WINNIPEG – For years, glyphosate-based herbicides have had the reputation of being immune to weed resistance.
So while prairie farmers carefully rotate chemicals that kill only certain types of weeds, they often apply non-selective herbicides like Roundup and Touchdown year after year to the same fields.
“The most often cited statistic is we’ve been doing a huge field experiment for the last 25 years around the world,” said Harvey Glick, a spokesperson for Monsanto Canada.
“And to date, there have been no confirmed cases of resistant weeds developing to Roundup.”
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But a weed scientist at the University of Saskatchewan said farmers should pay attention to reports from Australia on what may be the first documented case of weeds resistant to glyphosate.
Remote possibility
“I have to be honest with you,” said Malcolm Devine. “I had come to the conclusion that while it could happen, (resistance) was very unlikely.”
This summer, researchers at southwestern Australia’s Charles Sturt University in Wagga Wagga announced they found annual ryegrass in a farmer’s field that survived applications of the herbicide.
The farmer used glyphosate at least once a year in the field since 1981. Using minimum-till equipment, he planted sunflowers, wheat, chickpeas, faba beans, tomatoes and dill in the field. Between crops, he stocked the field with sheep or cattle.
In 1995, the farmer sprayed the field with glyphosate in the spring, but did not plant a crop. Patches of annual ryegrass survived the treatment.
So researchers tested the weed seed under controlled conditions and discovered some of the weeds resisted even high amounts of the chemical.
In Canada, gly-phosate is the active ingredient in herbicides such as Monsanto’s Roundup and Zeneca’s Touchdown.
Devine said annual ryegrass doesn’t grow in Canada, but if one weed adapted to the chemical after continuous use, so could other weeds.
Canadian farmers should make sure they rotate glyphosates the same way they rotate other groups of herbicides, he said.
Steve Meister, spokesperson for AgrEvo which manufactures Liberty, a non-selective herbicide that has a composition and mode of action different from glyphosate, agrees. AgrEvo also believes it is extremely unlikely that resistance to its new herbicide could naturally develop, but suggests farmers use common sense.
“I think it’s still a very good idea for farmers to use resistant-management strategies. We’re not going to say that it can’t happen or that it’s so highly unlikely that we should rule it out. I think that it is a fact of life that farmers need to keep this in mind with whatever methods of controlling weeds, diseases or insects they’re using,” Meister said.
Used in zero-till
Jim Pratley, lead researcher at the Australian university, said it’s crucial for farmers to preserve glyphosate’s life because it is widely used in zero-till and minimum-till operations.
“There has been an air of caution since the announcement but no sign of panic,” wrote Pratley, who responded to questions by fax.
“It is not doom and gloom, but it does suggest that we ought not take anything for granted.”
Pratley said the resistant ryegrass could be an isolated case, but said other resistant weeds may have gone unnoticed because they are killed when soil is disturbed during seeding or a selective herbicide may kill a glyphosate-resistant plant during the growing season.
