Managing Herbicide-resistant Weeds

Carol Mallory-Smith
Revised: 
March 2015

Adapted from Herbicide-Resistant Weeds and Their Management, PNW 437, a Pacific Northwest Extension publication (University of Idaho, revised 2011). Authors are Joan Campbell, University of Idaho; Carol Mallory-Smith and Andy Hulting, Oregon State University; Donn Thill, University of Idaho. Online at http://www.cals.uidaho.edu/edComm/pdf/pnw/pnw0437.pdf

Herbicide resistance is the inherited ability of a plant to survive a herbicide application to which the wild-type was susceptible. Resistant plants occur naturally within a population and differ slightly in genetic makeup, but remain reproductively compatible with the wild type.

Herbicide-resistant plants are in a population in extremely small numbers. Repeatedly using one herbicide allows these few plants to survive and reproduce. The number of resistant plants then increases in the population until the herbicide no longer effectively controls the w­eed.

Resistant plants likely will persist in infested fields for many years, even in the absence of any additional selection with the herbicide. There is no evidence that herbicides cause the genetic mutations that lead to herbicide resistance.

Resistant plants may be resistant to other herbicides (imidazolinones as well as sulfonylureas, for example) that kill plants in the same way (same site of action or same group). This is called cross-resistance.

Weeds also can be resistant to herbicides with different sites of action (aryloxyphenoxy propanoates as well as sulfonylureas, for example). In Oregon, a biotype of Italian ryegrass is resistant to at least four different herbicide groups. This is called multiple-resistance.

Herbicide resistance is not the natural tolerance that some species have to a herbicide. For example, wheat is tolerant to Discover because it can rapidly deactivate it. Wild oat can only slowly deactivate Discover, so the herbicide can be used selectively to remove wild oat from wheat.

The first identified herbicide-resistant weed—spreading dayflower (Commelina diffusa), resistant to 2,4-D—was identified in 1957, in a sugarcane field in Hawaii. Since then, more than 238 weed species resistant to one or more herbicides have been identified worldwide. For current information on the status of herbicide-resistant weeds, see: http://WeedScience.org/

Herbicide-resistant weeds are common in the Pacific Northwest:

  • Kochia, prickly lettuce, and Russian thistle resistant to sulfonylurea herbicides (Glean, Amber, Ally, and other Group 2 inhibitors)
  • Wild oat and Italian ryegrass resistant to Group 1 ACCase inhibitors
  • Powell amaranth resistant to triazines and other Group 5 Photosynthetic inhibitors
  • Yellow starthistle resistant to Tordon and other Group 4 Auxin
  • Prickly lettuce resistant to 2,4-D Group 4 Auxin
  • Kochia and Italian ryegrass resistant to Group 9 glyphosate

The appearance of herbicide-resistant weeds is strongly linked to repeated use of the same herbicide, or herbicides with the same site of action, in a monoculture cropping system (for example, wheat after wheat) or in non-crop areas (railway or road rights-of-way, for example). Managing herbicides to delay and prevent the appearance of herbicide-resistant weeds, requires understanding which chemical family a herbicide belongs, and which herbicides have the same site of action.

The table below lists herbicides by group number and site of action, chemical family, common name, and trade name. It also gives examples of resistant weeds. Herbicide families that have the same site of action have the same group number.

A herbicide program to prevent resistance does not use herbicides from the same group more than once in 3 years.

Tank mixing herbicides is not always an effective resistance management strategy. If herbicides in the tank-mixture control different weed species and have different soil residual characteristics, resistant weed biotypes will continue to be selected. For example if a long-residual (Glean) or a short-residual (2,4-D) herbicide are tank mixed, both herbicides may control emerged broadleaf weeds. However, Glean will continue to control weeds throughout the growing season and could continue to select for resistant plants. Tank mix only when a herbicide combination is required to control the weed spectrum or will reduce herbicide use rates. Tank mixing for other reasons is not economically or ecologically sound.

Management can prevent or delay the appearance of herbicide-resistant weeds. The following practices can be used with the information on herbicide families provided in the table to form a herbicide resistance management strategy.

Preventing herbicide-resistant weeds

Herbicide rotation Avoid year-after-year use of herbicides that have the same site of action. At one time this meant not using herbicides from the same chemical family, but this is no longer the case. For example, two chemically different groups of herbicides, the sulfonylureas and imidazolinones, have the same site of action. For another example, Discover and Poast belong to different chemical families but kill susceptible grasses in the same way.

Short-residual herbicides Using herbicides that do not persist in soil for long periods and are not applied repeatedly within a growing season reduces the selection of herbicide-resistant weeds. However, repeated applications within a growing season of a herbicide with no soil activity (e.g., Gramoxone) has resulted in weeds resistant to the herbicide.

Crop rotation Because different crops may require different herbicides, rotating crops can increase herbicide rotation. But with the large number of sulfonylurea and imidazolinone herbicides available for use in many different crops, crop rotation alone may not be enough to avoid weed resistance to herbicides. This also is true for other herbicides with the same site of action.

Cultivation In row crops, cultivation can be an effective tool for eliminating weed escapes that may represent the resistant population. Fallow tillage controls herbicide-resistant and herbicide-susceptible weeds equally as long as seedlings of the two biotypes emerge at the same time. Do not use the same site-of-action herbicide in fallow as was used to control weeds in the crop.

Accurate record keeping To have an effective herbicide rotation or tank-mix system to prevent resistance, you must know which herbicides have been used in the past, at what rate, and how often.

Clean seed Plant certified seed to prevent introducing herbicide-resistant weed seeds.

Integrated weed management This concept is important for all weed control, not just management of herbicide-resistant weeds. Integrated weed management uses all the tools available to control weeds, including cultural, mechanical, and chemical methods. An integrated approach to weed management, whether it is in crop or non-crop land, is an important environmental and economic consideration.

Dealing with herbicide-resistant weeds

Monitor fields for weed escapes Weed escapes are not necessarily resistant, but they may be. A resistance problem may not be visible until 30% or more of the weeds are no longer controlled. Determine whether escapes are only one species or a mixture. If they are a mixture, the problem is more likely related to environment or application. If they are only one species, the problem is more apt to be resistance, especially if the herbicide controlled the species in the past, and if the same herbicide has been used repeatedly in the field.

Keep weeds from spreading Prevent known resistant weeds from flowering and producing seed. After using machinery in fields or areas with known or suspected infestations of herbicide-resistant weeds, thoroughly clean the equipment to reduce the spread of resistant weeds from one field or area to another. Always plant clean seed.

Change crops and tillage systems Crop rotation and altered tillage practices can affect the weed populations. Alternating spring and winter crops means that the field will be tilled at different times each year. During one of the field preparation operations, resistant as well as susceptible weeds will be killed.

Change herbicide program If weed resistance occurs, herbicides with other sites of action and other weed management practices must be used.

Recognizing herbicide-resistant weeds

Irregular patches of a single weed species in the field are an indicator of herbicide resistance, especially when:

  • No other application problems are apparent.
  • Other weed species are controlled adequately.
  • There are no, or minimal, herbicide symptoms on the single weed species not controlled.
  • There has been a previous failure to control the same species in the same field with the same herbicide, or a herbicide with the same site of action.
  • Records show repeated use of one herbicide or of herbicides with the same site of action.

What to do if you suspect herbicide resistance

  1. Do not re-spray the field with the same herbicide, or a herbicide with the same site of action.
  2. Report your suspicion to university research or Extension personnel, or to the Extension educator in your county.
  3. Collect plants or seed that can be used to confirm resistance has developed.

Managing herbicide-resistant crops

Crops resistant to specific herbicides have been developed through genetic engineering and through traditional selective breeding techniques. Examples include Clearfield wheat, which was selected for resistance to imazamox, and Roundup Ready canola, which was genetically engineered to be resistant to glyphosate. Used properly, herbicide-resistant crops can be valuable tools to manage difficult weeds, but they also have two inherent risks that need to be considered before planting: the emergence in subsequent growing seasons of herbicide-resistant volunteers, and the potential for herbicide-resistant crops to cross with weedy relatives.

Volunteer herbicide-resistant crops as weeds Consider whether the herbicide-resistant crop typically is a volunteer crop in years after its cultivation and, if so, whether herbicide options are available in the crop rotation to remove herbicide-resistant volunteers. For example, glyphosate is commonly used to control volunteer crops before planting a rotational crop. Glyphosate will not control Roundup Ready crops; therefore a herbicide with a different site of action or a non-chemical control measure is required to control glyphosate-resistant volunteers. Evaluate the impact of using these other herbicides or non-chemical control measures for your operation. Impacts could be increased cost, or increased soil erosion or moisture loss due to increased tillage.

Volunteer crops are considered a problem largely within 1 year of harvest. However, certain species have extended dormancy, which could result in multiple years of a herbicide-resistant volunteer crop problem, even without new seed inputs.

Gene flow from herbicide-resistant crops to weedy relatives Rarely, the trait that confers herbicide resistance in the crop can move into weedy relatives through cross-pollination, resulting in a herbicide-resistant weed. Consider nearby weedy and native relatives of the herbicide-resistant crop as well as their propensity to cross-pollinate. Self-pollinating crops, such as soybean, are considered low-risk for gene flow to weeds or other crops. But a crop such as Roundup Ready, Clearfield, or Liberty Link canola could pollinate nearby herbicide-susceptible canola as well as weedy relatives of canola, resulting in volunteer canola plants or weeds that may be resistant to several herbicide families.

Crops that may pose problems Herbicide-resistant crops at risk for gene flow or volunteer-management problems would include some or all of the following traits:

  • The crop cross-pollinates with nearby relatives that are problem weeds, or with other crops.
  • Crop seed shatters or vegetative propagules are left in the ground after harvest, resulting in volunteer crops in subsequent years (for example, canola or potato).
  • Herbicides for managing volunteer crops are limited to ones in the same family to which the crop is resistant.
  • Crop seed is viable in soil for several cropping seasons.
  • Using the herbicide-resistant crop increases your reliance on herbicide families that would be applied multiple times per season or several times during a cropping system.

Herbicide Rotation

To avoid selecting for herbicide-resistant weeds, do not use herbicides from the same group more than once in 3 years. Rather, rotate to a different group every year of the production system.

Group Number and Site of Action1

Chemical Family

Common Name

Trade Name(s)

Resistant Weeds in the PNW

States with Resistant Weeds

Group 1

Acetyl CoA carboxylase (ACCase) inhibitors

cyclohexanediones

clethodim

Select Max, Envoy, several others

Italian ryegrass

ID, OR

sethoxydim

Poast, several others

Italian ryegrass

ID, OR

tralkoxydim

Achieve

Italian ryegrass

ID

aryloxyphenoxy propionates

clodinafop

Discover NG

Italian ryegrass

ID, OR

wild oat

ID

diclofop

Hoelon

wild oat

ID, OR, WA

Italian ryegrass

ID, OR, WA

fenoxaprop

Puma, Acclaim

wild oat

ID, OR

fluazifop

Fusilade DX

downy brome

OR

quizalofop

Assure II, Targa

Italian ryegrass

ID

phenylpyrazoline

pinoxaden

Axial

Italian ryegrass

OR

Group 2

Acetolactate synthase (ALS) inhibitors

imidazolinones

*imazamox

Raptor, Beyond, Clearmax (Beyond + MCPA), Clearcast

downy brome

OR

spiny sowthistle

WA

*imazapic

Plateau, Oasis

*imazapyr

Arsenal, Chopper, several others

*imazethapyr

Pursuit

prickly lettuce

ID

kochia

ID

spiny sowthistle

ID

black mustard

ID

mayweed
chamomile

ID

Group 2 (continued)

Acetolactate synthase (ALS) inhibitors

(continued)

sulfonylureas

*chlorsulfuron

Glean, Telar

prickly lettuce

ID, OR, WA

kochia

ID, OR, WA

Russian thistle

ID, OR, WA

Italian ryegrass

OR

mayweed
chamomile

ID, WA

smallseed falseflax

OR

*chlorsulfuron + metsulfuron

Finesse

smallseed falseflax

OR

*halosulfuron

Sandea

mesosulfuron

Osprey

Italian ryegrass

ID

mesosulfuron + propoxycarbazone

Olympus Flex

*metsulfuron

Ally, Escort, Cimarron

prickly lettuce

ID, OR

kochia

OR

Russian thistle

OR

smallseed falseflax

OR

*nicosulfuron

Accent

*primisulfuron

Beacon

downy brome

OR

*prosulfuron

Peak

*rimsulfuron

Matrix

*sulfometuron

Oust, Spyder

Italian ryegrass

OR

*sulfosulfuron

Maverick, Outrider, Certainty

downy brome

OR

thifensulfuron

Harmony

spiny sowthistle

WA

prickly lettuce

ID

mayweed
chamomile

ID

thifensulfuron + tribenuron

Harmony Extra, Affinity

*thifensulfuron + tribenuron + metsulfuron

Canvas

*triasulfuron

Amber

prickly lettuce

ID, OR

kochia

OR

Russian thistle

OR

Italian ryegrass

ID

tribenuron

Express

prickly lettuce

ID

mayweed
chamomile

ID

*triflusulfuron

UpBeet

sulfonylaminocarbonyl- triazolinones

flucarbazone

Everest

Italian ryegrass

ID, WA

propoxycarbazone

Olympus

triazolopyrimidines

florasulam

Orion (contains MCPA); Goldsky

Group 3

Microtubule assembly inhibitors

dinitroanalines

benefin

Balan

*ethalfluralin

Sonalan, Curbit

*oryzalin

Surflan

*pendimethalin

Prowl H2O, Pendulum, several others

*prodiamine

Barricade, Endurance, several others

*trifluralin

Treflan, Trust

benzamides

*pronamide

Kerb

wild oat

OR

Group 4

Synthetic auxins

phenoxy acetic acids

2,4-D

several

prickly lettuce

WA

2,4-DB

several

MCP

several

prickly lettuce

WA

mecoprop (MCPP)

several

benzoic acids

*dicamba

Banvel, Clarity, several others

kochia

ID, WA

pyridines

*aminopyralid

Milestone

*clopyralid

Stinger, Transline, Clopyr Ag

*fluroxypyr

Starane, Vista, Spotlight

*picloram

Tordon K, Tordon 22K

yellow starthistle

WA

*triclopyr

Garlon, Remedy, Renovate, several others

quinoline carboxylic acids

*quinclorac

Paramount, Drive

Group 5

Photosystem II inhibitors

triazines

*atrazine

AAtrex, Atrazine

common lambsquarters

ID, OR, WA

pigweed spp.

ID

common groundsel

OR, WA

annual bluegrass

OR

kochia

ID

Italian ryegrass

OR

*simazine

Princep, Simazine

common groundsel

WA

as-triazines

*hexazinone

Velpar, others

shepherd’s purse
Italian ryegrass

OR

*metribuzin

Sencor, Metri DF

Italian ryegrass

OR

uracils

*bromacil

Hyvar X, Hyvar X-L

*terbacil

Sinbar

common groundsel

OR

pigweed spp.

OR, WA

common lambsquarters

OR, WA

Group 6

Photosystem II inhibitors (same site as groups 5 and 7 but different binding behavior)

benzothiadiazoles

bentazon

Basagran

nitriles

bromoxynil

Buctril, Bromox, Bronate (contains MCPA), several others

common groundsel

OR

Group 7

Photosystem II inhibitors (same site as groups 5 and 6 but different binding behavior)

ureas

*diuron

Karmex, Direx

annual bluegrass

OR

common lambsquarters

OR

*linuron

Lorox, Linex

*tebuthiuron

Spike

Group 8

Lipid synthesis inhibitors but not ACCase inhibitors

thiocarbamates

butylate

Sutan, Sutan + (contains safener)

cycloate

Ro-Neet

EPTC

Eptam

EPTC + safener

Eradicane

triallate

Far-Go

wild oat

ID

Group 9

EPSP synthase inhibitors

glycines

glyphosate

Roundup, several others

Italian ryegrass

OR

kochia

ID, OR

Group 10

Glutamine synthase inhibitors

phosphinic acids

glufosinate

Rely, Liberty, Ignite, several others

Italian ryegrass

OR, WA

Group 14

Inhibitors of protopor-phyrinogen oxidase (Protox)

diphenylethers

*oxyfluorfen

Goal, several others

N-phenylphthalimides

flumiclorac

Resource

*flumioxazin

Chateau, Valor, SureGuard

triazinones

carfentrazone

Aim, several others

*sulfentrazone

Spartan, Portfolio

phenylpyrazole

pyraflufen

ET, Edict

Group 15

Inhibitors of very long chain fatty acid synthesis

chloroacetamides

acetochlor

Harness, Surpass, several others

alachlor

MicroTech, others

dimethenamid-P

Outlook

metolachlor

Stalwart, others

S-metolachlor

Dual Magnum, Dual II Magnum

oxyacetamides

flufenacet

Define, Axiom (contains metribuzin)

Italian ryegrass

ID, OR, WA

acetamides

napropamide

Devrinol

Group 16

Unknown

benzofurans

ethofumesate

Nortron, several others

annual bluegrass

OR

Group 17

Unknown

organoarsenicals

MSMA

several

Group 20

Inhibitors of cell wall synthesis Site A

nitriles

*dichlobenil

Casoron, Barrier

Group 22

Photosystem I electron diverters

bipyridiliums

diquat

Reglone, Reward

paraquat

Gramoxone Inteon, Firestorm, several others

Group 26

Unknown

pyrazoliums

difenzoquat

Avenge

wild oat

ID

carboxylic acids

pelargonic acid

Scythe

Group 27

Inhibitors of 4-hydroxy- phenyl-pyruvated-dioxygenase (4-HPPD)

isoxazoles

pyrasulfotole

Huskie (contains bromoxynil)

triketones

mesotrione

Callisto

topramezone

Impact

1 Herbicide classification according to primary site of action.
* Can persist in soil most of the growing season or from one growing season to the next.