Weeds outsmart glyphosate - A 25-year retrospective study reveals

By Andi Anderson

Glyphosate, once a 'silver bullet' for weed control, now faces a major setback. A quarter-century after its introduction in genetically modified corn and soybean crops, its effectiveness is waning, as revealed in a comprehensive study by the USDA Agricultural Research Service (USDA-ARS) and the University of Illinois Urbana-Champaign.

This retrospective analysis, published in PNAS Nexus, compiled data from numerous U.S. and Canadian universities. The findings are alarming: all seven major weed species studied showed rapid adaptation to glyphosate, diminishing its efficacy drastically within a decade.

Chris Landau of USDA-ARS, the study's lead author, highlights this as one of the largest studies demonstrating how weed communities have evolved due to widespread glyphosate use. Co-author Aaron Hager, an Illinois Extension specialist, emphasizes the unexpected adaptation of nature, contradicting initial expectations.

The study's data, collected from 1996 to 2021, involved nearly 8 million observations across North America. It focused on seven key weed species, including Palmer amaranth and waterhemp. The results were stark: a decline in glyphosate responsiveness by up to 31.6%, with a consistent downward trend.

The addition of pre-emergence herbicides showed a significant improvement in glyphosate's performance, stressing the importance of integrated weed management approaches. Hager and his colleagues have long advocated diversifying weed control strategies, including crop rotation and mechanical controls, to prevent resistance buildup.

The study also delves into the broader implications of glyphosate's declining effectiveness. While resistance is a likely factor, other potential causes like herbicide pressure and climate change might also contribute, affecting species without confirmed glyphosate resistance.

The study warns against relying solely on single solution weed control methods and advocates for a combination of chemical, cultural, and mechanical strategies for effective weed management, crucial for maintaining agricultural productivity and environmental health.

Photo Credit: gettyimages-ianchrisgraham