B-60816/99Herbicides

How They Work andthe Symptoms They Cause

Amino AcidSynthesisInhibitors

Growth Regulator Herbicides

LipidSynthesis Inhibitors

Photosynthesis Inhibitors

Pig

men

t Inh

ibito

rs

Seedling Grow

th Inhibitors

Cell Mem

brane Disruptors

and Organic A

rsenicals

Paul A. Baumann, Professor and Extension Weed SpecialistPeter A. Dotray, Associate Professor and Extension Weed Specialist

Eric P. Prostko, Assistant Professor and Extension Agronomist

The Texas A&M University System

Whether you are producing agricultural crops ortending a lawn or home garden, weed control willbe important to your success. Weeds can be con-trolled mechanically, culturally, biologically andchemically, and all these methods may be importantin an integrated weed control program that is eco-nomical and friendly to the environment.

Chemical control with herbicides has been animportant tool for managing weeds in crops andhome landscapes for many years. Many of today’sherbicides are more effective and selective. Thesetraits make them less harmful to the environmentwhen they are used properly. Although herbicidesare widely used, few people understand how theywork to control undesirable plants.

Herbicide applicationGenerally speaking, herbicides are applied eitherpreemergence or postemergence.That means theyare applied either before or after weeds emergefrom the soil and begin to grow. Preemergenceherbicides kill weeds shortly after they germinateor emerge through the soil surface. Postemergenceherbicides control weeds that are already growingand easily visible.

Some herbicides are applied to the soil and aretaken up by seedling plant roots or shoots.They aresaid to have soil activity. Herbicides that are appliedto plant foliage have foliar activity. Some herbicideshave both. Herbicides with a high degree of soilactivity usually are applied preemergence.

SelectivitySelectivity is the process by which a herbicide con-trols or kills certain plants but leaves othersunharmed. Selectivity may be as simple as control-ling broadleaf plants but not grass plants. Many newherbicides have more sophisticated selectivity thatdifferentiates between several broadleaf and/orseveral grass plants.

Herbicides with no selectivity, such as RoundupUltra®, are called non-selective.These products killall types of plants. Selectivity usually depends onthe time or placement of the herbicide applied.Most herbicides can be harmful, even to normallytolerant plants, if the dose is too high.

TranslocationSome herbicides move (translocate) within theplant. Systemic herbicides translocate once they aretaken up by the leaves, stems or roots. Herbicidesthat do not move after they enter the plant arecalled contact herbicides. Some products can beeither contact or systemic herbicides, depending onthe way they are applied.

Mode-of-actionMode-of-action refers to the effect a herbicide hason a plant. Herbicides work in many different ways.If we understand a herbicide’s mode of action, wewill know what symptoms it produces at lethal orsub-lethal doses.

Other problems such as disease, nutrient deficien-cy, and insect damage may mimic the effects of her-bicides.These other possibilities must be ruled outbefore herbicide injury is diagnosed.The followingmode-of-action categories cover most of the herbi-cides used in Texas.

Introduction

Today’s

herbicides are

more effective

and selective.

These herbicides are widely used to controlbroadleaf weeds in grass crops such as wheat, corn,sorghum, forages and turf grasses. One member ofthis group, 2,4-D, was one of the first selective her-bicides developed. Growth regulator herbicidesupset the normal hormonal balance that regulatesprocesses such as cell division, cell enlargement,protein synthesis, and respiration. That is why thisgroup of herbicides is sometimes called the “hormone herbicides.” These herbicides are veryversatile for weed control.They usually are appliedto the foliage, but are also effective in the soil. Anyherbicide that falls on the soil instead of the foliagecan be percolated into the soil with rain or irriga-tion and will be taken up by weed roots.

Herbicides in this categoryPhenoxy growth regulator herbicides have the leastplant activity and soil residual activity, while thecarboxylic acids generally have the most. Broadleafcrops and turf grasses should not be planted intosoils recently treated with these herbicides becausethey severely inhibit seedling emergence.

SymptomsThe most common symptoms for these herbicidesare leaf and stem malformations. In broadleaf plantsstems curl, twist and droop, while leaves arecupped, crinkled or have a “drawstring” appearancecaused by irregular growth at the leaf edges. In cot-ton plants, points develop on leaf edges. Over-application or application at the wrong stage ofdevelopment in corn can cause leaf rolling and crin-kling, brace root malformation, sterile flowers andmissing grain (blasting). Blasting and malformedseed heads are common symptoms of ill-timedapplications in wheat.

Special considerationsVapor from these products can easily drift to desir-able plants, so they must be applied carefully.Equipment should be cleaned according to labelinstructions before it is used to treat susceptiblecrops with other herbicides.

Growth Regulator Herbicides

These

herbicides

are very

versatile

for weed

control.

Dicamba drift onto soybeans

Dicamba injury to corn

Phenoxy herbicide driftonto cotton

GrowthRegulator

Herbicides

Family Common TradeName Name(s)

phenoxy 2,4-D Several2,4-DB Butyrac,®

Butoxone®

MCPA SeveralMCPP SeveralMCPB Several

benzoic acid dicamba Banvel,®

Clarity®

carboxylic acid picloram Tordon 22K®

clopyralid Stinger,®

Reclaim®

triclopyr Remedy,®

Grandstand®

quinclorac Facet®

Herbicides in this category inhibit photosynthesis,the process by which all green plants convert lightenergy from the sun into sugars (food).Photosynthesis inhibitors are broadleaf herbicides,but also control annual grasses to some extent.

Herbicides such as Buctril,® Basagran® and Tough®

are used as foliar, selective, postemergence prod-ucts. Others such as atrazine, Bladex,® Caparol® orCotoran® are generally used as preemergence her-bicides, but are sometimes used postemergence.Their selectivity when applied postemergencedepends on the crop and application method.

Herbicides in this categorySeveral herbicides in this category are critical tocotton, corn and rice production in Texas.

SymptomsSymptoms depend on the product’s mobility with-in the plant. Herbicides in the triazine, triazone,uracil and urea families move within the plant andexhibit these symptoms in older leaves first: yel-lowing between the leaf veins or in the veins; andyellowing of the leaf margins that eventually turnbrown and die. Herbicides in the benzothiadiazole,nitrile, phenyl-pyridazine and amide families arenot mobile within the plant and affect only treatedleaves. Symptoms include speckling, spotting, andyellowing or bronzing that may kill affected tissue.

Special considerationsSoil pH higher than7.2 can make injuryfrom the triazineand triazone familiesmore severe whenused preemergence.

Photosynthesis

inhibitors are

broadleaf

herbicides.

Photosynthesis Inhibitors

Triazine damage to cotton

Metribuzin damage to soybeans

Bromoxynil damage oncorn

Triazine carryover damage to cotton

Photosynthesis

Inhibitors

Family Common TradeName Name(s)

triazine atrazine AAtrex,® others cyanazine Bladex,® others prometryn Caparol,®

otherspropazinesimazine Princep,®

others

triazinone hexazinone Velpar®

metribuzin Sencor,®

Lexone®

uracil terbacil Sinbar®

bromacil Hyvar®

bromacil + diuron Krovar®

nitrile bromoxynil Buctril®

benzothiadiazole bentazon Basagran®

phenyl-pyridazine pyridate Tough®

urea diuron Karmex,®

othersfluometuron Cotoran,®

others linuron Lorox,® others siduron Tupersan®

tebuthiuron Spike®

amide propanil Stam®

Herbicides classified as pigment inhibitors destroythe green pigment (chlorophyll) in leaf tissue.Chlorophyll is necessary for photosynthesis; with-out it plants die. These herbicides are oftendescribed as “bleaching herbicides” because theycause new leaves to appear yellow or white.

These herbicides are absorbed by roots andtranslocate to the shoot tissue where they inhibitthe production of carotenoids – substances thatprotect the chlorophyll molecules that make plantsgreen. Without carotenoids, chlorophyll isdestroyed. These herbicides do not destroycarotenoids already formed, but prevent the forma-tion of new ones.

Herbicides in this categoryThere are three families of herbicides that bleachplant tissue.

SymptomsInjured leaves turn yellow or white, then oftentranslucent. New growth is yellow to white withsometimes a hint of purple or pink. These symp-toms can be found on cotyledons to the newestleaves of susceptible plants. Zorial® initially causesbleaching within veins, while Command® initiallycauses bleaching between veins.

Special considerationsIn order to use Command® in cotton, anorganophosphate insecticide (Thimet® or Di-Syston®)must be used in-furrow first. If the insecticide isplaced incorrectly or applied at the wrong rate, cottonmay be injured. Some formulations of Command® arevolatile and should be used with care. Consult thelabel for further precautions.

These

herbicides

are often

described as

“bleaching

herbicides.”

Clomazone damage inpeanuts

Pigment Inhibitors

Clomazone damage in cotton

Norflurazon carryover damage in corn

PigmentInhibitors

Family Common TradeName Name(s)

isoxazol clomazone Command®

isoxaflutole Balance®

pyridazinone norflurazon Zorial,® Evital,®

Solicam®

triazole amitrole Amitrol®

Some herbicides act on seedling weeds shortly afterthey germinate and before they emerge.These her-bicides work beneath the soil so their effects areseldom seen. If over-applied, however, they mayinhibit growth of weed or crop seedlings that doemerge through the soil surface. These herbicidescan be divided into two groups – root inhibitorsand shoot inhibitors.

Root inhibitors. These herbicides interrupt celldivision, which stops root growth in seedlingweeds. Plants die because they can not take upenough water and nutrients to sustain growth.Theroot inhibitors are most effective on small-seededbroadleaf and grass weeds. Large-seeded weedsand crops generally survive normal dosagesbecause their roots and shoots grow through theherbicide-treated zone in the soil.

Shoot inhibitors. The seedling shoot growthinhibitors also act on newly germinated weed seeds.They are absorbed by the seedling shoots of grassesand roots of broadleaf plants, and disrupt cellgrowth.They are most effective at controlling small-seeded grass and broadleaf weeds. Large-seededcrops and weeds are not usually affected. Once tol-erant or susceptible plants emerge they can general-ly overcome the effects of the herbicide.

SymptomsRoot inhibitors. Injury to tolerant plants is causedby root damage. Grass crops may be stunted and havea purple discoloration because roots can not take upenough phosphorus. Root systems appear stubby andthick, especially the lateral roots. Broadleaf plantsmay have swollen and cracked hypocotyls. If theseherbicides are incorporated shallowly or applied pre-emergence they sometimes cause callus tissue(tumors) to form on the plant stem at the soil sur-face. This weakens the stem and causes lodging.Dinitroaniline herbicides applied postemergence tobroadleaf crops may cause stunting.

Shoot inhibitors. Symptoms caused by the shootinhibitors are much different than those of the rootinhibitors. Over-application or extended periods ofcool, wet weather shortly after planting may some-times cause injury to tolerant crops such as corn orsorghum. Symptoms include leafing out under-

ground, improper leaf unfurling (shepherd’s crook-ing), buggy whipping (tightly rolled leaves), and leafcrinkling. In broadleaf plants, the center vein (mid-rib)may draw in the leaf edge in a draw-string effect. Leafpuckering is also a symptom on broadleaf plants.There may also be stunting that persists until the soilwarms enough to promote plant growth.

Herbicides in this categoryThese products are widely used in Texas row crops,turfgrass, and horticultural crops.

Special considerationsThe growth inhibitor herbicides have nopostemergence activity; therefore, the timingof application is critical. Dinitroanilines havevarious requirements for incorporation intothe soil. Consult the individual product labelfor specific recommendations. Grain sorghumtreated with Concep® seed safener is tolerantto the acetamide herbicides.

If over-applied,

they may

inhibit growth

of weed or

crop seedlings.

Seedling Growth Inhibitors

Acetamide herbicide damage to corn

SeedlingGrowth

Inhibitors

Family Common TradeName Name(s)

dinitroanilines benefin Balan®

(root inhibitors) ethalfluralin Sonalan,® Curbit®

oryzalin Surflan®

pendimethalin Prowl,®

Pentagon®

trifluralin Treflan,® othersoryzalin + benefin XL®

trifluralin + benefin Team®

acetamides acetochlor Harness,®

(shoot inhibitors) Surpass®, Topnotch®

alachlor Lasso,®

MicroTech®

butachlor Machete®

metolachlor Dual,® Dual II,®

Dual Magnum,®

otherspronamide Kerb®

propachlor Ramrod®

dimethenamid Frontier®

napropamide Devrinol®

Dinitroaniline herbicide damage to soybeans

Dinitroaniline herbicide damage tocorn seedling

The cell membrane disruptor postemergence herbi-cides control both grasses and broadleaf weeds bydestroying cell membranes and causing rapid dessi-cation of the plant.There are two types of cell mem-brane disruptor herbicides: the bipyridiliums and thediphenylethers. The bipyridilium herbicides requirethorough plant coverage to be effective, and theyhave no soil activity.The diphenylether herbicides actin a similar way but more slowly. Some of them aremore selective between crops and weeds.The herbi-cides Goal and Reflex have significant soil activity.

The organic arsenical herbicides DSMA and MSMAare often called contact herbicides as are the cellmembrane disruptors. However, their true modeof action is unknown. They are used to selectively control wide-leaved grasses such as crabgrass ordallisgrass in narrow-leafed grasses such asbermudagrass lawns.They are also very effective oncocklebur and common ragweed.These herbicidesbind tightly to soil clay and organic matter so theyhave no residual, preemergence activity.

Herbicides in this category

SymptomsPlants rapidly turn yellow or pale and may lookwater soaked; then they dry up. The effects of thebipyridilium herbicides are rapid. Even small

droplets that drift to non-target vegetation causespecks of burned tissue. Roots of perennial weedsare seldom killed because these herbicides do notusually translocate to the roots.

The organic arsenicals accumulate in root and leaftips and symptoms are first seen on leaf tips.Theyrapidly kill leaf and stem tissue. MSMA and DSMAare more effective on grass weeds than broadleafweeds, except for common ragweed and cockle-bur.When applied over cotton to control grasses orcocklebur, they sometimes cause speckled leaf burnand red stems on the cotton plants; however, thishas little effect on overall growth.

Special considerationsBe careful to prevent drift during application so thatnon-target plants are not harmed.Applying systemicherbicides shortly after cell membrane disruptors ororganic arsenicals is not advised. Paraquat and diquatare generally considered to be non-selective andharmful to both grassand broadleaf vegeta-tion. In peanuts, howev-er, some selectivity canbe achieved by usingparaquat at the crackingstage.Another bipyridili-um herbicide calledAvenge® is used in wheatand barley for selectivepost-emergence controlof wild oat.

The effects

of the

bipyridilium

herbicides

are rapid.

Diphenylether injury tosoybeans

Cell Membrane Disruptors and Organic Arsenicals

Paraquat drift onto corn

MSMA damage on Johnson-grass

Cell MembraneDisruptors

andOrganic

Arsenicals

Family Common TradeName Name(s)

bipyridiliums difenzoquat Avenge®

diquat Diquat,®

several othersparaquat Gramoxone

Extra,®

several others

diphenylethers acifluorfen Blazer®

fomesafen Reflex®

lactofen Cobra®

oxyfluorfen Goal®

organic arsenicals DSMA SeveralMSMA Several

Lipid synthesis inhibitors are unique because theyact only on annual and perennial grasses, not onbroadleaf plants. With the exception of diclofop,these herbicides are applied post-emergence andhave little or no soil activity. Crop oil concentrateor some other type of adjuvant must be used toincrease herbicide uptake into the leaf.To be mosteffective, these herbicides should be applied toactively growing grass weeds. If grass weeds arestressed and slow growing, these herbicides will beless effective.

These herbicides disrupt lipid biosynthesis in grassplants. All plants contain lipids, which are fattyacids essential for plants to function normally. Plantcells contain lipid membranes. Membranes help theplant cell regulate what moves in, what moves out,and what remains out. Because these herbicidesprevent the plant from producing fatty acids, mem-branes can not form. Leaves absorb these herbi-cides quickly and within an hour they can not beremoved by rain.

Herbicides in this categoryThere are two families of herbicides that disruptlipid biosynthesis.

SymptomsSymptoms develop slowly on grass plants and maynot appear for 7 to 14 days. Initial injury is seenwhere the newest leaves are developing. Theseregions usually turn pale or yellow and then die.The area at the base of new leaves quickly becomesmushy, has a rotted appearance, and new leaves inthe affected area can be pulled easily from the restof the plant. Reddish-blue pigmentation may alsobe observed on the stem sheath, leaf margins,and/or leaf blade.

Leaves absorb

these herbicides

quickly and

within an hour

they can not

be removed

by rain.

Fluazifop-P damage oncorn

Lipid Synthesis Inhibitors

Lipid SynthesisInhibitors

Family Common TradeName Name(s)

cyclohexanedione clethodim Select®

sethoxydim Poast,®

Poast Plus®

aryloxyphenoxy- fenoxaprop-P Whip 360,®

propionate Option II®

fluazifop-P Fusilade DX®

quizalofop-P Assure II®

fluazifop-P + fenoxaprop-P Fusion®

diclofop Hoelon®

Lipid synthesis inhibitor damage to corn. Note rotted appear-ance at the base of the leaf stem.

This new category of herbicides can be used atextremely low rates, controls both grasses andbroadleaf plants, has soil and foliar activity, and isessentially non-toxic to mammals and most non-vegetative life forms.

Amino acid synthesis inhibitors bind to a specificenzyme and prevent the development of aminoacids essential to plant life. The enzyme to whichthey bind is abbreviated ALS or AHAS, so these her-bicides are often called the AHAS/ALS herbicides.

Herbicides in this category

SymptomsWhen these herbicides are applied preemergence,symptoms do notusually appear untilthe plants haveemerged from thesoil. Symptoms forgrasses includestunting, purplecoloration, androot systems thatdevelop a “bottle-brush” appearance.On broadleaf plantssymptoms include red or purple leaf veins,yellowing of new leaf tissue, and sometimes black-ened terminals.

Special considerationsHerbicides in this category are very crop specific.The spray tank must be cleaned thoroughly beforethe sprayer is used on a potentially susceptiblecrop. It is very important that the susceptibility offuture rotational crops be considered before herbi-cides in this group are applied. High soil pHincreases the soil activity of sulfonylurea herbicidesand the potential for rotational crop damage.

These

herbicides

bind to

a specific

enzyme.

Imidazolinone carryover to corn

Amino Acid Synthesis Inhibitors

Amino Acid

SyntheisInhibitors

Family Common TradeName Name(s)

imidazolinone imazethabenz Assert®

imazamox Raptor®

imazapic Cadre,®

Plateau®

imazapyr Arsenal,®

Contain®

imazaquin Scepter,®

Image®

imazethapyr Pursuit®

sulfonylurea bensulfuron Londax®

chlorimuron Classic,®

Skirmish®

chlorsulfuron Glean®

halosulfuron Permit,®

Manage®

nicosulfuron Accent®

primisulfuron Beacon®

prosulfuron Peak®

rimsulfuron Matrix®

sulfometuron Oust®

thifensulfuron Pinnacle®

triasulfuron Amber®

tribenuron Express®

triazolopyrimidine chloransulam-methyl FirstRate®

diclosulam Strongarm®

lumetsulam Python®

pyrimidinylthio- pyrithiobac Staple®

benzoate Imidazolinone carryover in cotton

Sulfonylurea residue injury to cotton

The herbicides in this category also affect aminoacid synthesis but in a different way than the previ-ous group. These herbicides are non-selective andcontrol a broad range of annual and perennial grass-es, broadleaves and sedges. Roundup Ultra®, one ofthe most commonly used herbicides on the farmand around the home, is in this category.

Herbicides in this categoryHerbicides in this category have not yet been classified by family. Instead, they are grouped by theactive ingredient or common name.

SymptomsPlants treated with glyphosate or sulfosate turn yel-low in 5 to 7 days, then turn brown and die in 10to 14 days. Glufosinate acts more quickly, in 3 to 5days. An individual plant may have dead tissue, yel-low tissue and green tissue at the same time.Extremely low dosages of Roundup® cause leafpuckering.

Special considerationsBecause these herbicides are non-selective, it isvery important to protect desirable plants fromspray drift.These herbicides bind tightly to soil clayand organic matter and have no soil activity. Forthat reason they may be less effective when plantsare dusty or when application water is dirty.

Because these

herbicides are

non-selective,

it is very

important to

protect desirable

plants from

spray drift.

Glyphosate drift injury tosorghum

Other Herbicides That Inhibit AminoAcid Synthesis

OtherHerbicides

Family Common TradeName Name(s)

unknown glyphosate Ranger,®

Rodeo®

Roundup Ultra®

unknown sulfosate Touchdown®

unknown glufosinate Liberty,® Rely®, Remove,®

Finale®

Glyphosate injury to cotton

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Educational programs of the Texas Agricultural Extension Service are open to all people without regard to race, color, sex, disability, religion, age or national origin.

Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperationwith the United States Department of Agriculture. Chester P. Fehlis, Deputy Director,Texas Agricultural Extension Service,The Texas A&M University System.1.5 M, New WS

The information given herein is for educational purposes only. Reference to trade names is made with the understanding that no discrimination is intended and no endorsement by the Texas Agricultural Extension Service is implied.