Glyphosate: too much of a good thing spells doom

Stephen Powles, an Australian weed scientist and farmer, predicts that glyphosate will become a redundant herbicide in many parts of North America.  |  Robert Arnason photo

WASHINGTON, D.C. — Stephen Powles is not optimistic about the future of glyphosate in North America.

Powles, a farmer and weed scientist at the University of Western Australia, says the herbicide might have a shorter life span than farmers imagine.

“I’m going to make here a very bold prediction: glyphosate will be driven to redundancy, on driver weeds, in large parts of (North America),” Powles said at a herbicide resistance summit held in Washington Sept. 10.

“Driver weeds are the weeds that farmers make their weed control decisions. They’re the big weeds that we all know about, and glyphosate will be driven to redundancy.”

The Weed Science Society of America organized the summit, which attracted academics, representatives of the crop protection industry, commodity group members and farmers from the United States and Canada.

The objective was to find solutions to the herbicide resistance problem, but many participants agreed with Powles: the situation will get worse before it gets better.

“I would say that most growers (in Ontario) are reactive, not proactive, in implementing herbicide resistance management strategies,” Peter Sikkema, a University of Guelph weed scientist, said during a coffee break at the conference.

Mike Owen, an Iowa State weed specialist, polled the audience in Washington about farmers’ willingness to change their weed management habits.

More than 87 percent of attendees said it’s unlikely that farmers in the U.S. and Canada will adopt diverse weed management tactics “before resistance in fields reaches levels that decrease farm profits.”

“This (informal poll) obviously speaks loudly as to the challenges that we have,” said Owen.

Five percent of soybean fields in Iowa have weeds with resistance to five herbicides, but most growers remain reluctant to take preventative action, he added.

Powles said glyphosate could become a worthless herbicide be-cause North American farmers and the wider agricultural industry suffer from a condition he calls HOS: Herbicide Only Syndrome.

Powles said that when growers and ag consultants are faced with herbicide resistant weeds, the prescription almost always involves applying more herbicides and different modes of actions.

Crop science companies are introducing genetically modified soybean and corn crops with tolerance for glyphosate and other herbicides, such as 2,4-D and dicamba.

Powles said this technology will ultimately fail because growers will continue to apply glyphosate, causing increased selection pressure for glyphosate resistant weeds.

Preserving the functionality of glyphosate is a major concern, but weeds with resistance to multiple herbicides could change the game, said David Shaw, a Mississippi State University weed scientist.

“We have chosen to create a system … (that) is so simple, so economical and so convenient, that we are placing selection pressure like there never has been before,” he said.

“When we start running out of (herbicide) options … then we have to step back and rethink the entire production system.”

Powles said adopting diverse tactics to control weeds, such as weed seed destruction at harvest and strategic tillage, is the only way to maintain the long-term efficacy of gly-phosate and other herbicides.

“If HOS is replaced by diversity, then herbicides can be much more sustainable.”

Ohio farmer John Davis said weed experts shouldn’t be so negative. Farmers in his area are adopting diverse methods, such as cover crops, to combat herbicide resistance.

“The growers listen, probably more than you give them credit for,” he said. “And they apply what you tell them, more than you give them credit for.”

Powles said there is a chance that American and Canadian farmers can preserve glyphosate.

“If you are in an area of (North America) where glyphosate still works, do everything you can to intervene because glyphosate is a one in a 100 year chemical,” he said.

“Ladies and gentlemen … try to prove (my) prediction to be wrong. I would love to see action taken, and action can still be taken in many parts of (North America).”

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