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Herbicide-resistant weeds

When herbicide options become limited, there are significant economic and environmental consequences. Fortunately, there are things you can do to prevent and manage herbicide-resistant weeds.

Understanding herbicide resistance

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Preventing herbicide-resistant weeds

It’s difficult to predict exactly which weed species will have biotypes resistant to a given herbicide. However, we’ve learned from previous pesticide resistance problems that the occurrence of herbicide-resistant weeds is directly linked to:

  • The herbicide program used.
  • The weed species present.
  • The crop management practices employed.
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Prevention and management strategies

The North Central Weed Science Society (NCWSS) Herbicide Resistance Committee developed the following strategies for avoiding and managing problems with herbicide-resistant weed biotypes.

Keep in mind that relying on any one strategy isn’t likely to be effective. You must use the following strategies in carefully selected combinations to avoid or properly manage herbicide-resistant weed problems:

1. Only use herbicides when necessary

Where available, base herbicide applications on economic thresholds. Continued development of effective economic threshold models should be helpful.

2. Rotate herbicides (sites of action)

Don’t make more than two consecutive applications of herbicides with the same site of action to the same field unless the management system includes other effective control practices. Two consecutive applications could be single annual applications for two years, or two split applications in one year.

3. Apply herbicides that include multiple sites of action

Apply herbicides in tank-mixed, prepackaged or sequential mixtures that include multiple sites of action. For this strategy to be effective, both herbicides must have substantial activity against potentially resistant weeds.

Remember that in the past, weeds selected for herbicide resistance often weren’t the primary target species. It may be expensive to apply herbicide combinations that duplicate a wide spectrum of weed control activity. Many of the more economical herbicide combinations may not be adequate.

View the Take Action Herbicide Classification guide for a list of premixes and their corresponding sites of action.

4. Rotate crops, particularly those with different life cycles

An example is winter annuals such as winter wheat, perennials such as alfalfa and summer annuals such as corn or soybeans. Also, remember not to use herbicides with the same site of action in these different crops against the same weed, unless you also include other effective control practices in the management system.

5. Avoid more than two consecutive herbicide applications with herbicide-resistant crops

Planting new herbicide-resistant crop varieties shouldn’t result in more than two consecutive applications of herbicides with the same site of action against the same weed unless other effective control practices are also included in the management system.

6. Include mechanical weed control practices

Where feasible, combine mechanical weed control practices such as rotary hoeing and cultivation with herbicide treatments.

7. Consider primary tillage

Where soil erosion potential is minimal, consider primary tillage as a component of the weed management program.

8. Regularly scout fields to identify the weeds present

Quickly respond to changes in weed populations to restrict the spread of weeds that may have been selected for resistance.

9. Clean tillage and harvest equipment

Clean before moving from fields infested with resistant weeds to those that aren’t.

10. Encourage others to adopt prevention management strategies

Encourage railroads, public utilities, highway departments and similar organizations that use total vegetation control programs to use vegetation management systems that don’t lead to the selection of herbicide-resistant weeds.

Resistant weeds from total vegetation control areas frequently spread to cropland. Chemical companies, state and federal agencies and farm organizations can all help in this effort.

Jeff Gunsolus, Extension weed scientist

Acknowledgements

I’d like to acknowledge the members of the 1991 and 1992 North Central Weed Science Society (NCWSS) Herbicide Resistance Committees who developed the ten management strategies for avoiding and managing herbicide-resistant weeds and extensively reviewed the content on this page.

The members are: Thomas Bauman, T. Robert Dill, Ray Forney, R. Gordon Harvey (Chair, 1991), Nick Jordan, Rex Liebl, Michael Owen, Jamie Retzinger, Dave Stoltenberg, G. Chris Weed, Phil Westra, Gail Wicks and Bill Witt. It’s been my pleasure to serve as vice-chair and chair of this committee in 1991 and 1992, respectively, and to work with these dedicated people.

I’d also like to thank the American Cyanamid Company for the use of Figures 4 and 6, B.D. Maxwell for Figure 5 and J.J. Kells for Figure 7.

Reviewed in 2018

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