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Introduction to Weeds and Herbicides

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There are numerous denitions of aweed. Some common denitionsinclude:

a plant that is out of place and notintentionally sown

a plant that grows where it is notwanted or welcomed

a plant whose virtues have not yet

been discovered a plant that is competitive, persistent,

pernicious, and interferes negativelywith human activity

No matter which denition is used,weeds are plants whose undesirablequalities outweigh their good points,at least according to humans. Humanactivities create weed problems sinceno plant is a weed in nature. Thoughwe may try to manipulate nature for our

own good, nature is persistent. Throughmanipulation, we control certain weeds,while other more serious weeds maythrive due to favorable growing condi-tions. Weeds are naturally strong com-petitors, and those weeds that can bestcompete always tend to dominate.

Both humans and nature are involvedin plant-breeding programs. The maindifference between the two programs isthat humans breed plants for yield, whilenature breeds plants for survival.

Abundant Seed Production

Weeds can produce tens or hundredsof thousands of seeds per plant, whilemost crop plants only produce severalhundred seeds per plant. The follow-ing are some examples of approximatenumbers of seeds produced per weed:

giant foxtail10,000

common ragweed15,000

purslane52,000

lambsquarters72,000

pigweed117,000

What Are Weeds and Their Impacts?

Characteristics of Weeds

There are approximately 250,000 spe-cies of plants worldwide; of those, about3 percent, or 8,000 species, behaveas weeds. Of those 8,000, only 200 to250 are major problems in worldwidecropping systems. A plant is considereda weed if it has certain characteristicsthat set it apart from other plant species.Weeds possess one or more of the fol-lowing characteristics that allow them tosurvive and increase in nature:

abundant seed production

rapid population establishment

seed dormancy

long-term survival of buried seed

adaptation for spread

presence of vegetative reproductivestructures

ability to occupy sites disturbed byhumans

Penn State WeedManagement Website:

extension.psu.edu/pests/ weeds

Some weeds, such as pigweed, canproduce several hundred thousand seedsper plant under optimal growing conditions.

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Since most weeds deposit their seedsback to the soil, seed numbers in thesoil increase rapidly from year to yearif the weeds are not managed. Despitethat many weed seeds are either notviable, eaten by animals or insects, ordecompose within several months afterthey are deposited, hundreds of millionsof viable weed seeds per acre can still

be present and waiting to germinate.

Rapid Population Establishment

Most weeds can germinate and be-come established relatively quickly.They also produce viable seeds evenunder environmental and soil conditionsthat are not favorable for most cropplants. Under ideal conditions, denseweed populations can thrive and easilyoutcompete a crop if left unchecked.Under poor conditions, certain weedscan adapt and produce some viableseeds in a relatively short time period (6to 8 weeks).

Seed Dormancy

Dormancy is basically a resting stageor a temporary state in which the weedseeds do not germinate because ofcertain factors. Dormancy is a survivalmechanism that prevents germinationwhen conditions for survival are poor.

For example, seeds of summer annualweeds will generally not germinate inthe fall, preventing them from beingkilled by cold winter conditions. Thevarious factors that affect dormancy aretemperature, moisture, oxygen, light, thepresence of chemical inhibitors, toughseed coat, and immature embryos.There are several kinds of dormancy,but the most commonly used terms todescribe dormancy are innate, induced,and enforced.

Innate or primary dormancy inhibitsgermination at the time seeds are shedfrom the plant. After the seed shattersfrom the parent plant, time is requiredfor immature embryos to develop, natu-ral inhibitors to leach out, or extremesof temperature to crack hard seed coatsand allow germination to occur. Theseconditions cause innate dormancy, and,once lost, this type of dormancy cannotreoccur.

Induced dormancy is a temporarydormancy that occurs when a seed isexposed to hot or cold temperatures.It continues after temperatures changeand prevents germination during thewrong time of year. The dormancy isbroken by temperatures opposite ofthose that induced it.

Summer heat induces dormancy in sum-mer annual weeds such as yellow foxtailand pigweed, preventing germinationin the fall. Cold temperatures in fall andwinter break this dormancy (usually bymid-winter), and the seeds germinatein spring when conditions are right. Inwinter annual weeds, the process isreversed.

Dormancy can be induced in manyweed seeds when a crop canopy lterssunlight, shading the ground and reduc-

ing germination. Dormancy can beinduced over and over again for as longas the seeds remain viable.

Enforced dormancy takes place whenenvironmental conditionscold temper-atures, lack of moisture or oxygen, andoccasionally a high salt concentration inthe soilare unfavorable. When limita-tions are removed, seeds germinatefreely. Summer annual weed seeds losetheir induced dormancy by mid-winter

and, if not for the cold temperatures,would germinate at that time.

Seeds of different weed species havevarious temperature requirements forgermination. Common chickweed cangerminate under snow cover, whilecommon purslane will not germinateuntil the soil temperature reaches 70 to75F. Crop seeds are generally plantedat or near the optimum soil tempera-ture needed for quick germinationatemperature that is also ideal for some

weed seeds.

Seeds require water for germination.Seeds in dry soils may remain dormanteven when all other factors promotinggermination are favorable.

Oxygen availability also inuences aseeds ability to germinate. Water may llsoil pores and exclude air, limiting ger-mination in very wet soils. Soil compac-tion also may reduce the oxygen supplyand prevent seeds from germinating.Deep plowing, tillage, or hoeing canbring buried seeds to the surface, wherethey readily germinate upon exposure to

oxygen.

Long-Term Survival of BuriedSeed

If conditions are adequate, buried weedseeds have the potential to remainviable for 40 years or more. Broadleafweed seeds tend to last longer in thesoil than grassy weed seed since theyusually have tougher seed coats. Inmost cases, the majority of seeds onlyexist in the soil for a few years due togermination, decomposition, predatorfeeding, or other factors. However, withthe large number of seeds produced, asmall percentage may remain viable forlong-term survival.

Adaptation for Spread

Weeds have certain mechanisms foreasy dispersal of seeds. Most seedsor seed pods have special structuresthat allow them to cling, y, or oat.Common cocklebur and burdock seedpods have hooks that attach to animalfur or feathers; curly dock seeds havebladder-like structures that allow themto oat; and milkweed, dandelion, andthistle seeds have a feathery pappusthat allows them to be carried by thewind. Other weeds, such as jewelweedor snapweed, have pods that explodewhen the seeds are mature, projectingthem several feet from the parent plant.Weeds can also be spread when ani-mals or birds eat their fruit and depositthe seeds with their droppings. Weedseeds can be widely spread throughcrop seeds, grains, feed hay, andstraw. These and other human activitiesprobably account for the long-distancespreading of weeds.

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Vegetative ReproductiveStructures

Most perennial weeds possess specialvegetative structures that allow them toreproduce asexually and survive. Theseperennial structures contain carbohy-drates (food reserves, sugars), havenumerous buds in which new plants canarise, and include the following:

stolons aboveground, horizontalstems that root at the nodes (e.g.,crabgrass, bermudagrass, groundivy)

rhizomes belowground, thickenedstems that grow horizontally in theupper soil layers (e.g., quackgrass,Johnsongrass, wirestem muhly,Canada thistle)

tubers enlarged rhizomes withcompressed internodes located atthe ends of rhizomes (e.g., yellownutsedge, Jerusalem artichoke, po-tato)

bulbs modied leaf tissues for car-bohydrate storage that are located atthe base of the stem or below the soilline (e.g., wild garlic, onion)

budding roots modied roots thatcan store carbohydrates and growboth vertically and horizontally (e.g.,hemp dogbane, Canada thistle)

Despite these vegetative reproductivestructures, many perennials also repro-duce by seed. Some depend heavily onreproduction by seed (e.g., dandelion),while for others it is less important (e.g.,yellow nutsedge).

Ability to Occupy Disturbed Sites

Weeds are very opportunistic. Whenconditions are adequate, weed seedsgerminate and colonize if left un-checked. When a site is disturbed,weeds are usually the rst to emerge. Ifa weed becomes established rst, it hasthe competitive advantage over cropplants or desirable vegetation.

Problems with Weeds

Weeds are troublesome in many ways.Primarily, they reduce crop yield bycompeting for:

water

light

soil nutrients

space

CO2

The following are other problems associ-ated with weeds:

reducing crop quality by contaminat-ing the commodity

interfering with harvest

serving as hosts for crop diseases or

providing shelter for insects to over-winter

limiting the choice of crop rotationsequences and cultural practices

producing chemical substances thatcan be allergins or toxins to humans,animals, or crop plants (allelopathy)

producing thorns and woody stemsthat cause irritations and abrasions toskin, mouths, or hooves of livestock

being unsightly, dominant, aggres-sive, or unattractive

obstructing visibility along roadways,interfering with delivery of publicutilities (power lines, telephone wires),obstructing the ow of water in waterways, and creating re hazards

accelerating deterioration of recre-ational areas, parking lots, buildings,and equipment

invading exotic weed species thatcan displace native species in stabi-lized natural areas

Quackgrass reproduces primarily byshoots from underground rhizomes, but itcan also reproduce by seeds.

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Costs of Weeds

Weeds are common on all 485 millionacres of U.S. cropland and almost onebillion acres of range and pasture. Sinceweeds are so common, people generallydo not understand their economic im-pact on crop losses and control costs. In2003, it was estimated that the nonuse

of herbicides and the likely substitutionof alternatives (i.e., cultivation, hand-weeding) would result in a loss of $13.3billion in food and ber production.The total impact of herbicide nonusewould be an income loss of $21 billion,which includes $7.7 billion in increasedcosts for weed control and $13.3 billionin yield losses. In the early 1990s, theestimated average annual monetary losscaused by weeds, with current controlstrategies in the 46 crops grown in the

United States, was over $4 billion. Ifherbicides were not used, this loss wasestimated to be $20 billion. Losses ineld crops accounted for over 80 per-cent of this total. Other sources estimatethat U.S. farmers annually spend over$3.5 billion on chemical weed controland over $2.5 billion for cultural andother methods of control. The total costof weeds in the United States could ap-proach $15 to $20 billion. Weed control

and other input costs (e.g., seed, fertil-izer, other pesticides, fuel) vary with thecrop. For example, in the mid-1990s,herbicides for soybeans cost about$30 per acre, or about half of the totalper-acre purchased input. For corn, thecost was about $32 per acre, or about aquarter of the total per-acre purchasedinput. Weed control costs for wheat areabout $6 per acre, or about 5 percentof the total per-acre purchased inputs.

A decade later, these costs are aboutthe same. However, in most situations,herbicide use is still the most economi-cal means to control weeds. The USDAestimates that weed control costs fororganic vegetable growers in Californiacan be $1,000 per acre in comparison to$50 per acre that conventional growersspend on herbicides. Several factorshelp determine the relative costs ofherbicides from one crop to another,

including the competitive ability of thecrop, the weeds present, the contribu-tion of nonchemical control practices,the tillage method, management deci-sions, the type of crop seed used (e.g.,normal versus resistant GMO variety),and the value of the crop. Weeds notonly cause losses in crops, but also canaffect livestock production if poisonousweeds are present or weeds invade andrender the pasture useless.

Benets of Weeds

Despite the negative impacts of weeds,some plants usually thought of as weedsmay actually provide some benets,such as:

stabilizing and adding organic matterto soils

providing habitat and feed for wildlife

providing nectar for bees

offering aesthetic qualities

serving as a genetic reservoir forimproved crops

providing products for human con-sumption and medicinal use

creating employment opportunities

Weeds have a controversial nature. But

to the agriculturist, they are plants thatneed to be managed in an economicaland practical way in order to producefood, feed, and ber for humans andanimals. In this context, the negativeimpacts of weeds indirectly affect allliving beings.

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

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Origins of Weeds

Weeds are found throughout the world.However, all are not common in everyregion. Since weeds can be easilyspread, more and more are being dis-seminated to places where they werenot originally found. Only about 40 per-cent of the weeds found in the UnitedStates are native, while the remaining 60percent are considered exotic or import-ed. The following are some examples ofweeds and their origins:

United States common and giantragweed, common milkweed, fallpanicum, common cocklebur, poisonivy, marestail (horseweed), night-shade, wild or common sunower,and wild onion

South America pigweed speciesand prickly sida

Europe quackgrass, chickweed,Canada thistle, common lambsquar-ters, common purslane, wild garlic,and yellow foxtail

Asia or Africa Johnsongrass, wildcarrot, giant foxtail, velvetleaf, kudzu,and witchweed

Questions and AnswersRegarding Concerns aboutNonnative, Invasive Plants

Adapted from Swearingen, J., K.Reshetiloff, B. Slattery, and S. Zwicker.

2002. Plant Invaders of Mid-AtlaticNatural Areas. Washington, D.C.:National Park Service and U.S. Fish &Wildlife Service.

What Are Native Species?

A native species occurs naturally in aparticular place without human interven-tion. Species native to North Americaare generally recognized as those occur-ring on the continent prior to Europeansettlement. Nonnative plants are speciesthat have been introduced to an areaby people from other continents, states,

ecosystems, and habitats. Many non

native plants have great economic valuefor agriculture, forestry, horticulture, andother industries and pose little to nothreat to our natural ecosystems. Othershave become invasive and pose a seri-ous ecological threat.

What Are Invasive Plants?

Invasive plants reproduce rapidly, spread

over large areas of the landscape, andhave few, if any, natural controls, suchas herbivores and diseases, to keepthem in check. Many invasive plantsshare some important characteristicsthat allow them to grow out of control:(1) spreading aggressively by runners orrhizomes; (2) producing large numbersof seeds that survive to germinate; and(3) dispersing seeds away from the par-ent plant through various means such aswind, water, wildlife, and people.

Weed Ecology and Biology

Invasive plants, such as exotic honeysuckles, are aggressive, displace native species,reduce land value, and can be difcult and expensive to control.

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How Are Invasive PlantsIntroduced?

People introduce exotic plants to newareas, on purpose and by accident,through a variety of means. Some spe-cies (e.g., kudzu, kochia, multiora rose,Japanese knotweed, and Johnsongrass)are introduced for use in gardening andlandscaping or for erosion control, for-age, and other purposes. Others comein unknowingly on various importedproducts or in soil, water, and othermaterials used for ship ballast. Manyinvasive aquatic plants are introducedby dumping unwanted aquarium plantsinto waterways. Once established in anew environment, some exotic speciesproliferate and expand over large areas,becoming invasive pests.

How Do Invasive Plants Spread?

Invasive plants spread by seed, vegeta-tive growth (producing new plants fromrhizomes, shoots, tubers, etc.), or both.Seeds, roots, and other plant fragmentsare often dispersed by wind, water, andwildlife. Animals spread invasive plantsby consuming fruits and depositingseeds, as well as by transporting seedson their feet and fur. People also helpspread invasive plants by carrying seedsand other plant parts on shoes, clothing,

and equipment and by using contami-nated ll dirt and mulch. Invasive aquaticplants are often spread when plant partsattach to boat anchors and propellers.

Why Are Invasive Plants aProblem in Natural Areas?

Like an invading army, invasive plantsare taking over and degrading naturalecosystems. Invasive plants disrupt theintricate web of life for plants, animals,and microorganisms and compete forlimited natural resources. Invasive plantsimpact nature in many ways, includinggrowing and spreading rapidly over largeareas, displacing native plants (includingsome very rare species), reducing foodand shelter for native wildlife, eliminat-ing host plants of native insects, andcompeting for native plant pollinators.Some invasives spread so rapidly thatthey displace most other plants, chang-ing a forest, meadow, or wetland into alandscape dominated by one species.Such monocultures (stands of a singleplant species) have little ecological valueand greatly reduce the natural biologicaldiversity of an area.

Invasive plants also affect the type ofrecreational activities that we can enjoyin natural areas, such as boating, birdwatching, shing, and exploring. Someinvasives become so thick that access-ing waterways, forests, and other areasis impossible. Once established, inva-sive plants require enormous amountsof time, labor, and money to control

or eliminate. Invasive species cost theUnited States an estimated $34.7 billioneach year in control efforts and agricul-tural losses.

How to Prevent Spread ofInvasive Plants

Become familiar with invasive plant spe-cies in your area (Table 1). When select-ing plants for landscaping, avoid usingknown invasive species and those exoticspecies exhibiting invasive qualities.

Ask for native plant alternatives at yournursery. Obtain a list of plants native toyour state from your native plant society,state natural resources agency, or theU.S. Fish and Wildlife Service. If youhave already planted invasives on yourproperty, consider removing them andreplacing them with native species.

Table 1. List of selected invasive plantspecies common to the Northeast. Foradditional information about these andother invasive plants refer towww.invasive.org/eastern/midatlantic.

Aquatic Plants

Eurasian watermilfoil ( Myriophyllum spicatum )

Hydrilla ( Hydrilla verticillata )

Water chestnut ( Trapa natans )

Herbaceous Plants

Garlic mustard ( Alliaria petiolata )

Japanese knotweed ( Polygonum cuspidatum )

Japanese stiltgrass ( Microstegium vimineum )

Purple loosestrife ( 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 )

Multiora rose ( Rosa multiora )

Privets ( Ligustrum species)

Winged burning bush ( Euonymus alata )

Buttery bush ( Buddleja species)

Trees

Bradford pear ( Pyrus calleryana Bradford)

Norway maple ( Acer platanoides )

Tree of Heaven ( Ailanthus altissima )

Vines

Kudzu ( Pueraria montana v. lobata )

Mile-a-minute ( Polygonum perfoliatum )

Oriental bittersweet ( Celastrus orbiculatus )

Porcelainberry ( Ampelopsis brevipedunculata

Japanese honeysuckle ( Lonicera japonica )

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Classication of Weeds

Almost all plants are categorized bysome sort of plant classication systemand given a scientic name to identifythem anywhere in the world. Weedsare also classied by various means. Ingeneral, they can be classied by their

structure and appearance (for example,dicots [broadleaves] and monocots[grasses and sedges]), habitat, or physi-ology. A common categorization systemgroups them according to their life cycle(how long they live). The three major lifecycle groups are annuals, biennials, andperennials.

Annuals

Annuals are generally divided furtherinto summer annual and winter annualweeds. Summer annuals germinatein the spring, mature, produce seed,and die in one growing season. Largecrabgrass, giant foxtail, smooth pig-weed, common lambsquarters, commonragweed, velvetleaf, hairy galinsoga,and common purslane are examples oftroublesome summer annuals.

Winter annuals germinate in late sum-mer or fall, mature, produce seed, andthen die the following spring or sum-

mer. Examples of winter annuals includecommon chickweed, henbit, shep-herdspurse, downy brome, and annualbluegrass. (Some annual bluegrasssubspecies can occasionally function asa perennial.)

Biennials

Biennial weeds grow from seed any-time during the growing season. Theynormally produce a rosette of leavesclose to the soil surface the rst year,then ower, mature, and die duringthe second year. A true biennial neverproduces owers or seeds the rst year.

There are relatively few biennial weeds.Some examples include wild carrot,common burdock, bull and musk thistle,and poison hemlock.

Perennials

Perennial weeds live for more thantwo years and can be divided into twogroups: simple and creeping. Simple

perennials form a deep taproot andspread primarily by seed dispersal.Some examples of simple perennialsinclude dandelion, broadleaf plantain,

curly/broadleaf dock, and commonpokeweed. Creeping perennials may beeither herbaceous or woody and canspread by both vegetative structuresas well as by seed. Some commonherbaceous perennials include Canadathistle, common milkweed, hempdogbane, creeping buttercup, slenderspeedwell, ground ivy, quackgrass, andyellow nutsedge. Some examples ofwoody perennials include poison ivy,multiora rose, Japanese knotweed/ bamboo, brambles, wild grape, and

Virginia creeper. Creeping perennialsbecome established by seed or by veg-etative parts. Since perennial weeds liveindenitely, their persistence and spreadis not as dependent upon seed as theother two weed groups.

Common chickweed can be a problemin eld crops, gardens, lawns, and manyother areas.

Biennials, such as wild carrot, are con-trolled more easily during their rst year ofgrowth.

For perennial weed control, the besttime to either mow or apply an effec-tive herbicide is during the bud to bloomgrowth stage and/or in the fall.

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Since weeds are so prevalent in manyareas of the landscape, managementtechniques are necessary to maintainorder. Weed management is most suc-cessful when it involves an integratedapproach using a variety of methods.The common methods used to manageweeds include prevention and cultural,mechanical, biological, and chemicalmeans.

Weed Management Techniques

Prevention

Preventative methods are used to stopthe spread of weeds. Preventing theintroduction of weeds is usually easierthan controlling them after establish-ment. Preventative practices includecleaning tillage and harvesting equip-ment of weed seeds and vegetativestructures; planting certied, weed-freecrop seed; and controlling weeds inbarnyards, around structures, and alongfencerows, roadways, and ditch banks.

Cultural

Cultural and crop management tech-niques provide a healthy crop to bestcompete with weeds. Crop competitioncan be an inexpensive and effectiveaid to weed management if used to itsfullest advantage. Examples of culturaltechniques include following soil testrecommendations for fertilizer andlime; selecting the best crop varieties;planting dense crop populations at theproper timing; scouting elds regularlyfor weeds, insects, and diseases andcontrolling them when necessary; andincluding crop rotations in the system.Composting, ensiling, or feeding weedsor weed-infested crops to livestock candestroy the viability of weed seeds. Theheat and/or digestive acids break downthe majority of weed seeds. However,

some seeds pass through livestockunharmed and can germinate if spreadback onto the land.

Preventing weed spread includes controlling weeds around barns and along fences, roads,ditches, and woodlands.

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Mechanical

Mechanical or physical techniques eitherdestroy weeds or make the environmentless favorable for seed germination andweed survival. These techniques includehand-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 impedeweed growth.

Biological

Biological weed control involves theuse of other living organisms, such asinsects, diseases, or livestock, for themanagement of certain weeds. In theory,biological control is well suited for anintegrated weed management program.

However, the limitations of biologicalcontrol are that it is a long-term under-taking, its effects are neither immediatenor always adequate, only certain weedsare potential candidates, and the rate offailure for past biological control effortshas been fairly high. There have beena few success stories of weed speciesbeing managed with insect or diseasebiocontrol agents. Herbivores such assheep and goats can provide success-ful control of some common pastureweeds. Research continues in this areaof weed management.

Chemical

Herbicides can be dened as crop-protecting chemicals used to kill weedyplants or interrupt normal plant growth.Herbicides provide a convenient,economical, and effective way to helpmanage weeds. They allow elds to be

planted with less tillage, allow earlierplanting dates, and provide additionaltime to perform the other tasks that farmor personal life require. Due to reducedtillage, soil erosion has been reducedfrom about 3.5 billion tons in 1938 toone billion tons in 1997, thus reduc-ing soil from entering waterways anddecreasing the quality of the nationssurface water. Without herbicide use,no-till agriculture becomes impossible.However, herbicide use also carries risksthat include environmental, ecological,and human health effects. It is importantto understand both the benets anddisadvantages associated with chemicalweed control before selecting the ap-propriate control.

Herbicides may not be a necessity onsome farms or landscape settings,but without the use of chemical weedcontrol, mechanical and cultural controlmethods become that much moreimportant. There are many kinds of

herbicides from which to choose. Manyfactors determine when, where, and howa particular herbicide can be used mosteffectively. Understanding some of thesefactors enables you to use herbicides totheir maximum advantage.

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

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Herbicides can be classied severalways, including by weed control spec-trum, labeled crop usage, chemical fam-ilies, mode of action, application timing/ method, and others. For this publication,herbicides will be grouped according tomode and site of action, which are alsoimportant in understanding herbicideresistance in weeds.

Contact herbicides kill only the plantparts contacted by the chemical, where-as systemic herbicides are absorbedby the roots or foliage and translocated(moved) throughout the plant. Herbicideactivity can be either selective or non-selective. Selective herbicides are usedto kill weeds without signicant dam-age to desirable plants. Nonselectiveherbicides kill or injure all plants presentif applied at an adequate rate.

A common method of grouping her-bicides is by their mode of action.

Although a large number of herbicidesare available in the marketplace, severalhave similar chemical properties andherbicidal activity. Herbicides with acommon chemistry are grouped intofamilies. Also, two or more familiesmay have the same mode of action,and thus can be grouped into classes.Table 2 lists several groups of herbicidesand information related to their mode ofaction.

The following section provides a briefoverview of herbicide functions in theplant and associated injury symptomsfor each of the herbicide classes foundin Table 2.

Herbicides

Herbicide Mode and Site of Action

To be effective, herbicides must (1) ad-equately contact plants, (2) be absorbedby plants, (3) move within the plants tothe site of action without being deacti-vated, and (4) reach toxic levels at thesite of action. The term mode of actionrefers to the sequence of events fromabsorption into plants to plant death, or,in other words, how an herbicide worksto injure or kill the plant. The specicsite the herbicide affects is referred toas the site or mechanism of action.Understanding herbicide mode of actionis helpful in knowing what groups ofweeds are killed, specifying applicationtechniques, diagnosing herbicide injuryproblems, and preventing herbicide-resistant weeds.

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

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Table 2. Important herbicide groups and examples for agronomic and horticultural crops, turf, forestry, and industrial areas inPennsylvania.

Mode of Action(Class) Site of 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, Status, Vanquish

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

auxin transport 19 phthalamates naptalam Alanapinhibitors semicarbazones diufenzopyr component of Distinct,

Status

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

imazapyr Arsenalimazaquin Scepter

imazethapyr Pursuit sulfonylurea chlorimuron Classic

chlorsulfuron Glean, Telarforamsulfuron Optionhalosulfuron Permit, Sandea,

SedgeHammeriodosulfuron Autumn, Equipmetsulfuron Cimarron, Escortnicosulfuron Accentprimisulfuron Beaconprosulfuron Peakrimsulfuron Matrix, Resolvesulfometuron Oustsulfosulfuron Maverick, Outriderthifensulfuron Harmony GT

triasulfuron Ambertribenuron Express

sulfonylaminocarbonyl- ucarbazone Everesttriazolinones propoxycarbazone Olympus

triazolopyrimidine cloransulam FirstRate(sulfonamides) umetsulam Python

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

many others

Fatty acid (lipid) ACCase enzyme 1 aryloxyphenoxy clodinafop Discoverbiosynthesis propionates diclofop Hoeloninhibitors fenoxaprop Acclaim, Puma

uazifop Fusiladequizalofop Assure II

cyclohexanediones clethodim Selectsethoxydim Poasttralkoxydim Achieve

phenylpyrazoline pinoxaden Axial

continued on next page

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Table 2. continued

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

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

pendimethalin Prowl, Pendulumprodiamine Barricade, Endurancetriuralin Trean, 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 Intrrodimethenamid Outlookmetolachlor Dual, Pennantpropachlor Ramrod

oxyacetamides ufenacet Dene

lipid synthesis 8 thiocarbamates butylate Sutaninhibitors EPTC Eptam

pebulate Tillamvernolate Vernam

cell division inhibitors 8 phosphorodithioates bensulide Prefar

15 acetamides napropamide Devrinol

VLCFA inhibitor 15 isoxazoline pyroxasulfone Zidua

Photosynthesis photosystem II 5 triazines ametryn Evik inhibitors atrazine Atrazine(mobile 1) propazine Milo Pro

prometon Primito

simazine Princep triazinones hexazinone Velpar

metribuzin Sencor

uracils bromacil Hyvarterbacil Sinbar

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

Photosynthesis photosystem II 6 nitriles bromoxynil Buctrilinhibitors (nonmobile; benzothiadiazoles bentazon Basagranrapid-acting)

phenyl-pyridazines pyridate Tough

continued on next page

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Table 2. continued

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

Cell membrane PPO enzyme 14 diphenyl ethers aciuorfen Ultradisrupters fomesafen Reex, Flexstar

lactofen Cobraoxyuorfen Goal

N-phenyl-phthalimides umioxazin Valorumiclorac Resource

oxadiazoles oxadiazon Ronstar

pyrimidinediones saufenacil Sharpen

thiadiazoles uthiacet Cadet

triazolinones carfentrazone Aimsulfentrazone Authority, Spartan

photosystem I 22 bipyridyliums diquat Reward, Regloneparaquat Gramoxone

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

4-HPPD enzyme 27 isoxazoles isoxautole Balance

triketones mesotrione Callisto

tembotrione Laudistopramezone Impact

Phosphorylated GS enzyme 10 amino acid derivatives glufosinate Liberty, Finale, Relyamino acid (phosphinic acids)(N-metabolism disrupters)

Unknown ? ? dazomet Basamidendothall Aquatholfosamine 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.

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Plant Growth Regulators (PGRs)

These herbicides are effective on an-nual and perennial broadleaf plants andusually have no activity on grasses orsedges, except at high application rates.They produce responses similar to thoseof natural, growth-regulating substancescalled auxins. Application of articial

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

Cells of leaf veins rapidly divide andelongate, while cells between veinscease to divide. This results in long,narrow, strap-like young leaves.

Water content increases, makingtreated plants brittle and easily bro-ken.

Cell division and respiration rates in-crease, and photosynthesis decreas-

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

Roots of treated plants lose their abil-ity to take up soil nutrients, and stemtissues fail to move food effectivelythrough the plant.

The killing action of growth-regulatingchemicals is not caused by any singlefactor but results from the effects ofmultiple disturbances in the treatedplant.

Injury SymptomsBroadleaf plant leaves become crinkled,puckered, strap shaped, stunted, andmalformed; leaf veins appear parallelrather than netted, and stems becomecrooked, twisted, and brittle, withshortened internodes. If injury occursin grasses (e.g., corn), new leaves donot unfurl but remain tightly rolled inonion-like fashion, and stems becomebrittle, curved, or crooked, with shortinternodes. A lesser effect in corn is thefusion of brace roots, noticed later in theseason.

Amino Acid BiosynthesisInhibitors

These herbicides are effective mostly onannual broadleaves, while a few in thislarge group have activity on grasses,nutsedge, and/or perennial plants.(Glyphosate [Roundup], for example,is a broad-spectrum herbicide and hasactivity on all types of plants.) Theseherbicides work by interfering with oneor more key enzymes that catalyze theproduction of specic amino acids inthe plant. When a key amino acid is notproduced, the plants metabolic pro-cesses begin to shut down. The effect islike that of an assembly line worker notdoing his or her job. Different herbicidesaffect different enzymes that catalyzethe production of various amino acids,but the result is generally the samethe shutdown of metabolic activity witheventual death of the plant.

Injury SymptomsPlants that are sensitive to these herbi-cides stop growth almost immediatelyafter foliar treatment; seedlings die intwo to four days, established perenni-als in two to four weeks. Plants becomestraw colored several days or weeksafter treatment, gradually turn brown,and die.

Fatty Acid (Lipid) BiosynthesisInhibitors

These herbicides are rapidly absorbedby grasses and are translocated tothe growing points, where they inhibitmeristematic activity, stopping growthalmost immediately. They have no activ-ity on broadleaf plants and are mosteffective on warm-season grasses suchas Johnsongrass, shattercane, corn, fallpanicum, giant foxtail, and crabgrass.Cool-season grasses such as quack-grass, annual and perennial ryegrass,orchardgrass, timothy, and small grainsare not as sensitive as the warm-seasongrasses. Some of these herbicides areweaker on perennial species than otherproducts. They are frequently referred toas post-grass herbicides.

Injury Symptoms

Growing points are killed rst, resultingin the death of the leaves inner whorl.Older, outer leaves of seedlings ap-pear healthy for a few days, and thoseof perennials for a couple of weeks,but eventually they also wither and die.

After several weeks, the growing pointsbegin to rot, allowing the inner leavesto be pulled out of the whorl. Sensitivegrasses commonly turn a purplish colorbefore dying.

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Photosynthesis Inhibitors(Nonmobile; Rapid-Acting)

Herbicides in this group have activityon primarily annual and some perennialbroadleaves and are applied to the plantfoliage. The mode of action is the sameas the mobile photosynthesis inhibitors.

Injury SymptomsTheir activity within the plant is similar tothat of the mobile photosynthesis inhibi-tors, except the injury occurs at the siteof contact, causing leaf burning andeventual death of the plant.

Cell Membrane Disrupters

These herbicides control mostlybroadleaves. Certain products havesome activity on grasses, and paraquat(Gramoxone) provides broad-spectrum

control of many different species.These herbicides are referred to ascontact herbicides and they kill weedsby destroying cell membranes. They ap-pear to burn plant tissues within hoursor days of application. Good coverageof the plant tissue and bright sunlightare necessary for maximum activity. Theactivity of these herbicides is delayed inthe absence of light.

Injury Symptoms

All contact herbicides cause cellularbreakdown by destroying cell mem-branes, allowing cell sap to leak out.Effected plants initially have a water-soaked appearance, followed by rapidwilting and burning, or leaf specklingand browning. Plant death occurs withina few days.

Pigment Inhibitors

These herbicides provide control ofmany annual broadleaves and somegrasses. These products are referred toas bleachers since they inhibit carote-noid biosynthesis or the HPPD enzymeby interfering with normal chlorophyllformation.

Injury SymptomsSymptoms are very evident and easyto identify. Effected plants either do notemerge or emerge white or bleachedand eventually die. Older leaf tissue isaffected rst.

Phosphorylated Amino Acid(Nitrogen Metabolism)Disrupters

This herbicide provides broad-spectrumcontrol of most annual grasses andbroadleaves and some perennials. Itaffects growth by disrupting nitrogenmetabolism, thus interfering with otherplant processes. It is a contact herbicidewith slight translocation throughout theplant. Good spray coverage and sunlightare important for maximum efcacy.

Injury SymptomsInjury is similar to that of the cell mem-brane disrupter herbicides. Sensitiveplants show leaf burning, yellowing

and browning, and eventual death aftera week or so. Perennials generally takelonger for symptoms and death to occur.

Unknown Herbicides

This category contains miscellaneousproducts for which the mode of ac-tion and family are unknown. Dazomet(Basamid) and metam (Vapam) are con-sidered soil fumigants. These productsare applied to the soil and covered witha gas-tight tarp; there, they are con-

verted to gases and penetrate the soilto kill weeds, diseases, and nematodes.Endothall (Aquathol) is used for aquaticweed control. Fosamine (Krenite) is usedin noncrop areas to control perennialweeds and brush. Other compoundssuch as pelargonic acid (Scythe), afatty-acid herbicide, and clove oil andvinegar are contact, nonselective,broad-spectrum, foliar-applied productsthat are sometimes used for weed con-trol in organic crop production settings.However, because they basically burnonly the plant tissue they contact, thereis potential for plant regrowth.

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Herbicide Resistance

A number of weed species that wereonce susceptible to and easily managedby certain herbicides have developedresistance. These weeds are no longercontrolled by applications of previ-ously effective herbicides. As a result

of repeatedly using a certain type ofherbicide on the same land, many dif-ferent species of weeds have developedresistance to these chemicals. Cur-rently, about 225 weed species (morethan 400 weed biotypes) worldwide areresistant to about ten different herbi-cide families. It is believed that withinany population of weeds, a few plantshave sufcient tolerance to survive anyherbicide that is used. Since only thesurvivors can produce seed, it is onlya matter of time until the population ofresistant weeds outnumbers the suscep-tible type. Depending on the herbicidefamily and weed species, resistancecan occur within 5 to 20 years. Certainprecautions, such as tank-mixing, croprotations, and a combination of weedmanagement techniques, must be takento prevent resistance.

Growers, consultants, and those work-ing with herbicides to manage weedsshould know which herbicides are best

suited to combat specic resistantweeds. The Weed Science Society of

America (WSSA) developed a groupingsystem to help with this process. Her-bicides that are classied as the sameWSSA group number kill weeds usingthe same mode of action. WSSA groupnumbers can be found on many herbi-cide product labels and can be used asa tool to choose herbicides in differentmode of action groups so mixtures orrotations of active ingredients can be

planned to better manage weeds and re-duce the potential for resistant species.Refer to Table 2 (pp. 1214) for WSSAmode of action group numbers and cor-responding herbicides.

Times of Application

The following terms describe herbicidesbased on when they are applied:

Preplant incorporated: applied to soiland mechanically incorporated intothe top 2 to 3 inches of soil beforethe crop is planted

Preplant: applied to soil before thecrop is planted

Preemergence: applied after the cropis planted but before it emerges

Postemergence: applied after cropemergence

Although these terms normally refer toapplication in relation to crops, theymay also imply application in relationto weeds. Always be certain whetherreference is being made to the crop orto the weed. In no-till situations, it ispossible for an herbicide application tobe preplant or preemergence to the cropbut postemergence to weeds. Someherbicides must be preplant or preemer-gence to the weed for maximum activity.

Methods of Application

The following terms refer to the ways

herbicides can be applied: Broadcast: applied over the entire

eld

Band: applied to a narrow strip overthe crop row

Directed: applied between the rowsof crop plants with little or no herbi-cide applied to the crop foliage

Spot treatment: applied to small,weed-infested areas within a eld

Product Formulations

Herbicides are not sold as pure chemi-cals, but as mixtures or formulationsof one or more herbicides with variousadditives. Adjuvants (surfactants, emul-siers, wetting agents, etc.) or variousdiluents may increase the effectiveness

of a pure herbicide. The type of formula-tion determines toxicity to plants, uni-formity of plant coverage, and stabilityin storage. Herbicides are formulated topermit uniform and easy application asliquid sprays or dry granules.

Some everyday household produts areformulated similarly to herbicide prod-ucts. These similarities will be noted inthe sections below.

Emulsiable concentrates (EC or E) are

liquid formulations with an active ingre-dient that is dissolved in one or morepetroleum-based solvents. An emulsieris added to cause oil to form tiny glob-ules that disperse in water. The formula-tion then will mix readily with water forproper application. Emulsiable concen-trates usually contain between 2 and 8pounds of active ingredient per gallon.Dual II Magnum, Pennant, Acclaim, andProwl are generally emulsiable herbi-cide formulations. (Household product

with similar formulationPine-Sol.)Emulsiable gels (EG or GL) are herbi-cides that traditionally are emulsiableliquids formulated as gels. The gels typi-cally are packaged in water-soluble bags(WSB) and are stable at temperaturesranging from 20 to 500C. In addition,the gelling process reduces the needfor nonaqueous solvents, compared tostandard nonaqueous EC-type formula-tion processes. Currently, few herbicidesare formulated as gels.

Wettable powders (WP or W) are nelyground, dry particles that may be dis-persed and suspended in water. Theycontain from 25 to 80 percent activeingredient. Suspensions of wettablepowders appear cloudy. Wettable pow-ders are nearly insoluble and require agi-tation to remain in suspension. Atrazine,Kerb, and Dacthal are formulated aswettable powders. (Household products

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with similar formulationcocoa mix andour.)

Soluble liquid (S) and soluble powders (SP) dissolve in water to form a true so-lution. Once the soluble liquid or powderis dissolved, the spray mixture requiresno additional mixing or agitation. Fewherbicides are available as solublesbecause most active ingredients ofherbicides are not very soluble in water.2,4-D amine and Roundup are examplesof soluble liquid herbicide formula-tions. (Household products with similarformulationgrape juice concentrateand Kool-Aid mix.)

Dry owables (DF), also called water-dispersible granules (WDG or WG) ordispersible granules (DG) are wettable

powders formed into prills so they poureasily into the sprayer tank withoutclumping or producing a cloud of dust.Nearly insoluble, they require agitationto remain in suspension. Many herbi-cides are now formulated in this fashion.

Atrazine, Accent, Gallery, and Pendulumare examples of products formulated aswater-dispersible granules. (Householdproducts with similar formulationgritsand 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 aliquid carrier so they can be poured orpumped from one tank to another. Theyusually contain at least 4 pounds ofactive ingredient per gallon of formula-tion. Flowables are nearly insoluble in

water and require agitation to remainin suspension. Suspoemulsion (SE) is acombination formulation of an SC andan oil-based emulsion (E). Atrazine,Princep, and Callisto are formulated asowables or SCs. (Household productswith similar formulationPepto-Bismoland V8 vegetable juice.)

Microencapsulated (ME or MT) and cap- sule suspension (CS) herbicides are en-cased in extremely small capsules thatcan be suspended in a liquid carrier andpumped and applied with normal equip-ment. Microencapsulated formulationsare nearly insoluble in water and requireagitation to remain in suspension. Micro-Tech, Prowl H2O, and Command are for-mulated in microcapsules, allowing theactive ingredient to be slowly releasedover a period of time. This extends thesoil activity and improves weed control.(Household product with similar formu-lationolder versions of Contac coldcapsules.)

Granules (G) are formulated with apremixed carrier that contains a low per-centage of active ingredient. The carriermay be fertilizer, clay, lime, vermiculite,or ground corn cobs. These herbicidesare applied directly (dry) to the soilwithout further dilution. The performanceof granulated herbicides compared withthat of sprayable formulations varieswith the herbicide. Granular forms gen-erally require more rainfall for activation

than do sprayable formulations. Granuleherbicides are used often in turf andornamental settings. Some examplesinclude Balan and Ronstar. (Householdproducts with similar formulationcatlitter and Grape-Nuts cereal.)

Pellets (P) are like granules but are com-pressed into larger cylinders about inch long. Herbicides formulated as pel-lets usually contain from 5 to 20 percentactive material and are hand-applied to

control clumps of brush. They also maybe applied with cyclone-type spinnerspreaders mounted on helicopters oraircraft to control brush in forests or per-manent pastures. Pellets gradually breakdown from rainfall and leach into the soilfor root uptake. Spike is an example of apelleted herbicide. (Household productwith similar formulationguinea pig/

rabbit pellets.)

Premixes are not formulations, but twoor more herbicide active ingredientsmixed into one product by the manufac-turer. The actual formulation can be anyof those discussed above and com-monly combines two or more herbicidesthat are already used together. Theprimary reason for using premixes isconvenience. Many herbicide productsare now marketed as premixes.

Trade Name and FormulationNotations

In certain publications, many herbicidesare listed by trade name (or productname) and formulation (for example,Roundup 4S or Accent 75WDG).Roundup is the trade name, and 4Sstands for 4 pounds of active ingredi-ent (glyphosate) per gallon of productin a soluble (S) formulation. Accent isformulated as a water-dispersible gran-

ule with each granule (or certain unit)containing 75 percent active ingredient(nicosulfuron). The remaining parts ofthe formulation contain inert ingredients,which have no effect on weed control.

Additional information about formula-tion and ingredients can be found on theproducts label and MSDS sheet.

Herbicide Spray Additives(Adjuvants)

Additives or adjuvants are substances in

herbicide formulations or that are addedto the spray mixture to improve herbicid-al activity or application characteristics.More than 70 percent of all herbicidesrecommend using one or more adju-vants in the spray mixture. In general,there are two types of adjuvants: formu-lation and spray. Formulation adjuvantsare already in the container from themanufacturing process. These help with

Most herbicides are sold in a liquid or dryformulations.

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mixing, handling, effectiveness, andproviding consistent performance.

Spray adjuvants can be divided intospecial purpose adjuvants and activatoradjuvants. Special purpose adjuvantsinclude compatibility agents, bufferingagents, antifoam agents, drift retardants,and others that widen the range of con-

ditions for herbicide use. Activator ad- juvants are commonly used to enhancepostemergence herbicide performanceby increasing herbicide activity, absorp-tion, and rainfastness and by decreasingphotodegradation. These include surfac-tants (i.e., surface active agents), cropoil concentrates, vegetable oil concen-trates, wetting agents, stickers-spreaders, N-fertilizers, penetrants, andothers. Commonly used surfactants arenonionic surfactants and organosili-cones and are typically used at a rateof 1 quart per 100 gallons (0.25 percentv/v) of spray mixture. Crop oil concen-trates are 80 to 85 percent petroleumbased plus 15 to 20 percent surfactant,while vegetable oil concentrates containvegetable or seed oil in place of petro-leum oil. Oil concentrates are typicallyincluded at a rate of 1 gallon per 100gallons (1 percent v/v) of spray mixture.In general, oil concentrates are hotterthan surfactants, so they provide betterherbicide penetration into weeds underhot/dry conditions, but they are morelikely to cause greater crop injury undernormal growing conditions. Nitrogenfertilizers, such as UAN (a mixture ofammonium nitrate, urea, and water) or

AMS (ammonium sulfate), are used incombination with surfactants or oil con-centrates to increase herbicide activityand/or reduce problems with hard water.Many blended adjuvants are availablethat include various combinations ofspecial purpose adjuvants and/or activa-

tor adjuvants.

Be sure to include the proper adjuvant(s)for the herbicide being used. Most herbi-cide labels specify the type and amountof additive to use. Failure to follow therecommendations can result in poorweed control or excessive crop injury.

Mixing and Applying

Be aware that improper sprayer calibra-tion, nonuniform application, calculationerrors, or use of the wrong chemicalscan cause herbicide injury to the crop.

Apply only the recommended amount ofherbicide. Slight increases in rates couldresult in crop injury or leave residuesthat might injure succeeding crops.

Recalibrate sprayers frequently to adjustfor increased output resulting fromnormal nozzle wear. Be sure there is

sufcient agitation in the sprayer tankto prevent settling of wettable powders,dry owables, or owables.

Do not stop in the eld with the sprayeron, spill herbicide when loading, ordump unused herbicides into anythingexcept a holding tank.

Take the following steps when mixingherbicides:

Always be sure the sprayer has beencalibrated properly for application atrecommended rates.

Calculate the amount of herbicide toadd to the sprayer tank based on theactive material in each gallon of her-bicide concentrate or the percentageof active ingredient of dry herbicideformulation.

Read and follow the instructions onthe manufacturers label pertaining topersonal hazards in handling.

Fill the sprayer tank with at least halfthe volume of water or fertilizer solu-tion you will ultimately need.

Start with moderate agitation andkeep it going.

Add compatibility agents, ammoniumsulfate, or other mixing adjuvants,if needed. For maximum benet,

they must be in the solution beforeherbicides are added. (To determinepesticide compatibility, see the nextsection.)

If tank-mixing different types ofherbicide formulations and adjuvants,be sure to add them in the followingorder:

1. Add, mix, and disperse dry her-bicides (wettable powders, dryowables, or water-dispersible

granules). These formulations con-tain wetting and dispersing agentsthat aid in mixing.

2. Add liquid owables and mixthoroughly. Liquid owables alsocontain wetting and dispersingagents.

3. Add emulsiable concentrates ormicroencapsulated herbicides andmix thoroughly.

4. Finish by adding water-solubleformulations (2,4-D amine, etc.).

5. Add any adjuvants (surfactants,crop oil concentrates, drift inhibi-tors, etc.) last. Crop oils, especial-ly, do not mix and disperse well ifadded rst.

6. Add the remainder of water or liq-uid fertilizer and maintain agitationthrough spraying procedure untiltank is empty.

Caution: Never mix concentratedherbicides in an empty tank. Neverallow a sprayer containing mixedchemicals to stand without agita-tion because heavy wettable pow-ders may clog nozzles or settleinto corners of the sprayer tankwhere they are difcult to remove.

Effect of a surfactant on the spread andpenetration of spray solution across andthrough the leaf surface.

without surfactantwith surfactant

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Compatibility

Pesticides are not always compatiblewith one another or with the water or liq-uid fertilizer carrier. Lack of compatibilitymay result in the formation of a gel, pre-cipitate, or sludge that plugs up screensand nozzles. However, extreme incom-patibility may produce a settling out of

material that can harden like concretein the bottom of the tank and in hoses,pumps, and other internal parts of thesprayer. The result may be total loss ofthe pesticide and use of the sprayer.

Herbicides may be combined with liquidfertilizers to minimize trips over the eld.However, little information exists con-cerning the compatibility of herbicideswith specic fertilizer solutions. Herbi-cide-fertilizer solution combinations mayform a gel or precipitate that settles tothe bottom of the sprayer tank or willnot ow through the sprayer equipment.Incompatibility of tank mixtures is morecommon with suspensions of fertilizersand pesticides.

Tank-mixing several pesticides, althoughconvenient, may create other problems.Foliar activity may be enhanced andcould result in crop leaf burn or the re-duction in activity of one or more of thepesticides (antagonism).

To prevent the main water tank or liquidfertilizer measuring tank from becomingcontaminated, commercial applicatorsmay want to mix the herbicides andother ingredients in a separate hold-ing tank. The herbicide mixture is thensucked into the main line as the trucktank is being lled, and thorough mix-ing is provided by the trucks agitationsystem. Compatibility problems aremore likely to result when concentratedherbicides are mixed together, so a

compatibility test should be done beforenew mixtures are tried.

Use only labeled tank mixtures ormixtures recommended by experiencedscientists whose recommendations arebacked by research. For all unlabeledtank mixtures, a jar test for compatibilityis strongly recommended. The compat-ibility of herbicide-fertilizer combinations

should be tested before large batchesare mixed. In some cases, adding acompatibility agent (Blendex, Combine,Unite, or comparable adjuvant) may aidin maintaining component dispersion.

The following two-jar test proceduremay be used to test the compatibil-ity of herbicides with one another, or

herbicides and other pesticides withliquid fertilizers. Should the herbicide-carrier mixture prove compatible in thistest procedure, it may be applied tothe eld. The following test assumes aspray volume of 25 gallons per acre. Forother spray volumes, make appropriatechanges to the ingredients.

1. Add 1 pint of carrier (water or liquidfertilizer) to each of two one-quart

jars. (Note: Use the same source ofwater that will be used for the tank

mix and conduct the test at the sametemperature the spray mixture will beapplied.)

2. To one of the jars, add 0.25 tea-spoon (1.2 ml) of compatibility agent.To both jars, add the appropriateamount of pesticide(s), in their relativeproportions, based on recommendedlabel rates. If more than one pesticideis used, add them separately with dryformulations rst, owables next, andemulsiable concentrates last. Aftereach addition, shake or stir gently tothoroughly mix.

3. When all ingredients are added, putlids on and shake both jars for 15seconds and let stand for 30 minutesor more. Then inspect the mixture forakes, sludge, gels, heavy oil lms, orother signs of incompatibility.

If, after standing for 30 minutes,the components in the jar con-taining no compatibility agent

are dispersed, the herbicides arecompatible and no compatibilityagent is needed.

If the components are dispersedonly in the jar containing the com-patibility agent, the herbicide iscompatible only if a compatibilityagent is added.

If either mixture separates but canbe remixed readily, the mixturecan be sprayed as long as goodagitation is used.

If the components are not dis-persed or show signs of incompat-ibility in either jar, the herbicide-carrier mixture is not compatibleand should not be used.

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Herbicide Selectivity

Were it not for the fact that most herbi-cides can be applied just before cropplanting or emergence, and even overthe top after crop emergence withoutexcessive injury, herbicides would beof little value. Most of the herbicideslabeled for use today will selectivelyremove most of the weeds without injur-ing the crop. Selectivity is accomplishedprimarily by two methods: selectivity byplacement and true selectivity.

Selectivity by Placement

Selectivity accomplished by avoid-ing or minimizing contact between theherbicide and the desired crop is calledselectivity by placement. An example iswiping or directing an herbicide such asglyphosate on a weed without expos-ing the desired plant. Selectivity by thismeans is as good as any, as long asthe excess herbicide is not washed offthe weeds and leached into the rootzone where it might be absorbed by theroot. Selectivity by placement also isaccomplished when an herbicide thatdoes not readily leach is applied to thesoil surface for control of shallow-rootedweeds, but does not leach into the rootzone of a more deeply rooted crop suchas fruit trees or established alfalfa.

True Selectivity

Selectivity that is true tolerance as aresult of some morphological, physio-logical, or biochemical means is referredto as true selectivity. The herbicide canbe applied to the foliage of the crop orto the soil in which the crop is grow-ing without danger of injury. Althoughtrue tolerance may be the best type ofselectivity, it is not perfect. Such thingsas crop growth stage, cuticle thickness,hairiness of the leaf surface, locationof the growing point, air temperatureand humidity, spray droplet size, andthe surface tension of spray droplets allcan inuence herbicide activity. Whenconditions are ideal for herbicide activity,even true selectivity may not adequatelyprevent crop injury.

Morphological differences include plantcharacteristics such as size and orienta-tion of the leaf, waxiness or hairiness ofthe leaf surface, location of the growingpoint, and rooting depth. Generally, themore waxy or hairy the leaf surface, themore difcult it is for a foliar-appliedherbicide to penetrate. The more pro-tected the growing point (as in grasses),

the less likely it is that foliar herbicideswill reach the growing point. The moredeeply rooted the crop is, the more dif-cult it is to get a soil applied herbicideto the crop roots and the less likely thatthere will be sufcient uptake for injury.

Physiological differences can includevarious processes that affect the activityand/or the breakdown of the herbicide.In certain situations, herbicides may be

transported differently across the

plasma lemma, translocated differently within the

plant,

combined with some componentwithin the cell wall,

integrated with something in the cellcytoplasm, or

channeled into sinks where theherbicide will have no effect.

These factors all can contribute to toler-

ance, but any one factor will seldomprovide tolerance by itself.

Metabolic factors include genetic insen-sitivity due to an altered site of herbicideaction that prevents herbicide activity.For example, Roundup Ready soybeansproduce an excess of the enzyme thatglyphosate (Roundup) normally inhibits,so Roundup Ready soybeans are notaffected, even though normal amountsof the herbicide are absorbed by thecrop plant. Corn plants metabolizeand convert atrazine to an innocuousmetabolite so rapidly that the herbicidedoes not have time to inhibit photosyn-thesis, which provides crop toleranceas long as the metabolic system is notoverwhelmed by an excess of the pes-ticide or a combination of pesticides. Inthe case of corn treated with an organo-phosphate insecticide and followed witha post treatment of Accent, Beacon, or

some other ALS-inhibiting herbicide,both the insecticide and herbicide arebeing metabolized by the same path-way. This pathway is unable to rap-idly metabolize both the herbicide andinsecticide, so corn injury may result.Metabolic insensitivity and/or the abilityto metabolize the herbicide usually arethe best types of true tolerance.

Safe Herbicide Use

Use herbicides only when necessary,only at recommended rates and timesof application, and only for those cropsand uses listed on the label. Correct useis essential to ensure that chemical resi-dues on crops do not exceed the limitsset by law. Recommended herbicidesdo not generally injure people, livestock,wildlife, or crops if used properly and ifrecommended precautions are ob-served. However, any herbicide is poten-tially dangerous if improperly handled orused.

Follow these basic pesticide safetyprocedures:

Make sure that you are familiar withcurrent federal and state pesticidelaws and regulations and that youhave a license, if required.

Avoid drift of spray or dust that mayendanger other crops or animals.Cover feed pans, troughs, and water-ing tanks in livestock areas; protectbeehives.

To protect yourself and others, followall safety precautions on the label.Know and observe the general rulesfor safe pesticide use, and record thedate, time, location, and amount ofeach pesticide used.

Wear protective clothing and useprotective equipment according toinstructions on the pesticide label.

Never eat, drink, or smoke while ap-plying pesticides.

Avoid spilling spray materials onskin or clothing. If such an accidentoccurs, wash immediately with soapand water.

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3. Add more water and rinse the systemagain by using a combination of agi-tation and spraying. Remove nozzles,screens, and strainers and cleanseparately in a bucket of cleaningagent and water.

4. Rinse and ush the system onceagain with clean water.

Drift

Drift is the movement of any pesticidethrough the air to areas not intended fortreatment. During application, dropletor particle drift occurs as spray drop-lets or dust particles are carried by airmovement from the application area

to other places. Vapor drift takes placeafter application as herbicides evaporate(volatilize) and yield fumes (gases) arecarried on wind currents and depositedon soils or plants in untreated areas.

Drift may injure sensitive crops, orna-mentals, gardens, livestock, wildlife, orpeople and may contaminate streams,lakes, or buildings. It may contaminatecrops and cause illegal or intolerableresidues. Excessive drift may mean poorperformance in the desired spray areabecause the application rate is lowerthan expected.

Highly active chemicals present thegreatest drift hazard because extremelysmall amounts can cause severeproblems. For example, growth regula-tor herbicides such as 2,4-D, dicamba,and picloram at a rate of 1 ounce peracre can deform sensitive crops such astobacco, grapes, or tomatoes.

Vapor drift from Command (cloma-zone) that has not been incorporatedcan cause bleaching of chlorophyll insensitive plants within a quarter mile ofapplication. Vapor drift problems canoften be avoided by using nonvolatileformulations. Essentially, no vapor drifthazard is involved in the use of amineformulations of 2,4-D. Soil incorporation

of Command and a microencapsulatedformulation greatly reduces vapor loss ofthis herbicide.

Particle drift depends on the size ofthe particle or droplet, and droplet sizedepends on pressure and nozzle design.

Very small particles of fog or mist pres-ent the greatest drift hazard. To minimizeparticle drift, calibrate equipment tocreate droplets about the size of lightrain. Most nozzles can be adjusted toa pressure that permits droplet forma-tion as a result of surface tension. Ifnozzles are operated at this pressure, aminimum of mist-sized droplets will beformed. For some nozzles, this pressuremay be as little as 15 psi; for others, itmay be 30 psi.

The distance particles will drift increaseswith the height of release. Wind ve-locities usually are lower close to theground. Therefore, sprays should bereleased as close to the soil surface

or vegetation as adequate coveragepermits.

Pesticide drift is inuenced by wind, air temperature, boom height, and spray droplet size.

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Drift hazard usually is minimized ifprevailing winds are blowing away fromsensitive crops, but a sudden shift inwind direction could result in seriousdamage. If possible, do not apply pes-ticides when wind speed is greater than5 mph.

High temperatures increase the loss

of volatile herbicides. Esters of 2,4-Drapidly evaporate at temperatures above800F. The use of such ester formula-tions should be restricted to fall, winter,and early spring because sensitiveplants are not present and lower tem-peratures reduce vapor drift hazard.

Drift control should be considered witheach pesticide application. You canprevent severe drift problems by

using sprayer nozzles especially

designed for drift reduction; using low volatile or nonvolatile for-

mulations;

using low spray-delivery pressures(1530 psi) and nozzles with a largerorice;

using drift-inhibiting adjuvants in thespray mixture when spraying underless-than-ideal conditions;

using nozzles that allow for loweredboom height;

avoiding application of volatile chemi-cals at high temperatures;

spraying when wind speed is low(less than 5 mph) or when the wind isblowing away from areas that shouldnot be contaminated;

spraying during the early morning orevening hours when there is usuallyless wind;

leaving border areas unsprayed if

they are near sensitive crops.

Evaluating Herbicide Injury

Insects, diseases, severe weather (hail,lightning, drought, ooding), fertilizerburn, and nutrient deciencies areamong the causes of symptoms oftenattributed to herbicide injury. Cool, wetweather can increase the potential for

injury, particularly with preemergenceherbicides. When evaluating crop injury,careful consideration of the following willhelp you diagnose the problem:

1. What is the pattern in the eld ofplant injury or uncontrolled weeds?

A pattern of injury that starts onone side of an area and diminishesgradually and uniformly away fromthat area is typical of applicationdrift.

A pattern of injury occurring inirregular patches that follow airdrainage could indicate herbicidevolatilization and movement ofvapors.

Strips of injured areas or surviv-ing weeds at predictable intervalsindicate possible skipping oroverlapping application.

Poor control at the edges of a eldcan result from only half coverage

by the last nozzle on the boomand/or more sunlight availabilityalong the edge of the eld.

Injury limited to the end rows orends of the eld is usually due tooverlapping applications or highherbicide rates in the turnaroundareas at the ends of the rows.

A denite break between the nor-mal or uninjured part of the eldand the rest of the eld usuallyindicates some major differencein soil type or pH between the twosides.

A pattern of obvious overapplica-tion as indicated by bare ground(both crop and weeds killed),followed by improved crop survivaland appearance with good weedcontrol, followed by lack of cropinjury or weed control, indicatesinadequate or poor agitation in the

sprayer tank. The evidence is evenstronger if this pattern repeatsitself at intervals that correspondto each new load.

2. What is the history of the problemareafertility program, croppingsequence, land preparation, soil pH,soil texture and organic matter, and

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

rainfall, and prevailing wind at andimmediately following herbicide ap-plication?

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Plant Uptake

Herbicides may be absorbed by plantroots or leaves and inactivated withinthe plant. This effect generally accountsfor a relatively small amount of herbicideremoval.

Crop Removal

If a crop is harvested or removed fromthe treated area before rain has washedthe herbicide off the foliage or beforethe plant has had time to metabolize theresidue, the herbicide will be removedwith the crop. This seldom happensbecause herbicides are not commonlyused close to harvest. However, if grassclippings are collected shortly aftertreatment and used to mulch a garden,there may be enough herbicide on thegrass to damage the garden plants.

Toxicity

Toxicity usually is measured as LD 50 (lethal dose), which is the amount of atoxicant required to kill 50 percent of thetest animals. The lower the LD 50, the lesspesticide it takes to kill the animal. Aswith any chemical, whether naturally oc-

curring or synthetic, it is the dose thatmakes the poison. Below is a list of themost commonly available herbicides, aswell as other commonly used substanc-es, in order of decreasing oral toxicity.

Highly Toxic Herbicides(LD 50 < 50 mg/kg)

The probable lethal dose of a highlytoxic herbicide for a 150-pound personis a few drops to 1 teaspoon. The labelcontains the signal words Danger/Poi-

son and has a skull and crossbones.

metham (Vapam)sodium arsenite a,b

Moderately Toxic Herbicides(LD 50 = 50 to 500 mg/kg)

The probable lethal dose of a moderate-ly toxic herbicide for a 150-pound per-son is 1 teaspoon to 1 ounce. The signalword on the label reads Warning.

bromoxynil (Buctril)caffeinecopper sulfate (bluestone)difenzoquat (Avenge)diquatendothall (Aquathol, Des-i-cate)gasolinekerosenenicotineparaquat (Gramoxone)

Slightly Toxic Herbicides(LD 50 = 500 to 5,000 mg/kg)

The probable lethal dose of a slightlytoxic herbicide for a 150-pound personis 1 ounce to 1 pint or 1 pound. The sig-nal word on the label reads Caution.

aspirinethyl alcoholsodium chloride (table salt)acetochlor (Harness, Topnotch)aciuorfen (Blazer) c

alachlor (Micro-Tech, Lasso) c

ametryn (Evik) c,d

atrazine (various) d

bensulide (Betasan)bentazon (Basagran)butylate (Sutan+) d

CAMA (various)clodinafop-propargyl (Discover)clomazone (Command)

clopyralid (Stinger, Transline) ccloridazon (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)diclofop-methyl (Hoelon)dimethenamid (Frontier, Outlook) d

diuron (Karmex) d

DSMA (various)d

EPTC (Eptam, Eradicane)d

fenoxaprop-P-ethyl (Acclaim, Puma) d

uazifop-P-butyl (Fusilade)ufenacet (Dene)glufosinate (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)quizalofop-P-ethyl (Assure II)

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sethoxydim (Poast)sodium chloratedsulfentrazone (Authority)tebuthiuron (Spike)terbacil (Sinbar)thiobencarb (Bolero)topramezone (Impact)tralkoxydim (Achieve)triallate (Far-Go)

triclopyr (Garlon) dvinegar (acetic acid)

Almost Nontoxic Herbicides(LD 50 > 5,000 mg/kg)

The probable lethal dose of an almostnontoxic herbicide for a 150-pound per-son is more than 1 pint or 1 pound. Thesignal word on the label readsCaution.

asulam (Asulox)

benen (Balan)benzsulfuron-methyl (Londax)bromacil (Hyvar X) d

chlorimuron-ethyl (Classic) d

chlorsulfuron (Glean, Telar)clethodim (Select)DCPA (Dacthal)desmedipham (Betanex)dithiopyr (Dimension) c

ethaluralin (Sonalan)ethofumesate (Prograss)ucarbazone (Everest)

umetsulam (Python)umiclorac (Resource)uometuron (Cotoran)fomesafen (Flexstar, Reex)foramsulfuron (Option)fosamine (Krenite)glyphosate (Roundup, Touchdown,Rodeo, various)halosulfuron (Permit, Sempra)iodosulfuron (Autumn)imazamethabenz (Assert)imazamox (Raptor)imazapic (Cadre, Plateau)imazapyr (Arsenal, Chopper) d

imazaquin (Scepter, Image)imazethapyr (Pursuit)isoxaben (Gallery)isoxautole (Balance)lactofen (Cobra)metsulfuron-methyl (Cimarron, Escort)mesotrione (Callisto)napropamide (Devrinol)

Prepared by Dwight D. Lingenfelter,agronomy extension associate in weedscience, and Nathan L. Hartwig, profes-sor emeritus of weed science.

Penn State College of Agricultural Sciencesresearch and extension programs are fundedin part by Pennsylvania counties, the Com-monwealth of Pennsylvania, and the U.S.Department of Agriculture.

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

This publication is available in alternativemedia on request.

The Pennsylvania State University is com-mitted to the policy that all persons shallhave equal access to programs, facilities,admission, and employment without regard topersonal characteristics not related to ability,performance, or qualications as determinedby University policy or by state or federalauthorities. It is the policy of the University tomaintain an academic and work environmentfree of discrimination, including harassment.The Pennsylvania State University prohib-its discrimination and harassment againstany person because of age, ancestry, color,disability or handicap, genetic information,national origin, race, religious creed, sex,sexual orientation, gender identity, or veteranstatus and retaliation due to the reportingof discrimination or harassment. Discrimi-nation, harassment, or retaliation againstfaculty, staff, or students will not be toleratedat The Pennsylvania State University. Directall inquiries regarding the nondiscriminationpolicy to the Afrmative Action Director, ThePennsylvania State University, 328 BouckeBuilding, University Park, PA 16802-5901; Tel814-863-0471.

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Code UC175 Rev5M10/13payne

nicosulfuron (Accent)norurazon (Zorial, Solicam)oryzalin (Suran)oxadiazon (Ronstar) d

oxyuorfen (Goal)pendimethalin (Prowl, Pendulum)prodiamine (Barricade)picloram (Tordon)primisulfuron-methyl (Beacon)

pronamide (Kerb) dprosulfuron (Peak)rimsulfuron (Matrix, Resolve)siduron (Tupersan)simazine (Princep)sodium boratedsulfometuron-methyl (Oust)sulfosulfuron (Maverick)thifensulfuron-methyl (Harmony GT) d

triasulfuron (Amber)triuralin (Trean)tribenuron-methyl (Express)

Dermal response:a. Absorbed and poisonousb. Causes burns and blistersc. Moderately irritatingd. Mildly irritating