2000 North Dakota Weed Control Guide

X1. Herbicide resistance (R) occurs from repeated use of a herbicide that eliminates susceptible (S) weed species and allows R weed species to increase in the absence of competition from the S plant species. Genetically diverse weed species may contain a small percentage of plants that are R to herbicides having the same mode of action. Repeated exposure of a weed population to a herbicide may result in a rapid buildup of weed resistance to that class of herbicides. R types may dominate over time due to this selection pressure.

Risk of weed resistance increases by using herbicides that provide near 100% weed control (ALS inhibitors). Growers should not rely on one herbicide class in a crop rotation system, like using trifluralin in small grains and broadleaf crops. Plants with a wide genetic diversity that have developed herbicide resistance are kochia, pigweeds/water hemps, cocklebur, sunflower, green/giant foxtail, and wild oat. Models estimate resistance in kochia and other species to occur at 1 resistant plant in 10,000 to 100,000 plants.

Types of Resistance

Altered site of action - ALS inhibitors and other herbicides act on one specific site in a plant which may allow diverse plant species to be resistant to the herbicide or mutate to express resistance. Herbicides that affect one enzyme in a plant usually are prone to altered site of action resistance.

Altered herbicide metabolism - Plants prevent herbicide toxicity by rapid degradation. Corn degrades atrazine by this mechanism. This type of resistance is more serious than altered site of action type resistance because it involves several plant processes. Plants with altered metabolism resistance can degrade several unrelated herbicides of different modes of action through multiple genes controlling metabolic processes.

Plants having altered site of action resistance are not affected by herbicide concentration. Plants having altered metabolism resistance are affected by herbicide rate. As herbicide rate increases, the plant eventually reaches a point where it cannot degrade the herbicide faster than the herbicide is absorbed.

Cross and Multiple Resistance

A plant resistant to different herbicide chemistries having the same mode of action is cross resistant. Examples are:

Photosynthetic inhibitors: Atrazine and Sencor/Lexone.

ALS inhibitors: Sulfonylurea, imidazolinone, or triazolopyrimidine herbicides inhibit the ALS enzyme but at different sites on the enzyme. A weed can be resistant to all or any combination of the three herbicide chemistries. A plant resistant to different herbicide chemistries having different modes of action is multiple resistant.

Herbicide resistant weed species in ND:

1. Green foxtail to the DNA herbicides.

2. Wild oat to ACCase inhibitor herbicides.

3. Wild oat to ALS (Assert) herbicides.

4. Kochia to ALS herbicides.

5. Kochia to 2,4-D and dicamba.

6. Redroot pigweed to IMI herbicides.

7. Waterhemp to ALS herbicides.

8. Wild mustard to ALS herbicides.

9. Eastern black nightshade to imidazolinone herbicides.

Trifluralin (DNA) resistant green foxtail is found in areas of ND where trifluralin is used consecutively in small grain crops, row crops, and fallow. Continuous small grains, small grain/fallow, or small grain/sunflower rotations allows continuous DNA use. Also, trifluralin is usually applied at high rates in sunflower and residue may partially control green foxtail in the small grain crop planted the next year. Continuous use of and residue from high DNA rates increase selection pressure for DNA resistant green foxtail. DNA resistant foxtail has been documented in the RRV of ND. Resistant green foxtail and be expressed after 8 to 12 herbicide applications.

ACCase resistant wild oat is found within ND and MN from over 2,600 lines tested. Hoelon and fenoxaprop (Puma, Dakota, Tiller, and Cheyenne) resistance has been documented in every county sampled in ND. Resistance has varied from complete resistance to recovery from near death and all possible responses in between. Wild oat resistance to Assure II has been documented in the RRV of ND and MN. Resistant wild oat can be expressed after 8 to 12 herbicide applications. Wild oat resistance to Poast has not been documented in ND.

Assert (ALS) resistant wild oat has been documented from approximately 200 wild oat lines tested from ND/MN. No wild oat plants were unaffected but several lines exhibited less than 20% control when evaluated 4 weeks after application. Wild oat biotypes resistant to Assert may or may not be resistant to one or more of the other grass-killing ALS inhibiting herbicides.

ALS resistant kochia can be found across ND and developed originally in northern, western, and southwestern regions where Glean and SU herbicides were used extensively. Wide spread use of SUs in small grains, Pursuit in soybean and dry beans, Accent in corn, and Upbeet in sugarbeet have caused ALS resistant kochia to increase in the eastern ND. Resistant kochia has spread throughout the RRV and has escaped control from those using Raptor for the first time.

ALS herbicides consist of SU, imidazolinones (Imi), and triazolopyrimidines (TPS) chemistries. Repeated use of any ALS herbicide will contribute to kochia resistance. Kochia resistance c an be slowed following a resistance management program. Resistant kochia populations can be significant after 3 to 5 herbicide applications.

Plant growth regulator (PGR) resistant kochia was discovered in a survey conducted in 1993. Resistance was evaluated primarily against dicamba. Some kochia types were found resistant up to 0.5 pt/A of dicamba. Even though some plants survived, they were reduced in growth and may not compete well with vigorous growing crops.

IMI resistant redroot pigweed has been documented in Cass county. Continued use of Pursuit and Raptor in soybean and dry bean have caused resistant populations to establish.

ALS resistant waterhemp has been documented in the RRV of ND. Waterhemp commonly infests the midwest and plains states. Continued use of Accent in corn, Pursuit and Raptor in soybean and dry bean and SUs in small grains have caused resistant waterhemp populations to establish.

ALS resistant wild mustard has been documented in the RRV of ND. This is the first documented case of wild mustard resistance to ALS herbicides in the U.S. Continued use of Accent in corn, Pursuit and Raptor in soybean and dry bean and SUs in small grains have caused resistant wild mustard populations to establish.

ALS resistant Eastern black nightshade has been documented in the RRV of ND. This is the first documented case of eastern black nightshade resistance to ALS herbicides in the U.S. Continued use of Accent in corn, Pursuit and Raptor in soybean and dry bean and SUs in small grains have caused resistant eastern black nightshade populations to establish.

Weeds have developed resistance to herbicides throughout the U.S. Below are some examples:

ALS Mode of action:

- Yellow foxtail R to IMI but not SU herbicides (MN only).

- Giant foxtail R to ALS herbicides (MN only)

- Wild oat R to ALS (Assert) (MN only).

- Waterhemp R to ALS herbicides.

- Sunflower R to ALS herbicides.

- Common cocklebur R to ALS herbicides.

- Wild mustard R to ALS.

- Common ragweed R to ALS.

- Giant ragweed R to ALS.

ACCase Mode of Action:

- Green foxtail R to ACCase herbicides.

- Giant foxtail R to ACCase herbicides (WI).

- Wild oat R to Achieve (dim) resistance in W. MN.

Growth regulator mode of action:

Wild mustard R to growth regulator herbicides.

Triazines:

- Kochia resistance (R) to triazine herbicides.

Other:

- Wild oat R to Avenge and Far-Go (MT only).

For a comprehensive list of resistant weeds in the U.S. and world see web site: www.weedscience.com/

Herbicide resistant weeds are most likely to develop by using:

1. Single site of action herbicides.

2. Long residual herbicides.

3. Same mode of action herbicides applied over several consecutive years or multiple times during a growing season.

4. Herbicides used as "stand alone" products without using other weed control options, such as cultivation.

Weeds species most likely to develop resistance are genetically variable and have a rapid life cycle with short seed dormancy such as kochia and Russian thistle.

Genetically engineered crops resistant to Roundup and Liberty are now available. Roundup and Liberty have separate and distinct modes of actions from each other and from all other herbicides. Roundup and Liberty in bioengineered resistant crops will help manage current resistant weeds and will add another mode of action to reduce risk of resistance to other herbicides. However, effective and inexpensive weed control from Roundup and Liberty may cause a heavy selection pressure toward any weed with increased tolerance.

Weeds expressing some natural tolerance to glyphosate (Roundup/Touchdown) are quackgrass, wild buckwheat, nutsedge, nightshade, smartweed, kochia, dandelion, horseweed (marestail), common mallow, and velvetleaf. Weeds expressing some natural tolerance to Liberty include lambsquarters and yellow nutsedge. It is unlikely for weed resistance to glyphosate and Liberty to develop because of the lack of the resistant gene in the plant kingdom. However, as with all weed control strategies, herbicide and crop rotation should be used as a primary defense in the development of resistant weeds.

For a thorough discussion on weed resistance and management strategies, refer to the NCR Extension Publication 468, Herbicide Resistant Weeds. Copies are available through the the NDSU Distribution Center.

STRATEGIES TO MINIMIZE HERBICIDE RESISTANT WEEDS

The following strategies should be effective in reducing problems with herbicide tolerant and resistant weed biotypes, but no single strategy is likely to be totally effective.

1. Use herbicides only when necessary. Herbicide use should be based on economic thresholds.

2. Rotate herbicides with different modes of action in consecutive years.

3. Apply herbicides in tank-mix, prepackage or sequential mixtures that include multiple modes of action. Two or more herbicides in the tank-mix must have substantial activity against potentially resistant weeds. Many commercial premixes on the market do not contain herbicides that are targeted for the same weed species.

4. Rotate crops, particularly those with different life cycles, e.g., winter annual crops (winter wheat), perennial crops (alfalfa), summer annual (spring wheat, corn or beans). Do not use herbicides with the same mode of action in the different crops unless other effective control practices are also included.

5. Combine mechanical weed control practices like rotary hoe and cultivation with herbicide treatments. Hand removal of surviving weeds will prevent seed production of resistant plants.

6. Use pre-crop, in-crop, and post-crop tillage.

No weed has become resistant to steel!

7. Scout fields regularly and identify weeds that escape herbicide treatments. Monitor changes in weed populations and restrict spread of potentially resistant weeds that match the field history and a herbicide use pattern.

8. Clean tillage and harvest equipment before moving from fields infested with resistant weeds to those that are not.

9. Non-agricultural organizations (railroads, public utilities, highway departments) implementing total vegetation control programs should be encouraged to use vegetation management systems that do not lead to resistant weeds. Resistant weeds from total vegetation control areas frequently spread to cropland. Chemical companies, state and federal agencies, and farm organizations can help in this effort.

Herbicide Classification and Mode of Action for Resistant Weed Management
Mechanism of Action	Common Name	Herbicide Tradename	Premix Tradenames
ACC-ase Inhibitors (1) "Fops"	clodinafop-P diclofop fenoxaprop-P fluazifop-P quizalofop	Discover/Horizon Hoelon Puma Fusilade DX Assure II	Cheyenne, Dakota, Fusion, Tiller Fusion
"Dims"	clethodim sethoxydim tralkoxydim	Select/Prism Poast Achieve	Rezult
ALS Enzyme Inhibitors (2) Imidazolinones	imazamethabenz imazamox imazapic imazapyr imazaquin imazethapyr	Assert Raptor Plateau Arsenal Scepter Pursuit	Lightning, Sahara Backdraft, Detail, Squadron, Steel, Tri-Scept Extreme, Lightning, Pursuit Plus, Resolve, Steel
Sulfonylureas	chlorimuron chlorsulfuron ethametsulfuron foramsulfuron halosulfuron metsulfuron nicosulfuron oxasulfuron primisulfuron prosulfuron rimsulfuron sulfometuron sulfosulfuron thifensulfuron triasulfuron tribenuron triflusulfuron	Classic Glean/Telar Muster - Permit Ally/Escort Accent Expert Beacon Peak Matrix Oust Maverick Harmony GT, Pinnacle Amber Express UpBeet	Canopy XL, Reliance STS, Synchrony STS Finesse Canvas, Finesse Accent Gold, Basis, Basis Gold, Celebrity Plus Exceed, NorthStar, Spirit Exceed, Spirit Accent Gold, Basis Gold Basis, Canvas, Cheyenne, Harmony Extra, Reliance, Synchrony Rave Canvas, Cheyenne, Harmony Extra
Triazolopyrimides	chloransulam diclosulam flumetsulam	FirstRate StrongArm Python (Broadstrike)	FrontRow Accent Gold, Broadstrike+Dual/Treflan, FrontRow, Hornet, Scorpion III
Sulfonylamino- carbonyltriazolinone	flucarbazone propoxycarbazone	Everest Olympus
Root Inhibitors (DNA) (3)	benefin ethalfluralin oryzalin pendimethalin trifluralin	Balan Sonalan Surflan Prowl/Pendimax/others Trifluralin/Treflan/others	Team Rout Pursuit Plus, Squadron, Steel Broadstrike+Treflan, Buckle, Freedom, Passport, Salute, Tri-Scept
Growth Regulators (4) Phenoxys	2,4-D 2,4-DB MCPA	2,4-D/others Butyrac MCPA/others	Campaign, Crossbow, Curtail, Landmaster BW, Scorpion III, Shotgun, Starane Plus Salvo, Tiller, Weedmaster Bronate, Cheyenne, Curtail M, Dakota, Starane Plus Sword, Tiller
Benzoic acids	dicamba	Banvel/Clarity/Sterling	Celebrity Plus, Distinct, Fallow Master, Marksman, NorthStar, OpTill, Rave, Resolve, Spirit, Weedmaster
Pyridines	clopyralid fluroxypyr picloram	Stinger/Reclaim/Transline Starane Tordon 22K	Accent Gold, Curtail, Curtail M, Hornet, Scorpion III Starane + Salvo, Starane + Sword
Quinolines	quinclorac	Paramount/Drive
Photosystem II Inhibitors (5) (different than 6) Triazines	atrazine cyanazine simazine	Atrazine/other Bladex Princep	Axiom AT, Basis Gold, Bicep/II/Lite, Buctril+Atrazine, Bullet, Contour, Cy-Pro AT, Degree Xtra, Extrazine II, FieldMaster, FulTime, Guardsman, Harness Xtra, Laddok, Lariat, LeadOff, Liberty ATZ, Marksman, Readymaster ATZ, Shotgun Cy-Pro AT, Extrazine II Derby
Triazones	hexazinone metribuzin	Velpar Lexone/Sencor	Axiom DF, Axiom AT, Canopy, Domain, Epic, Salute, Turbo
Uracils	bromacil terbacil	Hyvar X Sinbar	Krovar I, WeedBlast
Phenyl-carbamates	desmedipham phenmedipham	Betanex	Betamix, Progress Betamix, Progress

Mechanism of Action	Common Name	Herbicide Tradename	Premix Tradenames
Photosystem II Inhibitors (6) (different than 5)	bromoxynil	Buctril/Broclean/Moxy	Bison, Bromac, Bronate, Buctril + Atrazine, Brozine, Moxy AT, Rezult
	bentazon	Basagran	Galaxy, Laddok, Storm
	pyridate	Tough
Photosystem II Inhibitors (7) (diff. than 5 or 6)	diuron linuron tebuthiuron	Diuron/Direx/Karmex Lorox, Linex, Linuron Spike	Krovar, Sahara, WeedBlast
Amide	propanil	Stampede	Stampede CM
Lipid Synthesis Inhibition (8) Thiocarbamates	cycloate EPTC triallate	Ro-Neet Eptam/Eradicane Far-Go	DoublePlay Buckle
Difenzoquat	difenzoquat	Avenge
EPSP Synthase Inhibitors (9)	glyphosate-ipa	Roundup	Backdraft, Bronco, Campaign, Extreme, FieldMaster, Landmaster BW, FallowMaster, ReadyMaster ATZ Some commercial names for glyphosate: Roundup/RT, Roundup Custom, Roundup Ultra/RT, Roundup Ultra Max, Roundup Original/RT, Rodeo, Glyphos, Glyphomax, Glyphomax Plus
	glyphosate-NH3	Roundup Ultra Dry
	glyphosate-tms	Touchdown
Glutamine Synthetase Inhibitors (10)	glufosinate	Liberty/Finale/Rely	Liberty ATZ
Pigment Inhibitors (13)	clomazone	Command	Commence
PPO Inhibitors (14) Diphenylethers	acifluorfen fomesafen lactofen oxyfluorfen	Blazer/Blazer Ultra/Status Flexstar/Reflex Cobra Goal	Conclude, Galaxy, Scepter OT, Storm Tornado, Twister Stellar
N-phenylthalimides	fluathiacet flufenpur flumiclorac flumioxazin	Action Resource Valor	Stellar
Trizolinones	carfentrazone sulfentrazone	Aim/Affinity Authority/Spartan	Canopy XL
Action unknown (15) Acetamides	acetochlor alachlor dimethenamid flufenacet metolachlor propachlor	Harness/Surpass/TopNotch/Degree Lasso/others Frontier Dual/Dual II/Magnum Ramrod	Degree Xtra, DoublePlay, FieldMaster, FulTime, Harness Xtra Bronco, Bullet, Freedom, Lariat Detail, Guardsman, LeadOff, OpTill Axiom, Axiom AT, Domain, Epic Bicep/II/Lite, Broadstrike+Dual, Turbo Ramrod and Atrazine
Auxin Inhibitor (19)	diflufenzopyr		Distinct, Celebrity, Celebrity Plus
Photosystem I Inhibitors (22)	diquat paraquat	Diquat Gramoxone Extra/Starfire
Carotenoid Inhibitors (28)	isoxaflutole	Balance	Epic
	mesotrione
Cold, Hard STEEL (29)	Plow, cultivator, rotary-hoe, etc.
Adapted from Weed Sci. Soc. of Am. (WSSA) Herbicide Classification System For Resistant Weed Management. Weed Technol. 11:384-389.