Weed Management

which species will populate a particular field thefollowing season. Most weed species in Illinois agronomic
cropping systems are either broadleaves or grasses.
Broadleaf species are generally easier to differentiate
than grasses, especially at early growth stages. Many
excellent identification references are available,
including the several listed here; one or more should bepart of every weed management practitioner’s library.

Aaron Hager
Numerous plant species are considered weeds inagronomic cropping systems. Weeds have many
attributes undesirable to crop producers, not theleast being the ability to reduce crop yields throughcompetition for resources such as sunlight, water,
nutrients, and space. Weeds also may harbor insects
and provide a host for certain plant pathogens. Someweed species, such as wild garlic and eastern blacknightshade, can reduce the quality of the harvested
crop. Eliminating or reducing the deleterious effects ofweeds on agronomic crops is the ultimate goal of weed
management. Integrated weed management includes allpractices that enhance a crop’s competitive ability anddecrease weeds’ ability to reduce yield.

Weeds of the North Central States (B772). Available
from the University of Illinois(www.pubsplus.illinois.edu)
Weeds of the Great Plains (ISBN-10: 0939870002;
ISBN-13: 978-0939870004). Available from the
Nebraska Department of Agriculture, (402) 471-2394
Weeds of the Northeast (ISBN-10: 0801483344;
ISBN-13: 978-0801483349). Available from Cornell
University Press.

Successful weed management requires identifying
relevant species and understanding their biologicalcharacteristics so that management can be tailoredto the weeds present in individual fields. Accurate
identification is critical: identification of seedling weeds
is necessary for selecting an appropriate postemergenceherbicide, while identifying mature weeds often indicates
Annuals

Most weeds of agronomic cropping systems areherbaceous, but a few species that can become
established in reduced-tillage fields are woody (such as
maple trees). Weeds can be categorized according totheir life cycle, or how long they live: annual, biennial,
and perennial (Table 12.1). Knowledge of life cycles isimportant to reducing the potential for weeds to produce
Perennials

Biennials

WinterSummer

SimpleSpreadingbutterweedbarnyardgrassbull thistle

common milkweed

Canada thistle

common chickweedburcucumber

common burdock

curly dockhemp dogbane

downy bromecommon cocklebur

musk thistle

dandelion

Jerusalem artichokefield pennycresscommon lambsquarters

poison hemlock

field bindweedjohnsongrass
perennial sowthistle

henbitcommon ragweedteaselhedge bindweed

horseweed

crabgrasswild carrot

honeyvine milkweed

quackgrass

little barley

giant foxtail

horsenettle

swamp smartweed

prickly lettuce

giant ragweed

pokeweedtrumpetcreeper

purple deadnettle

green foxtail

smooth groundcherry

shepherd’s-purse

jimsonweed

yellow rocket

wirestem muhly
yellow nutsedge

kochiashattercane
smartweedsmooth pigweed
tall morningglory
velvetleaf
waterhemp
yellow foxtail

Illinois Agronomy Handbook

Table 12.1. Examples of weed species by life cycle.

1Table 12.2. Characteristics of weed life cycles.

Weed type

Duration oflife cycle

Overwintering
state

Method of
reproduction

Annual

1 yr

Seed

Seed

Biennial

2 yr

RosetteSeed

Perennial

>2 yr

Seed,
vegetative
propagule

Seed,
vegetative
propaguleviable seed or vegetative structures that aid in weed
dispersal (Table 12.2).
Annual plants complete their life cycle (from seed toseed) in one year; they are sometimes further divided
into winter annuals and summer annuals. Summer
annual weeds emerge in the spring, grow in spring andsummer, then flower and produce seed during latesummer or early fall (Figure 12.1). These species are themost common weeds that grow in agronomic crops.
Summer annual weeds can be controlled by various
soil-applied herbicides before they emerge; they areeasiest to control with post-emergence herbicides when
they are small (about 4 inches or less). In general, mostweeds become progressively harder to control withherbicides as they become larger.
Winter annual weeds emerge during late summer orfall, overwinter in a vegetative state, then flower andproduce seed the following spring (Figure 12.2). They
are common in fields where no tillage is done after
harvest and in fall-seeded small grains and forages.
Controlling winter annual weeds with herbicides may
be accomplished during late fall or early spring. It isbest to control all existing weed vegetation (including
winter and summer annuals) before planting corn orsoybean in the spring or before fall-seeding small grains
or forages.
Biennial plants complete their life cycle over two years.
Biennials emerge in the spring or summer, overwinterin a vegetative stage (often referred to as a rosette),
then resume growth the following spring (Figure 12.3).
Elongation of the flowering stalk (bolting) and seedproduction can vary by species; it occurs during thespring, summer, or fall of the second year. Biennial
2

Figure 12.1. Summer annual weed life cycle.​

Figure 12.2. Winter annual weed life cycle.​

Figure 12.3. Biennial weed life cycle​

Chapter 12: Weed Management

new growth, so most of the food at this time of year ismoving upward from the roots to support new vegetative
development. Because of this upward movement, it isoften difficult to get sufficient herbicide into the rootwhen applications are made in early spring.
Better control of perennial broadleaf species canbe achieved when postemergence translocatedherbicides are applied about the time the plants begin
to flower. Another good time to treat perennial weed
species is early to mid-fall. As day length shortens andtemperatures fall, perennial plant species begin to
move food back into their roots, and more translocatedherbicide moves to the root as well.
Figure 12.4. Perennial weed life cycle​

weeds are often best controlled with postemergenceherbicides during the rosette stage of growth. Theirsusceptibility to herbicides generally decreases rapidly
after the onset of bolting.

Figure 12.5 depicts a generalized representation of postemergence herbicide effectiveness on annual, biennial,
and perennial weeds as influenced by stage of weed
growth at application.

Perennial species live longer than two years—
theoretically, indefinitely (Figure 12.4). Some species
reproduce almost exclusively by seed and are referred toas simple perennials. Other species can reproduce by bothseed and various types of vegetative propagules (creeping
roots, rhizomes, tubers, etc.). These types of perennials arereferred to as creeping, or spreading, perennials.
Perennial weed species often become established in
no-till production fields and can cause great frustration
with respect to how best to control or eradicate them.
Without the option of mechanical weed control (i.e.,
tillage), perennial weed species are generally best
controlled with post-emergence translocated herbicides.
Which translocated herbicide is used, as well as when theapplication is made, can impact the success achieved.
Perennial weed species are frequently difficult tocontrol because they store food reserves in their rootsystems or underground storage structures. Controlling
only what is above ground is usually not sufficient forsatisfactory, long-term control; what is undergroundmust be controlled as well. Translocated herbicides(those that can move into the roots) are usually the mosteffective chemical option to control perennial weeds,
but when they are applied is very important. In thespring, perennials rely on stored food reserves to initiate
Illinois Agronomy Handbook

Figure 12.5. Postemergence herbicide effectiveness on annual,biennial, and perennial weeds as influenced by stage of weed growthat the time of application.​3

Weed Interference1
Weed management strategies attempt to limit thedeleterious effects weeds have when growing with crop
plants. Most common is competition with the crop foravailable growth factors (light, water, etc.). Whatever
quantities weeds use are unavailable for use by the crop. If
weeds can use a sufficient amount of some growth factor,
crop yield can be, and often is, adversely impacted.
Currently, the most common method of managing weeds
is herbicides. Many options are available, each with
distinct advantages and disadvantages. There are also
several methods by which herbicides can be applied.
Whatever the herbicide or method of application, thegoal is to prevent weeds from contributing to crop yield
loss by reducing the amount of competition exerted by
the weeds.

Figure 12.6. Emergence sequences for weed species common in
midwestern corn and soybean​

Scout agronomic production fields for weeds several
times each season. In no-till fields, determine which
winter annual or early-emerging summer annual
species are present prior to any herbicide application
so that herbicide selection and application rates canbe optimized for the species present before planting.
Knowing when weed species begin to emerge can vastly
improve your management program if you practicetimely scouting and subsequent control tactics. Weedemergence can, and often does, vary somewhat fromyear to year. Weeds such as smartweed and kochia
emerge during early spring, while morningglory species
can emerge during mid-summer (see Figure 12.6 foremergence sequences for weed species common in cornand soybean). Some species, such as velvetleaf, tend tohave a relatively short period of emergence, whereas
others, such as waterhemp, tend to emerge over a
relatively long part of the growing season.

The concept of competition between weeds and crops
has received a great deal of attention from farmers andherbicide manufacturers alike. A particular point ofinterest focuses on when competition (from weeds) should
be removed so that yields (of corn and soybean primarily)
are not adversely impacted. Soil-applied residual
herbicides can be used to eliminate any early-season weedcompetition, but some farmers would rather use only
postemergence herbicides to control weeds.
Is one method better than another at reducing weed
interference? What research is needed to determine how
and when competition reduces crop yield? How should
results of such research be interpreted?
Those involved in managing weeds have long recognized
their harmful effects on crop growth and productivity
through competing for light, moisture, nutrients, andspace as well as hampering harvest operations, reducing
quality of the harvested crop, and producing propagulesthat lead to future problems. Numerous experiments
over the years have compared weed species anddensities in various crops and assessed the importanceof the duration of competition and the time of weed
1 Some text in the “Weed Interference” section has been modified
from L. M. Wax, 1998, “Factors to consider when interpreting cropweed competition studies,” Proceedings of the Illinois Agricultural
Pesticides Conference (https://hdl.handle.net/2142/89666)

4

Chapter 12: Weed Management

removal. From those studies, some general guidelines
evolved regarding the relative competitiveness of weeds
with various crops, the weed-free time needed following
crop emergence, and the appropriate time of weed
removal with postemergence treatments to preclude
loss of crop quantity and quality. However, as tillage,
planting, and weed management practices have changed
over the years, the once-accurate guidelines regardingcrop–weed competition should be revisited, and insome instances modified, as new findings are reported.
The following text reviews crop–weed competition
research, both past and present, and offers guidelines forinterpreting related data.
Cropping and Cultural Practices.
Crops vary greatly in their ability to compete with weeds,
from providing essentially no competition to competingvery aggressively. This text focuses on the major field
crops of Illinois, corn and soybean. Early studies, witha variety of weed species, tended to show nearly equal
competitive ability of corn and soybean, with somedifferences. Very tall-growing weeds, if left for the entire
season, were sometimes less competitive in corn than insoybeans, mainly because they could overtop soybeans
and cause greater losses from shading. Weeds that rarely
grew taller than soybeans often caused less yield loss insoybeans than in corn, again due to the excellent shading
provided by a healthy stand of soybean.
Crop varieties and hybrids can vary substantially inresponse to weed competition, with those that canopy
earlier and provide more shading being the mostcompetitive. For the most part, this aspect has notbeen exploited to any great degree, but it is currently
being investigated in crops where a limited number of
herbicide options exist, such as sweet corn. A numberof studies have shown that increasing crop populations
within the row, up to a point, can increase thecompetitive ability of the crop, with no deleterious effecton crop growth or yield.
Row spacing and time of planting can greatly influencea crop’s competitive ability. Especially for soybean,
narrow row spacings have enhanced the ability tocompete with weeds, so that under current production
practices, soybean may be more competitive than
Illinois Agronomy Handbook

corn. When planted in wide rows, soybeans and corn
are probably closer to equal in their competitiveness.
Time of planting for both corn and soybeans is earlier
now than several decades ago, but this does not always
enhance competitive ability. Very early planting,combined with reduced or no tillage, allows for greater
weed competition as well as for a different suite of weed
species to be present than historically has been common.
Clearly, weeds that are established at the time of crop
emergence begin to compete with the crop earlier thanweeds that emerge only after the crop emerges.
With modern production practices and herbicides,
do corn and soybeans differ in the ability to competewith weeds? Conclusive evidence is lacking, but many
speculate that there is probably not much difference
in most instances. However, soybeans, especiallywhen vigorous varieties are grown at high populationsin narrow rows, usually have an edge over corn incompetitive ability, assuming that complete weed
control is achieved with herbicides prior to crop canopy
closure and that neither crop will be cultivated.
Weed Variables.
Weeds have been able to reproduce, survive, andcompete for centuries, at least partly due to theirdiversity. Species of weeds, and sometimes biotypes
within species, can vary greatly in growth habits
and ultimately in their ability to compete withcrops. Germination patterns differ markedly andsometimes erratically, causing differences in potential
for competition, which can vary from year to year.
Emergence and growth also vary from slow to even rapid
and almost unpredictable. Different species and biotypes
appear to respond differentially to various environmental
conditions—only some years are a so-called “nightshadeyear” (or smartweed year or nutsedge year), whereas
in most areas of Illinois, every year is a foxtail orvelvetleaf year. Most recent years could be described aslambsquarters and pigweed years, and few could disputethe increased prevalence of waterhemp years acrossmuch of the state.
Obviously, as demonstrated in many competition studies,
weeds produce markedly differing amounts of growth
per individual plant and reach widely varying heights.
5

These studies have allowed the development of relativecompetitive indices that can be somewhat helpful
in determining the severity of problems presented
by stands of various weed species. For example, it
obviously requires more foxtail plants than cockleburor giant ragweed plants to produce the same degree ofcompetition with corn or soybean.
The density or population of weeds required to cause
a consistent yield reduction in crops has been difficult
to establish. Many research studies have addressed
this issue and helped establish some of the thresholds
and guidelines currently available. In general, cornand soybeans can withstand low populations of weeds
throughout the season without suffering yield or harvest
losses; losses tend to increase linearly with increases
in weed population up to some population level above
which further yield reductions tend to subside (see Figure12.7 for an illustration of the impact of giant ragweed
density on soybean yield).
Establishing consistent thresholds or numbers of weedsthat cause a specific yield reduction is difficult across
many locations, years, and weather patterns. A synthesisof competition experiments conducted across several
states and over many years suggests that improved
techniques may be needed to establish and refine
thresholds, since variation across locations and years
almost always occurs and can be considerable. This should
not be surprising, and it is most likely due to differencesin environmental conditions, with special emphasis on
weather patterns. General threshold guidelines would bepossible, as long as a range of likely responses is given,and could cover a majority of situations.

Figure 12.7. The impact of giant ragweed density on soybean yield​

6

Lessons from Research. Numerous experiments
over the years have attempted to define the criticalduration of weed competition in corn and soybean
and to determine the optimal time to implement
weed management practices. One type of experimentis designed to determine the early-season weedfree interval needed before the crop can effectively
compete with later emerging weeds and then progress
independently for the remainder of the season, withno crop quantity, quality, or harvesting losses. Suchexperiments are especially useful in determining how
much time a soil-applied herbicide needs in order to beeffective after planting.
In general, for many of the weed species encountered incorn and soybean production systems of the northern
U.S., the interval ranges from 3 to 6 weeks, with 4 to
5 weeks being the most frequent range needed. It is
important to note that some of these studies initiatedthe interval at planting, while others began at crop
emergence (a potentially significant difference,
depending on the season and the weather). Most studies
were conducted with healthy crop stands in 30 to 40-inch
rows, with the objective being to obtain 4 to 5 weeks
without weed competition, after which the weeds were
kept under control by crop shading and one or more“lay-by” cultivations. In sharp contrast, cultivators are
not used today nearly as much as they once were, andweed management after crop emergence is administeredin the form of postemergence herbicides if soil-applied
treatments do not last sufficiently long.
It is also important to note that these rules of thumbwere developed with good crop stands and, for the mostpart, with the most common row crop weeds, most ofwhich tend to emerge fairly uniformly, not in multiple
flushes well into the season. As mentioned here andagain later, a review of available data indicates that inmost studies, there has been considerable variation fromyear to year, probably due to differing environmental
conditions, so it is very difficult, if not impossible, to set
a specific weed-free interval that is acceptable with all
species and across all locations and years.
Another factor to consider is that many of these studies
were conducted either by seeding unimbibed weed
Chapter 12: Weed Management

seeds at various times after crop planting or by removing
natural weed populations as needed for a specified
period. These two methods effect different results, andhow these results compare with a herbicide treatmentthat lasts the same amount of time is not defined. Doesa lower dosage of a herbicide still cause some growth
inhibition of later emerging weeds? These and otherunanswered questions suggest extreme caution about
pronouncing exact periods that are to apply over a wide
variety of conditions.
Another type of experiment is designed to determine
how long weeds can remain in the crop and eventually
be removed with no resultant deleterious effects on
quantity and quality of crop yield. In previous years thiswas important so that producers would know how early
one needed to cultivate between the rows, as manyolder herbicides were applied only in a band over therow. With the growing prevalence of broadcast, selective
postemergence herbicides, these types of studies became
relatively more important for providing guidelines intiming postemergence herbicide applications. Until fairly
recently, such competition studies were often conductedby growing various populations of weeds from crop andweed emergence until the weeds were removed either
mechanically or by hand. The weeds were removed
at some time after either crop planting or emergenceor until certain weed heights or stages. As a general
guideline, many of these studies tended to show that amoderate population of weeds could remain growing
with the crop for up 3 to 6 weeks after planting, and once
removed, cause little or no crop yield loss. These types ofexperiments also have considerable variation in results, so
again it is difficult to set specific intervals that will be valid
over widely diverse conditions.
In assessing these experiments, one needs to consider
the weed species involved and their respective
populations. In general, denser weed populations should
be removed earlier, while less dense populations canbe left to compete longer. From an applied standpoint,
a problem with many competition studies is that only
one weed species is considered, whereas producers’
fields often contain a number of species with varying
populations. Experience would suggest that moreemphasis should be placed on total weed biomass
Illinois Agronomy Handbook

present at crop flowering and fruiting as the bestindicator of loss likely to result from competition.
However, this is generally well past the stage whencontrol is possible or even feasible, and herbicides
undoubtedly should be applied before this stage inmost instances. To reiterate, these types of studies are
influenced greatly by the environment, which makes
establishing concrete intervals arduous.
The results of these experiments should also be closely
examined with respect to how the competition (weeds)
was removed. Some removed the weeds by hand but
allowed any weeds that emerged afterward to grow,while others were hand-weeded throughout the season
to simulate season-long control. Modern-day studies
tend to focus on controlling either single species or a
mixture growing at whatever population is present in thefield, by applying selective postemergence herbicides atvarious weed sizes or growth stages. In interpreting theresults of these studies, it is important to note whether
the herbicide(s) used possessed any soil bioactivity that
may have provided some control of weeds emerging
following application. Additionally, the population andmixture of weeds are important to note; of particularimportance is to note how effectively the weeds were
controlled. This is important since any yield reduction
noted and attributed to pre-application competition
stress might actually have been partially due to postapplication stress from weeds that were not controlled orfrom weeds that emerged after application.
Invariably, these experiments lead to a range of intervals
for weed removal that work effectively under various
conditions. Recommendations often tend to suggest
removing competition at the average or even slightly
earlier time because potentially adverse conditions
might cause delays in herbicide application, resulting
in weeds that would be very difficult to control. Thismay become especially important when dealing
with weed species where later emergence might be a
problem with herbicides that lack soil residual activity.
Under this scenario, the conservative approach might
involve adding a herbicide with soil residual activity
to the mixture. As will be noted in the next section,
environmental conditions can cause significant variationin the results of these types of experiments.

7

More than any other factors, soil and air temperatureand soil moisture and rainfall before, during, andafter initiation of competition experiments probably
contribute most to the variation in results. Even thebest-planned and best-conducted studies can varyconsiderably from location to location and year to year,
often because of environmental conditions. These
conditions affect weed emergence and growth, herbicide
effectiveness, the competitive interaction between crop
and weed, and the ability of the crop to recover fromearly weed competition once the weeds have been
removed. Primarily because of environmental conditions,
one should be very cautious in setting precise guidelines
for crop/weed competition, including thresholds fordensity, duration of weed-free intervals, and times ofcompetition removal. It would seem prudent to establish
ranges of densities, times, and the like and/or to operateon the conservative side in these matters.
The total effect of weeds on crop plants is more correctly
termed interference, which is the total of competition
plus allelopathy. Allelopathy (the suppression ofplant growth due to release of natural plant-derived
substances) can and has been demonstrated, but withmost of the soils and cropping situations in the Corn Belt,
it is thought to be relatively minor and is very difficult todemonstrate. Thus this discussion has focused primarily
on weed competition, which many consider significantly
more important because it deals with plants competingfor light, moisture, nutrients, and perhaps space.
However, in dense infestations of weeds (such as grasses
in corn), allelopathy could be a contributing factor toyield loss in addition to competition.
Competition for light may be one of the most importantfactors in reducing yields, especially with weeds thatgrow taller than the crop. Moisture stress, especiallyduring and after removal of a very dense population of
weeds, may be extremely important in how well the cropis able to recover. Many do not consider nutrient stress
to be as important in the rich, fertile soils across much of
the Corn Belt, but in coarse-textured soils and soils withlow fertility, it may be more significant. Some researchhas demonstrated that weeds can exhibit “luxury
consumption” of certain nutrients, such as nitrogen, tothe detriment of the crop.
8

Those involved with developing weed management
systems need to remember that the whole subject ofcrop–weed competition, while seemingly not simple,
is even more complex in the marketplace. The fact thatweed management decisions are made not only basedon true crop–weed competition but on other factors aswell is widely recognized. Yield and quality loss are notthe only issues being considered by decision makers.
Harvest difficulties and additions of weed seed to the soil
seedbank are genuine concerns often not addressed in
traditional competition research. Esthetic thresholds, as
related to landowner perceptions, often necessitate weed
control at much higher levels than what is required based
simply on yield losses. Product guarantees and respray
programs have also contributed to extraordinarily highlevels of weed management expectations.
In summary.
Numerous experiments have investigated crop–weed
competition from a variety of aspects. The results ofthese studies can be helpful to those making decisions
about weed management, as guidelines can be preparedthat indicate in general the relative competitive ability ofvarious weeds at various densities in the major crops of
the Midwest. These experiments also provide guidance
for the duration of weed-free conditions needed after
crop emergence and for when weeds should be removed
with postemergence herbicides. Other concerns, such as
producer, neighbor, and landlord perceptions, may be
as important as yield loss indications from crop–weed
competition studies in determining the types of weed
management systems implemented.

Weed Management Practices
Effective weed management practices include those that
reduce the potential for weeds to adversely impact crop
growth and yield. These practices often allow the crop
to utilize all available resources necessary to achieve
its yield potential. Weeds require many of the same
resources for growth as crop plants, and any resource
utilized by the weed is unavailable for use by the crop.
The most common weed management practices inIllinois agronomic crops include cultural, mechanical,
and chemical approaches.

Chapter 12: Weed Management

Cultural weed management practices allow thecrop to become established without experiencing any
negative effects of weed interference. Proper crop variety
selection and planting date, adequate soil fertility andpH, and crop row spacing are examples of factors thatcan be manipulated to improve the competitive ability of
the crop.
Mechanical weed management involves physicaldisturbance of the weeds, through activities including
pulling weeds, tilling the soil before or after weeds
emerge, and mowing.
Chemical Weed Control
Herbicides are often the primary tools of choice for weed
management across most acres of the Midwest. Manydifferent herbicides and herbicide formulations arecommercially available, including soil-applied and foliarapplied products, selective and nonselective products,
products with long soil persistence, and products with nosoil residual activity. The selection of which herbicide touse should be based on multiple factors, including soils,
cropping rotations, tillage practices, and weed species.
Sole dependence on herbicides may not necessarily
provide the most economical or sustainable weed
management. Integrating multiple practices reduces
the likelihood of poor weed control due to unfavorable
environmental conditions and reduces the intensity of
selection for herbicide-resistant weeds.
Product Labels. Every herbicide product commerciallyavailable is required by law to have a label. The label
provides a great deal of information about the product,
including how it is to be applied, where, and in whatquantity. The label is considered a legal document; usinga herbicide in a manner inconsistent with its labeling isillegal. Herbicide labels change frequently, so be sure to
consult the most current label when using a product. All
pesticide products for sale in Illinois must be registeredwith the state government.
Application Rates. Herbicides applied at labeled rates
should provide good weed control during the season ofuse while minimizing the potential for in-season crop
injury and carryover into the following season. Herbicide
application rates can vary according to many factors.
Illinois Agronomy Handbook

Rates for soil-applied herbicides are greatly influenced
by soil characteristics, such as organic matter content,
texture, and pH. In general, heavy-textured soils highin organic matter often require a higher application
rate than coarse-textured soils lower in organic matter.
Application rates of postemergence herbicides are often
determined by weed species and weed and crop size.
For some postemergence products, higher application
rates are suggested when certain weed species arepresent and/or when one or more weed species exceed a
specified height or number of leaves.
Often several different commercially available
formulations or premixes contain the same herbicide
active ingredient. Much of the following text will
demonstrate how to determine product equivalents andhow to calculate amounts of active ingredient applied.
Keep in mind that just because two or more products
contain the same ingredient(s) does not necessarilymean they are applied at the same rates. Always
consult the respective product label to determine theappropriate application rate.
Nomenclature. Across its lifetime a herbicide active
ingredient may be sold by one or more companies andidentified by one or more names. The three most common
categories of names are trade, common, and chemical.
Trade names. The trade name is the name under whicha product is commercially sold; it is often the name mostfamiliar to users. Examples of trade names include Valor,
Raptor, Yukon, Basagran, and Cobra. These names are
typically trademarked by the manufacturer so that noother company can use them. Trade names come and go,
and sometimes they are recycled (for example, Option
was once the trade name of a soybean herbicide butthen became the trade name of a corn herbicide). You
thus cannot always rely on the trade name to know what
active ingredient(s) a product contains.
Common names. Each common name is unique to aparticular active ingredient. Common names are listedon the product label, usually in the active ingredientsection. Flumioxazin, imazamox, halosulfuron plus
dicamba, bentazon, and lactofen are the common namesof the active ingredients contained in the commercial
9

products Valor, Raptor, Yukon, Basagran, and Cobra,
respectively. While more than one trade name may be
used for a particular active ingredient, common names
remain constant irrespective of trade names.
Chemical names. Herbicide chemical names may not
be as familiar as trade names or common names.
Like common names, a chemical name is unique to aparticular active ingredient, describing its chemicalcomposition. For example, Salvo is the trade name ofa herbicide with the active ingredient known by thecommon name 2,4-D, whose chemical name in turn is
2,4-dichlorophenoxyacetic acid.
Active Ingredients. The active ingredient of a pesticide
formulation is the component responsible for its toxicity(phytotoxicity in the case of herbicides) or its ability tocontrol the target pest. The active ingredient is always
identified on the pesticide label, either by common name(for example, atrazine) or chemical name (for example,
2,4-dichlorphenoxyacetic acid). The active ingredientstatement may also include information about how
the product is formulated and the amount of active
ingredient contained in a gallon or pound of formulated
product. For example, the Basagran label indicates thatthe active ingredient (bentazon) is formulated as thesodium salt, and 1 gallon of Basagran contains 4 pounds
active ingredient.
Usually when a herbicide trade name is followed bya number and letter designation (4L, 75DF, 7EC, etc.),
the number indicates the pounds of active ingredientin a gallon (for liquid formulations) or a pound (for dry
formulations) of the formulated product. So, for example,
Basagran 4L contains 4 pounds of active ingredient(bentazon) per gallon of formulated product, AAtrex 90DF
contains 0.90 pounds of active ingredient (atrazine) per
pound of formulated product, and Prowl 3.3EC contains
3.3 pounds of active ingredient (pendimethalin) per
gallon of formulated product.
Many herbicide labels restrict the maximum amount of
product to be used per application and/or per year. Thesemaximum rates are generally presented in terms of thetotal amount of active ingredient that can be applied peracre and/or per year. Several calculations can be used to10

determine the amount of active ingredient applied at agiven product use rate. This is one of the easiest:

So, if we apply this equation to Basagram 4L, the amountof active ingredient (bentazon) applied at 2 pints (0.25
gallon) of product per acre is:

Types of Formulation. There are several ways to define
formulation, but in essence it consists of the active
ingredient and all associated components that makeup the commercially available product. The active
ingredient is responsible for controlling target weeds,
but it rarely is the only component in a gallon or apound of commercial herbicide. Other ingredients servevarious functions, such as making the active ingredientsafer and easier to handle, allowing the active
ingredient to easily mix with water, and aiding herbicide
uptake through plant leaves. These other components
of a herbicide formulation are generally listed as inert
ingredients on the product label, although they haveimportant functions in making the active ingredientwork as intended.
Several types of herbicide formulations are available,
and a given herbicide active ingredient may beavailable in more than one formulation. Formulations
are often designated on product labels as single or
two-letter abbreviations. The more common herbicide
formulations, along with their abbreviations, arepresented in Table 12.3.
Acid equivalents. In some instances, the number preceding
the formulation designation (L, EC, DF, etc.) indicates
not pounds of active ingredient per gallon or pound, butrather acid equivalent per gallon or pound. Acid equivalent
may be defined as that portion of a formulation (as in
the case of 2,4-D ester, for example) that theoretically
could be converted back to the corresponding or
parent acid. Another definition is the theoretical yield
Chapter 12: Weed Management

Table 12.3. Common examples of herbicide formulations.

Type of formulation

Description of formulation

Flowable or aqueous suspension (F, L, or AS)

Liquid formulation containing finely ground solids suspended in a liquid

Water-soluble concentrate

Liquid formulations that form a true solution when added to water

Emulsifiable concentrate (EC or E)

Liquid formulation containing solvents and emulsifiers that disperse the active ingredient in water

Water-dispersible granule or dispersible granule(WDG or DG)

Dry formulation in which the active ingredient is sorbed onto aggregated granular particles

Dry flowable (DF)Dry formulation very similar to water-dispersible granules

Wettable powder (WP or W)

A finely ground dry formulation (often mineral clays) onto which the active ingredient is sorbed

Granule (G)Capsule suspension (CS)Dry formulation in which the active ingredient is coated onto an inert granule, ready to use without
diluting in a liquid carrier
A stable suspension of capsules in a fluid, normally intended for dilution with waterof parent acid from a pesticide active ingredient thathas been formulated as a derivative (esters, salts, andamines are examples of derivatives). For example, theacid equivalents of the isooctyl and ethyl acetate ester
formulations of 2,4-D are 66% and 88%, respectively. Why
would a herbicide (one that has the acid as the parent
molecule) be formulated as a derivative of the parent acid?
An illustration using 2,4-D follows.
The herbicide active ingredient 2,4-D, originallydiscovered in the 1940s, continues to show utility acrossa diversity of landscapes. The herbicide is a popular tool
among homeowners for selectively controlling certain
broadleaf weed species in turf, and it is frequently acomponent of burndown herbicide applications in notill agronomic cropping situations. Many commerciallyavailable 2,4-D formulations and trade names exist, but
not all formulations and products are identical.
One characteristic of 2,4-D-containing products ofparticular importance is the type of formulation. Mostoften, 2,4-D products are available as one of three
formulations: acid, amine, or ester. Each type has uniquecharacteristics that can influence where and how aparticular product is used.
Figure 12.8 illustrates the chemical structure of 2,4D. The molecule is considered a weak acid because
the carboxyl hydrogen atom (the one to the far right)
can dissociate, imparting a net negative charge to themolecule. In the dissociated (negatively charged) form,
Illinois Agronomy Handbook

the acid molecule is very soluble in water but is notreadily absorbed through a plant leaf. The waxy cuticle
that covers the leaf surface is composed of many noncharged substances that reduce the ability of a charged
molecule to penetrate and enter the plant. Somehow
altering the parent acid form can influence how quickly
and thoroughly it enters a plant through the leaf. These
alterations produce derivatives that have physical andchemical properties different from the parent acid, suchas increased ability to penetrate through a waxy leaf orincreased water solubility for enhanced root uptake. The
two most common derivatives of 2,4-D acid are aminesand esters.
Esters are formed by reacting the parent acid with an
alcohol, while amine salts are formed when the parent
acid is reacted with an amine. The isooctyl ester is a very
common ester formulation of 2,4-D, and the ammonium
salt is perhaps the most common amine formulation.
Other esters and amine salt formulations, however, arecommercially available.

Figure 12.8. 2,4 D parent acid.​11

Table 12.4. Comparisons between amine and ester formulations of 2,4-D.

Amine salt

Ester

High water solubility

Generally insoluble in water

Low solubility in oils and waxes

Higher solubility in oils and waxes

Slow absorption into plant leaves

Faster absorption into plant leaves

No or very low volatility potential

Low to high volatility potential

Clear or slightly amber colored in water

Milky when mixed in water

Does not mix well with liquid fertilizers

More compatible with liquid fertilizers

Less preferred formulation for no-till burndown applications

Preferred formulation for no-till burndown applications

Reduced probability of crop injury following postemergence application

Greater probability of crop injury following postemergence application

Preferred formulation for in-crop (i.e., corn) applications when air
temperatures exceed 85 °F

Less preferred formulation for in-crop (i.e., corn) applications when air
temperatures exceed 85 °F

As previously mentioned, these different types ofderivatives impart different characteristics to theformulation. For example, the isooctyl ester formulation
is more soluble in hydrophobic (“water-avoiding”)
substances, like waxes, while amines are more soluble
in hydrophilic (“water-loving”) substances. In practicalterms, esters are better able than amines to penetrate
the waxy leaf surface of weeds, whereas amines are
more easily moved into the soil by rainfall for root
uptake (an important characteristic in certain brushcontrol applications). Table 12.4 provides some generalcomparisons between the amine and ester formulationsof 2,4-D. These comparisons are somewhat relative
since the specific type of amine salt or ester chain
length can influence some characteristics. For example,
ester formulations are considered more volatile (thechange from a liquid state to a vapor state) than amine
formulations, but the actual volatility potential of theester formulation is influenced by the length of the ester
chain (the number of carbon atoms). Also, rememberthat different derivatives can impact the amount ofactive ingredient contained in a quantity of formulated
product. To accurately compare among various products,
calculations of “equivalency” should be based on theamount of acid equivalent contained in the formulation
rather than the amount of active ingredient. An example
follows of how to calculate acid equivalents, using ester
formulations of 2,4-D as examples.

2,4-D can be formulated as various esters. The chain
length of the ester can vary, but it is most commonly
eight carbon atoms long (isooctyl ester). For thisexample, consider two ester formulations of 2,4-D: thefirst has only two carbon atoms forming the ester, andthe second has eight carbons forming the ester. The
parent acid is the same in these two formulations; theonly difference is the length of the ester. These can bevisualized in several diagrams.
Figure 12.8 illustrates the parent acid of 2,4-D; Figure
12.9 shows the parent acid formulated with a two-carbon
side chain, and Figure 12.10 shows an eight-carbon side
chain. While the carbon atoms of the side chain may
modify some aspect of herbicide performance, it is theparent acid (Figure 12.8) that acts at the target site within
the plant. The additional carbon atoms of the ester side
chain add weight to the formulation and may increase
the amount of active ingredient of a formulation, butthese atoms do not increase the amount of parent acid in
the formulation. If these formulations were commercially

Figure 12.9. 2,4-D ethyl acetate ester.​

12

Chapter 12: Weed Management

The formula that can be used to calculate the amount ofacid equivalent in a gallon of formulated product is:

Figure 12.10. Isooctyl ester of 2,4 D.​available, and someone wanted to know how much of
the parent acid each contained, the calculation would bebased on the acid equivalents, not the active ingredients,of the formulations.
Assume that both the two-carbon and eight-carbon
ester formulations (Figures 12.9 and 12.10, respectively)
are commercially available and that each formulation
contains 4 pounds of active ingredient per gallon. The
application rate for both products is 1 pint per acre. Since
the application rates and the pounds of active ingredientper gallon are identical for the two formulations, theamount of active ingredient applied would be the samefor each. Verify this by performing the calculations
previously illustrated for determining the amount ofactive ingredient applied. Even though the amountsof active ingredient applied are the same for the two
formulations, the amounts of acid applied are not thesame. Remember, it is the parent acid that binds to thetarget site to control the weed; the ester portion of theformulation is not involved in binding to the target site.
What, then, is required to determine the amount of acidapplied (i.e., the acid equivalent)?
The first step is to determine the amount of acidequivalent in a gallon of formulated product. Some labels
indicate the amounts of both active ingredient and acidequivalent in a formulation, while others list only active
ingredient. If the pounds acid equivalent is specified
on the product label, all one need do to determine thepounds acid equivalent applied per acre is to substitutepounds acid equivalent for pounds active ingredient in
the equation presented previously for calculating thepounds active ingredient applied. For this example,
assume that neither 2,4-D label indicates the amount ofacid equivalent.

Illinois Agronomy Handbook

Some molecular weights (i.e., how much the moleculeweighs) are needed to complete these calculations. The
molecular weight of the parent 2,4-D acid is 221.04. The
molecular weight of the two-carbon ester formulation is
29.02 (weight of the two carbons and five hydrogens) +221.04 (weight of the parent acid) = 250.06. The
molecular weight of the eight-carbon ester formulation
is 333.25. The acid equivalent of the two-carbon ester
formulation is:Thus, the amount of acid equivalent in one gallon of
formulated product is:

The acid equivalent of the eight-carbon ester
formulation is:Thus, the amount of acid equivalent in 1 gallon of
formulated product is:Again, each product is applied at 1 pint (0.125 gallon) per
acre, and because each formulation contains 4 pounds
active ingredient per gallon, the amounts of active
ingredient applied are equal. The amounts of acid (thatpart of the formulation that actually controls the weed)
applied for each formulation are not equal.

13

The amount of acid applied per acre with the twocarbon ester formulation is:

The amount of acid applied per acre with the eightcarbon formulation is:

This example demonstrates that more 2,4-D acid isapplied with the two-carbon ester formulation thanwith the eight-carbon formulation. In practical terms,
more of the part of the formulation that actuallycontrols the weeds was applied with the two-carbon
ester formulation. To compare the herbicidally active
portion of two ester, salt, or amine formulations, productequivalents should be calculated on the acid equivalent.
If only one formulation of a salt or ester product iscommercially available, it would not really matter if
one calculated active ingredient or acid equivalent.
For example, Pursuit is formulated as the ammonium
salt of imazethapyr, but currently this is the only salt
formulation commercially available for use in agronomic
crops. There are, however, several commercial
formulations of 2,4-D and glyphosate. Not all ofthese formulations contain the same amount of acidequivalent, so to determine equivalent rates among
different formulations, calculations should be based onacid equivalent rather than active ingredient.
Since the commercialization of glyphosate-resistantsoybean varieties in 1996, the number of glyphosatecontaining products commercially available has
increased dramatically. More than 50 such products areregistered for use in Illinois agronomic crops, and that
number is expected to continue increasing. Keeping trackof product names and formulations can be daunting.

much acid equivalent (ae) does the formulation contain?
Should a spray additive (such as nonionic surfactant)be added to the tank, or does the formulation contain a“built-in” additive system? Are factors such as rain-freeinterval and toxicity category similar in the products
you are considering? Once these questions have beenanswered and you have narrowed down the list of
products you are interested in purchasing, how can you
compare costs? Should price comparisons be based
simply on cost per gallon of formulated product? As in
determining equivalent application rates, producers
should compare prices on an acid equivalent basis.
To compare prices among glyphosate-containing
products you need to do a few simple calculations. First,
determine what rate to apply based on weed spectrum
and size. For well-timed applications, a rate of 0.75 lb ae/acre can be very effective on many broadleaf and grass
species. Once you have determined the application rate,
calculate how many fluid ounces of each product areneeded for this rate. Next, convert the price per gallon foreach product to price per fluid ounce. Finally, multiply
the number of fluid ounces needed to achieve the 0.75 lb
ae/acre rate for each product by the cost per fluid ounce.
An example to illustrate these calculations follows.
You decide to apply a glyphosate-containing product at
0.75 lb ae/acre when most broadleaf weeds are 4 inchestall. You are deciding between two glyphosate-containing
products and want to know which offers the lowest cost
per acre (for purposes of this example, assume additive
requirements, if any are required by label, are identicalfor each product). “Glyfo A,” a potassium salt, contains
4 lb ae per gallon and costs $23 a gallon. “Glyfo B,” anisopropylamine salt, contains 3 lb ae per gallon and costs$21.75 a gallon.
Start by calculating how many fluid ounces are needed
for an application rate of 0.75 lb ae/acre:Glyfo A:

When selecting one of these products for weed control,
keep several important considerations in mind: How
14

Chapter 12: Weed ManagementGlyfo B:

Determining how many pounds of acid equivalent arecontained in a given formulation may seem the mostdaunting part of this exercise, but several references
are available that list the amount of acid equivalent inmany commercially available glyphosate formulations.
Table 12.5 compares a number of glyphosate-containing
products based on the amount of acid equivalent per
gallon. The table also lists the amount of product (influid ounces) needed to apply a range of acid equivalents(0.375-1.5 lb per acre).

Next, divide the price per gallon by 128 to determine
price per fluid ounce:
Glyfo A:

Herbicide isomers. Herbicide isomers may not be very
familiar to weed management practitioners, but they
are becoming increasingly common in the marketplace.
In essence, herbicide isomers are variations ofa molecule, put together in slightly different
arrangements. One isomer of a particular active
ingredient is generally much more herbicidally active
than the other isomer. A small amount of chemistry canhelp explain stereoisomers and how they are relevant intoday’s weed management arena.

Glyfo B:

Finally, multiply cost per fluid ounce by the number of
fluid ounces needed to achieve an application rate of
0.75 lb ae/acre:Glyfo A:A good starting point might be to define the term
stereoisomer. Stereoisomers are molecules that havethe same atoms bonded to each other but differ in
how the atoms are arranged in space. Figure 12.11 andFigure 12.12 will serve as examples for the following

Glyfo B:So, while a gallon of Glyfo A costs $1.25 more than agallon of Glyfo B, calculating costs on an acide equivalentbasis reveals that the per-acre cost is $1.13 less with Glyfo
A than with Glyfo B.

Figure 12.11. A 5-carbon ring with two chlorine atoms, one
positioned above the plane of the ring and the other below.​

Table 12.5. Glyphosate-containing herbicides.
Product rate equivalent to (lb acid equivalent/A)

Activeingredient/
gal

Acid
equivalent/
gal

0.375

4

3

16

24

0.56

0.75

1.13

1.5

32

48

64

fl oz

5

3.68

13

19.5

26

39

52

5.4

4

12

18

24

36

48

5.14

4.17

11.5

17

23

35

46

5.5

4.5

11

16

21

32

43

6.16

5

10

14

19

29

38

Illinois Agronomy Handbook

15

Figure 12.12. The same 5-carbon ring as shown in Figure 12.11, buthere both chlorine atoms are positioned above the plane of the ring.​

discussion. Figure 12.11 illustrates a 5-carbon ringmolecule with two chlorine atoms attached to it; one
chlorine atom is positioned above the plane of the ring,
while the other is positioned below. Figure 12.12 shows
the same 5-carbon ring with the same two chlorine
atoms, but here both chlorine atoms are positioned
above the plane of the ring. Each molecule contains thesame number of atoms—5 carbon and 2 chlorine—but
the spatial arrangement of the chlorine atoms differs,
which is what differentiates this pair of stereoisomers.
An analogy of stereoisomers is a person’s two hands;
each hand consists of the same components, but they
are assembled differently. You cannot rotate your righthand to make it a left hand, and vice versa.
So how is a differential orientation of atoms or
substituent groups (i.e., stereoisomers) relevant toweed management? Even though two molecules
may have the same types and numbers of atoms anddiffer only in the orientation of one or more atoms orgroups, differential orientations can greatly affect thebiological activity of the molecules. If, for example, themolecules depicted in Figure 12.11 and Figure 12.12
were herbicides, the orientation of the chlorine atoms
in Figure 12.11 might cause that isomer to bind much
more effectively at the herbicide target site within theplant, whereas the orientation of the chlorine atoms
in Figure 12.12 might not allow this isomer to bind thetarget site at all. One might reason that if the molecule
depicted in Figure 12.11 is more herbicidally active
than the molecule depicted in Figure 12.12, it would bebetter to manufacture or use a product containing the
Figure 12.11 molecule only. While this notion is valid, theprocess used to manufacture certain herbicides results
in a combination of isomers (that is, a mixture of the twomolecules) in the commercially available formulation. An
example of stereoisomer chemistry in weed management
is the active ingredient metolachlor.
16

Metolachlor first became commercially available during
the 1970s and was sold under the trade name Dual.
The process used to manufacture Dual resulted in two
isomers of metolachlor present in the commercial
formulation. One isomer, designated the S-isomer, ismuch more herbicidally active than the other, designatedthe R-isomer. Dual and the subsequent product DualII each contained a 50:50 mixture of the active (S) andinactive (R) isomers of metolachlor. (Dual became Dual
II when a safener was added to the original formulation
to reduce the potential for adverse crop response.)
Application rates for these “nonresolved” formulations
were determined based on this 50:50 mixture of active
and inactive isomers.
In the 1990s, improvements in technology allowed
manufacturers to increase the amount of active (S)isomer in a formulation, and Dual II became Dual II
Magnum. The “Magnum” formulations (Dual II Magnum,
Bicep II Magnum, Bicep Lite II Magnum) still containthe same active ingredient(s) as always, but they now
contain a higher proportion of the active or resolved (S)isomer compared with the older formulations (Dual andDual II, Bicep and Bicep II, Bicep Lite and Bicep Lite II).
Specifically, the Magnum formulations contain an 88:12
mixture of the active (S):inactive (R) isomers comparedwith a 50:50 mixture of the active (S):inactive (R) isomers
found in the Dual and Dual II formulations. So what is apractical implication of having a formulation containing
more of the active isomer? Since a higher proportion of
the active isomer is present in the Magnum formulations,application rates are reduced approximately 35%
compared with the original formulation.
Perhaps another illustration will be of value. Say,
hypothetically, you were to count out 100 moleculesfrom a container of Dual II and 100 molecules from acontainer of Dual II Magnum. Assuming the rules ofprobability hold, the 100 molecules of Dual II would be
50 active molecules (the S or resolved isomer) and 50
inactive molecules (the R or unresolved isomer). The 100
molecules of Dual II Magnum would be 88 active and 12
inactive molecules.
Assuming the unresolved isomer does not contribute
Chapter 12: Weed Management

Figure 12.13. A droplet taken from a container of a non- resolved
metolachlor-containing herbicide (50:50 mixture of S and R
isomers). Note the equal numbers of S and R letters.​

Figure 12.14. A droplet taken from a container of a re- solved
metolachlor-containing herbicide (88:12 mixture of S and R
isomers). Note the higher proportion of S letters relative to R letters.​

much to weed control, it takes less Dual II Magnum thaneither Dual or Dual II to obtain the critical number of
S-metolachlor molecules needed for weed control. For
example, if 50 molecules of S-metolachlor (the active
isomer) are needed to achieve control of a particular
weed species, how many total molecules of Dual/Dual II
and Dual II Magnum would you need in order to apply atleast 50 molecules of S-metolachlor? You would need 100
total molecules of Dual or Dual II (50:50 mixture) to get
50 molecules of S-metolachlor, whereas you would need
only 57 total molecules of Dual II Magnum (88:12 mixture)
to get 50 molecules of S-metolachlor. Stated anotherway, if you were to apply the same product rate of Dualand Dual II Magnum, you would apply less active isomer
per acre from the Dual formulation.

of S and R). Each circle contains the same number of
total molecules (designated S and R), but a different
proportion of S and R isomers.
This information should help those who purchaseherbicides made up of stereoisomers better understandsome of the differences among commercially available
products. Currently there are many metolachlor andS-metolachlor products on the market, and there
appears to be some confusion about product equivalents
among these many formulations. For example,
equivalent rates may be defined several ways, including
equivalent amounts of active ingredients, equivalentamounts of active isomers, or simply the rates allowedby the respective product label. These are not always
synonymous or interchangeable.

Figure 12.13 and Figure 12.14 illustrate this concept.
The circles represent equal volumes of herbicide. Figure
12.13 was taken from a container of a nonresolved
metolachlor-containing herbicide (50:50 mixture of S andR isomers) while Figure 12.14 was taken from a containerof a resolved metolachlor formulation (88:12 mixture

Table 12.6 lists several examples of products containing
metolachlor or S-metolachlor. One should not assume
that applying the same rate of each product necessarily
results in applying the same amount of active ingredientor active isomer. In particular, it should be noted

Table 12.6. Metolachlor- and S-metolachlor-containing herbicides.

Product

Activeingredient

Activeingredient/gal

R:S mixture(ratio)If you apply(product/A):

Lb/ai

Lb active isomerDualmetolachlor

8 lb

50:50

2.5 pt2.5

1.25

You have applied

Dual II

metolachlor

7.8 lb50:50

2.5 pt

2.43

1.218

Dual Magnum

S-metolachlor

7.62 lb

88:12

1.67 pt

1.59

1.399

Dual II Magnum

S-metolachlor7.64 lb

88:12

1.67 pt

1.59

1.403

“Generic I” brandmetolachlor

8 lb

50:50

1.67 pt

1.67

0.835

“Generic II” brandmetolachlor

7.8 lb

50:50

1.67 pt

1.62

0.814

Illinois Agronomy Handbook

17

Table 12.7. Metolachlor- and S-metolachlor-containing herbicides.

Herbicide

Components(ai/gal or lb)

If youapply/A:

You haveapplied (ai):Product
equivalents:

Bicep II Magnum 5.5L

S-metolachlor = 2.4 lb
atrazine = 3.1 lb

2.1 qtS-metolachlor = 1.26
lb atrazine = 1.63 lb

Dual II Magnum 7.64E = 1.32 pt
AAtrex 4L = 3.26 pt

Harness Xtra 5.6L

acetochlor = 3.1 lb
atrazine = 2.5 lb

2.5 qtacetochlor = 1.94 lb
atrazine = 1.56 lb

Harness 7E = 2.21 pt
AAtrex 4L = 3.13 pt

that while applying the same product rates of an
S-metolachlor-containing product and metolachlorcontaining product can provide similar amounts of total
active ingredient, the amounts of the active isomer
applied can vary considerably.

active ingredient. Note here that while application rates
of commercial products are usually expressed in ounces,pounds, pints, or quarts of product per acre, active
ingredients are usually expressed in units of pounds
active ingredient or acid equivalent per acre.

Herbicide premixes. Herbicide premixes are commerciallyformulated products containing more than one
herbicide active ingredient. Combining two or more
active ingredients in a formulated product can provide
several advantages, including a broader weed control
spectrum than any individual component has alone,
reduced potential for physical or chemical incompatibility
problems, and reduced cost compared with purchasing
the components separately and mixing them.

The last column lists product equivalents for each premix
component when applied at the application rate listed in
the third column. The 2.5-quart rate of Harness Xtra 5.6L
provides the same amount of acetochlor and atrazine
contained in 2.21 pints of Harness 7E and 3.13 pints ofAAtrex 4L, respectively.

Herbicide premixes can be confusing with respect tocomponents, product equivalents, application rates, andother factors. Table 12.7 compares two commerciallyavailable corn herbicide premixes used in Illinois. The
first column lists the trade name and formulation of theherbicide, and the second provides the common names
for the components. For example, Harness Xtra (tradename) 5.6L (formulation) is composed of the active
ingredients acetochlor (common name) and atrazine(common name). The second column also indicatesthe amount of active ingredient (or sometimes acidequivalent) of each component per gallon or pound of
formulated product.
The third column lists an application rate for eachpremix, and the fourth column indicates how much ofeach active ingredient is applied at that application rate.
For example, 2.5 quarts of Harness Xtra 5.6L provides
1.94 lb acetochlor active ingredient and 1.56 lb atrazine
18

The application rate of Harness Xtra 5.6L listed in Table
12.7 is 2.5 quarts per acre. Instead of 2.5 quarts, suppose
someone would like to know how much acetochlor andatrazine are applied at a 2-quart rate of Harness Xtra 5.6L.
First, convert 2 quarts to gallons:

Next, calculate how much acetochlor and atrazine active
ingredient are contained in 0.5 gallon of Harness Xtra 5.6L.

Finally, determine product equivalents based on these
active ingredient amounts:

Chapter 12: Weed Management

Principles of Soil-Applied Herbicides. Soil-applied
herbicides remain an important part of weed
management programs in corn and soybean production
systems. Early preplant (EPP), preplant incorporated(PPI), and preemergence (PRE) surface are the mostcommon types of herbicide applications to soil. EPP
applications are typically made several weeks prior
to planting and are more common in corn fields thansoybean fields. PPI applications were once very common,but they have declined in recent years with the growing
adoption of conservation tillage. PRE applications
are generally made within one week of crop planting.
Regardless of when or how a herbicide is applied tothe soil, the effectiveness of soil-applied herbicides isinfluenced by several factors.
For a soil-applied herbicide to be effective, it needs to beavailable for uptake by the weed seedling (usually before
the seedling emerges, but some soil-applied herbicides
can control small, emerged weeds under certain
conditions). Processes such as herbicide adsorption tosoil colloids or organic matter can reduce the amountof herbicide available for weed absorption. Soil-applied
herbicides do not prevent weed seed germination;rather, they are first absorbed by the root or shoot of theseedling and then exert their phytotoxic effect. Generally,
this happens before the seedling emerges from the soil.
For a herbicide to be absorbed by weed seedlings, theherbicide must be in the soil solution or vapor phase(i.e., an available form). How is this achieved? The mostcommon methods for herbicides to become dissolved
into the soil solution are by mechanical incorporation or
precipitation. EPP applications in no-till systems attempt
to increase the likelihood that sufficient precipitation will
be received before planting to incorporate the herbicide.
If, however, no precipitation is received between
application and planting, mechanical incorporation(where feasible) will, in most instances, adequately
move the herbicide into the soil solution. Herbicide thatremains on a dry soil surface after application may notprovide much effective weed control and is subject tovarious dissipation processes, some of them described insubsequent paragraphs.
Many weed species, in particular small-seeded ones,
germinate from fairly shallow depths in the soil. The
Illinois Agronomy Handbook

top 1 to 2 inches of soil is the primary zone of weed
seed germination and should thus be the target areafor herbicide placement. Shallow incorporation canbe achieved by mechanical methods or precipitation.
Which method is more consistent? Precipitation
provides for fairly uniform incorporation, but mechanicalincorporation reduces the absolute dependence on
receiving timely precipitation. How much precipitationis needed and how soon after application it should bereceived for optimal herbicide performance depends on
many factors, but generally 1/2 to 1 inch of rain within 7
to 10 days is sufficient.
Herbicides remaining on the soil surface, or those placed
too deeply in the soil, may not be intercepted by the
emerging weed seedlings. Herbicides on the soil surface
are subjected to several processes that reduce theiravailability. Volatility (the change from a liquid to gaseousstate) and photolysis (degradation due to absorption ofsunlight) are two common processes that can reduce theavailability of herbicides remaining on the soil surface.
Volatility potential is determined by several properties ofthe soil and the herbicide formulation, while photolysis isdependent primarily on herbicide properties.
Dry soil conditions are conducive for planting, butthey may also reduce the effectiveness of soil-applied
herbicides. If herbicide applications are made prior
to planting and no precipitation is received between
application and planting, a shallow mechanicalincorporation prior to planting may help preserve muchof the herbicide’s effectiveness.
Principles of Postemergence Herbicides.
Postemergence herbicides are a key part of an integrated
weed management program. Applications made after
crops and weeds have emerged allow for identifying
the weed species present and assessing the severity ofinfestation so that herbicide selection can be tailoredto the particular field. Postemergence herbicide
applications minimize the interactions of the herbicidewith factors associated with soil (such as texture
and organic matter content), but they often magnify
interactions between the herbicide and prevailingenvironmental conditions.

19To achieve weed control with postemergence herbicides,
the herbicide must come in contact with the target, beretained on the leaf surface prior to absorption into theplant, be able to reach the site of action within the plant,
and finally induce some phytotoxic response. If for any
reason one or more of these steps is restricted or limited,
the level of weed control can be expected to decline.
The plant cuticle serves as an outer protective layer,or “barrier,” that restricts the amount of water lost by
the plant through transpiration. It also serves a variety
of other functions, and the cuticle is often considered
the primary barrier that limits herbicide absorption.
The cuticle is composed primarily of waxes and cutin,
substances that effectively limit water movement out of
the leaf (transpiration) or into it (absorption). The typeand amount of wax that comprises the cuticle influences
the degree of wetting that can be achieved, and thiscomposition can change with plant age and in responseto changes in the environment. Older plants and plants
under environmental stress generally have more wax
or a different structure of the wax comprising theircuticles and are thus more difficult to wet. One of themain functions of certain spray additives is to enhanceherbicide penetration through the cuticle.
Plant age and size, relative humidity, soil moisture, andtemperature are other factors that influence absorptionof postemergence herbicides. Younger, smaller plants
usually absorb herbicide more rapidly than older, moremature plants. Many postemergence herbicide labels
recommend applications be made when target weeds
are small and caution about reduced effectiveness
if applications are made to larger plants. Labels ofpostemergence herbicides may also suggest thatusers delay applications if weeds are under “adverse
environmental conditions.” Examples of such adverse
environmental conditions may include prolonged
periods without significant precipitation (resulting in drysoil) or low air temperatures. On the other hand, highrelative humidity, adequate soil moisture, and moderate
to warm air temperatures all favor enhanced herbicide
absorption. Remember that if conditions occur thatenhance herbicide absorption into weeds, conditions
also are favorable for enhanced absorption into the crop,which may result in crop injury.
20

Postemergence herbicides vary in their mobility
within the plant. Some demonstrate very limited
movement following absorption and are commonlyreferred to as “contact” herbicides. Others can moveextensively within the vascular elements of the plant
and are referred to as “translocated” herbicides.
Contact herbicides do show some movement
following absorption, but they do not move nearly as
extensively as translocated herbicides. Thorough spraycoverage of the plant foliage is very important withcontact herbicides but somewhat less important withtranslocated herbicides.
Almost every postemergence herbicide has a preharvestinterval specified on the label or a crop developmental
stage beyond which applications should not be made.
Labels of some products indicate both a developmental
stage and a preharvest interval. A preharvest interval
indicates the amount of time that must elapse
between herbicide application and crop harvest. Suchintervals are established to allow sufficient time forthe herbicide to be broken down or metabolized in theplant. Additionally, the preharvest interval reduces
the likelihood of herbicide residue remaining on theharvested portion of the crop. Failure to observe thepreharvest interval may result in herbicide residue inthe crop in excess of established limits. In addition topreharvest intervals, there are restrictions on many
postemergence herbicides labels about whether thetreated crop may be used for livestock feed or whether
treated fields may be grazed as forage.
Another interval that is important to observe is therotational crop interval. Nearly all herbicide labels, bothsoil-applied and postemergence, list rotational cropintervals that specify the time that must elapse between
herbicide application and planting a rotational crop.
This becomes particularly important with late-season
herbicide applications. Such intervals are established toreduce the likelihood that sufficient herbicide residues
will persist in the soil that could adversely affect therotational crop. Some herbicide rotational restrictions
are based solely on time, while others are influenced
by different factors, such as soil pH and the amount ofprecipitation received after herbicide application.

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Additives for postemergence herbicides. Additives arecompounds added to a herbicide formulation or spraymixture that in some way modify the characteristicsof the spray solution. Additives either are included in
the commercial herbicide formulation or are added
to the spray mixture prior to application. Different
types of spray additives perform different functions,such as improving herbicide uptake into the target
vegetation, reducing the number of very small droplets
so as to reduce physical drift, and enhancing herbicide
performance on certain weed species. Some of themost common additives for postemergence herbicides
are nonionic surfactants (NIS), crop oil concentrates(COC), and ammonium fertilizer salts. These are used toincrease the effect of the herbicide on the target plants.
Nonionic surfactants lower the surface tension of spray
droplets, thus increasing spray coverage, so they are
frequently referred to as spreaders or wetting agents.
Herbicide labels often specify that the NIS should containa minimum of 75% to 80% active ingredient or otherwise
use a higher rate of NIS. NIS is usually applied at 0.5 to 1
pint per acre, or 0.125% to 0.5% on a volume basis.
Ammonium fertilizer adjuvants are added to increaseherbicide activity on certain weed species, including
velvetleaf. The two most common ammonium fertilizers
used are ammonium sulfate (AMS) and urea ammonium
nitrate (UAN) solution (28-0-0). AMS is used at 8.5 to 17
pounds per 100 gallons of spray solution. UAN is used at
2 to 4 quarts per acre, or 2% to 4% by volume. Contact
herbicide labels may specify that fertilizer adjuvants
replace NIS or COC, while translocated herbicides often
specify the addition of UAN or AMS to NIS or COC.
Crop oil concentrates are phytobland oils withemulsifiers added to allow mixing with water. The oil
may be of petroleum (POC) or vegetable (VOC) origin. Oils
increase spray penetration through the leaf cuticle. Most
herbicide labels allow POC or VOC, but some may specify
one or the other only. COCs are used at 1 to 2 pints peracre, or 0.5% to 1% by volume.
Compatibility agents are spray additives that improvemixing, especially for soil-residual herbicides that are
applied with a liquid fertilizer spray carrier. Herbicide
Illinois Agronomy Handbook

labels often specify a “jar test” to determine the need for
a compatibility agent when mixing herbicides with liquid
fertilizer. The rate is usually 1 to 4 pints per 100 gallons ofspray mix.
Drift reduction agents are added to the spray tankto reduce small droplet formation and thus minimize
drift potential. The use rate per 100 gallons of spray is
generally 2 to 10 fluid ounces of concentrated forms and
2 to 4 quarts of dilute forms (1% to 2% active ingredient).
Buffer-surfactants or buffer-compatibility agents
contain organic phosphatic acids that provide anacidifying effect on spray mixes where a pesticide isaffected by alkaline water. Most herbicides do not needa buffering agent, and some sulfonylureas should not be
acidified because herbicide degradation is accelerated.
How Herbicides Work. Herbicides are frequently
categorized into families according to various similarities.
Examples of classification categories include modeof action, application timing, and chemical structure.
Herbicide mode of action describes the metabolic or
physiological plant process impaired or inhibited by theherbicide. Essentially, mode of action refers to how the
herbicide acts to inhibit plant growth. Herbicide site ofaction describes the specific location(s) within the plant
where the herbicide binds. Site of action thus identifies
the herbicide target site within the plant. Though themost common herbicide classification schemes utilize
mode of action, much ambiguity exists with respect tothat herbicide classification.
While understanding herbicide mode of action isbeneficial, classifying herbicides by site of action
may be more useful from the standpoint of resistancemanagement. Herbicide resistance in plants is often
due to an alteration of the binding site in the target
plant. Selecting herbicides based on these different
binding sites or sites of action may provide for morereliable classification, in contrast with the ambiguityof classification based on herbicide mode of action,
whose systems include anywhere from seven to 13
different categories. Some of these systems describemode of action categories as “cell membrane disruptors,”“seedling growth inhibitors,” and “amino acid synthesis
21

Table 12.8. Weed species in Illinois that include herbicide-resistant biotypes and the herbicide families to which the biotypes are resistant.

Species
Scientific nameCommon lambsquarters

Chenopodium album

Triazine

Smooth pigweed

Amaranthus hybridus

Triazine, ALS inhibitors

Kochia

Kochia scoparia

Triazine, ALS inhibitors

Common waterhemp

Amaranthus rudis

Triazine, ALS inhibitors, PPO
inhibitors, glyphosate, HPPD
inhibitors, Auxins, VLCFA inhibitors

Giant ragweed

Ambrosia trifida

ALS inhibitors

Common ragweed

Ambrosia artemisiifolia

ALS inhibitors

Common cocklebur

Xanthium strumarium

ALS inhibitors

Shattercane

Sorghum bicolor

ALS inhibitors

Giant foxtail

Setaria faberi

ALS inhibitors, ACCase inhibitors

Horseweed

Conyza canadensis

Glyphosate, ALS inhibitors

Palmer amaranthAmeranthus palmeri

ALS inhibitors, PPO inhibitors,
glyphosate

inhibitors.” Selecting herbicides based on thesecategories could cause confusion among growers. For
example, the mode-of-action category “amino acidsynthesis inhibitors” would place the herbicides Pursuit(imazethapyr) and Roundup (glyphosate) in the samefamily, whereas classification by site of action would
place these two herbicides into two distinctly different
families, allowing growers to more accurately selectherbicides for resistance management.
The United Soybean Board Take Action publication
Herbicide Classification presents a color-coded herbicideclassification system based on 17 sites of action. The
system is intended to enhance growers’ ability toselect herbicides based on site of action, in order toslow further development of herbicide-resistant weed
biotypes. The table, reproduced here on the next page,
separates herbicide sites of action into 17 “primary”
colors. Herbicide chemical families sharing a particularsite of action are coded in shades of the same color. The
table also can be used to determine the sites of action ofindividual herbicide premix components.

22

Resistant to herbicide(s)Common nameWeed Resistance to Herbicides. Herbicide-resistantweed biotypes continue to plague farmers across muchof Illinois. Biotypes are populations within a species thatpossess characteristics not common to the species asa whole. In this case, the “uncommon characteristic” isresistance to a particular herbicide. Understanding how
herbicide resistance develops is an important initial
step in designing effective weed-management strategies
that deter the selection for resistant biotypes. Table 12.8
provides a listing of weed species in Illinois that havebiotypes resistant to particular herbicides.
The terminology used when discussing herbicide
resistance can be confusing. The most common terms aredefined as follows:
Herbicide resistance: the inherited ability of a plantto survive and reproduce following exposure to adose of herbicide normally lethal to the wild type.
Herbicide tolerance: the inherent ability of a plant
species to survive and reproduce after herbicide
treatment.
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Notice in the definition of resistance, the word “plant”
is used, whereas “species” is used in the definition oftolerance. Stated another way, a resistant plant is amember of a species that, as a whole, is susceptible to agiven herbicide. The resistant plant is a biotype of thatspecies that is no longer susceptible to the herbicide.
Tolerance implies that a species has never beensusceptible to a given herbicide.
Origin of resistance. To slow the selection of herbicideresistant weeds, one should have a basic understandingof how a resistant weed population develops. The
natural-selection theory is widely regarded as the mostplausible explanation for the development of resistance.
The theory states that herbicide-resistant biotypes
have always existed at extremely low numbers within
particular weed species. When a herbicide effectively
controls the majority of susceptible members of a
species, only those plants that possess a resistance trait
can survive and produce seed for future generations.
What is meant by “selection pressure” in regard to
herbicide-resistant weeds? Herbicides are used to control
a wide spectrum of weeds. By controlling susceptible
members of a weed population, we are essentiallyusing herbicides as agents to “select for” biotypes thatare naturally resistant to the herbicide. When most of
the susceptible members of a weed population arecontrolled, the resistant biotypes are able to continue
growing and eventually produce seed. The seed fromthe resistant biotypes ensures that the resistance trait
carries into future seasons. if the same herbicide is used
year after year, or several times during a single season,
the resistant biotypes continue to thrive, eventuallyoutnumbering the normal (susceptible) population. In
other words, relying on the same herbicide (or herbicides
with the same site of action) for weed control creates
selection pressure that favors the development of
herbicide-resistant weeds.
The development of a herbicide-resistant weed
population can be summarized by the following
principle: The appearance of herbicide-resistant weeds
is often favored by using a herbicide with a single site ofaction year after year or by repeating applications of aherbicide during the growing season to kill a specific weed
Illinois Agronomy Handbook

species not controlled by any other herbicide or in any
other manner. This principle has three key components:
1.
2.
3.

A herbicide with a single site of action.
Repeated use of the same herbicide.
The absence of other control measures

By understanding these components and developing
weed-control systems with them in mind, producers cangreatly reduce the probability that herbicide-resistantweeds will develop in their fields.
Management Strategies to Minimize HerbicideResistant Weeds. The best solution for minimizing
herbicide-resistant weeds is to reduce the intensity of
their selection. In the past, as new weed problems werediscovered, the usual solution has been to develop new
herbicides. Today, the high cost of developing a new
herbicide makes good management practices the best
method for dealing with herbicide-resistant weeds.
The following strategies may help slow selection for
herbicide resistance:
•

•

•

•

•

Scout fields regularly to identify resistant weeds.
Respond quickly to changes in weed populations
to restrict the spread of plants that may havedeveloped resistance.
Apply herbicides in tank-mixed, prepackaged, orsequential mixtures that include multiple sites ofaction. Both herbicides in the mixture must havesubstantial activity against potentially resistantweeds, as well as similar soil persistence.
As new herbicide-resistant and herbicide-tolerant
crops become available, their use should still not
result in more than two consecutive applicationsof herbicides with the same site of action against
the same weed unless other effective practices areincluded in the management system.
Combine mechanical control practices (such as
rotary hoeing, cultivating, and even hand weeding)with herbicide treatments for a near-total weedcontrol program.
Clean tillage and harvest equipment before movingfrom fields infested with resistant weeds to fields
that are not infested.

23•Railroads, public utilities, highway departments, andsimilar organizations using total-vegetation-control
programs should be encouraged to use practices that
do not lead to the development of herbicide-resistantweeds. Resistant weeds resulting from areas of total
vegetation control frequently spread to cropland.
Chemical companies, state and federal agencies, andfarm organizations can help in this effort.

Several criteria may be used to diagnose a herbicideresistant weed problem correctly:
•
•
•

•

All other causes of herbicide failure havebeen eliminated.
Other weeds on the herbicide label (besides the one
in question) were controlled effectively.
The field has a history of continuous or repeated useof the same herbicide or herbicides with the samemode of action.
The weed species was controlled effectively inthe past. Weed control in the field has been based
entirely on herbicides, without mechanical control.

With these management strategies and diagnosis criteria
in mind, how does one go about correctly identifying
a resistant weed population? We know that initiallyresistant weed biotypes are present at extremely low
frequencies within a particular population. It stands
to reason, then, that because of such a low initialfrequency, resistance will most likely first be noticed
within a particular field as a few individual weeds thatwere not controlled. In other words, resistant weeds do
not usually infest an entire field within 1 year. Typically,
the resistant weed population is initially confined tosmall, isolated patches. If the same herbicide-control
program is followed repeatedly, these patches begin toencompass a larger and larger proportion of the field,
until finally the resistant weeds appear as the dominant
species. Therefore, a producer who encounters an entire
field of resistant weeds has most likely had a resistantpopulation in the field for more than 1 year.
Crop Injury and Herbicides. Crop response, meaning
injury, caused by herbicides applied for in-crop weed
control can range from no visible response to nearlycomplete crop loss. Determining the reason or reasons24

for observed crop injury can be challenging, as several
interacting factors may contribute to the severityof response. If the cause is readily discernible, theexplanation and prognosis also may be straightforward,
but if multiple factors contribute to crop injury, the processof assessment and prognosis may become less precise.
Crop genetics can influence the degree of injury response.
Certain corn hybrids, for example, are sensitive to 2,4-D(and other herbicides, for that matter) and may exhibit agreat deal of injury following herbicide application. The
labels of many corn herbicides, especially postemergenceherbicides, have precautionary statements about
the potential for certain hybrids to be more sensitive
than others to a particular active ingredient. If you areconcerned that a particular hybrid may be sensitive toa certain herbicide or herbicide family, contact the seed
company representative for information.
If more than one formulation of a particular active
ingredient is commercially available, the choice of
formulation, especially for postemergence applications,also can influence the occurrence of corn injury. For
example, ester formulations of 2,4-D tend to be absorbed
through the leaf surface faster than amine formulations.
Applying 2,4-D esters postemergence with additives suchas COC, or tank-mixing herbicides with formulations thatcan “behave” similarly to a spray additive, can increase
the rate of 2,4-D uptake into the corn, potentially leading
to enhanced corn injury.
The environment has a large influence on the severityof crop injury symptoms from both soil-applied andpost-emergence herbicides. High air temperatures andrelative humidity levels favor enhanced absorption ofpostemergence herbicides. Adequate soil moisture levels
and low relative humidity can enhance uptake of soilapplied herbicides. Rapid herbicide absorption into thecrop plant may temporarily overwhelm the plant’s ability
to break down the herbicide, leading to injury symptoms.
Apart from enhancing herbicide uptake, environmentinduced crop stress can enhance crop injury fromherbicides. Cool air temperatures and wet soil conditions
are good examples of environmental conditions that can
induce stress. Why is a crop under stress more likely to be
Chapter 12: Weed Management

injured by a selective herbicide? In most cases, herbicide
selectivity arises from the crop’s ability to metabolize(break down) the herbicide to a nonphytotoxic form
before it causes much injury. For example, a grasscontrol herbicide used in corn cannot discriminate
between giant foxtail and a corn plant; the herbicide
attempts to control the corn just as it attempts tocontrol the giant foxtail. When the corn is growing underfavorable conditions, it rapidly metabolizes the herbicide
before excessive injury occurs. If, however, the corn plantis under stress (which could be caused by a variety offactors), its ability to metabolize the herbicide may beslowed enough that injury symptoms develop.
The herbicide itself can influence the severity ofcrop injury, and spray additives applied with apostemergence herbicide or tank-mix combinations
may enhance crop response. Always read all labelsuggestions and precautions related to spray additives
that should be either included or avoided when
applying herbicides postemergence.

Illinois Agronomy Handbook

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Chapter #: Weed Management