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Corn+Soybean Digest

Will Glyphosate Fall to the Wayside as Resistance Grows?

Glyphosate's become one of the most widely used herbicides, eclipsing even atrazine as the workhorse of chemical weed control. So could glyphosate become practically useless for all but a few niche markets in the next few years? Steve Powles thinks that's a real possibility.

Powles, professor of plant biology at the University of Western Australia and director of the Western Australia Herbicide Resistance Initiative, gave that assessment during remarks at the Pan-American Weed Resistance Conference recently, attended by 284 scientists and media representatives from the Americas.

“Glyphosate will be driven to redundancy in the cotton, corn and soybean belt because many of the big driver weeds such as Palmer pigweeds, waterhemp, ragweed and johnsongrass will be resistant,” says Powles, a widely respected authority on herbicide resistance. “Outside of these areas of the U.S., glyphosate should continue to be effective because it is not massively used.

“There may be many weed species still controlled by glyphosate, but glyphosate will fail on the driver weeds; that means overall failure,” he says.

That glyphosate could fade to near-oblivion as a herbicide would be “lamentable,” he says, “because it's one of the world's greatest herbicides. It works on 140 weed species. It is a one-in-a-100-year discovery, and we'll never see another herbicide like it.”

Besides being a professor, Powles is a wheat, barley and canola farmer in western Australia. He urges his fellow weed scientists to do everything possible to preserve glyphosate's efficacy. “We need to learn from parts of the U.S. where it still works.”

Three different mechanisms of glyphosate resistance makes it different from what has traditionally happened when repeated herbicide applications have rendered them ineffective against specific weeds.

“In the 1970s, the first wave of resistance evolution was widespread triazine herbicide resistance in maize-growing regions of the U.S. and western Europe that were repeatedly treated with atrazine,” says Powles. “It was striking that that resistance was almost always due to the same mutation.

“Unfortunately, that experience continues to influence thinking on all herbicide resistance. That thinking has been wrong,” he adds.

SUBSEQUENT HERBICIDE-resistance developments have shown a wide range of resistance mechanisms and mutations, some with no impact on plant fitness, he says.

Glyphosate-resistance evolution will be a major issue in the coming decade because of massive glyphosate selection pressure in the large areas devoted to transgenic glyphosate-resistant crops, particularly in the Americas, he says.

Weed scientists have now documented cases of glyphosate resistance in rigid ryegrass across large areas of Australia and are encountering it in other weed species in different parts of the world.

L. rigidum (rigid ryegrass) and other weeds with multiple-herbicide resistance, including non-target-site-enhanced P450 metabolism of many herbicides, are difficult to control chemically,” says Powles. “Our current understanding of non-target-site-based herbicide resistance is very limited.

“There are no foreseeable new technologies to rival herbicides for weed management in world cropping. Herbicide sustainability is an imperative that must be achieved to help guarantee the world food supply,” Powles says.

He says diversity is the key to preserving herbicide compounds: diversity in cropping systems, modes of action and non-chemical weed-control measures.

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