Managing Herbicide-resistant Weeds* 
 *Adapted from Herbicide-resistant Weeds and Their Management, PNW 437, a Pacific Northwest Extension publication (University of Idaho, revised 2007). Authors are Carol Mallory-Smith, George R. Hyslop professor, Oregon State University; Donn Thill, professor of weed science, University of Idaho; and Don Morishita, Extension weed scientist, University of Idaho. 
Carol Mallory-Smith
Reviewed December 4, 2008

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 Australia, a biotype of annual ryegrass is resistant to at least five 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 Hoelon because it can rapidly deactivate it. Wild oat can only slowly deactivate Hoelon, 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 200 weeds resistant to one or more herbicides have been identified worldwide. Current information on the status of herbicide-resistant weeds can be found at http://WeedScience.org/in.asp
Herbicide-resistant weeds are now 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 Hoelon and other Group 1 (ACCase) inhibitors
•	Powell amaranth resistant to triazines and other Group 5 inhibitors
•	Yellow starthistle resistant to Tordon and other Group 4 inhibitors
 •	Wild oat resistant to Fargo and Avenge
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 noncrop areas (railway or road rights-of-way, etc.). To manage herbicides to delay and prevent the appearance of herbicide-resistant weeds, you must know in 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, and it gives examples of resistant weeds. Herbicide families that have the same site of action are the same group number.
A herbicide program to prevent resistance does not use herbicides from the same group more than once in 3 yr.
Tank-mixing herbicides is not 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) and 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, Hoelon 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 noncrop 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 percent or more of the weeds are no longer controlled. See whether escapes are only one species or a mixture. If a mixture, the problem is more likely related to environment or application. If 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: 
1.	No other application problems are apparent. 
2. Other weed species are controlled adequately.
3. There are no or minimal herbicide symptoms on the single weed species not controlled.
4. 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.
5. Records show repeated use of one herbicide or of herbicides with the same site of action.
What to do if you suspect herbicide resistance
•	Do not respray the field with the same herbicide.
•	Report your suspicion to university research or Extension personnel or to the Extension educator in your county.
•	Collect plants or seed that can be used to confirm resistance has developed.

Managing herbicide-resistant crops 
Herbicide-resistant crops are a recently developed tool for the control of weeds. These crops are resistant to herbicides that are lethal to nonresistant varieties of the same crop species. 
Crops resistant to specific herbicides have been developed through genetic engineering and through traditional selective breeding.  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 effectively control Roundup Ready crops, however, so some other herbicide or nonchemical control measure is required to control the glyphosate-resistant volunteers. Evaluate the impacts on your operation of using these other herbicides or nonchemical control measures. 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 yr of harvest. However, certain species have extended dormancy, which could result in multiple years of a herbicide-resistant volunteer crop problem even without seed production by the volunteer crop.
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 yr. Rather, rotate to a different group every year of the production system.
1Herbicide classification according to primary site of action.  
*Can persist in soil most of the growing season or from one growing season to the next.
MANAGING HERBICIDE-RESISTANT WEEDS (contd.)
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	
		sethoxydim 	Poast, several others	Italian ryegrass	ID	
		tralkoxydim 	Achieve	Italian ryegrass	ID	
	aryloxyphenoxy 
  propionates	clodinafop 	Discover NG	Italian ryegrass
wild oat	ID
ID	
		diclofop 	Hoelon	wild oat 
Italian ryegrass 	ID, OR, WA
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)	downy brome
spiny sowthistle	OR
WA	
		*imazapic	Plateau			
		*imazapyr	Arsenal, Chopper, several others			
		*imazethapyr	Pursuit	prickly lettuce
kochia
spiny sowthistle
black mustard
mayweed 
   chamomile	ID
ID
ID
ID

ID	
	sulfonylureas	*chlorsulfuron	Glean, Telar	prickly lettuce
kochia
Russian thistle
Italian ryegrass
mayweed 
   chamomile
smallseed falseflax	ID, OR, WA
ID, OR, WA
ID, OR, WA
OR

ID, WA
OR	
		*chlorsulfuron 
  + metsulfuron	Finesse	smallseed falseflax	OR	
		*ethametsulfuron 	Muster			
		*halosulfuron	Sandea			
		mesosulfuron	Osprey	Italian ryegrass	ID	
		mesosulfuron + propoxycarbazone	Olympus Flex			
		*metsulfuron	Ally, Escort, Cimarron	prickly lettuce
kochia
Russian thistle
smallseed falseflax	ID, OR
OR
OR
OR	
		*nicosulfuron	Accent			
		*primisulfuron	Beacon	downy brome	OR	
		*prosulfuron	Peak			
		*rimsulfuron	Matrix			
		*sulfometuron	Oust, Spyder			
		*sulfosulfuron	Maverick, Outrider, Certainty	downy brome	OR	
		thifensulfuron	Harmony	Spiny sowthistle
prickly lettuce
mayweed 
   chamomile	WA
ID

ID	
		thifensulfuron 
  + tribenuron	Harmony Extra, Affinity			
		*thifensulfuron 
  + tribenuron 
  + metsulfuron	Canvas			
		*triasulfuron	Amber	prickly lettuce
kochia
Russian thistle
Italian ryegrass	ID, OR
OR
OR
ID	
		tribenuron	Express	prickly lettuce
mayweed 
   chamomile	ID

ID	
		*triflusulfuron	UpBeet			
	sulfonylaminocarbonyl-
  triazolinones	flucarbazone	Everest	Italian ryegrass	ID	
		propoxycarbazone	Olympus			
	triazolopyrimidines	florasulam	Orion (contains MCPA); registration pending			
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			
		2,4-DB	several			
		MCP	several			
		mecoprop (MCPP)	several			
	benzoic acids	*dicamba	Banvel, Clarity,
several others	kochia	ID	
	pyridines	*aminopyralid	Milestone			
	*clopyralid	Stinger, Transline, Clopyr Ag			
	*fluroxypyr	Starane, Vista, Spotlight			
		*picloram	Tordon K, 
Tordon 22K	yellow starthistle	WA	
		*triclopyr	Garlon, Remedy, Renovate			
	quinoline carboxylic acids	*quinclorac	Paramount, Drive			
						
Group 5  
  Photosystem II 
  inhibitors	triazines	*atrazine	AAtrex	common 
  lambsquarters
pigweed spp. 
common groundsel
annual bluegrass
kochia 	ID, OR, 
WA
ID
OR, WA
OR
ID	
		*simazine	Princep, Simazine	common groundsel	WA	
	triazinones	*hexazinone	Velpar, Pronone			
		*metribuzin	Sencor, Metri DF			
	uracils	*bromacil	Hyvar X, Hyvar X-L			
		*terbacil	Sinbar	common groundsel
pigweed spp.
common 
  lambsquarters 	OR 
OR, WA

OR	
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 common  
  lambsquarters	OR 

OR	
		*linuron	Lorox			
		*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	
Group 10   
  Glutamine 
  synthase 
  inhibitors	phosphinic acids	glufosinate	Rely, Liberty, several others			
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, several others			
		dimethenamid-P	Outlook			
		metolachlor	Stalwart, several others			
		S-metolachlor	Dual Magnum, 
Dual II Magnun			
	oxyacetamides	flufenacet	Define, Axiom (contains metribuzin)	Italian ryegrass	ID, OR	
	acetamides	napropamide	Devrinol			
Group 16
  Unknown	benzofuranes	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, several others			
Group 26
  Unknown	pyrazoliums	difenzoquat	Avenge	wild oat	ID	
	carboxylic acids	pelargonic acid	Scythe			
Group 28
  Inhibitors of
  4-hydroxy-
  phenyl-
  pyruvated-
  dioxygenase
  (4-HPPD)	isoxazoles	pyrasulfotole	Huskie (contains bromoxynil)			
	triketones	mesotrione	Callisto			
		topramezone	Impact			

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