Wisconsin CCA Training Weed Management - Integrated Pest and ...

Wisconsin CCA Training 

Weed Management 

Vince Davis, University of Wisconsin-Madison 

vmdavis@wisc.edu (608) 262-1392

Topics 

• Weed Identification 

• Weed Competition 

• Herbicide Mode of Action 

• Herbicide Resistance

What is a weed? 

• A plant growing when and where it is not 

wanted. 

– Implications of this definition 

• There would be no weeds without humans 

• A plant may be a weed in some environments but not 

others; conversely it may be perceived as a weed by 

some people but not others. 

• “textbook” definition – Ross and Lembi 

– Plants that interfere with the growth of desirable 

plants and are unusually persistent. They damage 

cropping systems, natural systems, and human 

activities and as such are undesirable.

Biological traits of weeds 

• Persistent 

– Vegetative propagation, seed dormancy, 

high reproduction rates 

• Spread rapidly 

– High reproduction rates, multiple seed 

dispersal mechanisms 

• Competitive 

– Rapid growth and resource consumption 

• Widely adapted (high plasticity) 

– High genetic diversity, developmental 

plasticity

Weed Classification 

• There are several ways to classify weeds 

– Life histories 

– Modes of reproduction 

– Photoperiod sensitivity 

– Ecological 

– Legal 

– Botanical 

– Life cycles

• Annuals 

Life Cycles 

– Complete their life cycle in one year, 

includes summer and winter 

• Biennials 

– Complete their life cycle in two years. 

Often has rosette and vegetative growth 

only in year one, and reproduces in year 

two 

• Perennials 

– Lives more than two growing seasons

Why know life cycles? 

To plan weed management strategies. 

• In general, weeds are most difficult to 

control when they reach reproductive 

stages 

– Annuals: 

• always looking to limit seed production, and 

seed persistence for long-term management 

• We often attempt most control tactics at a time 

(early) to limit crop competition

Why know life cycles? 

• Biennials and Perennials are more 

susceptible to damage going into winter 

months 

• Control biennials in year one before root 

system becomes larger and reproduction 

occurs 

• Perennials also have different methods of 

reproduction 

– Simple perennials reproduce by seeds 

– Creeping perennials reproduce by seeds and 

spread by root stock (vegetative reproduction)

Vegetative reproduction of 

perennials 

• Stolons – aboveground stems that root 

at the nodes 

• Rhizome – underground stem 

• Tuber – underground storage structure 

at the end of a rhizome 

• Bulb – underground bud (storage in 

leaves) 

• Corm – enlarged underground stem 

(storage in stem)

Management Implications, or 

‘Who cares’? 

• Weed management requires controlling 

reproduction 

– Different strategies are needed to eliminate 

seed production versus vegetative 

reproduction 

– Would mowing be an effective strategy to 

control a biennial in the basal rosette 

stage? 

– Should a contact herbicide be used to 

control Canada thistle?

Management Implications 

• Cropping systems influence weeds 

present 

– Crops compete with weeds differently i.e. 

some crops favor some weeds more than 

others 

– Tillage favors annual weeds and aids in 

weed seedbank persistence 

– Shallow and minimal tillage systems can 

spread creeping perennial weeds 

– No-tillage can favor biennial and perennial 

weeds, winter annual weeds, small seeded 

annual weeds

Weed Identification 

• Why identify the weed? 

– You must know the pest before you can 

control the pest! 

– It enables you to seek accurate information 

• Life cycle and ecological niches 

• Herbicide susceptibility 

• Crop competitiveness 

• Interactions with other pests? 

• Natural enemies?

Plant Parts for Grasses and Broadleaves 

Ligule 

Margins 

Ross & Lembi 1999

Plant Families – Grasses 

POACEAE 

• Stems with distinct 

internodes, usually 

hollow 

• 2 ranked leaves, ligules 

usually present 

• Parallel venation 

• Seeds produce 1 

cotyledon 

• Foxtails 

• Barnyardgrass 

• Fescue 

• Bromes 

• Johnsongrass 

• Shattercane 

• Annual ryegrass

Plant Parts for Grasses

Plant Parts for Grasses 

Grass Stems

Plant Parts for Grasses


Grass Ligules


Grass Blades


Grass Special Features 

(Not a Grass..)


Grass Inflorescence


Grass Seed

Giant foxtail (Setaria faberi)

Giant foxtail (Setaria faberi)

Setaria Glauca 

• Dense spike like seed 

head, often yellow tint 

•long hairs at the base of 

the leaves 

• short hairs midway down 

the leaf 

• seed oval shaped with 

palea less than half way 

Palea 

Yellow foxtail

Yellow foxtail (Setaria glauca) 

Green foxtail (Setaria viridis)

Large Crabgrass (Digitaria sanguinalis)

Wild-proso Millet 

Ligule is hairy

Quackgrass (Elytrigia repens)

Quackgrass (Elytrigia repens)

• Wirestem muhly is a perennial, spreading 

by seed and rhizomes 

• Smooth branched stems give ‘bushy’ 

appearance 

• The rhizomes of wirestem muhly are very 

characteristic and are useful in 

identification. They are thick, scaly, and 

relatively short, unlike the long, smooth 

rhizomes of quackgrass.

Wirestem muhly (Muhlenbergia frondosa) 

Poaceae

Plant Parts for Broadleaves

Plant Parts for Broadleaves 

Broadleaf Leaf Arrangement


Broadleaf Petiole


Broadleaf Leaf Shape


Broadleaf Leaf Margin


Broadleaf Growth Type


Broadleaf Inflorescence


Broadleaf Special Features

Canada Thistle 

Cirsium arvense 

• Perennial 

• Flowers pink to purple, 

rarely white 

• Alternate and 

sessile leaves 

• Spiny margins 

• Base of leave surround stem, hairless

Canada thistle 

(Cirsium arvense) 

Bull thistle 

(Cirsium vulgare) 

Musk thistle 

(Carduus nutans)

Weed Families – Goosefoot 

CHENOPODIACEAE 

• Mealy white leaves, 

especially newly 

emerging leaves and the 

underside 

• Simple, alternate or 

opposite leaves 

• Grooved stem 

• Greenish, inconspicuous 

flowers 

• Common lambsquarters 

• Atriplex spp.

Chenopodium album 

• Summer annual 

• Young leaves have a 

mealy white 

• Alternate leaves 

• Stem has prominent 

venations 

• Sometimes has a 

purplish cast 

Common lambsquarters

Chenopodium album 

Common lambsquarters

Ambrosia artemisiifolia 

• Leaves 

alternate, 

opposite early 

• Once or twice 

compound 

• Can reach 6.5 

ft tall 

common ragweed

Ambrosia artemisiifolia 

Cotyledons are 

thick, dark green, 

rounded 

Flowers 

inconspicuous in 

cluster in terminal 

branches 

common ragweed

Common ragweed (Ambrosia artemisiifolia) 

Asteraceae

Common ragweed (Ambrosia artemisiifolia) 

Asteraceae

Giant ragweed (Ambrosia trifida) 

Asteraceae

Giant ragweed (Ambrosia trifida) 

Asteraceae

Plant Families – Nightshade 

SOLANACEAE 

• Flower forms a 5 – 

pointed star or are 

tubular 

• Fruiting body is berry or 

capsule 

• Foul odor 

• Cotyledons and leaf tips 

slightly pointed 

• Leaves often pointed 

• Horsenettle 

• Jimsonweed 

• Eastern black 

nightshade 

• Groundcherry; smooth 

and clammy

Eastern black nightshade (Solanum ptycanthum) 

Solanaceae

Eastern black nightshade (Solanum ptycanthum) 

Solanaceae

Plant Families – Pigweed 

AMARANTHACEAE 

• Simple, oblong or ovate, 

alternate or opposite 

leaves 

• Often reddish stems 

and roots 

• Flowers minute, green 

or red, in spikes 

• Small round, black, and 

shiny seed 

• Redroot pigweed 

• Smooth pigweed 

• Powell amaranth 

• Waterhemp 

• Spiny amaranth

Amaranthus spp. 

Pigweed

Redroot pigweed (Amaranthus retroflexus) 

Amaranthaceae

Redroot pigweed (Amaranthus retroflexus) 

Amaranthaceae

Plant Families – Mallow 

MALVACEAE 

• Simple, palmately lobed 

or veined leaves, 

alternate 

• Fibrous and often hairy 

stems 

• Flowers have 5 petals 

and sepals 

• Wedge-shaped seed are 

located in a sectioned 

capsule 

• Most have stipules 

• Common mallow 

• Venice mallow 

• Prickly sida 

• Velvetleaf

Abutilon theophrasti 

• Erect summer annual 

• Leaves heart shaped, and velvety to the touch 

• Fruit is a circular cup-shaped disk of 9-15 carpels 

Velvetleaf

Velveltleaf (Abutilon theophrasti) 

Malvaceae


Malvaceae


Malvaceae

Horseweed (Conyza canadensis) 

Asteraceae


Asteraceae


Asteraceae

What is the other weed category? 

Sedges (Cyperaceae) 

How do we ID a sedge 

compared to a grass? 

• Grass-like in 

appearance. 

• In ranks of 3 

• 3 – angled stems 

• Over 150 species in WI 

• Yellow nutsedge most 

common in crops

Cyperus esculentus 

• Can be confused with a 

grass. 

• Perennial with tubers 

Triangular 

stems 

yellow nutsedge

Yellow nutsedge (Cyperus esculentus) 

Cyperaceae (SEDGE, NOT A GRASS)

You can identify weeds, 

• Are they a problem in crops? 

– Competition for resources – yield reduction 

– Reduction in quality 

– Hosts for other pests 

• Are herbicides the only way? 

– NO, but they can be very effective when selectivity 

is matched with weed spectrum and applications 

are made at the right timing to the weed and crop 

– Cultural practices are very important 

• Crop rotation, tillage, timing

Weed-Crop Competition 

“Competition occurs when each of two organisms seek 

the measure they want of any particular factor or 

thing, when the immediate supply of the factor or 

things is below the combined demand of the 

organisms”. 

Donald (1963)

Factors for which Competition Occurs: 

Plants compete for factors which are limited in the environment. 

LIGHT 

WATER 

NUTRIENTS 

[Plants do not compete for heat, CO2 (rarely) or space (rarely)]. 

CO2 

Competition for CO2 can occur in dense canopies. Interestingly, C4 

plants may be more competitive under these conditions. 

SPACE 

Competition for space possible between competing roots. 

Competition from space in root crops eg carrots too closely planted. 

OXYGEN 

Competition for oxygen in germinating seed.

Competition for Nutrients: 

• The total biomass that can be produced upon a given piece of land is 

fixed by the amount of available resources. 

• This is called THE LAW OF CONSTANT FINAL YIELD.

Competition for Light: 

• Light is in relatively constant supply in the environment, it is 

fundamental to plant growth, and its limits are relatively constant. 

• Light is probably the factor for which competition is the most common 

and the most fierce. 

• Competition for light occurs as soon as plant canopies begin to overlap. 

• In cropping systems which are well irrigated and fertilized, light may 

be the only factor for which plants compete. 

• Plants that start growing early tend to be very competitive for 

light. 

• Plants which develop a large leaf area tend to be very competitive 

for light. 

• Plants which grow tall tend to be very competitive for light. 

• In general, broadleaf weeds are more competitive for light than grass 

weeds, and cause greater yield reductions.

Competition for Water: 

• Water may be more or less reliable in the environment, and may come 

into limited supply. 

• Different plant species have different water use efficiencies – although 

the outcome of competition for water may be difficult to determine. 

Species 

Sorghum (Sorghum 

bicolor) 

A 

Water 

Requirement 

Species 

304 Purslane (Portulaca 

oleracea) 

Corn (Zea mays) 349 Redroot pigweed 

(Amaranthus retroflexus) 

Wheat (Triticum aestivum) 542 Cotton (Gossypium 

hirsutum) 

Rice (Oryza sativa) 682 Lambsquarters 

(Chenopodium album) 

Water 

Requirement 

281 

305 

568 

658 

Smooth brome (Bromus 

inermis) 

977 Common ragweed 

(Ambrosia artemisiifolia) 

912 

A 

g water required to produce 1g plant dry weight.

Weed Species 

Modification of competition 

with soybean as water 

becomes limited. 

Cocklebur (Xanthium strumarium) - 

Sicklepod (Senna obtusifolia) + 

Jimsonweed (Datura stramonium) -, +, 0 

Common ragweed (Ambrosia artemisiifolia) - 

Redroot pigweed (Amaranthus retroflexus) + 

Velvetleaf (Abutilon theophrasti) - 

- Competition becomes less severe 

+ Competition becomes more severe 

0 Competition unchanged

Competition for Nutrients 

• Weeds can reduce yields by limiting nutrient 

acquisition by the crop. 

• Weeds often have greater response to nutrients so 

increasing fertilizer rates doesn’t help. 

• Timing, form and placement of fertilizer may be 

important

The Nature of Competition: 

Intraspecific competition: 

Competition between individuals of the same species 

Interspecific competition: 

Competition between individuals of two or more species. 

Competition is a major driver of selection/evolution when species use the 

same environment – NICHE OVERLAP. Species driven to utilize 

different parts of the environment – NICHE DIFFERENTIATION. 

Competitive ability is comprised of 2 components: 

COMPETITIVE EFFECT: The ability of a plant to deplete resources and 

make the unavailable to other plants. 

COMPETITIVE RESPONSE: The ability of a plant to grow, survive and 

reproduce despite the depletion of resources by competitors.

Effects of Plant Density: 

Plants respond to increasing density in one of two ways: 

1. Increased plasticity. 

2. Increased mortality. 

……….because……….. 

Final Yield is constant, but….. 

1. Is represented by a larger number of smaller plants. 

2. Yield can be represented in different plant parts (different plants 

respond with different patterns of resource allocation). 

3. Yield can be shared unequally between different plant species.

Weed Thresholds 

Crop Weed Species/Community Approx. Economic 

Threshold (plants m -2 ) 

Cereals Broadleaf weeds 40-50 

Grass weeds 20-30 

Wild oats (Avena fatua) 8-10 

Blackgrass (Alopecurus myosuroides) 30 

Corn Broadleaf weeds 5 


Velvetleaf (Abutilon theophrasti) 1 

Soybean Broadleaf weeds 1 


Velvetleaf (Abutilon theophrasti) 2 

Cotton 1-40 

Sunflower 1-40 

Tomato 0-5

Should I spray? 

• Competition threshold: weed density above which 

crop yield is reduced by an unacceptable, usually 

arbitrary (5-20%), amount. Used by most farmers. 

• Economic threshold: weed density at which the 

benefit derived from herbicide application (yield 

savings) just equals the cost of control. Varies with 

assumptions re: weed-free yield, commodity price, 

cost and efficacy of control. 

• No-seed threshold? 

• Cosmetic thresholds?

The Critical Period of Weed Competition: 

Is there a time period 

during which weeds 

compete with the crop? 

–or more importantly – 

Are there time periods 

during which weeds do not 

compete with the crop? 

Individual plants too far 

apart to be competing for 

the same resources? 

Large plants sufficiently well 

established to completely 

suppress newly emerging 

weeds?

5% yield loss

Yield Loss (% weed free) 

Soybean Yield Loss Influenced by the Timing of 

Weed Removal and Row Spacing 

40 

35 

30 

25 

20 

15 

7.5" 

15" 

30" 

10 

5 

0 

V1 V2 V3 V4 V5 V6 V7 R1 R2 R3 

Timing of weed removal (Soybean growth stage) 

Source: Knezevic et al. 2003.

You can identify weeds, 

• Are they a problem in crops? 

– Competition for resources – yield reduction 

– Reduction in quality 

– Hosts for other pests 

• Are herbicides the only way to control weeds? 

– NO, but they can be very effective when selectivity 

is matched with weed spectrum and applications 

are made at the right timing to the weed and crop 

– Cultural practices are very important 

• Crop rotation, tillage, timing

Management Practices: 

Survey of WI growers 

• Top three weed management practices: 

1. Herbicide application (84%) 

2. Crop rotation (55%) 

3. Mechanical cultivation (35%) 

• Top three herbicide selection criteria: 

1. Efficacy (85%) 

2. Cost (74%) 

3. Carryover potential (72%) 

Hammond et al. 2006

Management Practices: 

Survey of WI growers 

• Importance of weed management decision 

(ranked ‘very important’) 

1. Efficacy (91%) 

2. Low risk of crop injury (70%) 

3. Carryover potential to other crops in the rotation 

(64%) 

4. Cost of products (53%) 

5. Safety to applicator for farm family (52%) 

6. Development of herbicide resistance (44%) 

– All others

Herbicides 

How they work and the injury 

symptoms they cause

Web Resources 

• Crop Data Management Services 

– http://www.cdms.net/Home.aspx 

• Purdue Weed Science 

– http://www.ag.purdue.edu/btny/weedscience/Pag 

es/default.aspx 

• Weed Science Society of America (WSSA) 

– http://www.wssa.net/index.htm

Herbicide Mode and Site of Action 

• Mode of Action 

– the metabolic or physiological process impaired 

or inhibited by the herbicide 

• HOW the herbicide controls the plant 

• Site of Action 

– the physical location within the plant where the 

herbicide must bind to exert its MOA 

• WHERE the herbicide acts within the plant

Why Study Herbicide Mode of Action? 

• Planning weed management programs 

– match the herbicide to the weed problem 

– annuals vs. biennials vs. perennials 

• Correctly diagnose crop injury symptoms in the field 

• Better understand causes of reduced or enhanced 

herbicide performance 

– antagonism, synergism

Introduction Crop Injury 

• Crop injury caused by herbicides is common, but the 

reasons symptoms appear are frequently difficult to 

determine 

– why are some fields injured and others not, even when the 

same herbicide was applied to both? 

• Adding to the problem, injury symptoms from a given 

herbicide may be different, depending on how the 

crop was exposed 

– was the herbicide intentionally applied to the crop, or are 

symptoms related to “extended” soil persistence?

Most Cases of Crop Injury Occur from: 

• Carryover from applications made during previous 

seasons 

– soil factors (pH, organic matter), environmental factors, 

overlaps, etc. 

• Drift/volatility from adjacent fields 

– volatility dependent on the herbicide/formulation, drift 

dependent primarily on the applicator 

• Direct application of the herbicide to the crop 

– perhaps the most common scenario

Injury Following In-Crop Applications 

• Herbicides used in-crop must demonstrate 

selectivity to the crop in which they are used 

– herbicide resistant/tolerant crops are “engineered” to 

allow use of nonselective herbicides 

• If selectivity is compromised, crop injury symptoms 

will usually result 

– many factors can compromise herbicide selectivity 

• To understand why injury symptoms develop, we 

must understand the basis of herbicide selectivity

Basis of Herbicide Selectivity 

• Differential herbicide interception/absorption 

– rooting depth vs. herbicide placement, post-directed 

applications, leaf angle and composition 

• Differential susceptibility of the herbicide site of 

action 

– one form of ACCase in grasses, two in broadleaf species 

• Differences in rates or pathways of herbicide 

metabolism 

– probably the most common mechanism of herbicide 

selectivity

Herbicide Metabolism 

• The crop can metabolize (change) the herbicide 

fast enough to avoid phytotoxicity 

– detoxification of the herbicide 

• Plants use several processes for herbicide 

metabolism 

– hydroxylation, conjugation, cleavage of functional groups 

• Since herbicide metabolism is dependent on plant 

metabolism, factors that slow plant metabolism can 

also slow herbicide metabolism 

– herbicides cannot distinguish between weed and crop

Crop Injury Symptoms 

• In most cases, herbicide selectivity is achieved 

because the crop is able to metabolize the 

herbicide fast enough to avoid the phytotoxic effects 

of the herbicide 

• However, because the herbicide cannot 

differentiate crop and weed plants, the herbicide will 

attempt to control both crop and weed 

• Herbicide injury symptoms are an indication the 

herbicide was at least partially successful

Photosynthetic Inhibitors (primarily soil-applied) 

Triazines 

atrazine (AAtrex) 

simazine (Princep) 

metribuzin (Sencor) 

hexazinone (Velpar) 

ametryne (Evik) 

Uracils 

bromacil (Hyvar) 

terbacil (Sinbar) 

Phenylureas 

linuron (Lorox) 

tebuthiuron (Spike) 

diuron (Karmex)

Injury Symptoms of Soil-Applied PSI 

• Injury symptoms appear after the food reserves in 

the cotyledons have been exhausted 

• The injury symptoms will appear first on the oldest 

leaves of the plant 

• Since these herbicides move in the water 

conducting tissue (xylem), injury symptoms 

will follow the path of water movement in the plant 

Roots Stem Leaves Leaf margins

Injury Symptoms of Soil-Applied PSI 

• Injury symptoms appear as a yellowing (chlorosis) 

of the leaf tissue which, if severe 

enough, eventually becomes necrotic (dead) tissue 

• Initial chlorosis appears at the leaf margins as the 

herbicide follows the same path as water 

through the plant 

• Advanced chlorosis of the leaf tissue is interveinal 

(between the veins) 

– Veins remain green

Photosynthetic Inhibitors (primarily foliar-applied) 

Benzothiadiazoles 

bentazon (Basagran) 

Bipyridiliums 

Benzonitriles 

bromoxynil (Buctril) 

paraquat (Gramoxone Max) 

diquat (Diquat) 

Phenylpyridazines 

pyridate (Tough)

Injury Symptoms of Foliar Applied PSI 

• Foliar-applied PS inhibitors are primarily contact in 

nature so symptoms first develop on tissue directly 

contacted by herbicide 

– are mobile when soil-applied and symptoms are different 

• Injury symptoms consist of an initial water soaked 

area, bronzing or speckling of the plant tissue 

followed by chlorosis and necrosis 

– yellowing followed by death of injured tissue 

– how quickly symptoms develop depends on environment, 

spray additive, herbicide, etc.

Chlorophyll 

• Pigments are molecules that absorb light 

– Sunlight Pigments PS Food 

• Chlorophyll is the plant pigment responsible for the 

green color of plants 

• The synthesis of chlorophyll has many steps, some 

of which can be inhibited by herbicides 

• One family of herbicides that block a step in 

chlorophyll synthesis is the PPO inhibitors 

– Ultra Blazer, Cobra, Flexstar, Resource, Aim

Protoporphyrinogen Oxidase Inhibitors 

• These herbicides inhibit a step in the chlorophyll 

biosynthesis pathway 

– protoporphyrinogen oxidase = PPO inhibitors 

• Blockage of this pathway results in the production 

of free radicals which damage plant membranes 

– some refer to this family as the “membrane disruptors” 

– membrane damage results in characteristic leaf burn 

• When foliar-applied, these herbicides show limited 

movement in the plant

Protoporphyrinogen Oxidase Inhibitors 

Diphenyl Ethers N-Phenylphthalimides 

acifluorfen (Ultra Blazer) flumiclorac (Resource) 

lactofen (Cobra, Phoenix) flumioxazin (Valor) 

fomesafen (Reflex, Flexstar) 

Aryl Triazolinone 

sulfentrazone (Authority) 

carfentrazone (Aim)

Injury Symptoms of PPO Inhibitors 

• Foliar-applied PPO inhibitors show limited 

translocation following absorption 

– injury symptoms first appear on foliage directly contacted 

• Symptoms include an initial water-soaked 

appearance, followed by chlorosis and necrosis 

– symptoms are similar to those of foliar PS inhibitors 

• Speed of symptom development can depend on 

environmental conditions and spray additives 

– high relative humidity + high air temperature favor rapid 

development of injury symptoms, precipitation soon after 

application may “funnel” the herbicide into the whorl


• Soil-applied PPO inhibitors are mobile within 

the plant 

• Injury often related to multiple factors 

– variety, soil pH, tillage, environment 

• Symptoms often present on cotyledons and 

hypocotyl, older leaves 

– reddish lesions 

– “wrinkled” true leaves


• Fomesafen has soil residual activity, and 

degradation is primarily dependent on soil moisture 

– late applications followed by dry weather 

• Symptoms include veinal chlorosis, which results in 

a white striping of corn leaves 

– symptoms are similar to those of early Stewart’s wilt 

(without the bacterial ooze) and sunscald 

– roots show no injury symptoms

Lipid Synthesis Inhibitors 

• Lipids are composed of fatty acids, and are a major 

component of plant membrane systems 

• LSI herbicides block the formation of fatty acids 

– target the Acetyl-CoA carboxylase (ACCase) enzyme 

• These herbicides are used for postemergence 

grass control in broadleaf crops 

• Corn is exposed to this chemical family primarily by 

drift 

– at least one of these herbicides is labeled for directed 

application in corn

Lipid Synthesis Inhibiting Herbicides 

Aryloxyphenoxypropionates 

fluazifop (Fusilade DX) 

fenoxaprop (Puma) 

quizalofop (Assure II) 

fluazifop + fenoxaprop (Fusion) 

Cyclohexanediones 

sethoxydim (Poast Plus) 

clethodim (Select)

Injury Symptoms of LSI Herbicides 

• These herbicides translocate within grass plants, 

especially to areas of high fatty acid production 

– much fatty acid production occurs in meristems 

• The most common injury symptom is separation of 

the uppermost leaf from the whorl 

– will also eventually have death of roots, starting at the 

root tips 

• Injury symptoms are typically slow to develop 

– additional symptoms include a gradual necrosis of foliage

Amino Acid Synthesis 

• Amino acids are the basic structural units of 

proteins 

– amino acids proteins 

• Much of the amino acid synthesis in plants occurs 

in the areas of the plant undergoing 

rapid growth (meristems) 

– Apical ("top") and root ("bottom") meristems 

• One of the most common roles of plant proteins is 

to function as enzymes 

– amino acids proteins enzymes

Herbicides Inhibiting Amino Acid Synthesis 

• These herbicides inhibit key enzymes in various 

amino acid synthesis pathways 

– Synthesis pathway amino acids 

• Without amino acids, protein and enzyme formation 

and function are limited 

– Amino acids proteins enzymes 

• The enzymes inhibited include ALS (AHAS), EPSP 

synthase, and glutamine synthetase (GS) 

– ALS = imidazolinones, sulfonylureas, etc. 

– EPSP = glyphosate 

– GS = glufosinate (Liberty/Ignite)

Amino Acid Synthesis Inhibitors 

• Much of the amino acid biosynthesis in plants 

occurs in areas undergoing rapid growth 

(meristems) 

– apical and root meristems 

• Herbicides inhibiting amino acid biosynthesis target 

one of three pathways 

– acetolactate synthase (ALS) 

– 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase 

– glutamine synthetase (GS)

Herbicides Inhibiting Amino Acid Synthesis 

Imidazolinones 

imazethapyr (Pursuit) 

imazaquin (Scepter) 

Imazapyr (Arsenal) 

Imazamox (Raptor) 

Triazolo. Sulfonanilides 

Flumetsulam (Python) 

Cloransulam (FirstRate) 

Sulfonylureas 

chlorimuron (Classic) 

thifensulfuron (Harmony GT) 

nicosulfuron (Accent) 

primisulfuron (Beacon) 

halosulfuron (Permit) 

foramsulfuron (Option) 

chlorsulfuron (Glean) 

Phosphonos 

glyphosate (many) 

glufosinate (Liberty/Ignite)

Injury Symptoms of A.A. Synthesis Inhibitors 

• Injury symptoms are usually slow to develop 

– takes time to deplete the available amino acid pool 

• Above ground symptoms consist of: 

– stunted development, including leaf internode shortening, 

yellow flashing of whorl leaves, crinkling of leaf margins, 

purpling of leaf margins 

• Below ground symptoms (carryover) consist of: 

– inhibited root development (“bottlebrush”), where the 

shape of the root system often indicates where in the soil 

profile most of the herbicide is located


• These herbicides may cause soybean stunting and 

chlorosis of the leaf tissue 

– additives may enhance injury 

• Soybean leaf veins often appear red following the 

application of these herbicides 

– most noticable on under side of leaf 

• Severe injury may result in the death of the apical 

meristem 

– “tuning fork"


• Glyphosate also belongs in this herbicide family, 

and symptoms are somewhat different than those 

from direct application or carryover 

– exposure by drift, tank contamination 

• Foliar symptoms consist of white discoloration of 

leaf tissue, frequently beginning as small patches 

– often gives an indication of when the plant was exposed 

• Root symptoms include death of the roots 

– dead tissue generally is first noticed at the root tips

Plant Growth Regulators 

• This herbicide family includes some of the oldest 

selective herbicides 

– specific cause(s) of plant death remains to be determined 

• These herbicides tend to mimic the effects of 

endogenous plant hormones 

– hormones control numerous growth processes, but 

endogenous hormones act at very low concentrations 

• These herbicides are mobile within the plant, so 

symptoms on foliage and roots can occur

Plant Hormones 

• Plant hormones are organic compounds produced 

in one portion of the plant and 

translocated to another part where they cause 

some type of physiological response 

– abscisic acid, ethylene, gibberellins, 

indoleacetic acid 

• Hormones regulate numerous plant processes 

including cell division, flowering, and seed 

development

Growth Regulator Herbicides 

Phenoxys Benzoic Acids 

2,4-D 

dicamba (Banvel, 

MCPP 

Clarity, Distinct) 

MCPA 

2,4-DB (Butyrac) 

Pyridines 

picloram (Tordon) 

clopyralid (Stinger) 

triclopyr (Garlon)

Injury Symptoms of PGR Herbicides 

• Because these herbicides mimic the effects of plant 

hormones, corn injury symptoms can be variable 

– environment, crop growth stage can > or < crop response 

• Typical symptoms include: 

– failure of leaves to unfurl properly (soil or post) 

– mesocotyl twisting 

– leaf rolling 

– bent or elbowed stalks 

– root proliferation/malformation 

– improper ear set/lack of kernel set or development

Injury Symptoms of Plant Growth Regulators 

• Soybean injury symptoms include stem splitting, 

callus or gall tissue formation, leaf 

cupping, parallel leaf veination, and epinasty or 

twisting 

– 2,4-DB application, 2,4-D/dicamba,clopyralid drift or tank 

contamination 

• Small grain injury symptoms include twisting of the 

flag leaf, improper head emergence, and 

sterile seed heads

Carotenoid Synthesis Inhibitors 

• Carotenoids are “accessory” plant pigments that 

protect chlorophyll during photosynthesis 

– dissipate excess energy from excited chlorophyll 

molecules 

• Chlorophyll is the pigment responsible for the green 

color of plants 

– chlorophyll reflects the green wavelength of visible light 

• Carotenoid synthesis inhibitors block the formation 

of carotenoids 

– if carotenoids are absent, chlorophyll can be destroyed 

during PS and plants can lose their green color

Carotenoid Synthesis Inhibitors 

Isoxazalidinones 

clomazone (Command) 

Triketone 

mesotrione (Callisto) 

tembotrione (Laudis) 

tropramezone (Impact) 

Isoxazoles 

isoxaflutole (Balance)

Injury Symptoms of Carotenoid Synthesis 

Inhibitors 

• Corn foliage is white 

– lack of carotenoids leads to destruction of chlorophyll 

• Clomazone carryover – corn generally recovers 

• Isoxaflutole injury – corn does not always recover 

– formulation change from dry to liquid helped mixing 

problems 

– application problems, hybrid sensitivity issues are not 

resolved by formulation changes

Cell Reproduction 

• Cell division (mitosis) is the process by which cells 

reproduce 

• During cell division the genetic material 

(chromosomes) is replicated and distributed 

evenly to the two new daughter cells 

• Specialized structures called microtubules function 

to distribute the genetic material 

equally to the two new cells

Dinitroaniline Herbicides 

trifluralin (Treflan, others) 

pendimethalin (Prowl, Pentagon, Pendimax) 

ethalfluralin (Sonalan) 

benefin (Balan) 

oryzalin (Surflan) 

prodiamine (Barricade)

Injury Symptoms of DNA Herbicides 

• Corn seedlings are usually stunted with club-shaped 

roots and limited secondary root development 

– characteristic symptom of DNA, also corn nematodes 

• Foliage often with purple leaf margins, probably 

attributable to poor phosphorus uptake 

– limited root growth leads to limited nutrient absorption 

• Lateral roots of soybeans may be pruned from excessive 

rates of DNA herbicides. Hypocotyls may swell (callus) 

and crack, causing lodging late in the season

Chloroacetamide Herbicides 

• These herbicides inhibit primarily shoot 

development in susceptible species 

– root symptoms not common 

• Primary mechanism of plant death not completely 

understood 

– some speculate these herbicides interfere with cell 

division by inhibiting formation of very long chain fatty 

acids 

• Many formulated with safeners to reduce corn injury 

potential

Chloroacetamides and Thiocarbamates 

Chloroacetamides 

Thiocarbamates 

alachlor (Micro-Tech, IntRRo) butylate (Sutan +) 

S-metolachlor (Dual II Magnum) EPTC (Eradicane) 

dimethenamid (Outlook) 

acetochlor (Harness,TopNotch, etc) 

Oxyacetamides 

flufenacet (Define)

Injury Symptoms of Chloroacetamides 

• Injury symptoms are most frequent under cool, wet 

conditions 

– those conditions that slow corn growth 

• Corn seedlings may leaf-out underground. 

Following emergence, leaves may fail to unfurl 

correctly, and the leaves may appear crinkled 

– have observed leaf rolling on much taller corn, but is this 

more herbicide or environment related? 

• Symptoms typically dissipate when temperatures 

increase and soils dry

Injury Symptoms of Chloroacetamides 

• Corn seedlings may leaf-out underground. Following 

emergence, plant leaves may fail 

to unfurl and the leaves may appear crinkled 

– injury enhanced under adverse environmental conditions 

– cool, wet soils in early part of season 

• Soybean leaves may also appear crinkled or puckered. 

A shortened leaf mid-vein gives the 

leaf a heart-shaped or "draw string" appearance

Herbicide Resistance

Herbicide Resistance 

What’s the Big Deal?

What is Resistance? Definitions 

Susceptibility: 

Plant injury/death caused by a herbicide at acceptable use rates – 

the inherent plant response to a herbicide 

Tolerance: 

The ability of a plant population to remain uninjured at the 

herbicide at acceptable use rates – the inherent lack of plant 

response to a herbicide 

Resistance: 

The change in a plant population from a population that is initially 

predominantly susceptible to a population that is predominantly 

“tolerant” – the loss of effectiveness of a herbicide due to the 

evolution of resistance in a weed population

Definitions 

Resistance: 

The change in a plant population from a population that is initially 

predominantly susceptible to a population that is predominantly 

“tolerant” – the loss of effectiveness of a herbicide due to the 

evolution of resistance in a weed population. 

Cross Resistance: 

Resistance develops to two or more herbicides with the same 

mode of action. 

Multiple Resistance: 

Resistance develops to two or more herbicides from different 

chemical families with different modes of action.

‘Official’ source: 

International 

Survey of Herbicide 

Resistant Weeds 

www.weedscience.com

Timeline of herbicide resistance 

• The first weed biotype historically considered 

“resistant” was a biotype of common groundsel 

(Senecio vulgaris L.) documented in 1968 to have 

evolved triazine resistance in a Washington nursery 

(Ryan, 1970). 

• However, the ability for weed species to shift, and the 

ability for differential biotype selection to occur, due 

to herbicides was previously documented in the 

1950’s from the use of 2,4-D

Timeline of herbicide resistance 

• Before 1982 how many resistant weed biotypes? 

– Approx. 35 species 

• Between 1982 and 1990 the number of resistant 

weed biotypes increased by what factor? 

– Tripled = 107 species 

• From 1990 to 1995 the number of resistant weed 

biotypes increased by what factor? 

– Doubled = 214 species 

• Presently – 372 weed species resistant to herbicides

How do weeds become resistant? 

A population equilibrates (with a new biotype) 

that exhibits… 

• An altered site of action, 

• Enhanced metabolism, 

• Reduced uptake (foliar or root), 

• Reduced translocation 

– Compartmentalization 

– Sequestration/conjugation 

– Other 

• Other

Hypothetical Development of 

High-Level vs. Low-Level Resistance

Changes are driven from a present 

genetic mutation 

• A herbicide does NOT cause a genetic 

mutation 

– Thus, a ‘mutant’ plant biotype already exists 

within a population 

• Evolutionary rate of herbicide resistance 

is driven by a number of factors 

– Mutation rate 

– Biotype fitness 

– Population Dynamics 

– Selection Pressure 

Population Genetics 

Management can 

influence these two, but 

Selection Pressure is 

most important!

Changes are driven from a present 

genetic mutation 

• It’s a numbers game (Jasieniuk et al. 

1996) 

– With a mutation rate of 1 x 10 -6 in a dominate 

allele of a gene that regulate resistance, one 

resistant plant should occur in every 1.5 million 

plants 

• 1:1,500,000 

–With 1 weed acre -1 , that’s 1:1,500,000 

acres 

–With 5 weeds yard -2 , that’s 1:62 acres

Question 

• How rare would a resistant mutant have 

to be to avoid resistance evolution, if we 

provide selection pressure on 

• 74 million acres! 

– That’s just the U.S. Roundup Ready soybean acres! 

• To slow/stop resistance development, 

alternative methods of weed control must be 

used intermittently to limit selection pressure 

and continually mix up the Population 

Genetics

Alternating MOA’s why 

it’s important 

• Continuous selection pressure 

– 62 acre field, Field sprayed with herbX and 1 plant 

resistant to herbX went to seed. 

• Produced 300,000 seeds. 

• ONLY 1% (3000) of progeny germinate, emerge, and 

establish in crop next season 

• Second season, Spray HerbX or HerbZ? 

– Spray 3000 with HerbX again, 3000 survive and 

produce 300,000 seeds each. 

– Even if ONLY 1% of progeny from each plant 

establishes in next crop again, 

• that’s now 9,000,000 plants resistant to HerbX

Alternating MOA’s why 

it’s important 

• Continuous selection pressure 

– 62 acre field, Field sprayed with herbX and 1 plant 

resistant to herbX went to seed. 

• Produced 300,000 seeds. 

• ONLY 1% (3000) of progeny germinate, emerge, and 

establish in crop next season 

• Second season, Spray HerbX or HerbZ? 

– Spray 3000 with HerbZ which is 99% effective, 

only 30 plants survive and produce 300,000 seeds 

each. 

– If ONLY 1% of progeny from each plant establishes 

in next crop again, that’s now only 90,000 plants 

resistant to HerbX

Glyphosate- resistant weeds 

Followed Glyphosate-resistant crops 

Glyphosate-resistant (Roundup Ready ® ) 

released in 1996 

• Clean fields were common

1990 

1991 

1992 

1993 

1994 

1995 

1996 

1997 

1998 

1999 

2000 

2001 

2002 

2003 

2004 

2005 

2006 

Cumulative Number of Glyphosate-Resistant Species (bars) 

Percent of U.S. Hectares Planted to Glyphosate-Resistant 

Soybean, Corn, and Cotton (lines) 

10 

8 

6 

Cummulative glyphosate-resistant weed species 

Glyphosate use in soybean 

Glyphosate-resistant soybean 

Glyphosate use in corn 

Glyphosate-resistant corn 

Glyphosate use in cotton 

Glyphosate-resistant cotton 

100 

80 

60 

4 

40 

2 

20 

0 

0 

Year 

Johnson et al. 2009. EJA

So what happened? 

• Reduction in 

residual herbicide 

use 

• Delayed 

postemergence 

herbicide 

applications 

• No or minimal cost 

reductions 

• Glyphosateresistant 

weeds….. 

Preplant residual herbicide 

advertisement in the 1980 (Volume 12) 

publication of Weeds Today

Evolving yield robbers…. 

Davis, V.M.


Davis, V.M.

Giant Ragweed 

(Ambrosia trifida) 

Davis, V.M.

Palmer 

amaranth 

(Amaranthus 

palmeri) is in 

Wisconsin Crop 

Production Fields 

Wisconsin Crop 

Manager article 

10/13/11

Of most concern in WI: 

Glyphosate-Resistant Giant Ragweed 

• Others in the running 

– Common ragweed 

– Lambsquarters 

– Waterhemp 

– Palmer amaranth? 

– Shattercane?

Giant Ragweed 

(Ambrosia trifida)

Dr. Dave Stoltenberg 

Grant County Giant Ragweed 

September 2009

Rock County Giant Ragweed 

3.0 lb ae/acre glyphosate 

4 WAT 

RC-S 

RC-R 

Dr. Dave Stoltenberg

Avoiding Herbicide Resistance in 

Weeds A3615 – Take home message! 

• Rotate Herbicide MOA’s 

• Rotate Herbicide SOA’s and use tank-mixtures 

• Use herbicides with low risk of resistance 

development when possible 

• Rotate crops 

• Scout for weeds that escape herbicide control 

• Include mechanical control where 

environmentally feasible

Resistance, what to look for 

• Most of the time resistance is not detected until 

there is a large problem (more than one control 

failure) 

• Early warning signs: 

– Only one weed species escapes control 

– Plants will have a wide range of response to the 

herbicide 

• often dead plants next to alive plants, especially look for 

large dead plants next to small, injured alive plants 

• Injured plants with rapid regrowth 

– Arrangement of weed escapes in the field do not 

match to field equipment widths

THANKS and good luck. 

Vince Davis, University of Wisconsin-Madison 

vmdavis@wisc.edu (608) 262-1392

Wisconsin CCA Training Weed Management - Integrated Pest and ...

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