Unchecked use of glyphosate alone led to substantial and rapid loss of efficacy against the weeds it was designed to kill, according to a new USDA-ARS and ACES analysis

Source: University of Illinois

URBANA, Ill. — 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 years that followed, though, weeds targeted for eradication quietly fomented a rebellion.

A new PNAS Nexus study led by scientists from the USDA Agricultural Research Service (USDA-ARS) and the University of Illinois Urbana-Champaign takes a retrospective look at glyphosate efficacy after the engineered crops were commercialized. Amassing data from annual herbicide evaluation trials at land-grant universities across the U.S. and Canada, the researchers show a significant and rapid decline in glyphosate control for all seven major weed 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 weed control 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 loss of control, glyphosate efficacy became more variable over time.

“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.”

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