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Ontario Grain Farmer Magazine is the flagship publication of Grain Farmers of Ontario and a source of information for our province’s grain farmers. 

The global struggle


ontario’s growing problems with herbicide resistant weeds are just the tip of the iceberg in what is truly a global problem. The problem weed species, growing climates and affected crops change from country to country; even within Canada, the resistance situation differs from province to province. However, the root cause behind weed resistance – overuse of the same herbicide year after year – is the same issue for all farmers.


Carrot growers in Quebec have seen their crops get swallowed up by linuron-resistant common ragweed. In Ontario, carrot growers are battling linuron-resistant pigweed. Kristen Callow, who leads the Ontario Ministry of Agriculture and Food’s Weed Management Program in horticulture, says growers are getting desperate. In some cases, they have up and relocated their entire crop due to weed-saturated fields.

Unlike grain farmers, vegetable growers have a small weed control toolkit at their disposal. Now, carrot growers in Ontario and Quebec are in a desperate situation where they have pigweed and common ragweed resistant to their two backbone herbicides, linuron (Lorox) and prometryne (Gesagard).

“When a weed develops resistance it’s difficult for growers to find another herbicide that can be registered for use on their crops largely due to phytotoxicity issues. It’s just hard to find herbicides that will not damage the crop and still control the weeds,” says Callow. “So they basically have no other options.”

Callow and several other researchers from the University of Guelph and Agriculture and Agri-Food Canada are trying to find solutions and raise awareness of best practices that can help stem the tide of resistant pigweed and common ragweed. Though so far, it is proving to be a hard sell.

“We did workshops with the growers and 95 percent of them told us that they had at least one or more resistant weed species on their farm,” she says. “But they’re still doing the same practices and waiting for the next herbicide to come along.”

In Western Canada, growers have been dealing with group two resistant kochia and chickweed, as well as group one resistant wild oats and green foxtail for at least 10 years. Yet, it is only within the last couple of growing seasons that they have had to contend with resistance to a new herbicide group with glyphosate-resistant kochia.

According to Sonia Matichuk, a technical sales agronomist with DuPont Canada, the attitude towards dealing with them is markedly different than the situation in Ontario and Quebec. She credits increased media attention with bringing resistance issues to the forefront – and prompting many Western growers to begin seriously planning their crop and herbicide rotations well in advance.

“From what I’ve seen, I think a lot of western growers are coming to terms with it and thinking about resistance differently from what they may have thought three or four years ago,” says Matichuk.

She also suspects that there is a fear among growers of losing weed control tools, such as glyphosate, for good.

“I think they’re learning to manage it, because glyphosate is such a significant part of their operation,” says Matichuk. “So that means switching out to different modes of action and saving glyphosate for other crops that maybe need it more.”

north america
To get a wider North American perspective on the resistance problem, Stratus Agri-Marketing Inc., a Guelph, Ontario market research company, surveyed 2,028 farmers in Canada and the United States over three years. When they released their findings this past July, they came to the conclusion that more than one million acres of land across Canada could be infected with resistant weeds. In the United States, nearly half of farmers surveyed in 31 states said they had at least one type of glyphosate-resistant weed on their farms. Even more shocking are the numbers from the cotton-belt states, such as Georgia and Tennessee. According to the survey, a whopping 92 percent of Georgia farmers claimed to have at least one or two species of resistant weeds in their fields.

University of Guelph Plant Agriculture Professor François Tardif has studied resistance for over 20 years and says resistant weed infestations in the southern states often start small and quickly spiral out of control. The most reported resistant weed is Canada fleabane (known in the US as mare’s tail or horseweed) followed closely by Palmer pigweed.

“When a weed like Palmer pigweed starts, it doesn’t look or seem like much,” explains Tardif. “But it can quickly cover an entire field once it takes hold – especially in the summer when it’s warm and moist.”

In some of the most resistant weed-ravaged fields, Tardif says farmers have gone back in time to the turn of the century, manually hoeing their field crops.

“Now, cotton growers just pull out and chop the weeds, since their herbicides don’t work,” says Tardif. “Or, they go back to a weed control program that’s using nine different herbicides throughout the season.”

Even Midwest states are quickly losing ground to the spread of resistant weeds. Farmers in Nebraska have reported pockets of glyphosate-resistant giant ragweed, waterhemp and Canada fleabane.

Tardif recently attended a global summit on herbicide resistance in Australia and got a firsthand glimpse at the dire situations many farmers are facing there. The last new herbicide mode of action was introduced to Australia back in 1996. Since then, every weed control product has used variations of the same active ingredients. Ryegrass has been the number one weed problem for Australian farmers, since a strain resistant to Group 1 herbicides was identified more than 30 years ago.

Tardif says that herbicide resistance has become so severe that many Australian farmers are looking beyond chemistry to kill resistant weeds. A new machine, called the Harrington Seed Destructor, was launched last year and is quickly being adopted by farmers. The weed destructor is towed behind a combine and uses counter-rotating cages containing steel bars to crush any weed seeds from the chaff. Tardif says for many of the worst hit Australian farmers, the Seed Destructor machine is one last lifeline for their operations.

“It’s either using machinery like this and trying to keep the weed population as low as possible, or not farming all together,” he says.

Tackling resistance going forward is going to take far more than a weed seed crushing machine, or some clever crop rotations, warns Tardif. Rather, it is an entire world of growers that need to get on the same page about resistance, as the world’s food security and supply could be at stake.

“Even in other parts of the world, the grower mentality is, ‘I’m going to continue using this approach because it’s economical for me today’,” says Tardif. “So, there’s still a lot of education and training that needs to happen everywhere – because with global trade, it’s easy to inherit resistance problems from elsewhere.” •


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