Glyphosate resistant weeds in Canada – and what to do about them
March 18, 2025
Across Canada, we are seeing more glyphosate resistance in a variety of common weeds – a major concern for many farmers. Is this a problem that can still be managed?
A brief history of glyphosate resistance in Canada
When the topic of herbicide resistant weeds made its first appearances on farm meeting agendas in the late 1990s, it seemed a somewhat remote problem: that’s not an issue on my land, thought many a farmer. But it is now. Or at least it’s an issue no one can avoid as they make crop plans.
At the time, glyphosate was categorized as low risk for resistance development. Not zero risk, to be sure, but the science indicated that it would take a while for resistance to glyphosate to develop and there were bigger Group 1 and Group 2 fish to fry at the time.
Did you know that the first documented case of glyphosate resistance in Canada was in 2008?1 That was when a population of giant ragweed in Ontario was found to be resistant to glyphosate. That’s not bad when you consider that glyphosate was first registered here in the mid-1970s.
By the mid-1990s, the first glyphosate tolerant crops were planted in Canada. These early Roundup Ready® crops were a benefit for farmers, providing a new level of weed control while maintaining crop health and maximizing yield potential.
Unfortunately, the resulting increased use of glyphosate added to selection pressure on weeds, and a result was the 2008 discovery of glyphosate-resistant giant ragweed in Ontario. By 2024, there have been eight unique weed species resistant to glyphosate found across Canada – again, in comparison to the 59 there are globally.2
Glyphosate Resistance: Understanding the Challenge and Moving Forward
Glyphosate-resistant weeds are a growing concern in several provinces, including Alberta, Saskatchewan, Manitoba, Ontario, and Quebec. While the list of resistant weeds may seem limited, many of them pose significant management challenges due to multiple modes of action resistance. However, with proper resistance management strategies, growers can continue to benefit from the flexibility and effectiveness of glyphosate in their operations.
Key to managing resistance is incorporating diverse weed control practices, including crop rotation, the use of multiple herbicide groups, and integrated weed management approaches. By taking proactive steps, growers can maintain glyphosate’s role in their operations while effectively managing resistance in their fields.
Is glyphosate still effective?
Because glyphosate is such an effective weed control tool, farmers want to keep this valuable mode-of-action in their toolbox. It means finding ways to manage already resistant weed populations while slowing the onset of new resistance.
Such as building a truly integrated weed management (IWM) plan that includes an equal balance of chemical, cultural, physical and biological practices.3
How worried should we be about glyphosate resistance?
In a 2024 “temperature check” survey of 100 Prairie famers conducted for The Western Producer, 71 per cent of respondents said they were extremely concerned or very concerned about herbicide resistant weeds, while 88 per cent said they had dealt with at least one resistant weed problem on their farm.3
This survey revealed that expert advice from agrologists, researchers and field representatives over the years is being taken as nearly all of the respondents said they were rotating herbicide groups, using multi-mode-of-action herbicides and rotating crops to help combat the problem.
It’s a good start, but as herbicide resistance is growing, and as multiple resistances, including glyphosate resistance, are becoming more common, farmers can do more to protect their yield and crop quality.
What can be done about glyphosate resistance?
Managing glyphosate resistant weeds is best achieved through a multi-year, multi-pronged weed control strategy that involves crop and herbicide group rotations, using herbicide mixtures, proper product application, digital mapping, strategic use of crop traits and more.
The idea behind such a strategy is to create conditions that give you the best opportunity to control emerged weeds early, when they’re vulnerable, and control weed escapes before they can return seed to the weed seed bank and start the cycle again.
Start with good weed maps. You can’t fight what you can’t see, so develop good weed maps use digital mapping tools, like FieldView™, to ensure accuracy and get a handle what resistance problems you have, where you have them and when you’ve had them. Use this information as the foundation of crop and herbicide rotation strategies.
Be disciplined about rotations. It’s definitely a tough call to rotate away from a high-value crop, like canola, for a year or two but it’s an important tool for breaking weed cycles and for properly alternating herbicide groups. Spraying glyphosate, or any herbicide group, on the same field year after year only helps resistant weeds survive and thrive. Used together, crop and herbicide rotations will help slow or delay the onset of resistant weeds while preserving the efficacy of herbicides for as long as possible.
Choose competitive crop options in your rotations.
Use herbicide mixes for multiple mode-of-action weed control.
Layer herbicides – combine residual and over-the-top herbicides where possible.
Get creative with weed control. Use all methods at your disposal to control weeds – both herbicide resistant and susceptible – including:
Patch management – control concerning weed patches before they can set seed and spread.
Control weeds on non-cropland (ditches, yards, hedgerows, etc.).
Use strategic tillage, mowing, spot spraying or even hand-rouging where appropriate.
Use traits wisely. Glyphosate-tolerant crop systems, like TruFlex® canola or Roundup Ready(R) Xtend Crop System. can give you options and flexibility as to when and how much glyphosate you apply. These Bayer Traits systems have a role to play in a weed management plan when used wisely.
Pay attention to rates. Ensure you are applying an effective herbicide rate, which means use enough to control the weeds you have. Follow label directions always – cutting rates or pulling back on rates because you’re tank mixing multiple products only contributes to a weed’s ability to develop resistance.
The future outlook
No one is saying that herbicide-resistant weeds can be totally eradicated, but they can be effectively managed. Glyphosate resistance certainly adds a new challenge to an already challenging situation, but with the right products, the right crops and the right plan, it is manageable.
Sources:
1 Heap, I. The International Survey of Herbicide Resistant Weeds (weedscience.org). Page title: Herbicide Resistant Weeds in Canada (May 2024) https://www.weedscience.org/summary/Country.aspx
2 Manitoba Crop Alliance, Spread of waterhemp and Palmer amaranth in Manitoba (web). https://mbcropalliance.ca/directory/production-resources/spread-of-waterhemp-and-palmer-amaranth-in-mb/
3 The Western Producer, Prairie producers concerned about herbicide resistant weeds, March 2024, online. https://www.producer.com/opinion/prairie-producers-concerned-about-herbicide-resistant-weeds/
