Weeds to watch for in 2025

Thursday, May 22, 2025

Herbicide resistance in Ontario

By Emily McKinlay

Herbicide-resistant weeds are an increasing problem for many Ontario farmers, limiting their weed management options and increasing the importance of implementing diversified integrated weed management programs.

Peter Sikkema, a recently retired professor of field crop weed management at the University of Guelph, Ridgetown Campus, says that many farmers have had to adjust their weed management programs to compensate for herbicide resistance.

“I think many farmers in Ontario have adjusted their weed management programs because of the presence of herbicide-resistant biotypes on their farm,” says Sikkema.

“When I say they changed their weed management programs, chances are they’ve had to increase the cost of weed control because of herbicide resistance. I think it’s an issue for many Ontario farmers, and it’s increasing the cost of weed management.”

Mike Cowbrough, a weed specialist for OMAFA, says that in the past 25 years, more than 3,300 weed specimens have been sent for testing for herbicide resistance.

“That equates to approximately 132 weed samples each year. There is year-to-year variability in that average number since increased testing will happen for a newly found herbicide-resistant weed, or because a graduate student at the University of Guelph has a surveillance and monitoring project,” says Cowbrough.

“This past year, 57 weed specimens were submitted. It’s a topic of concern almost every year, with ebbs and flows based on the occurrence of novel findings.”

Which weeds are likely to cause trouble for farmers in 2025, and how should they be monitoring them?

Weeds to watch

The types of weeds and herbicide modes of action that are impacted by herbicide resistance change from year to year.

“More weed biotypes are evolving resistance to herbicides that are being used. Herbicide-resistant biotypes now occur on more farms across Ontario every year, and these weed biotypes are evolving resistance to more than one herbicide mode of action,” says Sikkema.

He notes that this results in fewer herbicide options for farmers.

Cowbrough says that the evolution of herbicide resistance can traditionally be linked to trends in herbicide use.

“In the 1980s, atrazine was commonly used in corn. Higher rates were labelled that no longer exist. At the time, it resulted in farmers planting continuous corn, and the selection of atrazine-resistant weeds became common,” says Cowbrough.

He says in the 1990s, Group 2 herbicides were commonly used with the planting of continuous soybeans, leading to Group 2-resistant weeds. In the 2000s, glyphosate-resistant corn and soybeans were introduced and the low cost and high effectiveness of glyphosate resulted in increased usage. This continued into the 2010s, when four glyphosate-resistant weed species were found.

In the most recent decade, new herbicide-resistant traits have been introduced for soybeans, and producers have been able to use herbicides like dicamba and 2,4-D to manage glyphosate-resistant weeds.

“One could predict that this will inevitably lead to the selection of dicamba and 2,4-D-resistant weeds,” says Cowbrough.

“Historically, the turnaround for selection of herbicide-resistant weeds may be as quick as five years – as in the case of Group 2 resistance – or it could take 10 to 15 years – as in the case of glyphosate.”

There are four weeds on Cowbrough’s radar for the 2025 season: Waterhemp, common ragweed, perennial ryegrass, and pigweed.

“With waterhemp, we continue to identify approximately 25 new fields each year,” says Cowbrough.

“The majority of specimens sent in are herbicide-resistant, including resistance to glyphosate, and they are also resistant to multiple modes of action. For example, many are also resistant to Group 2 herbicides, Group 5 herbicides, Group 14 herbicides and Group 27 herbicides.”

He recommends prioritizing a soil-applied herbicide to manage waterhemp, acknowledging that its ability to grow quickly makes it more difficult to manage with post-emergent herbicides.

“With common ragweed, we are finding more populations resistant to Group 14 herbicides, which are important tools to manage this weed in both dry beans and food-grade soybeans.

“This past year, two populations were found that are resistant to three modes of action: Groups 2, 5 and 14.”

Perennial ryegrass was one of the first species to evolve resistance to glyphosate.

“This is not typically a weed species that affects crop production in Ontario, but it is becoming more prevalent,” says Cowbrough.

“Three different glyphosate-resistant populations were found in 2024, and the Tardif Lab at University of Guelph has conducted a dose response test on these populations and found that one of the populations was also resistant to Group 1 herbicides.”

He notes that ryegrass patches can spread quickly with harvest equipment and can cause significant yield losses in cereals.

“For pigweed, some populations were found that are resistant to Group 14 herbicides, an important tool for management in dry beans and some horticultural crops,” says Cowbrough.

Sikkema says that challenges with herbicide resistance can be unique to the individual farm.

Monitoring weeds

Observing weed populations is an important step in scouting fields and monitoring for herbicide resistance.

“Monitoring for herbicide resistance is really important,” says Sikkema.

“Unfortunately, I think it’s going to primarily be a reactive situation. If you have weed escapes with a herbicide that in the past has provided good control of that specific weed species, growers should immediately follow up on that.”

To identify weed escapes, producers need to be scouting their fields after herbicide application.

“For herbicides applied prior to both weed and crop emergence, you may begin to see failures at 14 to 21 days after application,” says Cowbrough.

“This would be a good time to scout for such failures, as there is an opportunity to identify if the weed is herbicide resistant through molecular testing. Then you can avoid spraying the escaped weeds with a herbicide that won’t work.”

For post-emergence applications, weed death should be observed within seven to 14 days.

“You should seek testing any time you come across a weed that you feel should have been controlled by a herbicide, including if it’s on the herbicide label as a sensitive species, it was sprayed at the right stage and under ideal environmental conditions, and, for herbicides applied to emerged weeds, when you observe other sensitive species in close proximity that have been killed,” says Cowbrough.

Sikkema says that producers can contact their supplier and weed science extension experts in Ontario if they are concerned about weed escapes.

“There are quick genomic tests available for some herbicide-resistant biotypes and some modes of action. They can get that information back relatively quickly, within one to two weeks,” says Sikkema.

Cowbrough says that plant tissue should be collected for testing. In-season, leaf tissue can be sent to TurnKey Genomics using their Herbicide Resistance Testing Form.

“If the weed species and/or the herbicide group you wanted tested don’t exist, then you can collect seed from the surviving mature plant and submit it to the Tardif Lab at the University of Guelph,” recommends Cowbrough.

Mitigating further resistance

To take pressure off important chemical-based weed management strategies, there are other practices producers can use to control weeds in their fields.

“A management system that relies on multiple different strategies to control weeds will be the most resilient against the selection of herbicide-resistant weeds,” says Cowbrough.

Sikkema recommends introducing more diversity into crop rotations.

“An integrated weed management program starts with a diversified crop rotation,” says Sikkema.

“A minimum of three crops in a rotation is a good starting point. With more complex crop rotations, the opportunities to control weeds with different weed management tactics increases proportionately.”

Cowbrough adds, “Include fall-seeded and perennial crops in a rotation. Planting only crops that share a similar life cycle – such as corn and soybeans – will recruit the same weed species, which will increase their abundance and subject them to similar selection pressures. This increases the likelihood of selecting for a herbicide-resistant biotype.”

Once producers have diversified their rotation, Sikkema recommends using a variety of modes of action for herbicides, rather than relying too heavily on one.

Cowbrough also suggests ensuring crops are as competitive as possible to minimize weed pressure.

“Maintain adequate soil fertility and soil health. Crops canopy more quickly when grown in fertile, healthy soil, and a quick canopy closure reduces the window for weeds to germinate,” says Cowbrough.

“Use a two-pass weed control program system where a herbicide with residual activity is applied prior to crop emergence and a second application is made to control any escaped weeds just prior to crop canopy closure. This dramatically improves the level of weed control and then reduces the chance of a species surviving and setting seed.”

Sikkema says that there are numerous strategies to delay the evolution of herbicide resistance, but he believes it will be an ongoing challenge for Ontario crop farmers.

“There are lots of different strategies growers can implement on their farm to delay the evolution of herbicide-resistant weeds,” says Sikkema.

“I don’t know if we can ever eliminate it entirely, but we can delay it.” BF