How to identify and fight herbicide resistant weeds

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Shannon Chant M.Sc. PAg.

Regional Crops Specialist, Southwest Regional Services Branch

All herbicide-resistant weed infestations have developed following repeated use of a single herbicide (or herbicide group) for a number of years on the same field. Producers who have weed resistance on their farms, will typically see a single weed species that is normally controlled by a herbicide (or Group), survive the application of that herbicide. Individual resistant plants may survive three to 100 times or more the normal field rate of a herbicide.

Herbicide resistant weeds found in Saskatchewan

Weed - Herbicide group weed is resistant to

Chickweed - Group 2

Cleavers - Group 2

Green foxtail (wild millet) - Group 1, Group 3, Group 1 & 3

Kochia - Group 2, Group 9 (glyphosate)

Lamb’s-quarters - Group 2

Persian darnel - Group 1

Redroot pigweed - Group 2

Russian thistle - Group 2

Stinkweed - Group 2

Shepherd’s purse - Group 2

Wild mustard - Group 2

Wild oat - Group 1, Group 2, Group 8, Group 1 & 2, Group 1 & 8, Group 2 & 8,

Group 1& 2 & 8

Products in each group can be found in the front of the 2014 Guide to Crop Protection.

If you have herbicide resistant weeds, it is important to identify them before they spread across your field/farm. Resistant weed patches have been identified in fields where producers were unaware that they existed.

Herbicide resistance should be suspected under the following conditions:

- A single weed species that the herbicide has controlled in previous seasons now escapes the treatment, while other weeds that appear on the label continue to be controlled. It is nearly impossible for two weeds to develop resistance simultaneously.

- The weeds that escaped control cannot be explained by weather conditions or emergence after application (if a post-emergence product is in question).

- Irregular-shaped patches of a weed develop where the herbicide gives little or no control. Patches do not have sharply defined edges but may follow roughly square patterns associated with tillage or combine passes.

- Records of the past history of the field show repeated use of the same herbicide, or selection of herbicides, that kill the weed in question in the same way.

If suspect you have herbicide resistant weeds, contact your local agricultural office or crop protection company representative to help determine if weed escapes are resistant. Saskatchewan Agriculture’s Crop Protection Lab ( offers a testing service for herbicide resistance on a fee for service basis. They can also be contacted directly at 306-787-8130 or

The location of any patches should be marked by GPS or stakes so you will remember their location. After samples have been taken for resistance testing or resistance has been confirmed, the patches should be mowed or cultivated so resistant plants do not spread. Keep in mind that seed burial often prolongs seed survival and any equipment used should be cleaned before leaving each patch. To prevent the spread of seeds from herbicide resistant plants across the field or to other fields, harvest the patches separately, shut off all threshing systems while moving between patches and make sure all harvest, tillage or seeding equipment is cleaned thoroughly before leaving the field. Check the patches each year to monitor for possible spread of resistant weeds.  Keeping resistant weeds to a manageable patch is much easier that dealing with an entire field of resistant weeds. There may even be value in sacrificing the crop in smaller patches to ensure no seed production takes place within them.

The best (most effective and most economical) way to approach herbicide resistance management on your farm, is to prevent weeds from developing resistance and stopping or slowing their spread when they are present. A vigorous crop with a high plant density, the appropriate fertility and high yields will be very competitive with weeds. Diverse crop rotations will help prevent weeds from adapting to the fields on your farm.

Herbicide use should be planned for when planning your crop rotation. Pick herbicides for the crop with the least options first and progress to the crop with the most options. Avoid using herbicides from the same group more than once during your rotation.  If using the same Group cannot be avoided, include tank mixes to control high risk weeds where duplication occurs. Herbicide tank mixes have been shown to be more effective than rotation in the prevention of resistance as the chance of successful mutations occurring within a weed that confers resistance to two different groups at the same time is near impossible. Tank mixes only work to prevent the development of resistant weeds if both herbicides control the target weed. Once resistance to one of the mix partners is already present, the benefit of mixing is greatly reduced. You can also physically remove weeds from the field using mechanical means, like a chaff catcher or McLeod harvester, or rotate to forage crops.

If you suspect that you have a patch of herbicide resistant weeds, have it looked at by a Crops Specialist or chemical company rep or send in some samples for testing and then get rid of it! If you would like more information about this please contact Shannon Chant, Crops Specialist at 306-778-8291 or email

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