2013 Highlights: UC IPM Annual Report

Western workshops counter glyphosate resistance in weeds

Green weeds in photo are probably resistant to glyphosate, while brown weeds sprayed with glyphosate have been killed.

These weeds were sprayed with an herbicide but the green ones are likely resistant to it and weren’t killed. (Photo by B. Hanson.)


  • Herbicide resistance is an increasing problem when controlling weeds in California crops.
  • Three types of glyphosate-resistant weeds have been found in California.
  • UC IPM led workshops in 2013 on how to prevent resistance to glyphosate.

Herbicides are a common tool to manage weeds. But when weeds are resistant to an important common herbicide like glyphosate, they can be difficult to control, requiring other management methods or use of a different herbicide.

During five workshops for crop production advisors and consultants, participants learned how herbicide resistance occurs and ways to avoid it.

One hundred and forty people attended the workshops held in California, Oregon, and Washington. UC IPM Director and UCCE Specialist Kassim Al-Khatib, working with UCCE Specialist Brad Hanson and others from Oregon State University, USDA, and Washington State University, developed and conducted the workshops.

Weeds are a major factor limiting vineyard and orchard production throughout the western states. Over the past 35 years, the herbicide glyphosate has been the main pest management tactic in perennial crops. But using glyphosate repeatedly can contribute to resistance and a shift in the local weed species.

Already, of the 24 known species of glyphosate-resistant weeds, glyphosate-resistant rigid ryegrass, hairy fleabane, and horseweed have been reported in California.

Repeated use of glyphosate may lead to resistance. Individual plants in a population of weeds are naturally less susceptible to glyphosate and over time are more likely to survive a glyphosate application. These survivors will produce offspring that are also less susceptible. After repeated applications over several generations, the population consists primarily of less susceptible plants, and the weed population is, therefore, resistant to glyphosate.

In addition, repeated glyphosate applications in an area may cause a shift in local weed species because, over time, only those species that are able to survive the glyphosate applications will be found.

Educational materials and presentations are available for others to use in similar trainings. You can read about how resistance works, and about strategies for using glyphosate effectively while avoiding resistance.

The Western Region IPM Center funded the workshops.

Statewide IPM Program, Agriculture and Natural Resources, University of California
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