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2008 North Dakota Weed Control GuideW-253, January 2008 HERBICIDE RESISTANT WEEDS X1. Herbicide resistance occurs with repeated use of a specific herbicide for control of weed species that contain some plants in the population with a resistant gene. The resistant type will increase with each use of the herbicide because the gene pool in the field will shift from susceptible to resistant. This shift is permanent, assuming that the resistant type plants are equally "fit" in the cropping environment. Use of one herbicide from a group with one mode of action may give resistance to other herbicides with the same mode of action. However, weed specificity for resistance is known for different herbicides within a mode of action group. For example, wild oat resistant to Hoelon is often but not always controlled by other herbicides with similar chemistry and by Poast or Select of different chemistry, but all with the same mode of action. Weed plants with a wide genetic diversity develop resistance rapidly, especially for herbicides with one site of action. Kochia developed resistance rapidly in North Dakota to ALS herbicides because of kochia diversity and the SU single site of action. Kochia plants vary in resistance to various SUs, but in general kochia plants rapidly develop resistance to individual SU herbicides. Imidazolinone (Imi) herbicides are in the same action group (ALS inhibitors) as SUs, but weeds do not necessarily have cross resistance. For example, nightshade species exhibit natural tolerance to SUs, but only recently developed resistance to Imi herbicides. Table X1 lists herbicides within various mode of action groups as a guide for possible cross resistance. Types of Resistance: Altered site of action - ALS inhibitors and other herbicides act on one specific site in a plant selecting for resistant plants in diverse plant species. 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 complex 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 often are not affected by herbicide concentration, but plants having altered metabolism resistance are affected by herbicide rate. As 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 with a single resistance mechanism that enables survival when treated with different chemicals is cross resistant to those chemicals. Resistance that develops to one ALS herbicide often confers cross resistance to other ALS herbicides. The same is generally true with imidazolinones. In some cases, resistance that develops to a SU confers cross resistance to imidazolinones. A plant with two or more resistance mechanisms that survives treatment with different chemicals has multiple resistance. Kochia may be resistant to SUs and atrazine. Different resistance mechanisms are involved; therefore, a kochia plant that withstands treatment with SUs and atrazine has multiple resistance. Herbicide resistant weed species in ND: (#) = Herbicide mode of action, see pages 108-109. ACCase inhibitor herbicides (1): Wild oat (All ACCase herbicides except clethodim) Green foxtail (All ACCase herbicides except clethodim) Yellow foxtail (All ACCase herbicides except clethodim) Wild oat resistance has been documented in nearly every ND county. ALS inhibitor herbicides (2): Wild oat (Only Assert and Everest) Kochia (All ALS herbicides) E. black nightshade (Imi herbicides: Raptor and Pursuit) Redroot pigweed (Imi herbicides: Pursuit and Raptor) Waterhemp (All ALS herbicides) Wild mustard (All ALS herbicides) Common ragweed (TPS herbicide: FirstRate and SU herbicides) Marshelder (Imi - Pursuit and Raptor, and SU - Express) Mitotic inhibitor (3): Green foxtail (Treflan, Sonalan, Prowl) Growth regulator (4): Kochia (2,4-D, dicamba) Photosystem II inhibitor (5): Kochia (atrazine) Lipid synthesis inhibitor (8): Wild oat (Far-Go) Resistant wild oat biotypes were also found to be resistant to Avenge. Unknown mode of action (26): Wild oat (Avenge) Resistant wild oat biotypes were also resistant to Far-Go. Herbicide resistant weed species in the U.S. - not in ND: Other weeds present in ND that have developed resistance to herbicides in other areas of the nation are listed below. ALS inhibitor (2): Yellow foxtail, giant foxtail, waterhemp (ALS + Ps II - 2006, ALS + PPO + Glyt - 2006, ALS + Ps II + PPO - 2006), common lambsquarters, sunflower, common cocklebur, giant ragweed, and Russian thistle. Growth regulator (4): Wild mustard and field bindweed. Photosystem II Inhibitor (5): Yellow foxtail, redroot pigweed, common lambsquarters, and common ragweed. EPSP Synthase Inhibitor (9) (Glyphosate) - when discovered: - Ryegrass (1998) - Horseweed (Marestail) (2000) Glyt + ALS (2003) - Common ragweed (2004) Glyt + PPO (2006) Glyt + ALS + PPO (2006) - Common lambsquarters (2005) - Palmer amaranth (2005) - Waterhemp (2005) - Glyt @ 3 lb rate, 6 lb = 93% survivorship at 4 and 12 in tall plants Glyt + PPO + ALS (2006) - Canada fleabane (Canadian prairie provinces) (2005) KS has four glyphosate resistance weeds (2007) - Horseweed, Giant ragweed, Waterhemp, Common ragweed PPO inhibitor (14): Common ragweed (2004) PPO + ALS (2006) Waterhemp (2006) PPO + ALS + Glyt (2006) PPO + ALS + Ps II (2006) Weeds expressing some natural tolerance to glyphosate: cinquefoil clover common lambsquarters common mallow dandelion horseweed (marestail) kochia nightshade nutsedge prickly lettuce smartweed velvetleaf waterhemp wild buckwheat Weeds expressing some natural tolerance to Liberty: grasses common lambsquarters yellow nutsedge Genetically engineered crops resistant to glyphosate, Liberty, and Raptor (Imi herbicides) can be used to control weeds resistant to other herbicides. However, heavy selection pressure from these herbicides may cause resistant biotypes to occur.
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. General Guidleines: 1. Scout fields regularly and identify weeds that escape herbicide treatment. Monitor changes in weed populations and restrict spread of potentially resistant weeds that match the field history and herbicide pattern. 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. Most commercial premixes do not contain herbicides that target the same weed species. 4. Rotate crops, particularly those with different life cycles, e.g. winter annual crops (winter wheat), perennial crops (alfalfa), and summer annual crops (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. Weed resistance to herbicides cannot be prevented, but can be delayed. Herbicide and tillage rotations will only delay resistance by the length of time that the selection pressure for a given herbicide is removed by an alternative control method. The gene pool does not revert back in absence of the original selection, except when the resistant plants are poorly fit. Fitness has not been greatly different for resistant and susceptible biotypes and should not be relied on for resistance management. There are two options relative to resistance management: one is to use the desired herbicide until resistance occurs and then change to an alternative, and the other is to rotate control methods to delay the on-set of resistance. Method 1. Continued Herbicide Use - This approach allows for the use of the preferred treatment but will require more intense monitoring for resistance. The best resistance management strategy is early identification of resistant plants and then complete control (eradication) of the resistant plants while the infestation is small. Hand weeding, non-selective herbicides, cultivation, or combinations of methods can be used for eradication. Identification can be best accomplished with highly effective herbicide rates so that uncontrolled plants are obvious for early eradication. Elimination of the resistant plants will allow for continuous use of the herbicide. Advantages: 1. Allow use of preferred herbicide. 2. Allow for use of the herbicide best suited for weeds in a given field. 3. The above may save costs as a herbicide with a second mode of action may not be needed for the weeds present before resistance develops. Disadvantages: 1. Resistance will occur sooner and require earlier monitoring for resistance. 2. Does not save the herbicide for use in crops without alternatives. Method #2. Rotate Herbicides - This system will delay resistance, but may use unnecessary or less desirable herbicides in rotation or in mixture. Delaying resistance by alternative herbicides in the crop rotation is a means of keeping a herbicide for use in a crop that does not have an effective alternative. Advantages: 1. Monitoring for resistance probably can be delayed. 2. Herbicide mixtures may give control of more weed species and reduce the need for scouting to choose the appropriate herbicide for the field. Disadvantages: 1. May need to use herbicides other than the most desired. 2. Will select for multiple resistance. 3. Fewer herbicide options saved for future use. Testing weeds for herbicide resistance: Plant samples can be sent to Ag-Quest to test for weed resistance. Contact before sending to determine cost and packaging instructions. Ag-Quest, Inc. Haisheng Xie (Dr. Z), Ph.D #210 South Railway Street, Box 144 Minto, Manitoba, Canada, R0K 1M0 Office 204 776-5565 Fax 204 776-2250 haisheng.xie@agquest.com www.agquest.com ACCase or ALS (Group 1) herbicides = $80 CAN/sample Dinitroaniline (Group 3) herbicides = $45 CAN/sample
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