“U.S. farmers are heading for a crisis,” says Stephen Powles of the University of Western Australia, Crawley. Powles is an expert on herbicide resistance, a worsening problem in U.S. fields. Weeds resistant to glyphosate—the world’s most popular herbicide—are now present in the vast majority of soybean, cotton, and corn farms in some U.S. states.

Perhaps even worse, weeds that can shrug off multiple other herbicides are on the rise. Although the problem was highlighted here last week at an American Chemical Society (ACS) meeting symposium, chemists have little to offer: Few new weed killers are near commercialization, and none with a novel molecular mode of action for which there is no resistance.

Herbicide resistance has ebbed and flowed for decades. But because most herbicides could not kill all weeds, farmers had to continually rotate their crops and rotate herbicides to prevent resistant weeds from taking over their fields. That picture changed in the 1990s with the commercialization of transgenic crops resistant to glyphosate, marketed as Roundup by Monsanto. Glyphosate disrupts the ability of growing plants to construct new proteins. Because the transgenic crops didn’t suffer this fate, their use—and glyphosate’s—soared.”

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