Study in North America: Weeds are increasingly resisting glyphosate

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Pesticide use on a cotton field in California (symbolic image)

Photo: Gary Kazanjian/AP

As a miracle cure for weed-free agriculture with higher yields, glyphosate is likely to become less and less suitable in many regions of North America. This is suggested by a study published in the scientific journal "PNAS Nexus". According to the study, the effect as a weed killer on experimental fields there rapidly diminished after just a few years, because plants undesirable in agriculture developed resistance to the agent.

The authors, from the U.S. Department of Agriculture Research Service and the University of Illinois at Urbana-Champaign, analyzed data from several field trials over the past 25 years in the U.S. and Canada. During this time, the manufacturer Monsanto, which is now part of the German Bayer Group, established the glyphosate business under the brand name Roundup.

In North America, this includes genetically modified seeds designed by Monsanto in such a way that glyphosate cannot harm crops. Farmers can also apply the product to fields where crops are already planted and thus use it particularly intensively. Above all, the large-scale cultivation of corn and soybeans in the USA today depends on the herbicide.

To date, this application of green genetic engineering has not been approved in the European Union. Farmers here use glyphosate to clear fields of weeds before the crops are sown. As soon as corn grows in the field, for example, the product is no longer used, unlike on many arable land in North America, because the crops would then die. Therefore, the results of the current study are not directly transferable to the EU.

At first, it worked perfectly

"Nature has done exactly what we were trying to avoid," said co-author Aaron Hager, a crop researcher at the University of Illinois, referring to the results: "It has adapted."

Initially, the package solution for North America consisting of new poison and resistant seeds worked perfectly, according to the study. But over the years, more and more weed species have mutated to such an extent that they are proliferating again in fields sprayed with glyphosate. Two of the species studied – the velvet poplar and the white goosefoot – are still reliably controlled with glyphosate. But even in these cases, in view of the changes that have already been observed, it is only a matter of time before resistance occurs.

From 100 percent, the effectiveness of the weed killer has dropped to 50 or 30 percent or even less in many cases, according to ecologist Marty Williams of the Department of Agriculture's Research Service. The first signs of adaptation usually appear two to three years after the start of glyphosate use.

31.6 percent loss of control per decade

In the long run, the strategy of not relying solely on this one remedy was noticeably more successful. According to lead author Christopher Landau of the University of Illinois, glyphosate lost up to 31.6 percent of its weed control effect within a decade. If, on the other hand, an older weed killer was also used, weeds prevailed by a maximum of 4.4 percent more.

The researchers see early warnings of addiction confirmed – which, however, were mostly ignored in the initial glyphosate euphoria. Because of Monsanto's commercial success, research and development of other means has been shut down, so that there is now a lack of alternatives. "Unfortunately, we were right," Hager commented.

The research group's recommendation to farmers is to mix different methods, distribute agrochemicals alternately over the soil and over the leaves, grow different seeds in succession in a field, and remove weeds with mechanical tools. And they should never again believe the promise of a miracle cure.

In Germany, too, there are repeated discussions about glyphosate, for example on the question of whether it could be carcinogenic. An extensive investigation by the European Food Safety Authority (EFSA) recently found no unacceptable dangers, but pointed to data gaps in several areas. In November, the EU Commission extended the approval for a further ten years.

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