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The Silver Bullet That Wasn't: Glyphosate's Declining Weed Control Over 25 Years

By Lauren Quinn

It has been a quarter century since corn and soybeans were engineered to withstand the withering mists of the herbicide glyphosate. Initially heralded as a "silver bullet" for weed control, the modified crops and their herbicide companion were quickly and widely adopted across corn and soybean-growing regions of North America. In the following years, though, weeds targeted for eradication quietly fomented a rebellion.

Amassing data from annual herbicide evaluation trials at land-grant universities across the U.S. and Canada, the researchers show a significant and  in glyphosate control for all seven major  species they examined.

"Our analysis represents one of the largest cumulative measures of how weed communities have adapted to the simplified weed management tactics adopted at an unprecedented scale throughout North America," said Chris Landau, postdoctoral researcher for USDA-ARS and first author on the paper.

Although glyphosate provided superior  in the early years, most of the weeds in the dataset showed signs of adaptation to the chemical in just two to three years. Within a decade, weeds were up to 31.6% less responsive to glyphosate, with further linear declines as time went on.

"Nature did exactly what we were trying to help people avoid: it adapted," said co-author Aaron Hager, professor and faculty Extension specialist in the Department of Crop Sciences and Illinois Extension, part of the College of Agricultural, Consumer and Environmental Sciences (ACES) at U. of I.

In addition to , glyphosate efficacy became more variable over time.

For the current study, Landau winnowed the data down to fields that tested glyphosate annually, alone or in combination with a pre-emergence herbicide. He also narrowed the target weeds to seven major players: annual and giant ragweed, horseweed, lambsquarter, Palmer amaranth, velvetleaf, and waterhemp. In the end, the dataset represented trial data from 11 institutions.

After documenting patterns of glyphosate control and variability over time, Landau re-ran the analysis for plots in which a pre-emergence herbicide had been applied before glyphosate. The results were strikingly different.

"Adding a pre-emergence herbicide effective against the target weed species significantly improved control and reduced variability of glyphosate over time," Landau said. "The most we saw for any weed species was a 4.4% loss of control per decade, compared to 31.6% loss for glyphosate alone."

Hager isn't surprised. Along with several U. of I. colleagues, he has been cautioning against reliance on any single chemistry for 15 years. His 2008 recommendation aimed at avoiding glyphosate resistance included guidance to use a pre-emergence herbicide at the full rate. Far from common practice at the time, the recommendation was largely ignored.

"Having already seen loss of control with ALS- and PPO-inhibitors [other classes of herbicides], we eventually reached the point where we felt it necessary to come out with some very specific recommendations for glyphosate. Because if we didn't, we had a pretty good idea of where this was going to end up," Hager said. "And, unfortunately, we were right."

The dataset for glyphosate can only show patterns, not explanations. While herbicide resistance might be to blame—the issue has become a major problem in agricultural weeds in recent decades—it's not the only reason glyphosate may have loosened its grip.

Landau noted that two species he tracked in the analysis—velvetleaf and lambsquarter—have not yet had a confirmed case of glyphosate resistance anywhere in the world. Yet both followed the same trends as glyphosate-resistant species in the dataset. He said  pressure—or concurrent climate changes—over the past 25 years may have selected for larger leaf area or earlier emergence, both of which could help weeds survive glyphosate.

Regardless of the mechanism, the pattern is clear: silver bullets for weed control don't exist. The researchers urge diversification in chemistries, including soil- and foliar-applied products; crop rotation patterns; and mechanical controls.

And if another silver bullet is marketed in the future? Hager says the glyphosate story should serve as a cautionary tale.

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