Giant ragweed resists glyphosate

Friday, May 8, 2009

© AgMedia Inc.

by SUSAN MANN

Essex County soybean grower Leo Guilbeault isn’t surprised University of Guelph researchers found a type of giant ragweed that’s showing resistance to the popular herbicide glyphosate.

He’s always found giant ragweed tough to kill. “I know even the normal rate of Roundup traditionally hasn’t taken it down 100 per cent,” says Guilbeault, chair of the Ontario Soybean Growers.

The key is to catch the plants when they’re young, as seedlings, Guilbeault explains.

Trying to kill a mature giant ragweed that can grow to eight feet tall is almost impossible. “It’s like trying to kill a tree.”

But anytime a weed develops a resistance to glyphosate it sends off red flags for farmers, he adds.

Researchers Francois Tardif, a professor in the university’s plant agriculture department in Guelph, and Peter Sikkema, a plant agriculture professor at the university’s Ridgetown Campus, found that the giant ragweed plants they tested could still grow after an application of glyphosate at recommended levels. But normal susceptible ragweed plants were killed at those rates.

According to a press release from the University of Guelph, no weeds in Canada have been confirmed as resistant to glyphosate, the most often used herbicide globally. But in other countries around the world, 15 weed species

Tardif says their results are preliminary and so far the resistant type is only in that one corner of the province. In Ontario, giant ragweed can be found in fields mainly in Essex and Kent counties and in localized areas, such as along riverbanks, in other parts of the province.  

Research continues with Tardif and Sikkema looking at alternative herbicides this summer. “We’re going to try different tank mixtures and different timings to see what are the best options in soybeans and other crops,” Tardif says.

Ontario Ministry of Agriculture, Food and Rural Affairs weed specialist Mike Cowbrough says farmers may have look to alternative methods of weed control, such as cover crops or tillage.

Guilbeault says common ragweed and pigweed are bigger problems for farmers than giant ragweed. BF