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How to Spot Herbicide Resistant Weeds

By Nicole Montreuil, AAg, Crops Extension Specialist, Outlook

June 2022

Herbicide resistance has been an issue for many years but has recently been growing in severity. The agriculture industry has become accustomed to a very simple and efficient weed management strategy, but now we need to be working at developing more complex strategies to mitigate resistance.

Glyphosate Resistant Kochia
Glyphosate resistant kochia path from tumbling across field

Roughly 90 per cent of herbicide failures are due to factors other than resistance. It’s important to know the difference between herbicide resistance and herbicide failure, as proper identification and confirmation is imperative to timely mitigation.

Factors that can contribute to herbicide failure include:

  • Climatic conditions: rainfall patterns, microbial degradation, temperature, fields being too wet or dry, and time of day of application.
  • Herbicide application factors: timing, faulty sprayer, wrong rate, improper calibration, water quality, improper boom height, sprayer speed, herbicide antagonism and shading from other plants.
  • Weed factors: weeds under stress, high infestations, shading, size of weeds

Herbicide resistance should be suspected under the following conditions:

  • A weed species that the herbicide controlled in previous seasons now escapes the treatment, while other weeds on label continue to be controlled.
  • The escapes cannot be attributed to herbicide failure.
  • Surviving plants mixed with controlled plants of the same or different species.
  • Irregular-shaped patches of a weed develop where the herbicide gives little or no control.
  • Records of field history show repeated use of the same herbicide, or same combinations of herbicides.

Scouting your fields for suspicious patches and identification of herbicide resistance early on before they spread is a good first management strategy. Resistant weed patches have been identified in fields where producers were unaware of their existence. Begin scouting shortly after spraying and continue through July after the crop has headed out and weeds are more visible. If you find suspicious looking patches, contact your local Ministry of Agriculture office or crop protection company representative to assist you in confirming whether the weeds have herbicide resistance.

If resistance is suspected:

  • Mark the location of the patch.
  • Mow, cultivate or spot spray the patches so it does not produce seed.
  • Resistant patches should NOT be harvested with the rest of the field. Harvest separately and thoroughly clean all equipment before moving on. If possible, harvest fields with known or suspected herbicide-resistant weeds last to limit spread it other fields.
  • Monitor the patches year to year.

If you have further questions regarding herbicide resistant weeds, contact your local crops extension specialist or the Agriculture Knowledge Centre at 1-866-457-2377.

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