JEAN-FRANCOIS MONIER/AFP/GETTY IMAGES
“Total fear and shock.” That’s how Andrew Kniss, a weed scientist at the University of Wyoming in Laramie, describes the reaction of farmers to recent courtroom defeats suffered by a leading manufacturer of glyphosate, the world’s most widely used herbicide. Agricultural researchers are also worried, he says. They fear the loss of a compound that is crucial for controlling weeds and conserving soil. The scientists and farmers “are really nervous that these verdicts and public perception could cause them to lose this tool.”
Last week, a California jury awarded $2 billion to two property owners who claimed their non-Hodgkin lymphoma, a type of cancer, was caused by years of using Roundup, a common glyphosate-based herbicide. It was the third legal loss since August 2018 for Bayer AG in Leverkusen, Germany, which has seen its market value plummet to $52 billion, cut nearly in half since it acquired Monsanto a year ago. No national health agency has identified any cancer risk from glyphosate. But some scientists say that between public distrust and the rise of weeds resistant to Roundup and other herbicides, the moment is ripe to push for new and diverse forms of weed control. “We need to move to cropping systems that are less reliant on herbicides,” says Paul Neve, a weed scientist at Rothamsted Research in Harpenden, U.K.
It will be tough to compete with glyphosate, which accounts for about 25% of all herbicides sold worldwide. By targeting a universal plant enzyme crucial to making amino acids, it can kill a wide array of weeds. With transgenic crops engineered to resist glyphosate, such as Bayer’s Roundup Ready seeds, farmers can spray glyphosate while their crops are growing and control weeds without plowing them up, which saves fuel and conserves the soil.
The environmentally beneficial practice of using cover crops—planted when a field would otherwise be bare—also depends on glyphosate, says Helen Hicks of Nottingham Trent University in the United Kingdom. Planting rye, for example, prevents weeds from growing while also boosting soil carbon and retaining moisture. Farmers then spray glyphosate to kill the cover crop, and because the chemical quickly becomes inactive in the soil, they can immediately plant the field with any crop.
The recent controversy erupted after the International Agency for Research on Cancer (IARC) in Lyon, France, part of the World Health Organization, in 2015 listed glyphosate as a “probable carcinogen”—a label it has applied to dozens of chemicals, but also to red meat and very hot beverages. IARC decides only whether there is convincing evidence of a hazard; it does not evaluate the likelihood of getting sick at varying levels of exposure. That’s the role of national health agencies, which take into account not only some peer-reviewed studies, but also confidential industry data before deciding what exposures might be acceptable. European, U.S., and other agencies have concluded glyphosate is safe when used properly.
Nevertheless, the IARC decision unleashed a flood of lawsuits and increased the pressure from environmental groups to ban glyphosate. The groups point to hints of health risks from animal studies, which may not be large enough to pass muster with the health agencies. Campaigns against glyphosate are strongest in the European Union, where member nations in 2017 only narrowly reapproved a 5-year authorization of the compound.
If glyphosate is ever pulled from the market, farmers could turn to other herbicides, but all have drawbacks. U.S. corn growers would likely spray more atrazine, for example, but it is prone to contaminating groundwater and has been banned by the European Union. Certain crops have been engineered to withstand glufosinate, an herbicide that costs more and doesn’t work as well as glyphosate, especially in arid regions. Tolerance to dicamba has been added to crops as well, but this herbicide can drift in the wind and damage other crops.
Companies are engineering crops to tolerate multiple herbicides because more than 40 weed species worldwide have evolved resistance to glyphosate, spurred by its heavy use. Crop scientists hope it will be more difficult for weeds to simultaneously evolve resistance to a cocktail of herbicides. But it’s not impossible, and eventually new chemistries will likely be needed.
Ironically, glyphosate itself has suppressed the development of new herbicides. No compound with a new way of attacking weeds, or mode of action, has been commercialized for more than 30 years, because it was so hard for companies to compete with cheap glyphosate, says Franck Dayan, a weed scientist at Colorado State University in Fort Collins. But many companies are ramping up their R&D efforts. “I expect within the next decade we’ll see several new modes of action on the market,” says Stephen Duke, a plant physiologist with the U.S. Department of Agriculture in Oxford, Mississippi.
Some new candidates were described this week at a meeting of the International Union of Pure and Applied Chemistry in Ghent, Belgium, including a biopesticide Duke is studying called MBI-014. Under development by Marrone Bio Innovations in Davis, California, MBI-014 is made by bacteria and includes several compounds that attack plants in new ways, such as by interfering with RNA production, which disrupts protein synthesis.
Horst Steinmann, a weed scientist at the University of Göttingen in Germany, says the public debate over glyphosate could raise the profile of nonchemical alternatives. “Perhaps there is a turning point now,” he says. In Australia, after a long and intense battle with herbicide-resistant weeds, farmers have resorted to a laborious but effective way to keep weeds from proliferating: During harvest, the chaff is pulverized to crush any weed seeds, or burned in the field.
Mechanical weeders, pulled by tractors, have long been used as an alternative to herbicides, although they are impractical in vineyards on steep hillsides or in orchards with drip-irrigation tubing. Recently, engineers have added video cameras to the machines to help farmers target weeds more precisely.
For high-value crops, autonomous robots are becoming viable. Some spray tiny doses of herbicide directly onto weeds, whereas others use blades, lasers, or electricity to kill weeds. “There are potentially amazing technological advances occurring,” says Karla Gage, a weed ecologist at Southern Illinois University in Carbondale. But she advocates for cover cropping and other approaches that offer ecological side benefits.
In the evolutionary arms race between weeds and farmers, the need to refine all the tools, robotic or otherwise, will never end, Dayan says. “Plants will evolve resistance to anything,” he says. “Whatever we do, we’ll have to face the way nature works.”
“It will always win.”
Source: Science Mag