Herbicide Resistance Across the Prairies

Wednesday, April 23, 2025

Weeds to Watch for in 2025

By Emily Mckinlay

Herbicide-resistant weeds are becoming a common problem for Prairie farmers, limiting their options for chemical weed management and increasing the importance of non-chemical management strategies.

Charles Geddes, a research scientist of weed ecology and cropping systems for Agriculture and Agri-Food Canada, says that herbicide resistance has been on the rise globally.

“It’s a growing problem across the country, and it’s not just happening in Canada,” says Geddes.

“It’s a global issue and one that I think farmers will have to deal with every year. It has been increasing in the Prairies and in Eastern Canada. Just to put the problem into perspective, in Canada, we’re sitting in third place for having the greatest number of herbicide-resistant weed species globally.”

Geddes notes that approximately 70 per cent of fields surveyed in the Prairies have at least one type of herbicide-resistant weed, and that this trend has largely arisen in the past two decades.

Rob Gulden, a professor in the Department of Plant Science at the University of Manitoba, says that surveys, which are conducted across the Prairies on approximately five-year cycles, have been finding new types of resistance in different weeds.

“The biology of weeds and our management systems have created this issue,” says Gulden.

“Not only has the proportion of the already-resistant weeds increased, but every time the surveys run, they also identify biotypes with new resistant traits that haven’t been seen before.”

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

Weeds to watch

As the herbicide resistance problem has progressed, some affected weeds will be well known and common across the Prairies. New species and types of herbicide resistance are just starting to present problems for farmers in Western Canada.

“The concern about individual weeds is regional,” says Gulden.

“In Manitoba, wild oats are the major herbicide-resistant weed of concern. It’s one that we don’t have a lot of in-crop herbicide options for in cereal crops.”

Geddes says that Group 1 and Group 2 herbicide-resistant wild oats has become a problem across the Prairies.

“Group 1 and 2 resistance in wild oats has become an issue every year because it is so abundant across the region,” says Geddes.

“Group 1 resistance in wild oats is found in about 85 per cent of the fields tested and 46 per cent of fields surveyed across the Prairies.”

In recent surveys in Manitoba, Gulden says the proportion of herbicide resistance in wild oats has been increasing.

“In 2016, 80 per cent of the fields in Manitoba with wild oats had a Group 1 resistant plant in it. By 2022, that was up to 100 per cent,” says Gulden.

“It doesn’t mean the entire field is overrun, but we can make the assumption that most fields, if not all, have a Group 1 herbicide-resistant wild oat plant in them. Every time we apply a Group 1 herbicide in those fields, we will increase that proportion of resistance.”

Many farmers are also familiar with the challenge of managing kochia.

“Kochia is another big weed in the Prairies. Essentially, all kochia samples we screen are herbicide-resistant. About 75 per cent of samples are glyphosate-resistant as well,” explains Geddes.

“Herbicide-resistant kochia is estimated to be in about 30 per cent of fields surveyed on the Prairies.”

While most farmers will be familiar with herbicide resistance in wild oats and kochia, there are a few new weeds that risk becoming a problem.

“The other thing to keep in mind are the new herbicide-resistant weeds that have a high chance of spreading as well,” says Geddes.

“We have found Group 14 resistant kochia that has a high risk of spreading quickly. We have currently only confirmed this in two samples, but I suspect it will spread from there.”

Geddes notes that one case of glyphosate-resistant downy brome has been confirmed in Southern Alberta. He has also observed a low abundance of Group 2 resistant Japanese brome and Group 1 resistant foxtail barley. He warns that despite the current low prevalence, these weeds still have a risk of spreading with increased selection pressure.

In Manitoba, Gulden says this risk may also be present in waterhemp, which has been showing resistance to glyphosate and other herbicide groups.

“It’s been on our radar as it has been moving north from North Dakota. It’s been there for some time,” says Gulden.

Taking care to monitor for herbicide resistance can help mitigate further spread.

Monitoring for resistance

Most farmers are already spending time scouting their crops. By observing which weeds are present both before and after herbicide applications, producers may be able to catch cases of herbicide resistance earlier.

Geddes says that the time for post-application scouting will depend on the herbicide mode of action and application time.

“In the Prairies there is a heavy reliance on glyphosate as a pre-plant burndown. This allows us to practice reduced tillage or no-till systems. Farmers rely on glyphosate to control weeds going into seeding,” says Geddes.

“For glyphosate, scouting after the pre-plant application window is important.”

Scouting after application is also key for in-crop herbicides.

“Farmers should be monitoring for weeds throughout the growing season,” says Gulden.

“Spend time scouting pre-herbicide application to decide which herbicide to apply. There is also room for improvement in light of increasing herbicide resistance. I don’t think we do enough post-herbicide application scouting for in-crop applications.”

Geddes adds, “Group 1 and 2 herbicides are also used in a lot of crops. Typically, the best time to scout is three to four weeks after herbicide treatment, regardless of what it is. It is a good practice to get a handle on if you might be dealing with herbicide resistance.”

Gulden says you can typically identify herbicide resistance based on how the weeds behave after application. Start by looking for weeds that survive the herbicide.

“Is it resistant or not? Sometimes you can tell by looking at the patch and how it behaves, but sometimes you don’t know,” says Gulden.

Geddes suggests first looking for some key signs.

“One of the key telling factors is if you have plants of specific species that are living next to a plant of the same species that was killed. That could be a key telling factor of herbicide resistance.”

If a plant is suspected of being herbicide-resistant, the next step is to take samples.

“The first thing you should do is sample some plants and talk to your provincial weed specialists or people at universities to conduct an in-season diagnosis. We have molecular tests that are rapid and can test for commonly occurring mutations causing resistance,” says Gulden.

“It’s not perfect. There’s still a chance that there’s an alternate mutation. For tests, tissue has to be sent in. It can be any live plant tissue, but the DNA must be preserved, and provincial labs and weed specialists can assist in that.”

Gulden says for new cases of herbicide resistance, seed samples are also needed.

If resistance is identified or becomes a problem for a farm, how can the spread of the mutated weed be managed?

Mitigating spread

Land management can play a significant role in preventing the spread of herbicide-resistant weeds.

“There are quite a few ways to mitigate the spread of herbicide resistance,” says Geddes.

“It spreads two ways. There’s the initial evolution event, which is hard to prevent. Herbicide resistance occurs because of a natural process. It is selection pressure on a population that selects for plants that have a randomly occurring mutation that appears as resistance.

“From there, the other strategy is preventing those plants from going to seed. Anything farmers can do to help remove that weed from the field before they can produce seed and prevent that seed from going back into the soil seed bank will go a long way in preventing herbicide resistance.”

Gulden notes that to prevent the initial evolution, producers can implement strategies to decrease the selection pressure on that weed.

“It’s important to minimize the selection pressure for resistance. In whatever crop it is, producers should ensure they have a high enough density and narrow enough rows to have a competitive canopy,” says Gulden.

“It gives crops the capacity to compete with weeds and is a wonderful tool to manage weeds and also help herbicides.”

Gulden also recommends being cautious about herbicide choices.

“The biggest selection pressure is during in-crop application. Avoid multiple applications of the same herbicide back-to-back. If you have a dense canopy, hopefully one pass is all we need,” says Gulden.

“Where resistance already exists, go with a herbicide mix with multiple modes of action on target weeds to reduce selection pressure. Ideally, we do that all the time, but sometimes we don’t have multiple modes of action for certain weeds.”

If plants with resistance are present in a field, it’s essential to keep them from going to seed and spreading.

Gulden says, “Manage patches early to prevent seed production. How that is done depends on the size of the problem and the tools the producer is willing to use.”

Gulden says that spot spraying, tillage, hand-weeding and mowing are all potential options for dealing with a patch of herbicide-resistant weeds.

With careful observation and crop management, Prairie farmers can help minimize the growing problem of herbicide resistance. BF