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Herbicide Resistance: Coming to a Farm Near You

Posted by Blythe Howell | February 11, 2020
Herbicide resistance is a problem that is quickly spreading throughout the wheat growing regions of the inland PNW. The newly created Herbicide Resistance Weeds Map, located on the Wheat and Small Grains website, lets you see the results of Dr. Ian Burke’s Herbicide Resistance Testing Program at WSU.

After opening the Herbicide Resistance Weeds Map, select a county to see what herbicide-resistant biotypes have been identified in that county. Select “See screening results” in the popup box to see the number of samples that tested positive for resistance to the various herbicide active ingredients. You can also click on the active ingredient name or weed species name to learn more about the active ingredient or weed species.

In 2013, Dr. Ian Burke initiated his herbicide resistance testing program. There were 20 samples submitted the first year. Since that time, he and his research assistant, Rachel Zuger, have screened 195 samples submitted to the program.

They identified herbicide-resistant biotypes in six different grass species:

The grass species have complex resistance patterns to ACCase inhibiting herbicides and ALS inhibiting herbicides. A few downy brome samples have tested postitive for high level glyphosate resistance.

Resistance to the following herbicides were identified:

ACCase Inhibiting Herbicides (Group 1)

  • clethodim (Select Max)
  • clodinafop (Discover NG)
  • pinoxaden (Axial XL)
  • quizalofop (Assure II, Aggressor)

ALS Inhibiting Herbicides (Group 2)

  • imazamox (Beyond)
  • mesosulfuron (Osprey)
  • propoxycarbazone (Olympus)
  • pyroxsulam (PowerFlex HL)
  • sulfosulfuron (Outrider)

EPSP Synthase Inhibitors (Group 9)

  • Glyphosate (Roundup and many other trade names)

For broadleaf weeds, clopyralid resistance in mayweed chamomile (a.k.a. dog fennel) has been found and multiple resistance to ALS inhibiting herbicides has been identified in shepherd’s-purse.

The map will be updated annually to reflect results from the herbicide resistance testing program at WSU. The results of the broadleaf weed screening program will be added to the map in the near future.

The Herbicide Resistance Testing Program is currently a free service for any and all weed species. Seeds to be submitted for testing should be collected when mature. Here are some examples for popular species:

  • Downy brome seeds can mature in July through late August, typically seeds are mature as the wheat canopy begins to dry down. Seed should be golden brown or redden brown in color.
    • Avoid sending downy brome seed infected with head smut (black discoloration on the seed or in the seed head)
  • Wild oats are typically mature at the time of wheat harvest, although sometimes they can mature prior to that and will need to be monitored several weeks before that. Wild oat seeds will easily pull (or fall) from the glume. Seed should be brown to black in color.
  • Italian ryegrass seed will be pale green to beige in color and easily come off the stem (inflorescence). Maturity of Italian ryegrass can vary from early June to late August.
  • Mayweed chamomile seed should be brown to dark brown in color. If you take a flower or seed head between your fingers, the brown seeds will roll out. This does require some pressure on the seed head.

The time required to get results for resistance screening samples depends on the species and individual biotypes. Grass seeds often do not become germinable until 1 to 6 months after falling from the parent plant due to dormancy and/or seed aging (maturing). Wild oats and Italian ryegrass dormancy/seed maturity does not take as long (1 to 3 months, typically). Downy brome is a whole other story, due to differing dormancy scenarios, seed germination can take anywhere from 1 to 6 months.

We try to test as many herbicides used to control/suppress the species as possible, but specifics on the herbicide or herbicides of concern will help get the desired results if limited plants germinate.

The Herbicide Resistance Testing Program is currently working on methods to screen for resistance to preemergence herbicides. Hopefully, we will be able to effectively screen weeds for herbicides such as Zidua (pyroxasulfone) or metribuzin in the coming year.

More information on herbicide resistance can be found on the Herbicide Resistance Resources page.


For questions or comments, contact Drew Lyon via email at drew.lyon@wsu.edu, Ian Burke via email at icburke@wsu.edu, or Rachel Zuger via email at rachel.zuger@wsu.edu.

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