Herbicide resistance quietly growing problem in Manitoba

Multi-pronged weed control strategies that go beyond chemicals are urged by researchers

Glyphosate-resistant kochia is the latest warning sign for Manitoba farmers.

Farmers may lose the war against herbicide-resistant weeds if they don’t start using other forms of control besides chemicals, a University of Manitoba weed scientist says.

Rob Gulden
Rob Gulden photo: File

Herbicide resistance, common in other countries, is starting to appear in Western Canada and it’s just a matter of time before it becomes prevalent here too, Rob Gulden warns.

Years of continuous herbicide use to control weeds is starting to produce resistant biotypes which no longer respond to chemical treatment, Gulden said in a recent presentation to University of Manitoba agriculture students and staff.

For that reason, producers need to “use as many tools as possible” against persistent weeds, he said.

Right now, herbicide-resistant weeds are mostly distant thunder in Manitoba. In an interview, Gulden said the province is “so far, so good,” when it comes to herbicide resistance but producers still need to be vigilant against it.

“The battle continues but we keep losing modes of action, so we do have to be careful,” he said.

“We haven’t lost the war yet but we’re losing some battles for sure.”

Manitoba’s latest canary in the coal mine occurred in 2013 with the detection of glyphosate-resistant kochia in corn and soybean fields near Emerson and Elm Creek, following a provincial survey that fall.

Only two years earlier, the first case of glyphosate-resistant kochia in Western Canada was detected in southern Alberta. Its rapid progression across the Prairies shows how quickly resistant biotypes can spread, Gulden said.

In his presentation, Gulden showed slides to illustrate the point. In 2006, the first case of glyphosate-resistant giant ragweed was detected in south-central Minnesota. Seven years later, glyphosate-resistant weeds, including ragweed, kochia and water hemp, had spread throughout the state and into North Dakota.

In other parts of the United States, the situation is even worse. Gulden showed a slide stating that in Arkansas, where glyphosate-resistant cotton is a staple crop, 52 per cent of all cultivated hectares are now hand weeded because glyphosate is no longer effective.

In desperation, cotton farmers have increasingly turned to tillage to control resistant weeds. Gulden said the number of cultivated cotton acres rose from 13 per cent in 2004 to 32 per cent in 2010.

Here in Manitoba, glyphosate resistance is less of a problem because two-thirds of crop acres are devoted to wheat and canola, which compete well against weeds.

But there is a significant glyphosate-resistance problem with volunteer canola, which is glyphosate resistant. Gulden said volunteer canola is a particular problem in corn and soybeans, which are row crops and have space between rows for volunteer canola to grow.

Gulden recommended several management practices to limit the spread of resistant weeds. One involves using higher seeding rates and narrower row spacing to promote crop growth so that dense crop canopies can outcompete emerging weeds.

Another practice is simply to combine canola at a slower speed to prevent harvest losses and limit future crop losses to volunteer canola. One study found that volunteer canola can reduce average crop yields by six per cent.

Still another way to control weeds in fall is to actually encourage them to grow by slightly disturbing the soil with a light tillage pass. Gulden said this will trigger late emergence of weed seedlings, which are then killed by cold weather.

Gulden said a number of weed control methods used in conventional farming are standard practice in organic systems as well.

Organic producers often use higher seeding rates which produce more competitive crops. This also helps compensate for crop losses when growers do in-crop tillage for weed control.

“They’re aware that these tools work and they rely on them very heavily,” he said. “Cultural controls are critical to make an organic farm work.”

Gulden said cultural practices to control resistant weeds are essential because there have been no new modes of action for herbicides since 1990. Chemical companies admit there’s no new chemistry in the pipeline for the next 10 to 15 years and farmers must use any other methods they can.

Farmers have relied heavily on chemicals for too long and it’s time to use a complete tool box for weed control, he said.

“In the process of stepping away from combining those tools, we’re actually setting ourselves up to select for resistance more quickly because we’re not using the additional tools.”

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