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What’s up with weed resistance?

Palmer Amaranth
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Herbicide resistant weeds are adding up in Canada. According to weedscience.org Canada is home to 56 unique resistant weeds. So, what does that mean for Eastern Canadian farmers?

The rising incidence of resistant weeds means the problem isn’t going away. And farmers need to be proactive when it comes to their weed management strategies to maintain continued access to technologies that support farmers.

The summer of 2023 saw 2 new herbicide resistant weeds confirmed in Ontario alone – Palmer amaranth and Group-4 resistant green pigweed. Both discoveries are isolated incidents but underline the fact that reducing the emergence of herbicide resistant weeds is essential to ensure sustainable crop production in the future.

Learn more about herbicide resistance and how it has evolved from weed science experts, Drs. Peter Sikkema and François Tardif.

Problem weeds

Resistant weeds like waterhemp, common ragweed and Canada fleabane, are currently the most concerning. As of January 2023, herbicide tolerant waterhemp was confirmed in 18 counties across Ontario.1 Waterhemp’s ability to develop resistance to five chemical groups, including glyphosate, is most alarming. The same goes for common ragweed, where 25 counties across Ontario have identified resistant common ragweed as of April 2023.

The discovery of Palmer amaranth in Ontario this summer is also concerning. A pigweed species, Palmer amaranth, is an invasive weed that affects multiple crops, grows quickly, causes crop yield and quality damage, and can quickly develop herbicide resistance. U.S. farmers have been dealing with this problem weed for years, and weedscience.org confirms Palmer amaranth can be resistant to up to nine different modes of action. Many Palmer amaranth populations are multiple resistant, and in 2021 a population that was resistant to six different herbicide modes of action (multiple resistant to: WSSA groups 2, 5,6 ,9 ,14 and 27) was discovered in the United States.2

Take action

As herbicide tolerant weeds adapt, farmers need to take a program approach to reduce the risk of developing herbicide resistant weeds and manage any that do develop. Management strategies include following best management practices (BMPs) to manage resistance and protect crop yield and quality.

Adopting a program approach to weed management, like the responsible use and stewardship of the Enlist® Weed Control System that includes Enlist™ corn, Enlist E3™ soybeans, and Enlist™ herbicides will help ensure good agronomics, optimum yields and a sustainable system.  A program approach means using pre-emergence residual herbicides with different modes of action followed by Enlist™ herbicides post-emergence.

It’s critical to adopt best practices to manage resistance and protect crop yield and quality today, to ensure sustainable crop production for the future. Herbicide resistance costs Canadian growers an estimated $1.1 to $1.5 billion annually due to increased herbicide use and decreased yield and quality 3. Resistance best management practices (BMPs) can include a combination of cultural, mechanical, biological, and chemical control measures.

BMPs to consider include crop rotation, cover crops, mixing and rotating herbicides and actives, following label instructions like recommended rates and timing, tillage and diligent scouting. Testing is available for suspected herbicide resistance. Click here for sampling instructions and information.

1https://cropprotectionhub.omafra.gov.on.ca/weed-control/herbicide-resistant-weeds-database-and-maps
2https://weedscience.org/Pages/Species.aspx
3https://manageresistancenow.ca/weeds/ 

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