All About Herbicides

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Introduction to Weeds and Herbicides

College o Agricultural Sciences

Agricultural Research and Cooperative Extension



There are numerous denitions o a

weed. Some common denitions

include:

l a plant that is out o place and not

intentionally sown

l a plant that grows where it is not

wanted or welcomed

l a plant whose virtues have not yet

been discovered

l a plant that is competitive, persistent,

pernicious, and intereres negatively

with human activity

No matter which denition is used,

weeds are plants whose undesirable

qualities outweigh their good points,

at least according to humans. Human

activities create weed problems since

no plant is a weed in nature. Though

we may try to manipulate nature or our

own good, nature is persistent. Throughmanipulation, we control certain weeds,

while other more serious weeds may

thrive due to avorable growing condi-

tions. Weeds are naturally strong com-

petitors, and those weeds that can best

compete always tend to dominate.

Both humans and nature are involved

in plant-breeding programs. The main

dierence between the two programs is

that humans breed plants or yield, while

nature breeds plants or survival.

Abundant Seed Production

Weeds can produce tens or hundreds

o thousands o seeds per plant, while

most crop plants only produce several

hundred seeds per plant. The ollow-

ing are some examples o approximate

numbers o seeds produced per weed:

l giant oxtail—10,000

l common ragweed—15,000

l purslane—52,000

l lambsquarters—72,000

l pigweed—117,000

What Are Weeds and Their Impacts?

Characteristics o Weeds

There are approximately 250,000 spe-

cies o plants worldwide; o those, about

3 percent, or 8,000 species, behave

as weeds. O those 8,000, only 200 to

250 are major problems in worldwide

cropping systems. A plant is considered

a weed i it has certain characteristics

that set it apart rom other plant species.

Weeds possess one or more o the ol-

lowing characteristics that allow them to

survive and increase in nature:

l abundant seed production

l rapid population establishment

l seed dormancy

l long-term survival o buried seed

l adaptation or spread

l presence o vegetative reproductivestructures

l ability to occupy sites disturbed by

humans

Penn State Weed

manageent Web Site:

weeds.psu.edu

Some weeds, such as pigweed, can

produce several hundred thousand seeds

per plant under optimal growing conditions.



Since most weeds deposit their seeds

back to the soil, seed numbers in the

soil increase rapidly rom year to year

i the weeds are not managed. Despite

that many weed seeds are either not

viable, eaten by animals or insects, or

decompose within several months ater

they are deposited, hundreds o millions

o viable weed seeds per acre can still

be present and waiting to germinate.

Rapid Population Establishent

Most weeds can germinate and be-

come established relatively quickly.

They also produce viable seeds even

under environmental and soil conditions

that are not avorable or most crop

plants. Under ideal conditions, dense

weed populations can thrive and easily

outcompete a crop i let unchecked.

Under poor conditions, certain weeds

can adapt and produce some viable

seeds in a relatively short time period (6

to 8 weeks).

Seed Dorancy

Dormancy is basically a resting stage

or a temporary state in which the weed

seeds do not germinate because o

certain actors. Dormancy is a survival

mechanism that prevents germination

when conditions or survival are poor.

For example, seeds o summer annualweeds will generally not germinate in

the all, preventing them rom being

killed by cold winter conditions. The

various actors that aect dormancy are

temperature, moisture, oxygen, light, the

presence o chemical inhibitors, tough

seed coat, and immature embryos.

There are several kinds o dormancy,

but the most commonly used terms to

describe dormancy are innate, induced,

and enorced.

Innate or primary dormancy inhibitsgermination at the time seeds are shed

rom the plant. Ater the seed shatters

rom the parent plant, time is required

or immature embryos to develop, natu-

ral inhibitors to leach out, or extremes

o temperature to crack hard seed coats

and allow germination to occur. These

conditions cause innate dormancy, and,

once lost, this type o dormancy cannot

reoccur.

Induced dormancy is a temporary

dormancy that occurs when a seed is

exposed to hot or cold temperatures.

It continues ater temperatures change

and prevents germination during the

wrong time o year. The dormancy is

broken by temperatures opposite o

those that induced it.

Summer heat induces dormancy in sum-mer annual weeds such as yellow oxtail

and pigweed, preventing germination

in the all. Cold temperatures in all and

winter break this dormancy (usually by

mid-winter), and the seeds germinate

in spring when conditions are right. In

winter annual weeds, the process is

reversed.

Dormancy can be induced in many

weed seeds when a crop canopy lters

sunlight, shading the ground and reduc-

ing germination. Dormancy can beinduced over and over again or as long

as the seeds remain viable.

Enorced dormancy takes place when

environmental conditions—cold temper-

atures, lack o moisture or oxygen, and

occasionally a high salt concentration in

the soil—are unavorable. When limita-

tions are removed, seeds germinate

reely. Summer annual weed seeds lose

their induced dormancy by mid-winter

and, i not or the cold temperatures,would germinate at that time.

Seeds o dierent weed species have

various temperature requirements or

germination. Common chickweed can

germinate under snow cover, while

common purslane will not germinate

until the soil temperature reaches 70 to

75°F. Crop seeds are generally planted

at or near the optimum soil tempera-

ture needed or quick germination—a

temperature that is also ideal or some

weed seeds.

Seeds require water or germination.

Seeds in dry soils may remain dormant

even when all other actors promoting

germination are avorable.

Oxygen availability also infuences a

seed’s ability to germinate. Water may l

soil pores and exclude air, limiting ger-

mination in very wet soils. Soil compac-

tion also may reduce the oxygen supply

and prevent seeds rom germinating.

Deep plowing, tillage, or hoeing can

bring buried seeds to the surace, where

they readily germinate upon exposure to

oxygen.

Long-Ter Survival o Buried

Seed

I conditions are adequate, buried weed

seeds have the potential to remain

viable or 40 years or more. Broadlea

weed seeds tend to last longer in the

soil than grassy weed seed since they

usually have tougher seed coats. In

most cases, the majority o seeds only

exist in the soil or a ew years due to

germination, decomposition, predatoreeding, or other actors. However, with

the large number o seeds produced, a

small percentage may remain viable or

long-term survival.

Adaptation or Spread

Weeds have certain mechanisms or

easy dispersal o seeds. Most seeds

or seed pods have special structures

that allow them to cling, fy, or foat.

Common cocklebur and burdock seed

pods have hooks that attach to animal

ur or eathers; curly dock seeds have

bladder-like structures that allow them

to foat; and milkweed, dandelion, and

thistle seeds have a eathery pappus

that allows them to be carried by the

wind. Other weeds, such as jewelweed

or snapweed, have pods that “explode”

when the seeds are mature, projecting

them several eet rom the parent plant.

Weeds can also be spread when ani-

mals or birds eat their ruit and deposit

the seeds with their droppings. Weed

seeds can be widely spread through

crop seeds, grains, eed hay, and

straw. These and other human activities

probably account or the long-distance

spreading o weeds.



Vegetative Reproductive

Structures

Most perennial weeds possess special

vegetative structures that allow them to

reproduce asexually and survive. These

perennial structures contain carbohy-

drates (ood reserves, sugars), have

numerous buds in which new plants can

arise, and include the ollowing:

l stolons—aboveground, horizontal

stems that root at the nodes (e.g.,

crabgrass, bermudagrass, ground

ivy)

l rhizomes—belowground, thickened

stems that grow horizontally in the

upper soil layers (e.g., quackgrass,

Johnsongrass, wirestem muhly,

Canada thistle)

l tubers—enlarged rhizomes with

compressed internodes located at

the ends o rhizomes (e.g., yellow

nutsedge, Jerusalem artichoke, po-

tato)

l bulbs—modied lea tissues or car-

bohydrate storage that are located at

the base o the stem or below the soil

line (e.g., wild garlic, onion)

l budding roots—modied roots that

can store carbohydrates and grow

both vertically and horizontally (e.g.,

hemp dogbane, Canada thistle)

Despite these vegetative reproductive

structures, many perennials also repro-

duce by seed. Some depend heavily on

reproduction by seed (e.g., dandelion),

while or others it is less important (e.g.,

yellow nutsedge).

Ability to Occupy Disturbed Sites

Weeds are very opportunistic. When

conditions are adequate, weed seeds

germinate and colonize i let un-

checked. When a site is disturbed,

weeds are usually the rst to emerge. I

a weed becomes established rst, it has

the competitive advantage over crop

plants or desirable vegetation.

Probles with Weeds

Weeds are troublesome in many ways.

Primarily, they reduce crop yield by

competing or:

l water

l light

l soil nutrients

l space

l CO2

The ollowing are other problems associ-

ated with weeds:

l reducing crop quality by contaminat-

ing the commodity

l interering with harvest

l serving as hosts or crop diseases or

providing shelter or insects to over-winter

l limiting the choice o crop rotation

sequences and cultural practices

l producing chemical substances that

can be allergins or toxins to humans,

animals, or crop plants (allelopathy)

l producing thorns and woody stems

that cause irritations and abrasions to

skin, mouths, or hooves o livestock

l being unsightly, dominant, aggres-

sive, or unattractive

l obstructing visibility along roadways,

interering with delivery o public

utilities (power lines, telephone wires)

obstructing the fow o water in water

ways, and creating re hazards

l accelerating deterioration o recre-

ational areas, parking lots, buildings,

and equipment

l invading exotic weed species that

can displace native species in stabi-

lized natural areas

Quackgrass reproduces primarily by

shoots rom underground rhizomes, but it

can also reproduce by seeds.



Costs o Weeds

Weeds are common on all 485 million

acres o U.S. cropland and almost one

billion acres o range and pasture. Since

weeds are so common, people generally

do not understand their economic im-

pact on crop losses and control costs. In

2003, it was estimated that the nonuse

o herbicides and the likely substitutiono alternatives (i.e., cultivation, hand-

weeding) would result in a loss o $13.3

billion in ood and ber production.

The total impact o herbicide nonuse

would be an income loss o $21 billion,

which includes $7.7 billion in increased

costs or weed control and $13.3 billion

in yield losses. In the early 1990s, the

estimated average annual monetary loss

caused by weeds, with current control

strategies in the 46 crops grown in the

United States, was over $4 billion. Iherbicides were not used, this loss was

estimated to be $20 billion. Losses in

eld crops accounted or over 80 per-

cent o this total. Other sources estimate

that U.S. armers annually spend over

$3.5 billion on chemical weed control

and over $2.5 billion or cultural and

other methods o control. The total cost

o weeds in the United States could ap-

proach $15 to $20 billion. Weed control

and other input costs (e.g., seed, ertil-

izer, other pesticides, uel) vary with the

crop. For example, in the mid-1990s,

herbicides or soybeans cost about

$30 per acre, or about hal o the total

per-acre purchased input. For corn, the

cost was about $32 per acre, or about a

quarter o the total per-acre purchased

input. Weed control costs or wheat are

about $6 per acre, or about 5 percent

o the total per-acre purchased inputs. A decade later, these costs are about

the same. However, in most situations,

herbicide use is still the most economi-

cal means to control weeds. The USDA

estimates that weed control costs or

organic vegetable growers in Caliornia

can be $1,000 per acre in comparison to

$50 per acre that conventional growers

spend on herbicides. Several actors

help determine the relative costs o

herbicides rom one crop to another,

including the competitive ability o thecrop, the weeds present, the contribu-

tion o nonchemical control practices,

the tillage method, management deci-

sions, the type o crop seed used (e.g.,

normal versus resistant GMO variety),

and the value o the crop. Weeds not

only cause losses in crops, but also can

aect livestock production i poisonous

weeds are present or weeds invade and

render the pasture useless.

Benefts o Weeds

Despite the negative impacts o weeds,

some plants usually thought o as weeds

may actually provide some benets,

such as:

l stabilizing and adding organic matter

to soils

l providing habitat and eed or wildlie

l providing nectar or bees

l oering aesthetic qualities

l serving as a genetic reservoir or

improved crops

l providing products or human con-

sumption and medicinal use

l creating employment opportunities

Weeds have a controversial nature. But

to the agriculturist, they are plants thatneed to be managed in an economical

and practical way in order to produce

ood, eed, and ber or humans and

animals. In this context, the negative

impacts o weeds indirectly aect all

living beings.

Weeds reduce crop yield and quality and compete or necessary resources.



Origins o Weeds

Weeds are ound throughout the world.

However, all are not common in every

region. Since weeds can be easily

spread, more and more are being dis-

seminated to places where they were

not originally ound. Only about 40 per-

cent o the weeds ound in the United

States are native, while the remaining 60

percent are considered exotic or import-

ed. The ollowing are some examples o

weeds and their origins:

l United States—common and giant

ragweed, common milkweed, all

panicum, common cocklebur, poison

ivy, marestail (horseweed), night-

shade, wild or common sunfower,

and wild onion

l South America—pigweed species

and prickly sida

l Europe—quackgrass, chickweed,

Canada thistle, common lambsquar-

ters, common purslane, wild garlic,

and yellow oxtail

l Asia or Arica—Johnsongrass, wild

carrot, giant oxtail, velvetlea, kudzu,

and witchweed

Questions and Answers

Regarding Concerns about

Nonnative, Invasive Plants

Adapted rom Swearingen, J., K.

Reshetilo, B. Slattery, and S. Zwicker.

2002. Plant Invaders o Mid-Atlatic

Natural Areas. Washington, D.C.:

National Park Service and U.S. Fish &

Wildlie Service.

What Are Native Species?

A native species occurs naturally in a

particular place without human interven-

tion. Species native to North America

are generally recognized as those occur-

ring on the continent prior to European

settlement. Nonnative plants are species

that have been introduced to an area

by people rom other continents, states,

ecosystems, and habitats. Many non

native plants have great economic value

or agriculture, orestry, horticulture, and

other industries and pose little to no

threat to our natural ecosystems. Others

have become invasive and pose a seri-

ous ecological threat.

What Are Invasive Plants?

Invasive plants reproduce rapidly, spread

over large areas o the landscape, andhave ew, i any, natural controls, such

as herbivores and diseases, to keep

them in check. Many invasive plants

share some important characteristics

that allow them to grow out o control:

(1) spreading aggressively by runners or

rhizomes; (2) producing large numbers

o seeds that survive to germinate; and

(3) dispersing seeds away rom the par-

ent plant through various means such as

wind, water, wildlie, and people.

Weed Ecology and Biology

Invasive plants, such as exotic honeysuckles, are aggressive, displace native species,

reduce land value, and can be dicult and expensive to control.



How Are Invasive Plants

Introduced?

People introduce exotic plants to new

areas, on purpose and by accident,

through a variety o means. Some spe-

cies (e.g., kudzu, kochia, multifora rose,

Japanese knotweed, and Johnsongrass)

are introduced or use in gardening and

landscaping or or erosion control, or-

age, and other purposes. Others come

in unknowingly on various imported

products or in soil, water, and other

materials used or ship ballast. Many

invasive aquatic plants are introduced

by dumping unwanted aquarium plants

into waterways. Once established in a

new environment, some exotic species

prolierate and expand over large areas,

becoming invasive pests.

How Do Invasive Plants Spread?

Invasive plants spread by seed, vegeta-

tive growth (producing new plants rom

rhizomes, shoots, tubers, etc.), or both.

Seeds, roots, and other plant ragments

are oten dispersed by wind, water, and

wildlie. Animals spread invasive plants

by consuming ruits and depositing

seeds, as well as by transporting seeds

on their eet and ur. People also help

spread invasive plants by carrying seeds

and other plant parts on shoes, clothing,

and equipment and by using contami-nated ll dirt and mulch. Invasive aquatic

plants are oten spread when plant parts

attach to boat anchors and propellers.

Why Are Invasive Plants a

Proble in Natural Areas?

Like an invading army, invasive plants

are taking over and degrading natural

ecosystems. Invasive plants disrupt the

intricate web o lie or plants, animals,

and microorganisms and compete or

limited natural resources. Invasive plants

impact nature in many ways, including

growing and spreading rapidly over large

areas, displacing native plants (including

some very rare species), reducing ood

and shelter or native wildlie, eliminat-

ing host plants o native insects, and

competing or native plant pollinators.

Some invasives spread so rapidly that

they displace most other plants, chang-

ing a orest, meadow, or wetland into a

landscape dominated by one species.

Such “monocultures” (stands o a single

plant species) have little ecological value

and greatly reduce the natural biological

diversity o an area.

Invasive plants also aect the type o

recreational activities that we can enjoy

in natural areas, such as boating, bird

watching, shing, and exploring. Some

invasives become so thick that access-

ing waterways, orests, and other areas

is impossible. Once established, inva-

sive plants require enormous amounts

o time, labor, and money to control

or eliminate. Invasive species cost theUnited States an estimated $34.7 billion

each year in control eorts and agricul-

tural losses.

How to Prevent Spread o

Invasive Plants

Become amiliar with invasive plant spe-

cies in your area (Table 1). When select-

ing plants or landscaping, avoid using

known invasive species and those exotic

species exhibiting invasive qualities.

Ask or native plant alternatives at yournursery. Obtain a list o plants native to

your state rom your native plant society,

state natural resources agency, or the

U.S. Fish and Wildlie Service. I you

have already planted invasives on your

property, consider removing them and

replacing them with native species.

Table 1. List o selected invasive plant

species common to the Northeast. For

additional inormation about these and

other invasive plants reer to

www.invasive.org/eastern/midatlantic.

Aquatic Plants

Eurasian watermiloil ( Myriophyllum spicatum )

Hydrilla ( Hydrilla verticillata )

Water chestnut ( Trapa natans )

Herbaceous Plants

Garlic mustard ( Alliaria petiolata )

Japanese knotweed ( Polygonum cuspidatum )

Japanese stiltgrass ( Microstegium vimineum )

Purple loosestrie ( Lythrum salicaria )

Giant hogweed ( Heracleum mantegazzianum )

Bamboos, exotic ( Bambusa, Phyllostachys,and Pseudosassa species)

Spotted knapweed ( Centaurea biebersteinii )

Shrubs

Autumn olive ( Elaeagnus umbellata )

Bush honeysuckles, exotic ( Lonicera species)

Japanese barberry ( Berberis thunbergii )

Multifora rose ( Rosa multiora )

Privets ( Ligustrum species)

Winged burning bush ( Euonymus alata )

Butterfy bush ( Buddleja species)

Trees

Bradord pear ( Pyrus calleryana ‘Bradord’)

Norway maple ( Acer platanoides )

Tree o Heaven ( Ailanthus altissima )

Vines

Kudzu ( Pueraria montana v. lobata )

Mile-a-minute ( Polygonum peroliatum )

Oriental bittersweet ( Celastrus orbiculatus )

Porcelainberry ( Ampelopsis brevipedunculata

Japanese honeysuckle ( Lonicera japonica )



Classifcation o Weeds

Almost all plants are categorized by

some sort o plant classication system

and given a scientic name to identiy

them anywhere in the world. Weeds

are also classied by various means. In

general, they can be classied by their

structure and appearance (or example,dicots [broadleaves] and monocots

[grasses and sedges]), habitat, or physi-

ology. A common categorization system

groups them according to their lie cycle

(how long they live). The three major lie

cycle groups are annuals, biennials, and

perennials.

Annuals

Annuals are generally divided urther

into summer annual and winter annual

weeds. Summer annuals germinatein the spring, mature, produce seed,

and die in one growing season. Large

crabgrass, giant oxtail, smooth pig-

weed, common lambsquarters, common

ragweed, velvetlea, hairy galinsoga,

and common purslane are examples o

troublesome summer annuals.

Winter annuals germinate in late sum-

mer or all, mature, produce seed, and

then die the ollowing spring or sum-

mer. Examples o winter annuals includecommon chickweed, henbit, shep-

herdspurse, downy brome, and annual

bluegrass. (Some annual bluegrass

subspecies can occasionally unction as

a perennial.)

Biennials

Biennial weeds grow rom seed any-

time during the growing season. They

normally produce a rosette o leaves

close to the soil surace the rst year,

then fower, mature, and die during

the second year. A true biennial never

produces fowers or seeds the rst year.

There are relatively ew biennial weeds.Some examples include wild carrot,

common burdock, bull and musk thistle,

and poison hemlock.

Perennials

Perennial weeds live or more than

two years and can be divided into two

groups: simple and creeping. Simple

perennials orm a deep taproot and

spread primarily by seed dispersal.

Some examples o simple perennials

include dandelion, broadlea plantain,

curly/broadlea dock, and commonpokeweed. Creeping perennials may be

either herbaceous or woody and can

spread by both vegetative structures

as well as by seed. Some common

herbaceous perennials include Canada

thistle, common milkweed, hemp

dogbane, creeping buttercup, slender

speedwell, ground ivy, quackgrass, and

yellow nutsedge. Some examples o

woody perennials include poison ivy,

multifora rose, Japanese knotweed/

bamboo, brambles, wild grape, and

Virginia creeper. Creeping perennials

become established by seed or by veg-

etative parts. Since perennial weeds live

indenitely, their persistence and spread

is not as dependent upon seed as the

other two weed groups.

Common chickweed can be a problem

in eld crops, gardens, lawns, and many

other areas.

Biennials, such as wild carrot, are con-

trolled more easily during their rst year o

growth.

For perennial weed control, the best

time to either mow or apply an eec-

tive herbicide is during the bud to bloom

growth stage and/or in the all.



Since weeds are so prevalent in many

areas o the landscape, management

techniques are necessary to maintain

order. Weed management is most suc-

cessul when it involves an integrated

approach using a variety o methods.

The common methods used to manage

weeds include prevention and cultural,

mechanical, biological, and chemical

means.

Weed Management Techniques

Prevention

Preventative methods are used to stop

the spread o weeds. Preventing the

introduction o weeds is usually easier

than controlling them ater establish-

ment. Preventative practices include

cleaning tillage and harvesting equip-

ment o weed seeds and vegetative

structures; planting certied, weed-ree

crop seed; and controlling weeds in

barnyards, around structures, and along

encerows, roadways, and ditch banks.

Cultural

Cultural and crop management tech-

niques provide a healthy crop to best

compete with weeds. Crop competition

can be an inexpensive and eective

aid to weed management i used to its

ullest advantage. Examples o cultural

techniques include ollowing soil test

recommendations or ertilizer and

lime; selecting the best crop varieties;

planting dense crop populations at the

proper timing; scouting elds regularly

or weeds, insects, and diseases and

controlling them when necessary; and

including crop rotations in the system.

Composting, ensiling, or eeding weeds

or weed-inested crops to livestock can

destroy the viability o weed seeds. The

heat and/or digestive acids break down

the majority o weed seeds. However,

some seeds pass through livestockunharmed and can germinate i spread

back onto the land.

Preventing weed spread includes controlling weeds around barns and along ences, roads,

ditches, and woodlands.



mechanical

Mechanical or physical techniques either

destroy weeds or make the environment

less avorable or seed germination and

weed survival. These techniques include

hand-pulling, hoeing, mowing, plowing,

disking, cultivating, and digging. Mulch-

ing (straw, wood chips, gravel, plastic,etc.) can also be considered a mechani-

cal control means since it uses a physi-

cal barrier to block light and impede

weed growth.

Biological

Biological weed control involves the

use o other living organisms, such as

insects, diseases, or livestock, or the

management o certain weeds. In theory,

biological control is well suited or an

integrated weed management program.

However, the limitations o biologicalcontrol are that it is a long-term under-

taking, its eects are neither immediate

nor always adequate, only certain weeds

are potential candidates, and the rate o

ailure or past biological control eorts

has been airly high. There have been

a ew success stories o weed species

being managed with insect or disease

biocontrol agents. Herbivores such as

sheep and goats can provide success-

ul control o some common pasture

weeds. Research continues in this area

o weed management.

Cheical

Herbicides can be dened as crop-

protecting chemicals used to kill weedy

plants or interrupt normal plant growth.

Herbicides provide a convenient,

economical, and eective way to help

manage weeds. They allow elds to be

planted with less tillage, allow earlierplanting dates, and provide additional

time to perorm the other tasks that arm

or personal lie require. Due to reduced

tillage, soil erosion has been reduced

rom about 3.5 billion tons in 1938 to

one billion tons in 1997, thus reduc-

ing soil rom entering waterways and

decreasing the quality o the nation’s

surace water. Without herbicide use,

no-till agriculture becomes impossible.

However, herbicide use also carries risks

that include environmental, ecological,

and human health eects. It is important

to understand both the benets and

disadvantages associated with chemical

weed control beore selecting the ap-

propriate control.

Herbicides may not be a necessity on

some arms or landscape settings,

but without the use o chemical weed

control, mechanical and cultural control

methods become that much more

important. There are many kinds o

herbicides rom which to choose. Manyactors determine when, where, and how

a particular herbicide can be used most

eectively. Understanding some o these

actors enables you to use herbicides to

their maximum advantage.

Tine weeders and cultivators can be used to control weed seedlings.



Herbicides can be classied several

ways, including by weed control spec-

trum, labeled crop usage, chemical am-

ilies, mode o action, application timing/

method, and others. For this publication,

herbicides will be grouped according to

mode and site o action, which are also

important in understanding herbicide

resistance in weeds.

Contact herbicides kill only the plantparts contacted by the chemical, where-

as systemic herbicides are absorbed

by the roots or oliage and translocated

(moved) throughout the plant. Herbicide

activity can be either selective or non-

selective. Selective herbicides are used

to kill weeds without signicant dam-

age to desirable plants. Nonselective

herbicides kill or injure all plants present

i applied at an adequate rate.

A common method o grouping her-

bicides is by their mode o action.

Although a large number o herbicides

are available in the marketplace, several

have similar chemical properties and

herbicidal activity. Herbicides with a

common chemistry are grouped into

“amilies.” Also, two or more amilies

may have the same mode o action,

and thus can be grouped into “classes.”

Table 2 lists several groups o herbicides

and inormation related to their mode o

action.

The ollowing section provides a brie

overview o herbicide unctions in the

plant and associated injury symptoms

or each o the herbicide classes ound

in Table 2.

Herbicides

Herbicide mode and Site o

Action

To be eective, herbicides must (1) ad-

equately contact plants, (2) be absorbed

by plants, (3) move within the plants to

the site o action without being deacti-

vated, and (4) reach toxic levels at the

site o action. The term “mode o action”

reers to the sequence o events rom

absorption into plants to plant death, or,

in other words, how an herbicide works

to injure or kill the plant. The specic

site the herbicide aects is reerred to

as the “site or mechanism o action.”

Understanding herbicide mode o action

is helpul in knowing what groups o

weeds are killed, speciying application

techniques, diagnosing herbicide injury

problems, and preventing herbicide-

resistant weeds.

Herbicides provide a convenient, economical, and eective way to control weeds.



Table 2. Important herbicide groups and examples or agronomic and horticultural crops, tur, orestry, and industrial areas in

Pennsylvania.

Mode o Action

(Class) Site o Action WSSA Group Family Active Ingredient Trade Name(s)*

Plant growth IAA like 4 phenoxy 2-4-D variousregulators 2,4-DB Butyrac(PGR) MCPA various

MCPP (mecoprop) various

4 benzoic acid dicamba Banvel, Clarity, Distinct,

Vanquish

carboxylic acid (pyridines) aminopyralid Milestoneclopyralid Stinger, Lontrel, Translinefuoxypyr Staranepicloram Tordontriclopyr Garlon

auxin transport 19 phthalamates naptalam Alanapinhibitors semicarbazones difuenzopyr component o Distinct,

Status

Amino acid ALS enzyme 2 imidazolinone imazapic Plateaubiosynthesis imazamox Raptorinhibitors imazamethabenz Assert

imazapyr Arsenalimazaquin Scepter

imazethapyr Pursuit

sulonylurea chlorimuron Classicchlorsuluron Glean, Telaroramsuluron Optionhalosuluron Permit, Sandea,

SedgeHammeriodosuluron Autumn, Equipmetsuluron Cimarron, Escortnicosuluron Accentprimisuluron Beaconprosuluron Peakrimsuluron Matrix, Resolvesulometuron Oustsulosuluron Maverickthiensuluron Harmony GT

triasuluron Ambertribenuron Express

sulonylaminocarbonyl- fucarbazone Everesttriazolinones propoxycarbazone Olympus

triazolopyrimidine cloransulam FirstRate(sulonamides) fumetsulam Python

EPSP enzyme 9 amino acid derivative glyphosate Roundup, Touchdown,(glycines) Accord, Honcho,

many others

Fatty acid (lipid) ACCase enzyme 1 aryloxyphenoxy clodinaop Discoverbiosynthesis propionates dicloop Hoeloninhibitors enoxaprop Acclaim, Puma

fuaziop Fusiladequizaloop Assure II

cyclohexanediones clethodim Select, Prismsethoxydim Poast, Vantagetralkoxydim Achieve

phenylpyrazoline pinoxaden Axial

continued on next page



Table 2. continued

Mode o Action


Seedling growth microtuble inhibitors 3 dinitroanilines benen Balaninhibitors ethalfuralin Sonalan, Curbit (root and shoot) oryzalin Surfan

pendimethalin Prowl, Pre-M, Pendulumprodiamine Barricade, Endurancetrifuralin Trefan, Tri-4

pyridines dithiopyr Dimension

benzamides pronamide Kerb

benzoic acids DCPA Dacthal

carbamates asulam Asulox

cell wall biosynthesis 20 nitriles dichlobenil Casoron

inhibitors

21 benzamides isoxaben Gallery

Seedling growth unknown 15 chloroacetamides acetochlor Harness, Surpass,inhibitors (shoot) Topnotch

alachlor Micro-Techdimethenamid Frontier, Outlookmetolachlor Dual, Pennantpropachlor Ramrod

oxyacetamides fuenacet Dene

lipid synthesis 8 thiocarbamates butylate Sutaninhibitors EPTC Eptam, Eradicane

pebulate Tillamvernolate Vernam

cell division inhibitors 8 phosphorodithioates bensulide Prear

15 acetamides napropamide Devrinol

Photosynthesis photosystem II 5 triazines ametryn Evik

inhibitors atrazine Atrazine(mobile 1) propazine Milo Pro

prometon Primitosimazine Princep

triazinones hexazinone Velparmetribuzin Lexone, Sencor

uracils bromacil Hyvarterbacil Sinbar

(mobile 2) photosystem II 7 ureas diuron Karmexlinuron Loroxsiduron Tupersantebuthiuron Spike

Photosynthesis photosystem II 6 nitriles bromoxynil Buctril

inhibitors (nonmobile;benzothiadiazoles bentazon Basagran

“rapid-acting”)

phenyl-pyridazines pyridate Tough

continued on next page



Table 2. continued

Mode o Action


Cell membrane PPO enzyme 14 diphenyl ethers acifuoren Blazerdisrupters omesaen Refex, Flexstar

lactoen Cobraoxyfuoren Goal

N-phenyl-phthalimides fumioxazin Valorfumiclorac Resource

oxadiazoles oxadiazon Ronstar

triazolinones carentrazone Aimsulentrazone Authority, Spartan

photosystem I 22 bipyridyliums diquat Rewardparaquat Gramoxone, Boa

Pigment inhibitors diterpenes (carotenoid 13 isoxazolidinones clomazone Command(bleaching) biosynthesis) pyridazinones norfurazon Zorial

4-HPPD enzyme 27 isoxazoles isoxafutole Balance

triketones mesotrione Callistotembotrione Laudistopramezone Impact

Phosphorylated GS enzyme 10 amino acid derivatives gluosinate Liberty, Finale, Rely,amino acid (phosphinic acids) Ignite

(N-metabolism disrupters)

Unknown ? ? dazomet Basamidendothall Aquatholosamine Krenitemetam Vapampelargonic acid Scythecinnamon oilcitric acidclove oil Matrancorn gluten meanthyme oilvinegar (acetic acid)

*Only selected trade names appear. Certain active ingredients may have other trade names or be contained in prepackaged mixtures.



Plant Growth Regulators (PGRs)

These herbicides are eective on an-

nual and perennial broadlea plants and

usually have no activity on grasses or

sedges, except at high application rates.

They produce responses similar to those

o natural, growth-regulating substances

called auxins. Application o articial

auxins, such as 2,4-D, upsets normalgrowth as ollows:

l Cells o lea veins rapidly divide and

elongate, while cells between veins

cease to divide. This results in long,

narrow, strap-like young leaves.

l Water content increases, making

treated plants brittle and easily bro-

ken.

l Cell division and respiration rates in-

crease, and photosynthesis decreas-

es. Food supply o treated plants isnearly exhausted at their death.

l Roots o treated plants lose their abil-

ity to take up soil nutrients, and stem

tissues ail to move ood eectively

through the plant.

The killing action o growth-regulating

chemicals is not caused by any single

actor but results rom the eects o

multiple disturbances in the treated

plant.

Injury Symptoms

Broadlea plant leaves become crinkled,

puckered, strap shaped, stunted, and

malormed; lea veins appear parallel

rather than netted, and stems become

crooked, twisted, and brittle, with

shortened internodes. I injury occurs

in grasses (e.g., corn), new leaves do

not unurl but remain tightly rolled in

onion-like ashion, and stems become

brittle, curved, or crooked, with short

internodes. A lesser eect in corn is the

usion o brace roots, noticed later in the

season.

Aino Acid Biosynthesis

Inhibitors

These herbicides are eective mostly on

annual broadleaves, while a ew in this

large group have activity on grasses,

nutsedge, and/or perennial plants.

(Glyphosate [Roundup], or example,

is a broad-spectrum herbicide and has

activity on all types o plants.) These

herbicides work by interering with one

or more key enzymes that catalyze the

production o specic amino acids in

the plant. When a key amino acid is not

produced, the plant’s metabolic pro-

cesses begin to shut down. The eect is

like that o an assembly line worker not

doing his or her job. Dierent herbicides

aect dierent enzymes that catalyze

the production o various amino acids,

but the result is generally the same—

the shutdown o metabolic activity with

eventual death o the plant.

Injury Symptoms

Plants that are sensitive to these herbi-

cides stop growth almost immediately

ater oliar treatment; seedlings die in

two to our days, established perenni-

als in two to our weeks. Plants become

straw colored several days or weeks

ater treatment, gradually turn brown,

and die.

Fatty Acid (Lipid) Biosynthesis

Inhibitors

These herbicides are rapidly absorbed

by grasses and are translocated to

the growing points, where they inhibit

meristematic activity, stopping growth

almost immediately. They have no activ-

ity on broadlea plants and are most

eective on warm-season grasses such

as Johnsongrass, shattercane, corn, all

panicum, giant oxtail, and crabgrass.

Cool-season grasses such as quack-

grass, annual and perennial ryegrass,

orchardgrass, timothy, and small grains

are not as sensitive as the warm-season

grasses. Some o these herbicides are

weaker on perennial species than other

products. They are requently reerred to

as “post-grass” herbicides.

Injury Symptoms

Growing points are killed rst, resultingin the death o the leaves’ inner whorl.

Older, outer leaves o seedlings ap-

pear healthy or a ew days, and those

o perennials or a couple o weeks,

but eventually they also wither and die.

Ater several weeks, the growing points

begin to rot, allowing the inner leaves

to be pulled out o the whorl. Sensitive

grasses commonly turn a purplish color

beore dying.



Seedling Growth Inhibitors

(Root and Shoot)

Herbicides in this group prevent cell

division primarily in developing root tips

and are eective only on germinating,

small-seeded annual grasses and some

broadleaves.

Injury Symptoms

Seeds o treated broadleaved plants

germinate, but they either ail to emerge

or emerge as severely stunted seedlings

that have thickened, shortened lower

stems, small leaves, and short, club-

shaped roots. Seedlings o tap-rooted

plants, such as soybeans and alala, are

usually not aected, nor are established

plants with roots more than a couple o

inches deep.

Grass seeds germinate, but generally ail

to emerge. Injured seedlings have short,club-shaped roots and thickened, brittle

stem tissue. Seedlings die rom lack o

moisture and nutrients because o the

restricted root system.

Seedling Growth Inhibitors

(Shoot)

Herbicides in this class are most e-

ective on annual grasses and yellow

nutsedge. They are sometimes reerred

to as “pre-grass” herbicides. Depending

on the product, some will control small-seeded annual broadleaves. These

herbicides cause abnormal cell develop-

ment or prevent cell division in germi-

nating seedlings. They stop the plant

rom growing by inhibiting cell division

in the shoot and root tips while permit-

ting other cell duplication processes to

continue. Then ollows a slow decline in

plant vigor.

Injury Symptoms

Germinating grasses normally do notemerge. I they do, young leaves ail to

unold, resulting in lea looping and an

onion-like appearance. The tip o the

terminal lea becomes rigid, not ree

fapping (fag like). The leaves o broad-

leaved plants turn dark green, become

wrinkled, and ail to unold rom the bud.

The roots become shortened, thickened,

brittle, and club like.

Photosynthesis Inhibitors

(mobile)

These herbicides are eective primarily

on annual broadleaves, while some pro-

vide control o grasses as well. Photo-

synthesis-inhibiting herbicides block

the photosynthetic process so captured

light cannot be used to produce sugars.

In the presence o light, green plants

produce sugar rom carbon dioxide and

water. Energy is needed or carbon, hy-

drogen, and oxygen atoms to rearrange

and orm sugar. To supply this necessary

energy, electrons are borrowed rom

chlorophyll (the green material in leaves)

and replaced by electrons split rom

water. I chlorophyll electrons are not re-

placed, the chlorophyll is destroyed and

the plant’s ood manuacturing system

breaks down. The plant slowly starves to

death due to lack o energy.

As soil-applied treatments, these herbi-

cides permit normal seed germination

and seedling emergence, but cause

seedlings to lose their green color soon

aterward. With the seeds’ ood supply

gone, the seedlings die. These herbi-

cides are more eective on seedling

weeds than on established perennial

weeds. Herbicides such as prometon

(Primitol) and tebuthiuron (Spike) are

considered soil sterilants. Soil sterilants

are nonselective chemicals that can killexisting vegetation and keep the soil

ree rom vegetation or one or more

years.

Injury Symptoms

In broadleaved plants, early seedling

growth appears normal, but shortly

ater emergence (when energy reserves

in cotyledons are depleted), leaves

become mottled, turn yellow to brown,

and die. In most cases, the oldest leaves

turn yellow on the lea margins rst,

the veins remain green, and eventually

the plant turns brown and dies. Herba-ceous and woody perennials starve very

slowly because they have large energy

reserves in roots or rhizomes to live on

while photosynthesis is inhibited. The

herbicide may have to eectively inhibit

photosynthesis or a ull growing season

to kill trees or brush. This kind o death

may be slow, but it is certain.

Selective herbicides control weeds without causing injury to the crop or other desirable plants



Photosynthesis Inhibitors

(Nonobile; “Rapid-Acting”)

Herbicides in this group have activity

on primarily annual and some perennial

broadleaves and are applied to the plant

oliage. The mode o action is the same

as the mobile photosynthesis inhibitors.

Injury Symptoms

Their activity within the plant is similar to

that o the mobile photosynthesis inhibi-

tors, except the injury occurs at the site

o contact, causing “lea burning” and

eventual death o the plant.

Cell mebrane Disrupters

These herbicides control mostly

broadleaves. Certain products have

some activity on grasses, and paraquat

(Gramoxone) provides broad-spectrum

control o many dierent species.

These herbicides are reerred to as

contact herbicides and they kill weeds

by destroying cell membranes. They ap-

pear to burn plant tissues within hours

or days o application. Good coverage

o the plant tissue and bright sunlight

are necessary or maximum activity. The

activity o these herbicides is delayed in

the absence o light.

Injury Symptoms

All contact herbicides cause cellularbreakdown by destroying cell mem-

branes, allowing cell sap to leak out.

Eected plants initially have a “water-

soaked” appearance, ollowed by rapid

wilting and “burning,” or lea speckling

and browning. Plant death occurs within

a ew days.

Pigent Inhibitors

These herbicides provide control o

many annual broadleaves and some

grasses. These products are reerred to

as “bleachers” since they inhibit carote-

noid biosynthesis or the HPPD enzyme

by interering with normal chlorophyll

ormation.

Injury Symptoms

Symptoms are very evident and easy

to identiy. Eected plants either do not

emerge or emerge white or bleached

and eventually die. Older lea tissue is

aected rst.

Phosphorylated Aino Acid

(Nitrogen metabolis)

Disrupters

This herbicide provides broad-spectrum

control o most annual grasses and

broadleaves and some perennials. It

aects growth by disrupting nitrogen

metabolism, thus interering with other

plant processes. It is a contact herbicide

with slight translocation throughout the

plant. Good spray coverage and sunlight

are important or maximum ecacy.

Injury Symptoms

Injury is similar to that o the cell mem-

brane disrupter herbicides. Sensitive

plants show “lea burning,” yellowing

and browning, and eventual death atera week or so. Perennials generally take

longer or symptoms and death to occur.

Unknown Herbicides

This category contains miscellaneous

products or which the mode o ac-

tion and amily are unknown. Dazomet

(Basamid) and metam (Vapam) are con-

sidered soil umigants. These products

are applied to the soil and covered with

a gas-tight tarp; there, they are con-

verted to gases and penetrate the soilto kill weeds, diseases, and nematodes.

Endothall (Aquathol) is used or aquatic

weed control. Fosamine (Krenite) is used

in noncrop areas to control perennial

weeds and brush. Other compounds

such as pelargonic acid (Scythe), a

atty-acid herbicide, and clove oil and

vinegar are contact, nonselective,

broad-spectrum, oliar-applied products

that are sometimes used or weed con-

trol in organic crop production settings.

However, because they basically “burn”

only the plant tissue they contact, there

is potential or plant regrowth.



Herbicide Resistance

A number o weed species that were

once susceptible to and easily managed

by certain herbicides have developed

resistance. These weeds are no longer

controlled by applications o previ-

ously eective herbicides. As a result

o repeatedly using a certain type oherbicide on the same land, many di-

erent species o weeds have developed

resistance to these chemicals. Cur-

rently, about 180 weed species (more

than 300 weed biotypes) worldwide are

resistant to about ten dierent herbi-

cide amilies. It is believed that within

any population o weeds, a ew plants

have sucient tolerance to survive any

herbicide that is used. Since only the

survivors can produce seed, it is only

a matter o time until the population o

resistant weeds outnumbers the suscep-

tible type. Depending on the herbicide

amily and weed species, resistance

can occur within 5 to 20 years. Certain

precautions, such as tank-mixing, crop

rotations, and a combination o weed

management techniques, must be taken

to prevent resistance.

Growers, consultants, and those work-

ing with herbicides to manage weeds

should know which herbicides are best

suited to combat specic resistantweeds. The Weed Science Society o

America (WSSA) developed a grouping

system to help with this process. Her-

bicides that are classied as the same

WSSA group number kill weeds using

the same mode o action. WSSA group

numbers can be ound on many herbi-

cide product labels and can be used as

a tool to choose herbicides in dierent

mode o action groups so mixtures or

rotations o active ingredients can be

planned to better manage weeds and re-duce the potential or resistant species.

Reer to Table 2 (pp. 12–14) or WSSA

mode o action group numbers and cor-

responding herbicides.

Ties o Application

The ollowing terms describe herbicides

based on when they are applied:

l Preplant incorporated: applied to soil

and mechanically incorporated into

the top 2 to 3 inches o soil beore

the crop is planted

l Preplant: applied to soil beore the

crop is planted

l Preemergence: applied ater the crop

is planted but beore it emerges

l Postemergence: applied ater crop

emergence

Although these terms normally reer to

application in relation to crops, they

may also imply application in relation

to weeds. Always be certain whether

reerence is being made to the crop orto the weed. In no-till situations, it is

possible or an herbicide application to

be preplant or preemergence to the crop

but postemergence to weeds. Some

herbicides must be preplant or preemer-

gence to the weed or maximum activity.

methods o Application

The ollowing terms reer to the ways

herbicides can be applied:

l Broadcast: applied over the entire

eld

l Band: applied to a narrow strip over

the crop row

l Directed: applied between the rows

o crop plants with little or no herbi-

cide applied to the crop oliage

l Spot treatment: applied to small,

weed-inested areas within a eld

Product Forulations

Herbicides are not sold as pure chemi-

cals, but as mixtures or ormulations

o one or more herbicides with various

additives. Adjuvants (suractants, emul-

siers, wetting agents, etc.) or various

diluents may increase the eectiveness

o a pure herbicide. The type o ormula-tion determines toxicity to plants, uni-

ormity o plant coverage, and stability

in storage. Herbicides are ormulated to

permit uniorm and easy application as

liquid sprays or dry granules.

Some everyday household produts are

ormulated similarly to herbicide prod-

ucts. These similarities will be noted in

the sections below.

Emulsifable concentrates (EC or E) are

liquid ormulations with an active ingre-dient that is dissolved in one or more

petroleum-based solvents. An emulsier

is added to cause oil to orm tiny glob-

ules that disperse in water. The ormula-

tion then will mix readily with water or

proper application. Emulsiable concen-

trates usually contain between 2 and 8

pounds o active ingredient per gallon.

Dual II Magnum, Pennant, Acclaim, and

Prowl are generally emulsiable herbi-

cide ormulations. (Household product

with similar ormulation—Pine-Sol.)

Emulsifable gels (EG or GL) are herbi-

cides that traditionally are emulsiable

liquids ormulated as gels. The gels typi-

cally are packaged in water-soluble bags

(WSB) and are stable at temperatures

ranging rom –20 to 500°C. In addition,

the gelling process reduces the need

or nonaqueous solvents, compared to

standard nonaqueous EC-type ormula-

tion processes. Currently, ew herbicides

are ormulated as gels.

Wettable powders (WP or W) are nely

ground, dry particles that may be dis-

persed and suspended in water. They

contain rom 25 to 80 percent active

ingredient. Suspensions o wettable

powders appear cloudy. Wettable pow-

ders are nearly insoluble and require agi-

tation to remain in suspension. Atrazine,

Kerb, and Dacthal are ormulated as

wettable powders. (Household products



with similar ormulation—cocoa mix and

four.)

Soluble liquid (S) and soluble powders

(SP) dissolve in water to orm a true so-

lution. Once the soluble liquid or powder

is dissolved, the spray mixture requires

no additional mixing or agitation. Few

herbicides are available as solubles

because most active ingredients o

herbicides are not very soluble in water.

2,4-D amine and Roundup are examples

o soluble liquid herbicide ormula-

tions. (Household products with similar

ormulation—grape juice concentrate

and Kool-Aid mix.)

Dry owables (DF), also called water-

dispersible granules (WDG or WG) or

dispersible granules (DG) are wettable

powders ormed into prills so they poureasily into the sprayer tank without

clumping or producing a cloud o dust.

Nearly insoluble, they require agitation

to remain in suspension. Many herbi-

cides are now ormulated in this ashion.

Atrazine, Accent, Gallery, and Pendulum

are examples o products ormulated as

water-dispersible granules. (Household

products with similar ormulation—grits

and dry milk.)

Flowables (F or FL), suspension con-

centrates (SC), and aqueous suspension

(AS) are nely ground, wettable pow-

ders or solids already suspended in a

liquid carrier so they can be poured or

pumped rom one tank to another. They

usually contain at least 4 pounds o

active ingredient per gallon o ormula-

tion. Flowables are nearly insoluble in

water and require agitation to remainin suspension. Suspoemulsion (SE) is a

combination ormulation o an SC and

an oil-based emulsion (E). Atrazine,

Princep, and Callisto are ormulated as

fowables or SCs. (Household products

with similar ormulation—Pepto-Bismol

and V8 vegetable juice.)

Microencapsulated (ME or MT) and cap-

sule suspension (CS) herbicides are en-

cased in extremely small capsules that

can be suspended in a liquid carrier and

pumped and applied with normal equip-

ment. Microencapsulated ormulations

are nearly insoluble in water and require

agitation to remain in suspension. Micro-

Tech, Prowl H2O, and Command are or-

mulated in microcapsules, allowing the

active ingredient to be slowly released

over a period o time. This extends the

soil activity and improves weed control.

(Household product with similar ormu-

lation—older versions o Contac cold

capsules.)

Granules (G) are ormulated with a

premixed carrier that contains a low per-

centage o active ingredient. The carrier

may be ertilizer, clay, lime, vermiculite,

or ground corn cobs. These herbicides

are applied directly (dry) to the soil

without urther dilution. The perormance

o granulated herbicides compared with

that o sprayable ormulations varies

with the herbicide. Granular orms gen-

erally require more rainall or activation

than do sprayable ormulations. Granuleherbicides are used oten in tur and

ornamental settings. Some examples

include Balan and Ronstar. (Household

products with similar ormulation—cat

litter and Grape-Nuts cereal.)

Pellets (P) are like granules but are com-

pressed into larger cylinders about ¼

inch long. Herbicides ormulated as pel-

lets usually contain rom 5 to 20 percent

active material and are hand-applied to

control clumps o brush. They also may

be applied with cyclone-type spinner

spreaders mounted on helicopters or

aircrat to control brush in orests or per-

manent pastures. Pellets gradually break

down rom rainall and leach into the soil

or root uptake. Spike is an example o a

pelleted herbicide. (Household product

with similar ormulation—guinea pig/

rabbit pellets.)

Premixes are not ormulations, but two

or more herbicide active ingredients

mixed into one product by the manuac-

turer. The actual ormulation can be any

o those discussed above and com-

monly combines two or more herbicides

that are already used together. The

primary reason or using premixes is

convenience. Many herbicide products

are now marketed as premixes.

Trade Nae and Forulation

Notations

In certain publications, many herbicides

are listed by trade name (or product

name) and ormulation (or example,

Roundup 4S or Accent 75WDG).

Roundup is the trade name, and 4S

stands or 4 pounds o active ingredi-

ent (glyphosate) per gallon o product

in a soluble (S) ormulation. Accent is

ormulated as a water-dispersible gran-

ule with each granule (or certain unit)containing 75 percent active ingredient

(nicosuluron). The remaining parts o

the ormulation contain inert ingredients,

which have no eect on weed control.

Additional inormation about ormula-

tion and ingredients can be ound on the

product’s label and MSDS sheet.

Herbicide Spray Additives

(Adjuvants)

Additives or adjuvants are substances in

herbicide ormulations or that are addedto the spray mixture to improve herbicid-

al activity or application characteristics.

More than 70 percent o all herbicides

recommend using one or more adju-

vants in the spray mixture. In general,

there are two types o adjuvants: ormu-

lation and spray. Formulation adjuvants

are “already in the container” rom the

manuacturing process. These help with

Most herbicides are sold in a liquid or dry

ormulations.



mixing, handling, eectiveness, and

providing consistent perormance.

Spray adjuvants can be divided into

special purpose adjuvants and activator

adjuvants. Special purpose adjuvants

include compatibility agents, buering

agents, antioam agents, drit retardants,

and others that widen the range o con-

ditions or herbicide use. Activator adjuvants are commonly used to enhance

postemergence herbicide perormance

by increasing herbicide activity, absorp-

tion, and rainastness and by decreasing

photodegradation. These include surac-

tants (i.e., “surace active agents”), crop

oil concentrates, vegetable oil concen-

trates, wetting agents, stickers-

spreaders, N-ertilizers, penetrants, and

others. Commonly used suractants are

nonionic suractants and organosili-

cones and are typically used at a rate

o 1 quart per 100 gallons (0.25 percent

v/v) o spray mixture. Crop oil concen-

trates are 80 to 85 percent petroleum

based plus 15 to 20 percent suractant,

while vegetable oil concentrates contain

vegetable or seed oil in place o petro-

leum oil. Oil concentrates are typically

included at a rate o 1 gallon per 100

gallons (1 percent v/v) o spray mixture.

In general, oil concentrates are “hotter”

than suractants, so they provide better

herbicide penetration into weeds under

hot/dry conditions, but they are morelikely to cause greater crop injury under

normal growing conditions. Nitrogen

ertilizers, such as UAN (a mixture o

ammonium nitrate, urea, and water) or

AMS (ammonium sulate), are used in

combination with suractants or oil con-

centrates to increase herbicide activity

and/or reduce problems with hard water.

Many blended adjuvants are available

that include various combinations o

special purpose adjuvants and/or activa-

tor adjuvants.

Be sure to include the proper adjuvant(s)

or the herbicide being used. Most herbi-

cide labels speciy the type and amount

o additive to use. Failure to ollow the

recommendations can result in poor

weed control or excessive crop injury.

mixing and Applying

Be aware that improper sprayer calibra-tion, nonuniorm application, calculation

errors, or use o the wrong chemicals

can cause herbicide injury to the crop.

Apply only the recommended amount o

herbicide. Slight increases in rates could

result in crop injury or leave residues

that might injure succeeding crops.

Recalibrate sprayers requently to adjust

or increased output resulting rom

normal nozzle wear. Be sure there is

sucient agitation in the sprayer tankto prevent settling o wettable powders,

dry fowables, or fowables.

Do not stop in the eld with the sprayer

on, spill herbicide when loading, or

dump unused herbicides into anything

except a holding tank.

Take the ollowing steps when mixing

herbicides:

l Always be sure the sprayer has been

calibrated properly or application at

recommended rates.

l Calculate the amount o herbicide to

add to the sprayer tank based on the

active material in each gallon o her-

bicide concentrate or the percentage

o active ingredient o dry herbicide

ormulation.

l Read and ollow the instructions on

the manuacturer’s label pertaining to

personal hazards in handling.

l Fill the sprayer tank with at least hal

the volume o water or ertilizer solu-

tion you will ultimately need.

l Start with moderate agitation and

keep it going.

l Add compatibility agents, ammonium

sulate, or other mixing adjuvants,

i needed. For maximum benet,

they must be in the solution beoreherbicides are added. (To determine

pesticide compatibility, see the next

section.)

l I tank-mixing dierent types o

herbicide ormulations and adjuvants

be sure to add them in the ollowing

order:

1. Add, mix, and disperse dry her-

bicides (wettable powders, dry

fowables, or water-dispersible

granules). These ormulations con-tain wetting and dispersing agents

that aid in mixing.

2. Add liquid fowables and mix

thoroughly. Liquid fowables also

contain wetting and dispersing

agents.

3. Add emulsiable concentrates or

microencapsulated herbicides and

mix thoroughly.

4. Finish by adding water-soluble

ormulations (2,4-D amine, etc.).

5. Add any adjuvants (suractants,

crop oil concentrates, drit inhibi-

tors, etc.) last. Crop oils, especial-

ly, do not mix and disperse well i

added rst.

6. Add the remainder o water or liq-

uid ertilizer and maintain agitation

through spraying procedure until

tank is empty.

Caution: Never mix concentrated

herbicides in an empty tank. Never

allow a sprayer containing mixed

chemicals to stand without agita-

tion because heavy wettable pow-

ders may clog nozzles or settle

into corners o the sprayer tank

where they are dicult to remove.

Eect o a suractant on the spread and

penetration o spray solution across and

through the lea surace.

without suractant

with suractant



Copatibility

Pesticides are not always compatible

with one another or with the water or liq-

uid ertilizer carrier. Lack o compatibility

may result in the ormation o a gel, pre-

cipitate, or sludge that plugs up screens

and nozzles. However, extreme incom-

patibility may produce a settling out o

material that can harden like concretein the bottom o the tank and in hoses,

pumps, and other internal parts o the

sprayer. The result may be total loss o

the pesticide and use o the sprayer.

Herbicides may be combined with liquid

ertilizers to minimize trips over the eld.

However, little inormation exists con-

cerning the compatibility o herbicides

with specic ertilizer solutions. Herbi-

cide-ertilizer solution combinations may

orm a gel or precipitate that settles to

the bottom o the sprayer tank or willnot fow through the sprayer equipment.

Incompatibility o tank mixtures is more

common with suspensions o ertilizers

and pesticides.

Tank-mixing several pesticides, although

convenient, may create other problems.

Foliar activity may be enhanced and

could result in crop lea burn or the re-

duction in activity o one or more o the

pesticides (“antagonism”).

To prevent the main water tank or liquid

ertilizer measuring tank rom becoming

contaminated, commercial applicators

may want to mix the herbicides and

other ingredients in a separate hold-

ing tank. The herbicide mixture is then

sucked into the main line as the truck

tank is being lled, and thorough mix-

ing is provided by the truck’s agitation

system. Compatibility problems are

more likely to result when concentrated

herbicides are mixed together, so a

compatibility test should be done beorenew mixtures are tried.

Use only labeled tank mixtures or

mixtures recommended by experienced

scientists whose recommendations are

backed by research. For all unlabeled

tank mixtures, a jar test or compatibility

is strongly recommended. The compat-

ibility o herbicide-ertilizer combinations

should be tested beore large batches

are mixed. In some cases, adding a

compatibility agent (Blendex, Combine,

Unite, or comparable adjuvant) may aid

in maintaining component dispersion.

The ollowing “two-jar test” procedure

may be used to test the compatibil-

ity o herbicides with one another, or

herbicides and other pesticides withliquid ertilizers. Should the herbicide-

carrier mixture prove compatible in this

test procedure, it may be applied to

the eld. The ollowing test assumes a

spray volume o 25 gallons per acre. For

other spray volumes, make appropriate

changes to the ingredients.

1. Add 1 pint o carrier (water or liquid

ertilizer) to each o two one-quart

jars. (Note: Use the same source o

water that will be used or the tank

mix and conduct the test at the sametemperature the spray mixture will be

applied.)

2. To one o the jars, add 0.25 tea-

spoon (1.2 ml) o compatibility agent.

To both jars, add the appropriate

amount o pesticide(s), in their relative

proportions, based on recommended

label rates. I more than one pesticide

is used, add them separately with dry

ormulations rst, fowables next, and

emulsiable concentrates last. Ater

each addition, shake or stir gently to

thoroughly mix.

3. When all ingredients are added, put

lids on and shake both jars or 15

seconds and let stand or 30 minutes

or more. Then inspect the mixture or

fakes, sludge, gels, heavy oil lms, or

other signs o incompatibility.

l I, ater standing or 30 minutes,

the components in the jar con-

taining no compatibility agent

are dispersed, the herbicides arecompatible and no compatibility

agent is needed.

l I the components are dispersed

only in the jar containing the com-

patibility agent, the herbicide is

compatible only i a compatibility

agent is added.

l I either mixture separates but can

be remixed readily, the mixture

can be sprayed as long as good

agitation is used.

l I the components are not dis-

persed or show signs o incompat-

ibility in either jar, the herbicide-

carrier mixture is not compatible

and should not be used.



Herbicide Selectivity

Were it not or the act that most herbi-

cides can be applied just beore crop

planting or emergence, and even over

the top ater crop emergence without

excessive injury, herbicides would be

o little value. Most o the herbicides

labeled or use today will selectively

remove most o the weeds without injur-

ing the crop. Selectivity is accomplished

primarily by two methods: selectivity by

placement and true selectivity.

Selectivity by Placeent

Selectivity accomplished by avoid-

ing or minimizing contact between the

herbicide and the desired crop is called

selectivity by placement. An example is

wiping or directing an herbicide such as

glyphosate on a weed without expos-

ing the desired plant. Selectivity by this

means is as good as any, as long as

the excess herbicide is not washed o

the weeds and leached into the root

zone where it might be absorbed by the

root. Selectivity by placement also is

accomplished when an herbicide that

does not readily leach is applied to the

soil surace or control o shallow-rooted

weeds, but does not leach into the root

zone o a more deeply rooted crop such

as ruit trees or established alala.

True Selectivity

Selectivity that is true tolerance as a

result o some morphological, physio-

logical, or biochemical means is reerred

to as true selectivity. The herbicide can

be applied to the oliage o the crop or

to the soil in which the crop is grow-

ing without danger o injury. Although

true tolerance may be the best type o

selectivity, it is not perect. Such things

as crop growth stage, cuticle thickness,

hairiness o the lea surace, locationo the growing point, air temperature

and humidity, spray droplet size, and

the surace tension o spray droplets all

can infuence herbicide activity. When

conditions are ideal or herbicide activity,

even true selectivity may not adequately

prevent crop injury.

Morphological dierences include plant

characteristics such as size and orienta-

tion o the lea, waxiness or hairiness o

the lea surace, location o the growing

point, and rooting depth. Generally, the

more waxy or hairy the lea surace, the

more dicult it is or a oliar-applied

herbicide to penetrate. The more pro-

tected the growing point (as in grasses),

the less likely it is that oliar herbicideswill reach the growing point. The more

deeply rooted the crop is, the more di-

cult it is to get a soil applied herbicide

to the crop roots and the less likely that

there will be sucient uptake or injury.

Physiological dierences can include

various processes that aect the activity

and/or the breakdown o the herbicide.

In certain situations, herbicides may be

l transported dierently across the

plasma lemma,

l translocated dierently within the

plant,

l combined with some component

within the cell wall,

l integrated with something in the cell

cytoplasm, or

l channeled into “sinks” where the

herbicide will have no eect.

These actors all can contribute to toler-

ance, but any one actor will seldomprovide tolerance by itsel.

Metabolic actors include genetic insen-

sitivity due to an altered site o herbicide

action that prevents herbicide activity.

For example, Roundup Ready soybeans

produce an excess o the enzyme that

glyphosate (Roundup) normally inhibits,

so Roundup Ready soybeans are not

aected, even though normal amounts

o the herbicide are absorbed by the

crop plant. Corn plants metabolize

and convert atrazine to an innocuous

metabolite so rapidly that the herbicide

does not have time to inhibit photosyn-

thesis, which provides crop tolerance

as long as the metabolic system is not

overwhelmed by an excess o the pes-

ticide or a combination o pesticides. In

the case o corn treated with an organo-

phosphate insecticide and ollowed with

a post treatment o Accent, Beacon, or

some other ALS-inhibiting herbicide,

both the insecticide and herbicide are

being metabolized by the same path-

way. This pathway is unable to rap-

idly metabolize both the herbicide and

insecticide, so corn injury may result.

Metabolic insensitivity and/or the ability

to metabolize the herbicide usually are

the best types o true tolerance.

Sae Herbicide Use

Use herbicides only when necessary,

only at recommended rates and times

o application, and only or those crops

and uses listed on the label. Correct use

is essential to ensure that chemical resi-

dues on crops do not exceed the limits

set by law. Recommended herbicides

do not generally injure people, livestock,

wildlie, or crops i used properly and i

recommended precautions are ob-

served. However, any herbicide is poten

tially dangerous i improperly handled or

used.

Follow these basic pesticide saety

procedures:

l Make sure that you are amiliar with

current ederal and state pesticide

laws and regulations and that you

have a license, i required.

l Avoid drit o spray or dust that mayendanger other crops or animals.

Cover eed pans, troughs, and water-

ing tanks in livestock areas; protect

beehives.

l To protect yoursel and others, ollow

all saety precautions on the label.

Know and observe the general rules

or sae pesticide use, and record the

date, time, location, and amount o

each pesticide used.

l

Wear protective clothing and useprotective equipment according to

instructions on the pesticide label.

l Never eat, drink, or smoke while ap-

plying pesticides.

l Avoid spilling spray materials on

skin or clothing. I such an accident

occurs, wash immediately with soap

and water.



l Bathe ater applying pesticides

and change into reshly laundered

clothing. Wash clothing ater apply-

ing pesticides, keeping in mind that,

until laundered, such clothing must

be handled according to the same

precautions as the pesticide itsel.

Wash pesticide-contaminated cloth-

ing apart rom other laundry, and takecare in disposing o the wash water.

l Store pesticides in their original con-

tainers in a locked, properly marked

cabinet or storeroom, away rom ood

or eed.

l Do not store herbicides with other

pesticides; avoid the danger o cross-

contamination.

l Be sure to triple-rinse all empty con-

tainers beore recycling (in a special

recycling program only through thePa. Dept. o Agriculture; this is dier-

ent rom typical household curbside

recycling programs) or disposing o

them in an approved landll.

l I you suspect poisoning, contact

your nearest Poison Control Center,

hospital emergency room, or physi-

cian. Take the pesticide label and, i

possible, the MSDS sheet with you

and give it to the attending physician.

Livestock

When used properly and in accordance

with the use restrictions on the product’s

label, herbicides sprayed on plants usu-

ally are not toxic to livestock. Animals

can be poisoned by consuming unused

herbicides let in open containers or

by drinking water contaminated with

herbicides.

Certain unpalatable or poisonous plants

treated with herbicides may become

more attractive as orage to livestock.

Make sure livestock cannot get to poi-

sonous plants that have been sprayed

with herbicides.

The nitrate content o several kinds o

weeds may increase ater they have

been sprayed with 2,4-D, Clarity, or

similar herbicides. Livestock grazing on

these treated plants may become ill.Remove all animals rom sprayed areas

or several days, or until it has rained or

the weeds have died.

Gae and Fish

Controlled spraying may benet wildlie

by maintaining desirable cover. Her-

bicides recommended or control o

aquatic weeds usually have benecial

results or sh populations. Be sure to

properly apply these herbicides. Do not

drain or fush equipment where chemi-

cals may wash into ponds or streams,

and do not leave open containers where

curious animals might nd them.

Crop Saety

Farmers are occasionally concerned

about possible herbicide injury to crops.

Most injuries o this kind are caused by

misuse, contaminated equipment, or

drit. Unavorable weather conditions

combined with herbicide residues roma previous crop planting can potentially

injure crops.

Cleaning Containated

Equipent

Sprayer cleanout is necessary to prevent

crop injury rom spray contamination

and to preserve the lie o the sprayer.

Cleaning is very important, especially

when using a sprayer in dierent types

o crops. Many herbicides, even at lowconcentrations, may have the poten-

tial to injure crops or which they are

not labeled. Sprayers used to apply

2,4-D–type herbicides can be used to

apply other chemicals beore crops are

planted or beore crop plants emerge,

but this equipment must be thoroughly

cleaned beore applications are made on

emerged crops (except grasses). Ester

ormulations are harder to remove than

amine or salt ormulations.

The ollowing cleaning procedure is

recommended or all herbicides unless

the label species a dierent cleaning

procedure:

1. Add one-hal tank o resh water

and fush tanks, lines, booms, and

nozzles or at least 5 minutes us-

ing a combination o agitation and

spraying. Rinsate sprayed through

the booms is best sprayed onto

croplands to avoid accumulation o

pesticide-contaminated rinsate. Thor-oughly rinse the inside suraces o

the tank, paying particular attention

to crevices and plumbing xtures.

2. Fill the tank with resh water and add

one o the cleaning solutions below,

or a commercially available tank

cleaner, and agitate the solution or

15 minutes. Add one o the ollow-

ing to each 50 gallons o water to

make a cleaning solution: (a) 2 quarts

o household ammonia (let stand in

sprayer overnight or growth regulatoherbicides such as 2,4-D and Clarity)

or (b) 4 pounds o trisodium phos-

phate cleaner detergent. Operate the

spray booms long enough to ensure

that all nozzles and boom lines are

lled with the cleaning solution. Let

the solution stand in the system or

several hours or overnight. Agitate

and spray the solution onto an area

suitable or the rinsate solution.

Always wear the proper saety equipment

when working with herbicides or other

pesticides.



3. Add more water and rinse the system

again by using a combination o agi-

tation and spraying. Remove nozzles,

screens, and strainers and clean

separately in a bucket o cleaning

agent and water.

4. Rinse and fush the system once

again with clean water.

Drit

Drit is the movement o any pesticide

through the air to areas not intended or

treatment. During application, droplet

or particle drit occurs as spray drop-

lets or dust particles are carried by air

movement rom the application area

to other places. Vapor drit takes placeater application as herbicides evaporate

(volatilize) and yield umes (gases) are

carried on wind currents and deposited

on soils or plants in untreated areas.

Drit may injure sensitive crops, orna-

mentals, gardens, livestock, wildlie, or

people and may contaminate streams,

lakes, or buildings. It may contaminate

crops and cause illegal or intolerable

residues. Excessive drit may mean poor

perormance in the desired spray area

because the application rate is lower

than expected.

Highly active chemicals present the

greatest drit hazard because extremely

small amounts can cause severe

problems. For example, growth regula-

tor herbicides such as 2,4-D, dicamba,

and picloram at a rate o 1 ounce per

acre can deorm sensitive crops such as

tobacco, grapes, or tomatoes.

Vapor drit rom Command (cloma-

zone) that has not been incorporated

can cause bleaching o chlorophyll in

sensitive plants within a quarter mile o

application. Vapor drit problems can

oten be avoided by using nonvolatile

ormulations. Essentially, no vapor drit

hazard is involved in the use o amine

ormulations o 2,4-D. Soil incorporation

o Command and a microencapsulatedormulation greatly reduces vapor loss o

this herbicide.

Particle drit depends on the size o

the particle or droplet, and droplet size

depends on pressure and nozzle design.

Very small particles o og or mist pres-

ent the greatest drit hazard. To minimize

particle drit, calibrate equipment to

create droplets about the size o light

rain. Most nozzles can be adjusted to

a pressure that permits droplet orma-

tion as a result o surace tension. I

nozzles are operated at this pressure, a

minimum o mist-sized droplets will be

ormed. For some nozzles, this pressure

may be as little as 15 psi; or others, it

may be 30 psi.

The distance particles will drit increases

with the height o release. Wind ve-

locities usually are lower close to the

ground. Thereore, sprays should be

released as close to the soil surace

or vegetation as adequate coveragepermits.

Pesticide drit is infuenced by wind, air temperature, boom height, and spray droplet size.



Drit hazard usually is minimized i

prevailing winds are blowing away rom

sensitive crops, but a sudden shit in

wind direction could result in serious

damage. I possible, do not apply pes-

ticides when wind speed is greater than

5 mph.

High temperatures increase the loss

o volatile herbicides. Esters o 2,4-Drapidly evaporate at temperatures above

800°F. The use o such ester ormula-

tions should be restricted to all, winter,

and early spring because sensitive

plants are not present and lower tem-

peratures reduce vapor drit hazard.

Drit control should be considered with

each pesticide application. You can

prevent severe drit problems by

l using sprayer nozzles especially

designed or drit reduction;

l using low volatile or nonvolatile or-

mulations;

l using low spray-delivery pressures

(15–30 psi) and nozzles with a larger

orice;

l using drit-inhibiting adjuvants in the

spray mixture when spraying under

less-than-ideal conditions;

l using nozzles that allow or lowered

boom height;

l avoiding application o volatile chemi-

cals at high temperatures;

l spraying when wind speed is low

(less than 5 mph) or when the wind is

blowing away rom areas that should

not be contaminated;

l spraying during the early morning or

evening hours when there is usually

less wind;

l leaving border areas unsprayed i

they are near sensitive crops.

Evaluating Herbicide Injury

Insects, diseases, severe weather (hail,

lightning, drought, fooding), ertilizer

burn, and nutrient deciencies are

among the causes o symptoms oten

attributed to herbicide injury. Cool, wet

weather can increase the potential or

injury, particularly with preemergenceherbicides. When evaluating crop injury,

careul consideration o the ollowing will

help you diagnose the problem:

1. What is the pattern in the eld o

plant injury or uncontrolled weeds?

l A pattern o injury that starts on

one side o an area and diminishes

gradually and uniormly away rom

that area is typical o application

drit.

l A pattern o injury occurring in

irregular patches that ollow air

drainage could indicate herbicide

volatilization and movement o

vapors.

l Strips o injured areas or surviv-

ing weeds at predictable intervals

indicate possible skipping or

overlapping application.

l Poor control at the edges o a eld

can result rom only hal coverage

by the last nozzle on the boomand/or more sunlight availability

along the edge o the eld.

l Injury limited to the end rows or

ends o the eld is usually due to

overlapping applications or high

herbicide rates in the turnaround

areas at the ends o the rows.

l A denite break between the nor-

mal or uninjured part o the eld

and the rest o the eld usually

indicates some major dierence

in soil type or pH between the two

sides.

l A pattern o obvious overapplica-

tion as indicated by bare ground

(both crop and weeds killed),

ollowed by improved crop survival

and appearance with good weed

control, ollowed by lack o crop

injury or weed control, indicates

inadequate or poor agitation in the

sprayer tank. The evidence is even

stronger i this pattern repeats

itsel at intervals that correspond

to each new load.

2. What is the history o the problem

area—ertility program, cropping

sequence, land preparation, soil pH,

soil texture and organic matter, and

seed source?3. What was the temperature, moisture,

rainall, and prevailing wind at and

immediately ollowing herbicide ap-

plication?



Persistence

The residual lie or length o time an

herbicide persists in the soil is the length

o time it can be expected to control

weeds. Residual toxicity, i not consid-

ered, may injure the next crop planted in

a herbicide-treated eld.

Inactivation, breakdown, and disappear-

ance o herbicides are infuenced by the

ollowing actors.

microbial Degradation

Microorganisms eed on all types o

organic matter, including organic her-

bicides. Microbial degradation is the

primary means o herbicide breakdown.

Some herbicides are more readily at-

tacked by microorganisms than others,

oten because o minor dierences inchemical structure that permit rapid

decomposition in some cases and block

decomposition in others. Soil tempera-

ture, aeration, pH levels, organic matter,

and moisture levels avorable or mi-

crobial growth promote rapid herbicide

breakdown. Microbial degradation takes

place primarily in the top oot o soil,

where microbial activity is the greatest.

Cheical Degradation

Herbicides may be inactivated upon

reaction with salts, acids, and other

substances in the soil. These reactions

are aected by the same environmental

actors that infuence microbial break-

down. Chemical degradation can occur

anywhere in the soil prole and is the

primary process responsible or herbi-

cide dissipation below the top oot o

soil, where microbial activity is limited or

nonexistent.

Runo

Water moving over the surace o a eld

or treated area can carry herbicide with

it. The greatest loss o herbicide occurs

when the herbicide is applied to the soil

surace and is washed o by the rst

rain ater application. I the herbicide is

incorporated or leached into the soil with

light rains or irrigation, most loss occursonly with erosion ater the herbicide is

adsorbed to soil particles.

Leaching

Water carries herbicides into and ulti-

mately out o the root zone. The portion

lost to leaching depends on soil texture,

herbicide solubility, and amount and

intensity o rainall. As a rule, herbicides

leach most rom sandy soils and least

rom clay soils or soils high in organic

matter.

Adsorption

Ater application, herbicides may

become adsorbed (bound) to clay and

organic matter particles. The extent o

adsorption increases as the percentage

o organic matter and/or clay increases.

Adsorption reduces the amount o

chemical available to plants and slows

leaching. Herbicides are then degradedby various means.

Volatilization

Some herbicides may be rapidly lost as

vapors ater application. Loss as vapor

reduces the persistence o dinitroani-

line and thiocarbamate herbicides and

Command. The rate o vapor loss is

infuenced by soil moisture, temperature

and adsorption. Evaporation o herbi-

cides increases as sand content, soil

moisture, and soil temperature increase.Incorporation into soil immediately ater

application reduces this kind o loss.

Photodecoposition

Sunlight may inactivate herbicides—a

actor that may contribute to a decline in

eectiveness o unincorporated her-

bicides such as trifuralin (Trefan) and

benen (Balan). Exposure to light or two

or more hours reduces the eectiveness

o trifuralin and related herbicides and

can be avoided by soil incorporation.



Plant Uptake

Herbicides may be absorbed by plant

roots or leaves and inactivated within

the plant. This eect generally accounts

or a relatively small amount o herbicide

removal.

Crop Reoval

I a crop is harvested or removed rom

the treated area beore rain has washed

the herbicide o the oliage or beore

the plant has had time to metabolize the

residue, the herbicide will be removed

with the crop. This seldom happens

because herbicides are not commonly

used close to harvest. However, i grass

clippings are collected shortly ater

treatment and used to mulch a garden,

there may be enough herbicide on the

grass to damage the garden plants.

Toxicity

Toxicity usually is measured as LD50

(lethal dose), which is the amount o a

toxicant required to kill 50 percent o the

test animals. The lower the LD50

, the less

pesticide it takes to kill the animal. As

with any chemical, whether naturally oc-

curring or synthetic, it is “the dose thatmakes the poison.” Below is a list o the

most commonly available herbicides, as

well as other commonly used substanc-

es, in order o decreasing oral toxicity.

Highly Toxic Herbicides

(LD50

< 50 g/kg)

The probable lethal dose o a highly

toxic herbicide or a 150-pound person

is a ew drops to 1 teaspoon. The label

contains the signal words “Danger/Poi-

son” and has a skull and crossbones.

metham (Vapam)

sodium arsenitea,b

moderately Toxic Herbicides

(LD50

= 50 to 500 g/kg)

The probable lethal dose o a moderate-

ly toxic herbicide or a 150-pound per-

son is 1 teaspoon to 1 ounce. The signal

word on the label reads “Warning.”

bromoxynil (Buctril)caeine

copper sulate (bluestone)

dienzoquat (Avenge)

diquat

endothall (Aquathol, Des-i-cate)

gasoline

kerosene

nicotine

paraquat (Gramoxone)

Slightly Toxic Herbicides

(LD50

= 500 to 5,000 g/kg)

The probable lethal dose o a slightly

toxic herbicide or a 150-pound person

is 1 ounce to 1 pint or 1 pound. The sig-

nal word on the label reads “Caution.”

aspirin

ethyl alcohol

sodium chloride (table salt)

acetochlor (Harness, Topnotch)

acifuoren (Blazer)c

alachlor (Micro-Tech, Lasso)c

ametryn (Evik)c,d

atrazine (various)d

bensulide (Betasan)

bentazon (Basagran)

butylate (Sutan+)d

CAMA (various)

clodinaop-propargyl (Discover)

clomazone (Command)

clopyralid (Stinger, Transline)c

cloridazon (Pyramin)

cycloate (Ro-Neet)

2,4-D (various)

2,4-DB (Butyrac 200, various)

2,4-DP, dichlorprop (various)

dicamba (Banvel, Clarity, Vanquish)

dichlobenil (Casoron)

dicloop-methyl (Hoelon)

dimethenamid (Frontier, Outlook)d

diuron (Karmex)d

DSMA (various)d

EPTC (Eptam, Eradicane)d

enoxaprop-P-ethyl (Acclaim, Puma)d

fuaziop-P-butyl (Fusilade)

fuenacet (Dene)

gluosinate (Liberty, Finale, Rely)

hexazinone (Velpar)d

linuron (Lorox)c

MCPA (various)d

MCPB (Thistrol)

MCPP, mecoprop (various)

metolachlor (Dual, Pennant)

metribuzin (Sencor, Lexone)

molinate (Ordram)MSMA (various)d

pebulate (Tillam)

pinoxaden (Axial)

prometon (Primatol)d

prometryn (Caparol)d

propachlor (Ramrod)c

propanil (Stam, Stampede)

pyridate (Tough)

quizaloop-P-ethyl (Assure II)



sethoxydim (Poast)

sodium chlorated

sulentrazone (Authority)

tebuthiuron (Spike)

terbacil (Sinbar)

thiobencarb (Bolero)

topramezone (Impact)

tralkoxydim (Achieve)

triallate (Far-Go)

triclopyr (Garlon)dvinegar (acetic acid)

Alost Nontoxic Herbicides

(LD50

> 5,000 g/kg)

The probable lethal dose o an almost

nontoxic herbicide or a 150-pound per-

son is more than 1 pint or 1 pound. The

signal word on the label reads

“Caution.”

asulam (Asulox)

benen (Balan)benzsuluron-methyl (Londax)

bromacil (Hyvar X)d

chlorimuron-ethyl (Classic)d

chlorsuluron (Glean, Telar)

clethodim (Select)

DCPA (Dacthal)

desmedipham (Betanex)

dithiopyr (Dimension)c

ethalfuralin (Sonalan)

ethoumesate (Prograss)

fucarbazone (Everest)

fumetsulam (Python)fumiclorac (Resource)

fuometuron (Cotoran)

omesaen (Flexstar, Refex)

oramsuluron (Option)

osamine (Krenite)

glyphosate (Roundup, Touchdown,

Rodeo, various)

halosuluron (Permit, Sempra)

iodosuluron (Autumn)

imazamethabenz (Assert)

imazamox (Raptor)

imazapic (Cadre, Plateau)

imazapyr (Arsenal, Chopper)d

imazaquin (Scepter, Image)

imazethapyr (Pursuit)

isoxaben (Gallery)

isoxafutole (Balance)

lactoen (Cobra)

metsuluron-methyl (Cimarron, Escort)

mesotrione (Callisto)

napropamide (Devrinol)

Prepared by Dwight D. Lingenelter,

agronomy extension associate in weed

science, and Nathan L. Hartwig, proes-

sor emeritus o weed science.

Visit Penn State’s College o Agricultural Sci-ences on the Web: www.cas.psu.edu

Penn State College o Agricultural Sciences

research, extension, and resident educationprograms are unded in part by Pennsylvaniacounties, the Commonwealth o Pennsylva-nia, and the U.S. Department o Agriculture.

This publication is available rom the Publica-tions Distribution Center, The PennsylvaniaState University, 112 Agricultural Administra-tion Building, University Park, PA 16802. Forinormation telephone 814-865-6713.

Where trade names appear, no discrimina-tion is intended, and no endorsement by thePenn State College o Agricultural Sciencesis implied.

This publication is available in alter-native media on request.

The Pennsylvania State University is com-mitted to the policy that all persons shallhave equal access to programs, acilities,admission, and employment without regard topersonal characteristics not related to ability,perormance, or qualications as determinedby University policy or by state or ederalauthorities. It is the policy o the University tomaintain an academic and work environmentree o discrimination, including harassment.The Pennsylvania State University prohibitsdiscrimination and harassment against anyperson because o age, ancestry, color, dis-

ability or handicap, national origin, race, reli-gious creed, sex, sexual orientation, genderidentity, or veteran status. Discrimination orharassment against aculty, sta, or studentswill not be tolerated at The PennsylvaniaState University. Direct all inquiries regardingthe nondiscrimination policy to the Arma-tive Action Director, The Pennsylvania StateUniversity, 328 Boucke Building, UniversityPark, PA 16802-5901; Tel 814-865-4700/V,814-863-1150/TTY.

Produced by Ag Communicationsand Marketing

© The Pennsylvania State University 2007

Code # UC175 R6M04/08graphtech3650

nicosuluron (Accent)

norfurazon (Zorial, Solicam)

oryzalin (Surfan)

oxadiazon (Ronstar)d

oxyfuoren (Goal)

pendimethalin (Prowl, Pendulum)

prodiamine (Barricade)

picloram (Tordon)

primisuluron-methyl (Beacon)

pronamide (Kerb)dprosuluron (Peak)

rimsuluron (Matrix, Resolve)

siduron (Tupersan)

simazine (Princep)

sodium borated

sulometuron-methyl (Oust)

sulosuluron (Maverick)

thiensuluron-methyl (Harmony GT)d

triasuluron (Amber)

trifuralin (Trefan)

tribenuron-methyl (Express)

Dermal response:

a. Absorbed and poisonous

b. Causes burns and blisters

c. Moderately irritating

d. Mildly irritating

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