Manitoba farmers losing $77M a year to herbicide-resistant weeds, new study finds

A new study is shining a light on a growing problem Manitoba farmers are facing.

Herbicide-resistant weeds are continuing to spread across Manitoba farmland, costing the province’s farmers an estimated $77 million per year, according to a new report.

Published last month in the Canadian Journal of Plant Science, the study surveyed 155 annually cropped fields in Manitoba ahead of the 2022 harvest. Researchers found herbicide-resistant weeds in 72 per cent of fields surveyed, estimating they now occupy about 1.5 million hectares of annual cropland across the province.

The $77 million estimate was calculated using the average loss Manitoba farmers self-reported in a 2016 survey — about $27 per hectare in reduced yields, crop quality and weed management costs — applied to the total land area affected in 2022.

“Those are big numbers, but at the same time, to a lot of folks in the industry here in Manitoba, this isn’t entirely unexpected,” said Ashley Ammeter, a whole farm specialist with Manitoba Crop Alliance.

“We know that herbicide-resistant weeds are a growing issue, and we know that managing them can be costly from an economic point of view.”

The study, published by researchers from Agriculture and Agri-Food Canada and Manitoba Agriculture, shows the problem has grown steadily over two decades — from 32 per cent of surveyed fields in 2002, to 48 per cent in 2008 and 68 per cent in 2016.

Ammeter, whose organization represents wheat, barley, corn, flax and sunflower farmers across the province, said resistance develops when a plant evolves or inherits the ability to survive herbicide exposure that would normally kill it. Repeated herbicide application, she said, inadvertently selects for these resistant plants over time.

“Even though Manitoba farmers are diligent about using herbicides carefully and responsibly… weeds are going to find a way to adapt,” Ammeter said. “And that’s where it comes to us in agriculture — to make sure that we’re keeping one step ahead of them.”

She said farmers aren’t without options, pointing to a range of tactics known as integrated weed management — including mixing herbicides, diversifying crop rotations and strategic tilling — that can make it harder for weeds to adapt.

“I would really encourage farmers to consider how these kinds of practices fit on their farm to mitigate or manage the risk of herbicide-resistant weeds,” she said. “But this is also an opportunity to explore research into new techniques for controlling weeds, so that we can stay ahead of this problem.”

Since the first recorded cases of evolved herbicide resistance in the 1950s, resistance has been documented in more than 270 weed species worldwide, according to the report. Wild oat, green foxtail, common lambsquarters and redroot pigweed are among the most frequently uncontrolled weed species found in Manitoba fields before harvest.

Still, Ammeter says the study carries some good news. While more fields are being affected, the size of resistant weed patches has actually shrunk — down from about 2.2 million hectares in 2016 to 1.5 million hectares in 2022.

“It does indicate that farmers are really aware of this growing problem. And so, where these patches exist, they are strategically using alternative management practices to reduce the size of those weed patches,” she said.

The findings come as Manitoba’s farmers generated $10.4 billion in farm revenue in 2025, with crops accounting for more than half of that total, according to the provincial government.