Opponents say the weedkiller, intended to be used with crops genetically modified to tolerate the herbicide, would do little to stop the evolution of superweeds. “The minimal benefits of this expanded use of 2,4-D will very quickly be outweighed by its myriad negative impacts,” says Food and Water Watch, one of the groups. The Environmental Working Group said 5,600 schools are within 200 feet of corn and soybeans fields where the new herbicide could be used.