Glyphosate resistance spreads in southwestern Ontario

by DAVE PINK

The widening range of two glyphosate-resistant weed species in southwestern Ontario should be a signal to the province’s farmers to adopt a more diverse herbicide application strategy, says one of the two University of Guelph weed scientists who conducted recent field surveys.

Glyphosate is a widely-used, broad spectrum herbicide used to control broadleaf weeds and grasses. It is the active ingredient in Monsanto’s Roundup herbicides.

Initial surveys of weed resistance were conducted in 2008 by Peter Sikkema  and Francois Tardif of the university’s department of plant agriculture and graduate student Joe Vink. They were followed up in 2010 and again this year. The results are unmistakeable.

“In 2008 we had one field in Essex County that was resistant to giant ragweed. Now we have 48 in Essex, Kent and Lambton,” said Sikkema. “In 2008, we found no fields with resistance to Canada fleabane. Now we have 10 in Essex, Kent and Lambton.

“We now have more species that are resistant over a wider area.”

Asked if the number of glyphosate resistant varieties would grow and continue to spread, Sikkema responded: “If I were a betting man, I’d say probably. But I can’t say that with any form of certainty.”

He cautioned farmers to reassess their weed-control strategies, on a field-by-field basis.  “The single-most important word is diversity — diverse crops and diverse weed-control tactics.” Rotate crops, plant aggressive strains of seeds, and don’t be reliant on any single variety of herbicide, he advised.

“What it comes down to is just good agronomic practices,” said Sikkema. “Do everything you can to make that crop more competitive and you’ll help yourself out in the long run.

“And it’s imperative that you use other forms of weed management. Every field is different, and every weed management strategy has to be adjusted for each field.”

He concedes, however, these new tactics are “going to affect farm profitability.”

The study was partially funded by Monsanto Canada, the Grain Farmers of Ontario, and the Canadian Agricultural Adaptation Program. BF