Managing Herbicide-Resistant Weeds1

1 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 https://www.extension.uidaho.edu/publishing/pdf/pnw/pnw0437.pdf

Herbicide resistance is the inherited ability of a plant to survive an 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 frequency of resistant plants then increases in the population because all susceptible individuals are controlled.

Resistant plants likely will persist in infested fields for many years, even in the absence of any additional selection with the herbicide. This is because the competitive ability of the resistant biotypes does not seem to be dramatically compromised. To date, there is no evidence that herbicides cause the genetic mutations that lead to herbicide resistance.

Weed populations may evolve resistance to more than one herbicide. When populations evolve, a pattern of resistance to more than one herbicide within the same mechanism of action (imidazolinones as well as sulfonylureas, for example) is labeled cross-resistance. When populations evolve resistance to more than one herbicide from different mechanisms of action, this pattern is known as multiple-resistance. Some weed populations that have been exposed to continued selection pressure by herbicides may exhibit both patterns of cross- and multiple-resistance.

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

The first identified herbicide-resistant weed—wild carrot (Daucus carota), resistant to 2,4-D—was identified in 1957, in Ontario, Canada. Since then, more than 262 populations from distinct weed species evolved resistance to one or more herbicides 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, downy brome, and Russian thistle resistant to Group 2 herbicides
• Wild oat, downy brome, and Italian ryegrass resistant to several Group 1 herbicides
• Redroot pigweed and annual bluegrass resistant to Group 5 herbicides
• Prickly lettuce resistant to Group 4 herbicides
• Kochia and Italian ryegrass resistant to glyphosate (Group 9)

The evolution of herbicide-resistant weeds is strongly linked to repeated use of the same herbicide, or herbicides with the same mechanism of action, in a simplistic cropping system (for example, wheat after wheat) or in non-crop areas (railway or road rights-of-way, for example). Managing herbicides to delay the appearance of herbicide-resistant weed populations requires an understanding of which herbicide chemical families, and which herbicides have the same mechanism of action.

The table below lists herbicides by group number and site of action, chemical family, and common name. It also gives examples of resistant weed populations documented.

To delay selection of herbicide-resistant weed populations, it is recommended to follow good agronomic practices to manage weeds that integrate herbicide use with cultural, physical, and preventive strategies. When using herbicides, proper 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 an herbicide resistance management strategy.

Preventing herbicide-resistant weeds

Herbicide rotation and tank mixtures Avoid year-after-year use of herbicides that have the same mechanism of action. For example, two chemically different groups of herbicides, the sulfonylureas and imidazolinones, have the same mechanism of action (both Group 2), and this tankmix should be avoided. Similarly, fluazifop-P-butyl and clethodim (both Group 1) belong to different chemical families but control susceptible grasses in the same way. Tank mixtures and herbicide rotations from different mechanisms of action are preferred compared to single herbicide treatments. Postemergence herbicides should be used to eliminate all individuals in a population at the recommended rate and application stage. Using lower or higher than recommended rates, as well as treating weeds at growth stages that results in marginal control levels, pose selection pressure that select for the herbicide-resistant individuals within the population.

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 pressure on weed populations. Tankmixing soil-applied herbicides delays evolution of herbicide resistance, as long as the residual activities and the weed spectrum are similar. If herbicides in the tankmix have different soil residual characteristics, resistant weed biotypes will continue to be selected. For example, if a long-residual or a short-residual herbicide are tankmixed, both herbicides may control emerged and emerging broadleaf weeds. However, the herbicide with short residual activity will dissipate faster, thus the long-residual herbicide will continue, alone, to control weeds throughout the growing season and could continue to select for resistant plants.

Crop rotation Because different crops may require different herbicides, rotating crops can naturally reduce reliance on one herbicide or herbicides with similar mechanism of action. 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 crop 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 mechanism of action herbicide in fallow as was used to control weeds in the crop.

Accurate record keeping To have an effective herbicide rotation or tankmix system to delay 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 efforts, not just management of herbicide-resistant weeds. Integrated weed management uses all the tools available to control weeds, including cultural, mechanical, physical, 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 a considerable number of weed individuals in a population 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. Many weed species exhibit wind pollination, and herbicide resistance traits may be transferred to field populations from adjacent areas. Therefore, controlling weeds growing in field edges, fences, and roadsides is important to avoid gene flow into fields via pollen or seed movement.

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 controlled.

Change herbicide program If weed become resistant to a particular herbicide, 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 (or same geographical region) with the same herbicide, or an herbicide with the same site of action.
• Records show repeated use of one herbicide or of herbicides with the same mechanism of action.

What to do if you suspect herbicide resistance:

1. Do not re-spray the field with the same herbicide, or an herbicide with the same mechanism 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 evolved.

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 (Group 2), and Roundup Ready canola, which was genetically engineered to be resistant to glyphosate (Group 9). 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, an herbicide with a different mechanism 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 seed dormancy, which could result in multiple years of an 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 an herbicide-resistant weed population. 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).
• 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, rotate and tankmix herbicides from different sites of action.

Group Number and Site of Action1	Chemical Family	Common Name	Resistant Weeds in the PNW	States with Resistant Weeds
Group 1
Acetyl CoA carboxylase (ACCase) inhibitors	cyclohexanediones	clethodim	downy brome	OR
			Italian ryegrass*	ID
		sethoxydim	downy brome	OR
			Italian ryegrass	ID
			wild oats	WA
	aryloxyphenoxy propionates	clodinafop-propargyl	Italian ryegrass*	ID
			wild oats	ID
		diclofop-methyl	Italian ryegrass*	ID, OR
			wild oats	ID, OR, WA
		fenoxaprop-P-ethyl	wild oats	ID, OR, WA
		fluazifop-P-butyl	downy brome	OR
		quizalofop-P-ethyl	downy brome	OR
			Italian ryegrass*	ID
			wild oats	WA
	phenylpyrazolines	pinoxaden	wild oats	WA
Group 2
Acetolactate synthase (ALS) inhibitors	imidazolinones	imazamox	downy brome	OR
			spiny sowthistle	WA
		Imazethapyr	mayweed chamomile	ID, WA
			prickly lettuce	ID
	sulfonylureas	chlorsulfuron	kochia	ID, OR, WA
			mayweed chamomile	ID
			prickly lettuce	ID, OR, WA
			Russian thistle	ID, OR, WA
			small seed false flax	OR
		metsulfuron-methyl	kochia	OR
			prickly lettuce	ID, OR
			Russian thistle	OR
			smallseed false flax	OR
		primisulfuron-methyl	downy brome	OR
		sulfosulfuron	downy brome	OR
		triasulfuron	kochia	OR
			Italian ryegrass*	ID
			prickly lettuce	ID, OR
			Russian thistle	OR
		thifensulfuron-methyl	mayweed chamomile	ID, WA
			prickly lettuce	ID
		tribenuron-methyl	mayweed chamomile	ID, WA
			prickly lettuce	ID
			spiny sowthistle	WA
	triazolopyrimidines	cloransulam-methyl	mayweed chamomile	WA
Group 4
Synthetic auxins	benzoates	dicamba	kochia	ID
			prickly lettuce	WA
	phenoxy-carboxylates	2,4-D	prickly lettuce	WA
		MCPA	prickly lettuce	WA
	pyridine-carboxylates	picloram	yellow starthistle	WA
Group 5
Photosystem II inhibitors	nitriles	bromoxynil	common groundsel	OR
	triazines	atrazine	annual bluegrass	OR
		simazine	common groundsel	WA
	uracils	terbacil	common lambsquarters	WA
			Powell amaranth	WA
			redroot pigweed	OR, WA
	ureas	diuron	annual bluegrass	OR
	triazinones	metribuzin	annual bluegrass*	OR
			common lambsquarters	WA
			redroot pigweed	ID, WA
			shepherd’s purse	OR
		hexazinone	shepherd’s purse	OR
Group 8
Lipid synthesis inhibitors but not ACCase inhibitors	thiocarbamates	triallate	wild oats*	ID
Group 9
EPSP synthase inhibitors	glycines	glyphosate	Italian ryegrass*	OR
			kochia	ID, OR
			Russian thistle	OR, WA
Group 10
Glutamine synthase inhibitors	phosphinic acids	glufosinate-ammonium	Italian ryegrass*	OR
Group 15
Inhibitors of very long chain fatty acid synthesis	benzofurans	ethofumesate	annual bluegrass	OR
	α-oxyacetamides	flufenacet	Italian ryegrass*	ID, OR, WA
Group 16
Unknown	benzofurans	ethofumesate	annual bluegrass	OR
Group 26
Unknown	pyrazoliums	difenzoquat	wild oat	ID
Unknown	pyrazoles	difenzoquat	wild oats	ID

*Multiple resistant populations have been reported. See https://weedscience.org for more details.