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Ontario Weed Resistant To Glyphosate

The University of Guelph has confirmed that a population of giant ragweed in a field near Windsor, Ont., are Canada’s first glyphosate-resistant weeds.

The discovery is of no immediate threat to Manitoba farmers because giant ragweed is not common here, nor is the Ontario corn belt’s corn-soybean rotation, said Manitoba Agriculture, Food and Rural Initiatives’ weed specialist Nasir Shaikh.

However, the discovery is still a warning.

“From this we can learn to follow proper crop rotation,” he said. “I would say, have a clean start and use the right (herbicide) rate at the right time. In Manitoba we grow a wide variety of crops, so that helps to control the weeds one way or another.”

Because the field where the resistant weeds were found was near an airport, the farmer was restricted in what crops he could grow. Soybeans were grown repeatedly as a result. Rotating crops and herbicides is one way to prevent weeds from developing herbicide resistance, Shaikh said.

“For western growers I think the important message is that this is one field, one weed, one farmer, but it’s a reminder obviously for growers,” said Trish Jordan, spokeswoman for Monsanto Canada, whose American parent company invented glyphosate, a nonselective herbicide.

“That’s why we’re always harping on product stewardship and making sure farmers do the right things to lessen the likelihood of weed resistance developing. Farmers need to start

“For western growers I think the important message is that this is one field, one weed, one farmer, but it’s a reminder obviously for growers.”


with a clean field, use the right rate, the right herbicide at the right time, use crop rotation.”

Monsanto’s current best management practices include:

Start with a clean field with a burn-down herbicide or tillage to control weeds early.

Use Roundup Ready technology as the foundation of a total weed management program.

Add other herbicides or cultural practices, where appropriate, as part of the Roundup Ready cropping system.

Use the right herbicide rate at the right time.

Control weeds throughout the season and reduce the weed seed bank.

Rotation to other Roundup Ready crops will add opportunities for introduction of other modes of action.

Manitoba is almost 2,000 kilometres away from Windsor, but there are glyphosateresistant weeds – giant and common ragweed and water-hemp – just a few hundred kilometres to the south in North Dakota and Minnesota, according to North Dakota State University. In North Dakota’s Traill county, between the cities of Grand Forks and Fargo, 25 to 40 per cent of all common ragweed in soybean fields are believed to be resistant to glyphosate, according to an NDSU map.

The University of Guelph, which started investigating in 2008, says the giant ragweed in question meets the five requirements of herbicide resistance, including surviving increased rates of glyphosate and the ability to pass resistance to the next generation.

Meanwhile, the university, in co-operation with Monsanto, is testing other giant ragweed populations in southwestern Ontario to measure their susceptibility to glyphosate.

The university and Monsanto are also helping the farmer with strategies for controlling the glyphosate-resistant weeds in his field.

There are more than 30 other species of weeds in Canada that have developed resistance – mostly to Group 1 and 2 herbicides, Jordan said.

A total of 17 weed species – including giant ragweed – have been confirmed as resistant to glyphosate around the globe. Ten of those species are in the United States. All of these glyphosate-resistant weed biotypes have been managed with other herbicides and cultural practices, according to Monsanto.

“Roundup is the most widely used herbicide in the world, so growers don’t want to lose its use and neither do we obviously from a business perspective because we know it’s such an important and effective tool for growers,” Jordan said. [email protected]

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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