Controlling Glyphosate-Resistant Weeds

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Source: Dan Rajzer, Michigan State University Extension

The introduction of glyphosate-resistant crops such as soybeans, corn and cotton has led to a significant increase in the use of this herbicide. In the case of soybeans, as an example, the percentage of acres that were treated since its introduction in 1996, has risen from 25 to 91 percent in 2007, and is estimated to be near 96 percent in 2010. This rapid expansion has many growers, agribusinesses and researchers concerned about the potential for weeds to develop resistance.

Currently there are 19 weeds resistant to glyphosate world-wide with 12 of them located in the United States. Michigan has had glyphosate-resistant horseweed (marestail) since 2007. In addition, our neighboring states to the south are struggling with resistant waterhemp (Illinois), giant ragweed (Indiana and Ohio), and common ragweed (Ohio).

Herbicide resistance (in generic terms) can be influenced by many factors associated with both the weeds we are trying to control and the herbicides we use. Weed characteristics that favor resistance include:

  • High reproductive capacity producing a high number of seeds, as well as multiple generations per year.
  • Long emergence period over time eliminates more susceptible biotypes and allowing survival of resistant ones.
  • Seed dispersal mechanisms that allow the seeds to disperse over greater distances (wind vs water).
  • Genetic variability resulting in many biotypes with potential for one to show resistance.
Herbicide characteristics that influence weed resistance include:

  • Products with a single site of action.
  • The frequency of which an herbicide is used, particularly if used multiple times per year.
  • Herbicide efficacy or its ability to control the target weeds, with higher efficacy accelerating the development of resistance.
  • Herbicide persistence in the environment.
Scouting for glyphosate-resistant weeds in your fields offers a challenging opportunity because the symptoms are quite variable. You may see dead weeds next to live weeds of the same species that look normal but are stunted. Other symptoms include plants that are almost entirely dead with only one or two buds trying to grow, or plants where glyphosate has killed the main growing points, with nodes below this area beginning to grow new lateral branches. Keep in mind that similar symptoms can also be the result of herbicide failures from inaccurate rates, improper application, timing, weather and a host of other conditions. If you suspect herbicide resistance, it is best to have the weeds tested.

North Dakota State University Extension Service and the University of Minnesota Extension offer a short video on scouting your fields for glyphosate resistant weeds at: http://www.youtube.com/watch?v=OaKRLdEPqrU.

Managing glyphosate-resistant weeds is similar to other types of herbicide resistant weeds that we have experienced and includes practices such as: rotating herbicides with different sites of action; using tank mixes or sequential applications of herbicides with differing sites of action; recognize the importance of the herbicide’s characteristics (efficacy, persistence and frequency of use); and limiting the number of herbicide applications with the same site of action during the growing season. When you consider alternative herbicides, keep in mind that products in the ACCase, ALS, and photosynthesis inhibiting sites of action also pose a great risk of resistance development and should be used with vigilance.

Many university weed scientists from the corn and soybean growing area have developed a Web site where you can obtain the latest information and research on glyphosate resistance. Please visit http://www.glyphosateweedscrops.org/ to learn more.

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