Weed Control Guide for Ohio Field Crops

Weed Control Principles

Herbicide Resistance in Weeds

A number of crops and weeds exhibit tolerance to some herbicides by preventing their absorption and/or translocation, or by rapidly metabolizing the herbicide to a non-toxic form. These are the basic mechanisms of herbicide selectivity upon which modern herbicide use is based. Herbicide-resistant plants have biochemical differences in the site of action normally attacked by a herbicide in susceptible plants, thus leaving them unaffected by the herbicide. In weed populations throughout the world, large populations of single weed species can contain a relatively small number of less competitive biotypes that have slight genetic differences from the rest of the population. Experience since the herbicide revolution began in the 1940s indicates that some naturally occurring weed biotypes can be resistant to herbicides that are normally lethal to the majority of the population.

The phenomenon of resistance can be explained as follows. When a herbicide having a very site-specific mode of action (for example, a photosynthesis inhibitor) is applied to an area repeatedly over time, the portion of a weed population susceptible to that herbicide is gradually depleted. This creates an opportunity for other weeds naturally resistant to that herbicide (including resistant biotypes of species normally susceptible to the herbicide) to become established. If the same herbicide or others with an identical mode of action are applied on a year-to-year basis, there is no interruption of the resistant weeds' yearly reproductive cycle, and the population will continue to expand rapidly over time.

Common lambsquarters and pigweed species are examples of weeds that have developed populations resistant to the triazine herbicides. In Ohio, triazine-resistant populations are most prevalent in areas where atrazine and/or simazine have been applied annually in continuous corn-growing areas. Some wild carrot populations have developed resistance to 2,4-D in Ohio. Elsewhere in the United States, there are reports of triazine-resistant velvetleaf, triazine-resistant giant foxtail, dinitroaniline-resistant goosegrass, and ACCase resistant giant and green foxtail and johnsongrass. More recently, there is growing concern because of reports that some weed biotypes are showing resistance to the newer herbicides, especially ALS inhibitors. The latter is of special concern because both classes of herbicides attack exactly the same site of action in plants, and evidence is growing that weeds resistant to imidazolinones may also be resistant to sulfonylureas and sulfonamides, a phenomenon known as cross-resistance. OSU has identified populations in Ohio of common and giant ragweed, waterhemp, and Powell amaranth that have developed resistance to ALS inhibitors.

Prevention is the key to avoiding development of herbicide-resistant weed populations in agricultural land. The following management techniques help keep resistant populations from developing:

Crop rotation - Some weed problems are more easily managed in some crops than others because different control options may be available. Crop rotation also helps disrupt weed life cycles and helps prevent any single weed species from becoming firmly established.

Herbicide rotation - Herbicide rotation is generally practiced along with crop rotation, and as long as herbicides used in one crop have a different mode of action from those used in other crops in the rotation, it will be more difficult for resistant populations to become established. Herbicides should also be rotated in a continuous monoculture system. Where two herbicide applications are made to a crop in the same year, avoid using herbicides with the same mode of action in both applications. Be sure to understand the components of premix herbicides when planning herbicide programs.

Timely postemergence practices - In general, triazine-resistant weed populations appear to develop more rapidly in no-till cropping systems. Timely cultivation and/or posteómergence herbicide application are possible control options for conventional tillage. No-till growers have a number of postemergence herbicide options available for timely control of weed escapes. Control of escaped weeds is necessary to prevent reseeding and development of resistant weed populations.

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