22 December 2023

What if invasive weeds were winning the war against glyphosate?


That’s the hypothesis of an American study: in soils treated with glyphosate, plants targeted for eradication are spreading again. A phenomenon that dampens the enthusiasm that has partly surrounded the controversial herbicide since its launch

by Matteo Cavallito


In cropfields of North America planted with corn and soybeans and treated with glyphosate, one of the most notorious and controversial herbicides, weeds that were supposed to be eradicated are reportedly gaining ground again after developing some adaptability. This is supported by a study by the Agricultural Research Service of the U.S. Department of Agriculture (USDA-ARS) and the University of Illinois Urbana-Champaign.

The study

The survey was based on the collection of annual data from herbicide evaluation trials conducted at a number of universities in the United States and Canada. “Trials from 24 institutions from 1996 to 2021 were compiled into a single database,” the study states. The researchers tested the performance of glyphosate for each year, either alone or in combination with another herbicide. The investigation covered seven weeds.

Control ability in areas treated with glyphosate alone declined over time while that measured in areas subject to prior use of another herbicide remained relatively unchanged.

“These results,” the study points out, “illustrate the rapid adaptation of agronomically important weed species to the paradigm-shifting product glyphosate. Including more diversity in weed management systems is essential to slowing weed adaptation and prolonging the usefulness of existing and future technologies.”

Grasses adapted to glyphosate

According to Chris Landau, USDA-ARS researcher and lead author of the study, quoted in a statement released by the University of Illinois, the 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.” Although glyphosate proved particularly effective initially, most noxious weeds showed signs of adapting to the chemical in only two to three years.

Over a decade, the statement continues, the glyphosate reactivity of grasses declined by 31.6 percent in the worst cases. Evidencing further declines as time passed. The mechanism is not entirely understood. It seems clear, however, that the strategy based on the use of a single herbicide has failed.

A herbicide under fire

Particularly widespread, not only in America, glyphosate has long been in the sights of various critics who denounce its adverse effects on humans and the soil ecosystem. The World Health Organization, for example, has defined the substance as “probably carcinogenic to humans.” A recent survey conducted in Mexico by researchers at the Dutch University of Wageningen, moreover, found a negative correlation between the presence of glyphosate and the density of several categories of macroinvertebrates.

In 2017, the EU Commission granted a five-year approval for glyphosate. The approval expired on December 15, 2022, and was subsequently renewed for one year. On November 16, the European Commission committed to renew the authorization for 10 more years. The EU action is based on the favorable opinion of the European Food Safety Authority (Efsa) and came in response to the failure of representatives of the 27 EU countries to reach a qualified majority in a vote on the issue.