Knowledge is power when it comes to weed management

Today’s strategy for effective weed management is to stay informed and maintain a broad year-round focus.

By Treena Hein

No one needs to tell Canadian famers that weed control is changing. Where once they could rely on a pre-emergence burndown followed by an in-crop spray and maybe some post-harvest tidying up as needed, weed management is now much more complex. New weather patterns leading to warmer winters and more hot days in summer are upon us, all while herbicide resistance keeps expanding.

For example, Cornell University’s Weeds and Climate Change Project has found that today’s weather patterns are causing some weed species to produce larger root systems, which makes weed control through tillage potentially less effective. Other findings from this project reveal that climate change might render some herbicides less effective, not only through reduced absorption and translocation, but also through higher temperatures, which may negatively impact some active ingredients.

Indeed, it’s very possible that everything to do with controlling weeds — their ranges, dates of emergence, length of seed production period, growth patterns and more — might shift over time, says Charles Geddes, a weed scientist at Agriculture and Agri-Food Canada (AAFC) in Lethbridge.

Plus, the one thing we know for sure is that further herbicide resistance is a certainty. “We see increasing resistance across the Prairies,” says Geddes. “Herbicide-resistant wild oats have already reached all production areas in the region. The most-recent weed survey results for all Prairie provinces, from 2019 to 2023, show that 36 per cent of surveyed cropland area had some resistant weeds, and it has likely increased since.”

So, what’s a farmer to do? Cultivating a mindset of constant learning about what’s present in your fields, paying attention to “driver” weed trends, and taking comprehensive action throughout the year are key.

Shifts in weed range and resistance

Keeping your weed knowledge up to date is easier than it’s ever been because, since 2023, all the historic and new Prairie weed survey results are posted on the Prairie Weed Monitoring Network website (Prairieweeds.com). Led by Geddes and Julia Leeson, weed monitoring biologist at AAFC Saskatoon, this initiative involves annual weed surveys, staggered by province — the Saskatchewan survey is happening this year, Manitoba in 2026 and Alberta in 2027.

The survey has shown that, over time, kochia’s range has expanded massively. Historically, kochia was limited to the southern portions of the Prairies, says Geddes, but due to warmer temperatures, it’s recently been able to germinate and survive in fields as far north as the Alberta Peace region. In fact, surveys from 2019 to 2023 show that it’s now the sixth most abundant weed on the Prairies, with three-way resistance (groups 2, 4 and 9) in some populations and Group 14 resistance beginning to spread.

Recent surveys in Saskatchewan have found that yellow foxtail, which was traditionally limited to southern Manitoba, is now present in the southeastern part of the province, with some Group 1 resistance in both locations, says Geddes. “Waterhemp is confirmed in southern Manitoba, and while there is less palmar amaranth on the Prairies, they are both of concern because they’re at our doorstep, so to speak, with large populations across the U.S. border.”

Geddes also reports that in some areas with more precipitation, false cleavers are increasing in abundance. “Volunteer canola has also increased quite a bit over the last 20 years,” he adds. “And it’s now the second-most abundant weed, after green foxtail, found after in-season herbicide treatment, with wild oats being third. Canola has retained some weediness traits, such as seed dormancy, that allows seed survival in soil over a few years.”

In Ontario, weed management specialist Mike Cowbrough, along with colleagues at the Ontario Ministry of Agriculture, Food and Agribusiness, tested 57 fields in that province last year for herbicide-resistant weeds. They found continued expansion of resistant populations of waterhemp — indeed, the report states that since 2020, waterhemp has been found in 20 to 30 new fields annually. Researchers tested 21 populations of waterhemp, all of which were herbicide resistant with the majority having multiple resistance, including one in Chatham-Kent with four-way resistance (groups 2, 5, 9 and 14). Worryingly, they also found palmer amaranth in two new locations (Bruce and Haldimand counties).

The OMAFA weed survey also found single and multiple herbicide resistant populations of pigweed, common ragweed and ryegrass throughout the fields they tested. Farmers in the province can search for the best management recommendations for a given weed at the Ontario Crop Protection Hub.

Narrow the focus: what needs your attention most

While all this information is important for farmers in making weed management plans, it’s important to ask: Which weeds are moving up the ladder in terms of importance?; Which weeds need closer attention? And while some weeds may be more abundant, are others becoming more of a problem because of resistance or yield impact?

It’s important for farmers to determine which weeds are of particular concern on their farms. Think of them as “driver weeds” since they will drive herbicide decisions. One way to do this is to look at spray thresholds, but that’s not as straightforward as it is for insects.

“We recently did economic threshold analysis for kochia in canola and found that the density of kochia that causes five per cent yield loss is six to seven plants per square metre,” says Geddes. “But there are many other factors besides density that could affect yield. If the weeds emerge before the crop does, their impact will be greater. There’s also the weather, the weed species, the crop that’s being grown. Barley is very competitive with kochia for example, compared to lentils.”

But one straightforward action everyone should take, in Geddes’ view, is thorough in-season scouting. Scouting early in the growing season is important to know which herbicides to use but scouting after in-crop weed control is where you see weeds that might have resistance. “Send those in for testing,” Geddes says. “Knowledge is key. It’s not just a simple confirmation of resistance –— there’s more to it. Resistance traits can vary quite a bit, and the test results can help identify those present on your farm. That provides you with a lot of help in knowing which herbicides will work and which will not. Using multiple effective modes of action for each driver weed on your farm is critical.”

Geddes says farmers can do a lot to protect their fields against both weed spread and resistant weed spread through equipment sanitation alone. For example, a combine used for harvest in the U.S. that has not been completely cleaned presents big risks for introducing waterhemp and palmar amaranth (not only because they are more common across the border, but because their seeds are so small). Seeds with a herbicide-resistant trait can also be introduced from using uncleaned equipment in fields nearby each other as well.

Geddes also stresses careful fall weed control. “It helps prevent winter annuals and perennials getting a head start on the next growing season, but it’s also important for some summer annuals like kochia,” he says. “And don’t forget to check marginal areas where kochia could be present and deal with the patch before it goes to seed.”

As with insects and crop disease, knowledge is power when it comes to weed control. Stay up to date about what’s on your farm and try to identify changes in a weed’s range, growth pattern and resistance trends — digital farming tools can help here. But also shift your thinking so that you view weed control as an ongoing and comprehensive task throughout the year. Knowledge and more-frequent action will keep you one step ahead.