Managing Herbicide-resistant Weeds

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.