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Tool To Avoid Glyphosateresistant Weeds

“Two-and three-way mixes, are in my opinion, the answer if we’re going to use herbicides.”


Monsanto Co. has launched a web-site to help western Canadian farmers assess their risk of cultivating glyphosate-resistant weeds.

“We know it (glyphosate resistance) can happen so we really wanted to generate this tool so growers could see how they’re doing today for their risk for resistance and make any changes if needed,” Sean Dilk, Monsanto’s technology development lead, said in a interview. “And even though we know Roundup is low risk for developing resistance, growers are concerned.”

Earlier this year Canada’s first glyphosateresistant weeds, a population of giant ragweed, were confirmed in a field near Windsor, Ont.

A total of 17 weed species – including giant ragweed – have been confirmed as resistant to glyphosate around the globe. Ten are in the United States.

Monsanto says all g l y p h o s a t e -re s i s t a n t weed biotypes have been managed with other herbicides and cultural practices.


Researchers believe a few of the billions of weeds out there have a natural tolerance for certain herbicides. As the weeds that lack resistance succumb, increasingly what’s left are resistant weeds, which mate producing resistant offspring.

Farmers who visit www.weedtool.comwill be asked 10 questions about how often they apply Roundup (Monsanto’s brand name for glyphosate) and how effective it is.

After assessing the risk, the site offers farmers advice on how to reduce that risk.

One of the “best management practices” suggested is to increase tillage because it reduces the reliance on herbicides.

That’s not an option for zero-till farmers. But the site also says there are other practices that are more highly correlated with herbicide-resistant weeds such as short crop rotations consisting of few crops or repeated use of herbicides with the same “mode of action” for killing weeds.


The simplest way to delay glyphosate-resistant weeds is to use less glyphosate, according to Stephen Powles, a professor of plant biology at the University of Western Australia and a grain grower and glyphosate user. Farmers who don’t have glyphosate-resistant weeds still have a chance to avoid the problem, he wrote in the New York Times this spring.

“This will call for diversifying crops and giving glyphosate a rest by using other herbicides and non-chemical weed control tools that make sense,” he wrote.

Glyphosate needs to be treated like a “precious resource” so it can be used for current and future food production, Powles wrote.

“Glyphosate is the world’s greatest herbicide,” he wrote. “In my view glyphosate is a one-in-100-year discovery that is as important for global food production as penicillin is for global human health.”

With half the cropland in Alberta and Saskatchewan in zero till and 20 per cent in Manitoba, preserving glyphosate’s effectiveness is very important, said Gary Martens, a plant science instructor at the University of Manitoba.

“I agree for the farmers’ own good they should not be using this up,” he said. “They should be stretching it out by putting non-Roundup crops in between. But from Monsanto’s point of view, I don’t think it’s in their best interest to do that, monetarily.”


Glyphosate-resistant weeds are more prevalent in the southern United States where it’s common to sow Roundup Ready crops, which are tolerant to glyphosate annually and to spray those fields three to five times a year with the herbicide, Martens said.

Some Manitoba farmers might be doing the same given they have access to Roundup Ready corn, canola, and soybeans, he said. They could start with a pre-seeding burn-off, spray in-crop twice and then apply pre-and post-harvest.

While herbicide rota-t ion is promoted as one solution, Martens says that just delays the problem.

“A better solution that’s being talked about now quite a bit is tank mixes,” he said.

Instead of relying on one mode of action to kill weeds, use two. The odds of a weed being resistant to two both are lower, Martens said.

“Two-and three-way mixes, are in my opinion, the answer if we’re going to use herbicides,” he said. “Now of course if you don’t have to use a herbicide that’s a way of delaying resistance. If there is no selection pressure in a field there’s no way of developing resistance.”

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About the author


Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.



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