Events Icon


View Events
Events Icon

Smart Partner Program

Learn More
Subscriber Login

Weed Wisdom


Testing uncontrolled weeds will help with resistance management decisions next year

By Charles Geddes
Variable weather during spraying season this year helped some weeds escape herbicide control. These weeds can be misidentified as being herbicide-resistant, when in fact poor weather conditions during or after herbicide application were the cause. However, with a growing number of cases of herbicide-resistant weeds, herbicide-susceptibility can no longer be assumed; even in a year where ideal conditions during spraying were few and far between. A false assumption of herbicide-susceptibility can result in uninformed weed control decisions next year, and even large losses in profit or further spread of herbicide-resistant weed biotypes.

Suspicious Russian thistle

Several entities exist to help growers stay on top of their herbicide-resistant weeds. Diagnostic testing labs like the Crop Protection Lab, Pest Surveillance Initiative, or Ag Quest, among others, provide routine diagnostic testing for known herbicide-resistant weed biotypes (find more information here: Testing can help inform growers of herbicides which remain effective on a resistant weed biotype, or help confirm that a weedy escape was in fact due to a herbicide miss or weather event. Growers are encouraged to take advantage of these resources and have their weeds tested for resistance. Some diagnostic labs offer testing for a broad range of herbicide and weed species combinations; but in general, these tests are limited to known cases of resistance found previously in western Canada.

Young Russian thistle

Mutations resulting in herbicide insensitivity are a natural, but rare, phenomenon, and selection pressure imposed by recurrent herbicide application results in an increase of resistant weed biotypes in the weed population over time. Efforts in discovery, surveillance, and monitoring are necessary to stay on top of herbicide resistance. Herbicide resistance affects the farming community as a whole, not just individual farms; and thus the impacts of identifying a potentially resistant weed extend beyond the farm boundary. Early discovery of a herbicide-resistant weed biotype can aid in eradication efforts and limit the impact or spread of resistant weeds on or off the farm.

Discovery of novel herbicide-resistant weed biotypes requires more in-depth experimentation than routine diagnostic testing, meaning that it is considered research. The Prairie Herbicide Resistance Research Lab at AAFC-Lethbridge can help with identification of new types of resistance at no charge to the grower. This research lab will test weed samples using a series of experiments, which can aid in early identification of novel resistant weed biotypes. Growers are encouraged to contact Dr. Charles Geddes (Charles.Geddes at if they suspect they have a type of herbicide resistance which is uncharacterized in the Canadian Prairies. Testing requires viable seed, so samples must be collected either pre- or post-harvest depending on the weed species.

young Kochia plant

Some examples of weeds on the watch list are:

  • Glyphosate (group 9)-resistant waterhemp, Russian thistle, wild oat or cleavers
  • PPO inhibitor (group 14)-resistant kochia
  • Synthetic auxin (group 4)-resistant Russian thistle