The Small Grains Field Guide

Herbicide Resistant Weeds

Herbicide Resistant Weeds 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 2.1. These resistant weed species can no longer be controlled by these herbicides.

Table 2.1 Herbicide families used in small grains where resistant weed biotypes have developed

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. However, a 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 also is 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 resistant weed biotypes. This intense selection pressure allows 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.

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 develop.

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.

See the "Herbicide Resistant Weeds" publication for more details. This publication can be ordered from the University of Minnesota Extension Store as FO-6077-C.

Authors: Beverly R. Durgan and Richard Zollinger