Herbicide and Nonherbicide Injury Symptoms on Spring Wheat and Barley

Several common weed species in small grains have developed herbicide resistance. These weed species and the herbicides against which they have developed resistance are shown in Table 1. These resistant weed species can no longer be controlled by these herbicides.

Herbicide resistance develops through the selection of naturally occurring weed biotypes that have an inherent ability to tolerate the herbicides. The term "biotype" refers to plants within a species that have a slightly different genetic makeup from the general population. For example, "triallate resistant wild oat" is a biotype of wild oat that looks like any other wild oat plant. However, the resistant wild oat biotype can survive a herbicide rate several times higher than that needed to control susceptible biotypes. Resistance may arise due to the natural morphological or physiological characteristics of the species. It is also possible that resistance may develop in response to selection pressures due to farming practices or particular herbicide usage.

Herbicide resistance usually begins when a small number of resistant biotypes from a species survives an application from a particular herbicide. When a small grain field is sprayed with a herbicide, susceptible weeds die and resistant biotypes survive. The resistant biotype plants that mature and set seed become the source of future generations of resistant biotypes that eventually replace the susceptible weed species.

Three factors that intensify the selection process of resistant weed biotypes are herbicide efficacy, frequency of use, and duration. A highly effective herbicide acts like a screening process by removing the susceptible weeds and leaving the resistant weed biotypes. The greater the efficacy of a herbicide, the greater the selection intensity for selecting the resistant weed biotypes. This intense selection pressure allows the resistance weed biotypes to quickly establish themselves over a few growing seasons. Coupled closely with herbicide efficacy in this selection process is frequency of herbicide use. When herbicides with the same mode of action are applied over consecutive growing seasons to crops in rotation, pressure is placed on susceptible weed species and resistant weed biotypes are left. Also phytotoxicity from soil-applied herbicides that offer full season activity or from multiple applications made during the growing season can intensify the resistant weed biotype selection process. Although herbicides select for resistant biotypes, they probably do not directly cause the genetic mutation in the resistant biotypes.

Resistance mechanisms differ with herbicide families. Resistant biotypes may possess small biochemical differences from susceptible counterparts that eliminate sensitivity to specific herbicides. For example, in sulfonylurea-susceptible plants the herbicide binds to an enzyme (acetolactate synthase or ALS) that is responsible for disrupting amino acid biosynthesis (Figure 1). Plants resistant to sulfonylurea herbicide have a modified ALS enzyme that prevents herbicide binding.

Figure 1. Enzymes function as steps in biological processes. Enzymes are also extremely specialized in their function. As a result, different enzymes are involved with the many different biological processes that occur within a plant. Some herbicides can stop specific enzymes from functioning, resulting in a disruption of specific plant processes; this often leads to the death of the plant. This herbicide-enzyme relationship is very specific and any chemical modification of the herbicide or enzyme can eliminate herbicidal activity. Reprinted with permission from Herbicide Mode of Action, J. Gunsolus and W. Curran, NCR Extension Publication 377, University of Minnesota Extension Service, St. Paul, 1996. PLANTS ARE HEALTHY PLANTS ARE KILLED

Regardless of how resistance develops, it is important to know the herbicide mode of action to plan weed control programs that prevent the development and spread of resistant weeds. Weed control programs should incorporate a variety of strategies that emphasize prevention. Relying solely on a single strategy or one herbicide family for managing weeds increases the likelihood that herbicide resistance will development. Guidelines using an integrated approach for managing herbicide resistant weeds follow.

STRATEGIES FOR PREVENTING AND MANAGING HERBICIDE RESISTANT WEED PROBLEMS Scout fields to identify weed species present. Use herbicides only when necessary. Practice herbicide rotation using herbicides with different modes of action and herbicides from different chemical families. Use herbicide mixtures with different modes of action. Control weed escapes and sanitize equipment to prevent the spread of resistant weeds. Integrate mechanical, cultural, and chemical weed control methods.

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