. Leaf surface composition and the in?uence
Figure 1. Spray droplet spread on a leaf surface as
of surfactants on droplet spread over the leaf surface
in?uenced by leaf pubescence and waxiness.
(adapted from Hull, Davis, and Stolzenberg).

Table 1. Scepter selectivity due to differential metabolism in different plant species (Shaner and Robson, 1985, Weed
Science 33:469-471).

Scepter remaining

in plants


Plant species

after 3 days
in plants (days)
Common cocklebur



Very Susceptible






Spray additives can increase weed control, but
potentially can reduce selectivity by increasing the
Plant Cell
spray retention and herbicide absorption by the crop
more than by the weed. Spray additives should be
used only if recommended on the herbicide label.
Refer to K-State Research and Extension publication
Spray Adjuvants with Herbicides, MF-1043, for
more information on the function and use of spray
Herbicide Metabolism
Metabolism is one of the most important ways
a plant can escape the toxic effects of a herbicide.
Herbicide-tolerant plants often have the ability to
metabolize or break down the chemical to nonactive
compounds before it can build up to toxic levels at the
Figure 4. Site exclusion type of herbicide resistance.
site of action. Susceptible plants are unable to detoxify
Atrazine is ineffective on resistant biotype because a
herbicides. Selectivity of many herbicides is based
conformational change in the chloroplast prevents it from
on differing rates of metabolism. Table 1 illustrates
binding at the site of action (adapted from Gonsolus).
differential metabolism and tolerance of Scepter
among soybeans, velvetleaf, and common cocklebur.
Research and Extension publication Questions &
Answers on Managing Herbicide-Resistant Weeds,

Altered Site of Action
MF-926, for more information.
An altered site of action can result in dramatic
resistance to a herbicide. An altered site of action
Herbicide Translocation in Plants
refers to genetically different plant biotypes that
Systemic herbicides are translocated in plants,
have a structurally altered site of action that prevents
while contact herbicides are not translocated. To be
herbicide binding and activity. An altered site of
effective, contact herbicides must be applied to the
action can be visualized using the lock-and-key
site of action. Most foliar-applied contact herbicides
concept illustrated in Figure 4. Altered site of action
work by disrupting cell membranes. Thorough spray
has been the basis for many herbicide- resistant
coverage of a plant is essential with foliar-applied
weed problems. Kochia resistance to atrazine or
contact herbicides to kill the entire plant.
Glean is an example of herbicide resistance due to
Contact herbicides generally are ineffective
an altered site of action. A small percentage of the
for long-term perennial weed control. Contact
original weed population is genetically different
herbicides damage the top growth that the spray
and contains the resistant trait. Repeated use of the
solution contacts, but the underground portion of
herbicide or herbicides with the same site of action
perennial plants remains unaffected and can rapidly
results in removal of susceptible biotypes, while
initiate new growth.
resistant biotypes increase until the weed population
Contact herbicides often are more effective on
is no longer controlled effectively with that group
broadleaves than on grasses. The growing point of
of herbicides. Weeds that are resistant to a specific
young grasses is located in the crown region of the
herbicide often are also resistant to other herbicides
plant, which is at or below the soil surface, and thus,
with the same site of action. Refer to K-State
difficult to contact with the spray. In contrast, the

growing point on young broadleaf plants is exposed
Factors Affecting
to the spray treatment. Thus, paraquat may not kill
Herbicide Activity
all the growing points of a tillered grass plant, and
Factors influencing herbicide activity include
regrowth can occur.
application rate, application technique, plant
Systemic herbicides can be translocated to other
maturity, and environmental conditions. In addition,
parts of the plant either in the xylem or the phloem
soil characteristics can affect soil-active herbicides.
(Figure 5). The xylem is nonliving tissue through
Moisture and temperature are environmental
which water and nutrients move from the roots to
factors that influence activity of soil-applied
the shoots and leaves of plants. Translocation in the
herbicides. Precipitation is essential to move
xylem is only from the roots to the leaves. Phloem
surface-applied or preemergence herbicides
is a living, conducting system in which materials
into the soil and activate them. Mechanically
can move both upward and downward. The phloem
incorporated herbicides tend to provide more
transports the food that is produced in the leaves to
consistent weed control than surface-applied
the roots and to areas of new growth (Figure 5).
herbicides because the herbicide is in place, and
Herbicides can be translocated in the xylem,
adequate moisture usually is present in the soil
the phloem, or both. Translocation depends on
to activate the chemical. However, incorporation
the chemical and the plant species. Herbicides
too deep may dilute the herbicide so weed control
translocated only in the xylem are most effective
is poor. Improperly adjusted equipment, or
as soil-applied or early postemergence treatments
incorporation when soils are too wet, may result in
because translocation is only upward. Atrazine is
streaking and poor weed control.
a good example of a herbicide that is translocated
Soil moisture is important because it influences
only in the xylem. Phloem translocated herbicides
herbicide adsorption to soils. Therefore, the
that move downward and suppress root and rhizome
herbicide is unavailable for plant uptake. Adsorption
growth, as well as top growth, provide the best
occurs when herbicide molecules adhere to soil
perennial weed control. Tordon, 2,4-D, Banvel, and
particles and organic matter. While adsorbed,
Roundup are examples of systemic herbicides that
herbicide molecules are unavailable for absorption
will translocate in the phloem and provide good,
by plants. Water molecules compete with herbicide
long-term control of certain perennial weeds.
molecules for adsorption sites on soil particles and
organic matter. Therefore, herbicide adsorption is
Foliar Contact
highest under dry soil conditions, and lowest in
moist soils. Consequently, weed control is generally
best with moist soil conditions because more
herbicide is available for plant uptake in the soil
solution or gaseous phase.
Temperature affects the activity of soil-applied
herbicides primarily because of its influence on the
Xylem & Phloem
rate of seed germination, emergence, and growth.
Seedling plants tend to be more susceptible to
soil-applied herbicides under cool conditions than
under warm temperatures because plant emergence
is delayed and metabolism is slowed. On the other
hand, extremely high temperatures sometimes
increase crop injury simply by placing the plant
under multiple stresses.
Soil characteristics affecting herbicide activity
are texture, organic matter, and pH. Herbicide
adsorption is greater in fine-textured soils high in
organic matter than in coarse-textured soils low in
organic matter. Thus, a lower proportion of herbicide
is available in the fine-textured soils, so a higher
Figure 5. Herbicide translocation in plants.
herbicide application rate is required to provide the
same level of weed control as in a coarse-textured
soil. At the same time, the chance of crop injury

is greater on coarse-textured soils low in organic
Herbicide Sites of Action
matter because a higher proportion of the applied
Herbicides can work at various sites in plants.
herbicide is available for plant uptake. Soil-applied
They generally interfere with a process essential for
herbicide rates usually need to be adjusted according
normal plant growth and development. Herbicides
to soil texture and organic matter content.
can be classified by site of action based on how they
Soil pH influences the availability and
work and the injury symptoms they cause. The Weed
persistence of certain herbicides in the soil. Soil pH
Science Society of America (WSSA) has developed
can alter the ionic nature of the herbicide molecule,
a numbered classification system based on the
which influences adsorption, solubility, and rate
herbicide site of action. Knowledge of herbicide sites
of herbicide breakdown. The triazine herbicides
of action can allow proper selection and rotation of
(atrazine, Sencor, and simazine) and some of the
herbicides to reduce the risk of developing herbicide
sulfonylurea herbicides (Amber, Finesse, Peak,
resistant weeds. Classification of herbicides by site
Exceed, Oust, and Classic) are more active and
of action and the WSSA classification number (in
more persistent in high pH soils (> 7.0) than in low
parenthesis) are described below.
pH soils. Refer to K-State Research and Extension
publication Residual Herbicides, Degradation, and
Growth Regulators (4)
Recropping Intervals, C-707, for more information.
Environmental conditions can have a two-

2,4-D, 2,4-DB (Butyrac)
fold effect on the performance of postemergence

herbicides. Higher humidity and favorable
temperatures generally result in greater herbicide
Benzoic acid

dicamba (Banvel, Clarity,
absorption and activity in plants.

Environment also influences herbicide efficacy
by affecting plant growth. Plants are generally
Carboxylic Acid
picloram (Tordon)

most susceptible to postemergence herbicides when

clopyralid (Stinger)

actively growing. Extreme environmental conditions

fluroxypyr (Starane)
that slow plant growth and thicken leaf cuticles often

triclopyr (Garlon, Remedy,
increase plant tolerance to a herbicide. Crop injury

from a herbicide, however, can increase during poor
growing conditions because of slower metabolism

quinclorac (Paramount)
and detoxification of the herbicide. Thus, if crop
tolerance is based on the ability of the crop to
Growth regulator herbicides are used primarily
rapidly metabolize the herbicide, the potential
for controlling broadleaf weeds in grass crops and
for crop injury may increase and weed control
pastures and include some of the more effective
decrease if a herbicide is applied when plants are
chemicals for perennial broadleaf weed and
not growing actively. For this reason, most herbicide
brush control. Most growth regulator herbicides
labels caution against application during extreme
are readily absorbed through both roots and
environmental conditions.
foliage and are translocated in both the xylem and
Annual plants are usually more susceptible to
phloem (Figure 5). Translocation of foliar-applied
herbicides when they are small than when they
treatments, however, is more restricted in grasses
are mature. As they mature, plants develop thicker
than in susceptible broadleaves.
wax layers on leaf surfaces, reducing herbicide
These herbicides are called growth regulators
absorption. In addition, it is harder to achieve
because they mimic natural growth hormones, and
thorough spray coverage on large plants than on
thus, upset the natural hormone balance in plants.
small plants.
Growth hormones regulate cell elongation, protein
Established perennial weeds tend to be more
synthesis, and cell division. The killing action of
susceptible to herbicides if applied during the early
growth-regulating chemicals is not caused by any
flowering stage of growth or to actively growing
single factor, but rather by the disruption of several
plants in the fall, probably because application at
growth processes in susceptible plants.
these times results in the greatest translocation of
Injury symptoms on susceptible plants treated
the herbicide to the roots. However, true seedlings
with growth regulator herbicides include growth
are much easier to control than established
and reproduction abnormalities, especially on
perennial weeds.

of absorption and action is the emerging shoot and
growing point. The mechanism of action of these
herbicides is not well understood, but they seem to
interfere with normal cell development in the newly
developing shoot.
Corn tolerance to the carbamothioate herbicides
increases with use of dichlormid safener (formulated
in Eradicane and Sutan+), which increases
metabolism of the chemicals to nontoxic substances.
Dichlormid is unique because it can be applied with
the spray formulation at low rates and selectively
protects only corn against herbicide injury.
Photo 8. Late applications of plant growth regulator
Repeated use of the carbamothioate herbicides on
herbicides such as dicamba and 2,4-D can interfere
the same field results in a buildup of microbes that
with pollination and seed production, sometimes called
break down the herbicides, decreasing their residual
headblasting in sorghum. Cold night temperatures during
life and period of weed control. This phenomenon
pollination can also cause poor grain ?ll.
is known as “enhanced degradation” or soil
vapor drift. Vapor drift increases as temperatures
conditioning. Repeated use of one carbamothioate
increase, and may occur as late as several days after
herbicide also conditions the soil for enhanced
degradation of the other carbamothioates. The best
The phenoxies are relatively short-lived in the
way to avoid enhanced degradation of herbicides is
environment and have small pollution potential.
to rotate to a different class of herbicides and avoid
Tordon is water soluble and persistent in the soil.
application of carbamothioates in successive years.
Consequently, Tordon has a high leaching potential
Injury symptoms on grass plants include failure
and should not be used on coarse-textured soils
of the shoot to emerge from the coleoptile or
with a shallow water table, where groundwater
whorl of the plant, giving the plant a buggy-whip
contamination is most likely to occur.
appearance. Susceptible grass seedlings often fail to
emerge from the soil. Injury symptoms on broadleaf
Seedling Growth Inhibitors
plants include enlarged cotyledons, restricted growth
The seedling growth inhibitors work during
of the true leaves, and a dark green color, a symptom
germination and emergence and include
sometimes referred to as bud seal. The roots become
three groups: 1) the seedling shoot inhibitors
short, thick, brittle, and club-shaped.
(carbamothioates), 2) the seedling shoot and root
inhibitors (acetamides), and 3) the microtuble
Seedling Shoot and Root Inhibitors (15):
assembly inhibitors (dinitroanilines).
alachlor (Lasso, Micro-Tech,
Seedling Shoot Inhibitors (8):


S-metolachlor (Dual MAGNUM)
EPTC (Eradicane, Eptam)

propachlor (Ramrod)

butylate (Sutan+)

dimethenamid (Frontier)

triallate (Far-Go)

acetochlor (Surpass, Topnotch,

Harness, Degree)
Carbamothioate herbicides are soil-incorporated

flufenacet (Axiom, Define)
for control of annual grasses and some broadleaf

P-dimethenamid (Outlook)
weeds. All are volatile and need to be incorporated
immediately after application to avoid excessive
Acetamide herbicides are used preemergence or
vapor loss. Vapor loss of the carbamothioate
with shallow soil incorporation to control annual
herbicides is less when applied to dry soils than
grasses and some broadleaf weeds in a variety
when applied to moist soils. The carbamothioates
of crops. The acetamides do not control emerged
are absorbed from the soil solution or vapor phase
plants. The primary site of absorption and action of
through both roots and emerging shoots, but are
these herbicides on broadleaf species is the roots,
translocated only in the xylem. The primary site
while the primary site of absorption and action on

grass species is the emerging shoot. The acetamides
The new shoots fail to emerge from the coleoptile
are not readily translocated in the plant, so herbicide
and whorl of the shoot of grass species (Photo 9).
placement and availability are important. As with
Susceptible germinating grasses often fail to emerge
the carbamothioates, the mechanism of action of the
from the soil. Injury symptoms on broadleaf species
acetamides has not been well defined, but appears
include general stunting and a drawstring effect
similar to the carbamothioates. These herbicides
around the margins of the true leaves (Photo 10).
affect various biochemical processes in the plant and
interfere with normal cell development.
Microtubule Assembly Inhibitors (3):
Dinitroaniline trifluralin (Treflan)

pendimethalin (Prowl, Pendimax)

ethalfluralin (Sonalan)

benefin (Balan)
Dinitroaniline herbicides are generally applied
preplant incorporated to control annual grasses
and some broadleaf weeds in many crops. Treflan,
Sonalan, and Balan need to be incorporated to avoid
photodecomposition and volatility losses. Prowl is
less volatile than the other dinitroaniline herbicides
and can be applied preemergence, but generally
provides better weed control when soil-incorporated.
Photo 9. Acetamide herbicides can cause emergence
The dinitroaniline herbicides are absorbed by both
problems and distorted shoots of grasses, such as this
unsafened sorghum that was treated with Dual. Only

roots and shoots of emerging seedlings, but are
Concep treated sorghum seed should be planted if
not readily translocated (Figure 5). The emerging
acetamide herbicides will be used (except Ramrod).
shoot is the primary site of absorption and action on
grass species. These herbicides are mitotic poisons
Lasso, Micro-Tech, Partner, Dual MAGNUM,
that inhibit cell division. Thus, the meristematic
Frontier, and Outlook may be used in sorghum if
regions, such as the growing points of stems and
the seed is treated with Concep seed protectant. The
roots, are most affected. Selectivity may be based on
seed protectant increases sorghum tolerance to the
metabolism, as well as herbicide placement and type
acetamide herbicides by increasing metabolism of
of emergence of the grass species.
the herbicide to inactive compounds.
Injury symptoms on grass species include
Injury symptoms caused by the acetamides
short, swollen coleoptiles. Injured broadleaf plants
are similar to those caused by the thiocarbamates.
often have swollen hypocotyls. Preemergence
pendimethalin sometimes causes callus formation
Photo 10. Acetamide herbicides sometimes cause minor
stunting and distorted leaves such as the heart-shaped

Photo 11. Preemergence pendimethalin can cause callus
leaf on this soybean plant, which resulted from Lasso
formation and brittle stems on soybeans, resulting in
injury following cold, wet weather during emergence.
breakage and lodging.

Photosystem II, Site C (6):

bentazon (Basagran)

bromoxynil (Buctril, Moxy)

pyridate (Tough)
Photosynthetic inhibitor herbicides control
many broadleaf and some grass weeds. All of these
herbicides work by disrupting photosynthesis, but
there are three different binding sites. Binding site
A includes the triazines, triazinones, and uracils,
Photo 12. DNA herbicides like Tre?an and Prowl can
cause poor root development and short, stubby roots.

binding site B includes the phenylureas, and binding
site C includes Basagran, bromoxynil and Tough.
and brittle stems near the soil surface, which
The triazines, triazinones, uracils, and
may break over during the growing season
phenylureas are soil-applied or early postemergence
(Photo 11). Both grasses and broadleaves may have
herbicides in crops and noncropland sites. These
short, stubby secondary roots (Photo 12). As a
herbicides are absorbed by both shoots and roots,
consequence, the plants may be stunted and exhibit
but are translocated only in the xylem (Figure 5).
nutrient deficiency or drought symptoms because of
Basagran, Tough, and bromoxynil are used
the poorly developed root system.
primarily as early postemergence treatments. They
Environmental and use considerations.
are contact herbicides that are not translocated in
The carbamothioates and dinitroanilines are
the plant (Figure 5). Thorough spray coverage of the
characterized by low solubility in water and high
foliage is essential for good weed control with these
adsorption to soils. Thus, they are not readily
leached or moved in water. The acetamides are more
These herbicides block photosynthesis, the food
soluble and less adsorptive, but less persistent in the
production process in plants. Plants are not affected
by the herbicide until after they emerge and begin
photosynthesis. Even though photosynthesis is
inhibited, susceptible plants do not die simply from
Photosynthetic Inhibitors
starvation. Herbicide injury symptoms appear too
quickly and are not typical of starvation. Instead,
Photosystem II, Site A (5):
susceptible plants treated with a photosynthetic
inhibitor die from a buildup of highly reactive

molecules that destroy cell membranes.

simazine (Princep)
The selective action of triazine herbicides is

ametryn (Evik)

primarily determined by differential metabolism.

prometon (Pramitol)
Plant species such as corn and sorghum possess
the glutathione-S-transferase enzyme and can

metribuzin (Sencor)

selectively metabolize triazine herbicides into

hexazinone (Velpar)
nontoxic substances. Crop and weed selectivity to
urea herbicides, such as Lorox, is due primarily

terbacil (Sinbar)
to herbicide placement rather than metabolism or

bromacil (Hyvar)

differential physiological tolerance of plant species.
Injury symptoms from soil-applied treatments
Photosystem II, Site B (7):
will not appear until after photosynthesis begins.
Susceptible broadleaf plants will exhibit interveinal

linuron (Lorox, Linex)
chlorosis and necrosis beginning around the leaf

diuron (Karmex, Diurex)
margins and progressing toward the center of the

tebuthiuron (Spike)
leaves (Photo 13). Susceptible grasses will become
chlorotic and necrotic beginning at the leaf tips and