In a finding that undermines one key argument in favor of genetically modified (GM) crops, researchers at Iowa State University have discovered that a number of “superweeds” have developed a resistance to Monsanto’s widely used Roundup herbicide. Monsanto has engineered crops that are tolerant of Roundup, the idea being that the chemical would kill everything in a field but the desired crop, thereby freeing farmers from using additional herbicides and leading to an overall decrease in the use of chemicals. But if superweeds gain a foothold, farmers will again need other herbicides. “Companies like Monsanto have spun GM crops and their weed killers as having less impact on the environment, but the fact of resistant weeds undoubtedly means more weed killers, and means the impact on the environment will be greater,” said Pete Riley of Friends of the Earth. “These discoveries remove a central plank from the whole argument for GM crops.”