Herbicide Resistance Management
Management of Herbicide Resistant Weeds
A number of weed species that once were susceptible to and easily managed by certain herbicides have developed resistance. These weeds no longer are controlled by applications of previously effective herbicides. To date, at least 180 species of weeds are resistant to at least seven different herbicide families. Some well-known herbicides and resistant species are presented in Table 2.1-8 available in The Agronomy Guide.
Herbicide resistance probably developed through the selection of naturally occurring biotypes of weeds exposed to herbicides over a period of years. A biotype is a population of plants within the same species that has specific traits in common. Resistant plants survive, go to seed, and create new generations of herbicide-resistant weeds.
Mechanisms for resistance vary depending on the herbicide family. Resistant biotypes may have slight structural or biochemical differences from their susceptible counterparts that eliminate sensitivity to certain herbicides. For example, while photosynthesis is inhibited in triazine-susceptible biotypes, because of a slight change in a chloroplast protein, triazine-resistant biotypes are able to continue normal photosynthesis upon exposure to triazine herbicides. In sulfonylurea-susceptible plants, an herbicide attaching or binding to an enzyme (acetolactate synthase or ALS) is responsible for disrupting protein biosynthesis. Sulfonylurea-resistant plants have a modified ALS enzyme that prevents herbicide binding.
Regardless of the mechanism for resistance, becoming familiar with the herbicide mode of action can help design programs that prevent the introduction and spread of herbicide-resistant weeds. Management programs for herbicide resistance should emphasize an integrated approach that stresses prevention. Some guidelines for an integrated approach to managing herbicide-resistant weeds are given below. Dependence on a single strategy or herbicide family for managing weeds surely will increase the likelihood of additional herbicide resistance problems in the future.
A key to preventing herbicide resistance is to become familiar with herbicide mode of action, site of action, and classification systems. Refer to Table 2.1-9 available in The Agronomy Guide for a list important herbicide groups for corn, soybean, small grain, forages, and some vegetable crops. The WSSA herbicide group is a system of classifying herbicides developed by the Weed Science Society of America, based on mode and site of action, to help understand and plan for resistance management. Herbicide manufacturers are voluntarily including the WSSA herbicide groups on their labels. Table 2.1-10 available in The Agronomy Guide includes the common premix herbicides for agronomic crops in Pennsylvania along with their associated WSSA herbicide groups.
Consider the following management guidelines for dealing with herbicide-resistant weeds:
- Use herbicides only when necessary.
- Rotate herbicides with different sites of action. Do not make more than two consecutive applications of herbicides with the same site of action to the same field unless other effective control practices are included in the management system. Two consecutive applications may be either single annual applications for two years, or two split applications in one year.
- Apply herbicides in tank-mixed, prepackaged, or sequential mixtures that include multiple sites of action. Both herbicides must have substantial activity against potentially resistant weeds for this strategy to be effective.
- Rotate crops, particularly those with different life cycles (e.g., winter annuals such as winter wheat or barley, perennials such as alfalfa, summer annuals such as corn or soybeans); and do not use herbicides with the same site of action in these different crops against the same weed unless other effective control practices also are included.
- Planting new herbicide-resistant crop varieties should not result in more than two consecutive applications of herbicides with the same site of action against the same weed unless other effective control practices also are included.
- Combine, where feasible, mechanical weed control.
- Where soil erosion potential is minimal, include primary tillage as a component of the weed-management program.
- Scout fields regularly and identify weeds present. Respond quickly to changes in weed populations to restrict the spread of weeds that may have been selected for resistance.
- Clean tillage and harvest equipment before moving it from fields infested with resistant weeds to those that are not.
- Encourage railroads, public utilities, highway departments, and similar organizations that use total vegetation control (TVC) programs to use vegetation- management systems that do not lead to selection of resistant weeds. Resistant weeds for TVC areas could spread to cropland.