By: David Murray

One of the largest retrospective studies ever done on glyphosate effectiveness has cast further doubt on the paradigm of using herbicides like glyphosate — the key ingredient in Bayer’s Roundup — repeatedly to target weeds while sparing crops genetically engineered to resist them.

The study says the data show a rapid falling-off of glyphosate effectiveness against seven major weeds after the first couple of years. The authors conclude that intensive use of any one weedkiller like glyphosate year-after-year serves to speed up evolution in weeds to resist it.

 Titled, “The silver bullet that wasn’t: Rapid agronomic weed adaptations to glyphosate in North America,” the study was jointly conducted by the U.S. Department of Agriculture Agricultural Research Service and the University of Illinois Urbana-Champaign. It appeared in the journal PNAS Nexus Dec. 5. It looked at 25 years’ worth of data.

“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.

Problems with glyphosate generating increased week resistance have been recognized for years. In 2015, Harvard University published an article titled, “Why Roundup Ready crops have lost their allure.” Within a decade of first application, according to the new study, weeds were up to 31.6% less responsive to glyphosate, with further linear declines as time went on, according to the study.

“When glyphosate-tolerant crops were first adopted, weed control was high in every environment; however, year after year glyphosate performance became less consistent,” said co-author Marty Williams, an ecologist with the USDA-ARS and affiliate professor of crop sciences. “For example, glyphosate provided nearly 100% control of a given species in most plots in the mid-1990s. But over time, acceptable weed control became rarer, often deteriorating below 50%, 30%, and worse.”

Many farmers have come to recognize the drop-off in effectiveness from experience, and some vary their pesticides from year to year to try to keep weeds off-balance.

Bill Freese, science director of the Center for Food Safety, said the study provides richer data for a problem that’s been recognized for years. “What’s frustrating is that the weed scientists simply refuse to explicitly put the blame squarely where it belongs–on the whole herbicide-resistant crop paradigm. Turns out that when you make a crop resistant to a specific broad-spectrum herbicide, then charge a big premium for the HR seed, this constitutes a huge economic and convenience incentive for farmers to rely on the companion herbicide entirely.  And that’s a recipe for major resistance, even when the herbicide (like glyphosate) is not inherently resistance-prone,” Freese told High Plains Journal.

Freese said there were practically no glyphosate-resistant weeds from the 1970s until the introduction of Roundup ready crops. The same issue will emerge, he said, from the next promised “silver bullet” — dicamba-resistant crops. Freese said there were already signs that weeds that have developed resistance to glyphosate are also developing resistance to dicamba. 

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