Funding Available to Continue Testing Herbicide-Resistant Weeds

By: Kristen Obeid, OMAFRA Weed Management Specialist – Horticulture

Funding is available for producers, agri-business, consultants and extension personnel to submit weed samples to be tested for herbicide resistance again this year.  Sample collection kits with sampling procedures can be obtained from  These tests only require a small amount of leaf tissue from the suspected resistance weeds.  DNA is extracted from the leaf tissue to determine if there is a molecular change where the herbicide acts to kill the weed, making the weed resistant. 

There are 19 molecular tests (more in progress) to assist in identifying herbicide resistance in 14 weed species.  Tests are also available to differentiate between Brassica (mustard) and Amaranthus (pigweed) species.  Tests differentiating pigweed species have been instrumental in confirming new cases of waterhemp in Ontario, Manitoba and Quebec.  Once confirmed, the waterhemp is tested for Groups 2, 5, 7, 9 and 14 resistances.


  1. 121 new cases of herbicide resistance in Ontario crops since 2016.
  2. New tests developed in 2021 for:
    1. Groups 2, 5 and 7 resistant giant ragweed
    1. Group 9 resistant Italian ryegrass
    1. Group 9 resistant common ragweed
    1. Group 14 resistant green pigweed
  3. New mutation conferring resistance to Groups 5 and 7 was found in green pigweed.
  4. Continued increase in the number of fields with multiple resistant weed species:
    1. Common ragweed resistant to herbicide Groups 2 and 5 in pumpkins and 2, 5 and 7 in soybeans and sunflowers.
    1. Redroot and green pigweed resistant to herbicide groups 2 and 5 in tomatoes.
    1. Redroot and green pigweed resistant to herbicide Groups 5 and 7 in carrots and potatoes.
    1. Waterhemp resistant to herbicide Groups 2, 5, 9 and 14 in asparagus, peppers, corn, soybeans and white beans.
  5. Continued increase in the number of horticulture crops with glyphosate (Group 9) resistant Canada fleabane (apples, blueberries, carrots, grapes, onions, pumpkins and strawberries).

This testing has been instrumental in documenting new cases of herbicide resistant weeds.  In 2021, 96% of the fields tested in Ontario were resistant to at least one herbicide group. These tests deliver a result and recommendation to the producer within the same growing season, sometimes in as little as 5 days.  Traditional resistance testing in the greenhouse can take from three months to a year to get results. Once confirmed producers were provided the resistance profile enabling a change in management to help prevent spread.   Participates were pleased with the timely results, welcomed the in-season management recommendations, and highly value this service.

There are many more undocumented cases of herbicide resistant weeds in Canada. The resistance mechanism is unknown for most of them. The major concern is their distribution and economic impact for producers.  Knowing where resistant biotypes are located will improve management and maintain the longevity of our crop protection tools.

Table 1: Available Genetic Tests

Several of these tests were developed by other researchers and reproduced from the scientific literature.
*Soil applied/preemergence herbicides are still effective against Group 14 resistant species because they prevent the weed from germinating or kill the weed as it germinates.  However, much more research is required in this area.

Contact Kristen Obeid for sample collection kits, sampling procedures and how to submit samples to Harvest Genomics

Kristen Obeid



Thank-you to the project partners, that have provided funding and/or in-kind support: AAFC, AAFC-PMC, Bayer CropScience Inc., FMC Canada, FVGO, MAPAQ, OAG, OFVGA, OPVG, and Syngenta Canada Inc.

About Sean Westerveld

Ginseng and Medicinal Herbs Specialist, OMAFRA
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