Herbicide Resistance --
Prevention and Detection

Selective herbicide use began in the 1940's with the discovery of 2,4-D. This new miracle compound killed many broadleaf weeds without damage to grassy plants, adding a new dimension to crop production. Producers could easily and economically control broadleaf weeds in grass crops that previously had required mechanical or hand removal. Use of these materials spread rapidly and has continued to grow with the discovery and registration of new herbicides.

Weeds vary in susceptibility to herbicides. Some weeds tolerate herbicides and others do not. For example, morningglory and other broadleaf plants tolerate Fusilade DX, while annual and perennial grasses do not. Herbicide labels and weed response tables provide growers with this information, but there is also variation in control within a particular genus or species. Selected plants of a species normally controlled by a herbicide may require slightly higher application rates for an acceptable level of control. For example, in a particular field or year, Treflan may not control smooth pigweed as well as it does in other fields or years. Similarly, spiny amaranth (also known as spiny pigweed) may not be controlled as well as smooth pigweed. You rarely get 100 percent control of any weed species with any herbicide.

A number of factors influence weed control: target coverage, application method, herbicide rate, environmental conditions, and weed size and development at application time.

Lack of control may also be due to the genetic ability of a weed to tolerate or resist the herbicidal properties of the pesticide. The Weed Science Society of America has defined resistant weeds as "species or a biotype of a species that originally was controlled by a specific herbicide that is no longer effective."

Resistance may not be detected for many years, until a high percentage of the targeted species survives the herbicide treatment. The resistant-weed biotypes survive to produce seed, and the population grows. As the population of resistant weeds increases in relation to susceptible plants, you may suspect resistance, especially if this observation is made more than one year. Factors such as seed production and longevity, seed survival, germination rate, seedling hardiness, growth rate, and competitiveness of the susceptible and resistant biotypes influence the speed at which the resistant population grows.

Herbicide resistance has become an issue in Mississippi. The following cases have been found and confirmed:

• annual bluegrass resistant to the triazines [simazine (Princep, Simazine) and atrazine (several)]
• johnsongrass resistant to the acetyl-coenzyme A caroxylase (ACCase) herbicides [fluazifop-P (Fusilade DX and quizalofop-P (Assure II)]
• common cocklebur resistant to imidazolinone herbicides [imazaquin (Scepter) and imazethapyr (Pursuit)]
• common cocklebur resistant to arsenical herbicides [DSMA (DSMA Liquid, DSMA Slurry, Ansar 8100, and other trade names) and MSMA (Crabgrass, Killer, Ansar, Bueno, Daconate, and other trade names)]
• goosegrass and johnsongrass resistant to dinitroaniline herbicides [trifluralin (Treflan, Tri-4, Trilin, etc.) and pendimethalin (Prowl)]
• ryegrass resistant to sulfonylurea herbicide [sulfometuron (Oust)].

Confirming herbicide resistance in a weed population is a slow process. Seed or other propagation material must be collected, plants grown to treatment size in a controlled environment, treatments applied, and results evaluated. Collected seeds may require an after-ripening period or storage at freezing temperatures before germination occurs. Seedlings from a susceptible parent (preferably one that has never been exposed to the suspected herbicide) must be grown and treated with the resistant seedlings for comparison purposes. This process can require from several months to one year after the initial collection. Fortunately, there is an ongoing effort to develop techniques for quicker resistance detection.

Often, you must evaluate the situation in the field to determine the reason for lack of control. If you detect several weed species that should have been controlled by the herbicide, resistance probably is not the cause for lack of control. Likewise, if you can detect a pattern of no control, or if adverse environmental conditions existed at the time of application, the control failure was probably not caused by herbicide resistance. If, however, all weeds except one susceptible species were controlled, you might suspect herbicide resistance. When you suspect resistance, contact your county Extension agent to initiate resistance testing.

Proper herbicide selection and use can delay the development of a resistant-weed population. Crop rotation is often touted as the primary tactic against herbicide resistance because it requires use of different herbicides. Crop rotation may not be necessary, however, if several alternative herbicides are available so that you can use a herbicide with a different mode of action in that crop. For example, imidazolinone-resistant cocklebur can be controlled in soybeans with other herbicides such as bentazon (Basagran) that have a different mode of action. Although ACCase-resistant johnsongrass can be controlled with clethodim (Select) in cotton or soybeans, no one can determine if or when resistance to Select will occur. It would be wise to use glyphosate (Roundup), therefore, as a wiper treatment, spot treatment, or after harvest for johnsongrass control in cotton or soybeans rather than continued use of clethodim (Select) on those resistant populations.

Crop rotation will not delay weed resistance if herbicides with similar modes of action are used in the rotation crop. In the example just mentioned, rotation from cotton to soybeans would not help resistant johnsongrass because many of the same herbicides are used in both crops. Rotation to corn and use of nicosulfuron (Accent) or primisulfuron (Beacon) for johnsongrass control alternates the herbicide modes of action.

Use of herbicides that contain more than one active ingredient in the formulation, or premixes, may help control certain herbicide-resistant weeds. This will be an effective treatment alternative only if both ingredients were initially effective on that particular weed.

If you have detected a resistant-weed population, use all available control methods to avoid seed depositon in the field. Hand-removal following cultivation may be economical if the end result is to avoid spread of a herbicide-resistant weed population.

The table contains many of the herbicides routinely used in crop production in Mississippi, along with the modes of action of these herbicides. This information can be useful to plan weed control tactics that include herbicide rotation so herbicides with similar modes of action or in the same families are not repeatedly used year after year.

Mode of action, family, and labeled uses of commonly used herbicides.¹

¹This table is not intended for herbicide-use recommendations. Instead, refer to Extension Publication 1532 Weed Control Guidelines for Mississippi for recommendations. All herbicide uses were current at the time of preparation.
²In some cases herbicide symptomology is presented, since the primary mode of action is not known.
³Trade names are used for illustrative purposes only. The trade name mentioned in the table may not be labeled for all crop sites indicated. However, other trade names of that active ingredient are labeled for use in those crops.

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Publication 1907
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

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