A new PNAS Nexus study from USDA ARS and Crop Sciences researchers from the University of Illinois found glyphosate, when used alone, became up to 32% less effective within a decade of its introduction. Credit: University of Illinois
Key point:
- Within the last 25 years, weeds have adapted to glyphosate, making the herbicide much less effective.
- The study is based on data from 11 institutions over 25 years.
- The team cautions against relying on any single chemistry to solve a problem.
It has been 25 years since corn and soybeans were engineered to withstand the herbicide glyphosate. Once the crops were modified, glyphosate was quickly and rapidly adapted across North America for weed control. And it worked—until it didn’t.
A new retrospective study published in PNAS has shown a significant and rapid decline in glyphosate control for all seven major weed species examined.
In cooperation with 24 institutions throughout North America, researchers compiled a massive database representing nearly 8 million observations from herbicide evaluation trials from 1996 to 2021. They then narrowed the data down to fields that tested glyphosate annually, alone or in combination with a pre-emergence herbicide. The team focused on seven major weeds: annual and giant ragweed, horseweed, lambsquarter, Palmer amaranth, velvetleaf, and waterhemp. In the end, the dataset represented trial data from 11 institutions.
Although glyphosate was shown to provide superior weed control in the early years, most of the weeds in the dataset showed signs of adaptation to the chemical in just 2 to 3 years. Within a decade, weeds were up to 31.6% less responsive to glyphosate, with further linear declines as time went on. In addition to loss of control, glyphosate efficacy became more variable over time.
Two specific species 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. The team said herbicide 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. In general, reliance on a single chemistry is risky.
“Let's imagine a company says they have the next silver bullet,” said Chris Landau, postdoctoral researcher for USDA-ARS and first author on the paper. “It's going to enjoy a huge market share almost instantaneously. But it needs to come with a stewardship plan to better preserve that chemistry over time,” he said. “Why would we think nature would behave any differently? It won’t."