In the last 15 years, glyphosate has become one of the most widely used herbicides in the world, eclipsing even atrazine as the workhorse of chemical weed control in row crops and a myriad of other uses.

So could glyphosate, arguably one of the world’s most important herbicide compounds, become practically useless for all but a few “niche” markets in the next few years? Steve Powles thinks that is 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 in Miami Jan. 19. The conference was attended by 284 scientists and media representatives from North America and South America.

“Glyphosate will be driven to redundancy in the cotton, corn and soybean belt,” said Powles, a widely respected authority on herbicide resistance. “Outside of these areas of the U.S., then glyphosate should continue to be effective because it is not massively used.

“Within the cotton, corn and soybean belt the massive reliance on glyphosate means that it will be driven to redundancy because many of the big driver weeds such as Palmer pigweeds, waterhemp, ragweed and Johnsongrass will be resistant. There may be many weed species still controlled by glyphosate, but glyphosate will fail on the driver weeds and that means overall failure.”

Weed scientists from throughout the Western Hemisphere talked about the increasing tide of documented cases of glyphosate resistance during the conference, which was sponsored by Bayer CropScience, though none of the forecasts were quite as dire as that of Powles.

That glyphosate could fade to near-oblivion as a herbicide would be “lamentable,” he said, “because glyphosate is 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.”

Powles, who besides being a university professor is a wheat, barley and canola farmer in western Australia, urged his fellow weed scientists to do everything possible to preserve the efficacy of glyphosate. “There are some parts of the United States where it is already lost. In other parts, it still works. In those, we need to learn from them what has happened.”

Weed scientists have documented three different mechanisms of resistance to glyphosate, which makes glyphosate resistance different than what has traditionally happened when repeated applications of herbicides 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 United States and western Europe that were repeatedly treated with atrazine,” said Powles. “What was striking about triazine resistance evolution was that resistance was almost always due to the same mutation.

“Unfortunately, the experiences with triazines shaped and continues to influence thinking on all herbicide resistance. That thinking has been wrong.”

Subsequent herbicide resistance developments have shown a wide range of herbicide resistance mechanisms and mutations, some with absolutely no impact on plant fitness, according to Powles.

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.

“To date, a non-target site mechanism of restricted glyphosate translocation is most common. However, target-site glyphosate resistance evolution due to substitutions at EPSPS Pro-106 is occurring, as well as resistance due to EPSPS gene amplification.”

Prior to the introduction of Roundup Ready-tolerant crops in the mid-1990s, weed scientists said no cases of glyphosate resistance in weeds had ever been documented. That perception changed as first marestail and then Palmer amaranth or pigweed were found to be resistant to glyphosate in fields in Delaware, west Tennessee and Georgia.

“Clearly, even with a herbicide for which resistance evolution is difficult, if the selection pressure is intense and persistent, then resistance mechanisms will evolve in large weed populations,” Powles said in a paper distributed at the conference.

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 possessing 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.

“As there are no foreseeable new technologies which can 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 said diversity is the key to preserving herbicide compounds such as the triazines, ALS and ACCase inhibitors; that is, diversity in cropping systems, herbicide modes of action and non-chemical weed control measures.

He showed a photo of a large wheat field on his own farm in southern Australia. “This photo is screaming about the lack of diversity,” he said. “In Australia, economics have made wheat followed by wheat, followed by wheat the primary crop rotation system. I try not to do that on my farm, which may be why I’m losing money.”

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