WASHINGTON (AP) – The Environmental Protection Agency has withdrawn approval of a controversial new weed killer to be used on genetically modified corn and soybeans. The EPA announced the decision after receiving new information from manufacturer Dow AgroSciences that a weed killer called Enlist Duo, a combination of two popular older herbicides, is probably more toxic to other plants than previously thought.

It was originally approved a year ago and is designed to be used with new strains of genetically modified corn and soybeans. The agency says it needs to study whether wider buffer zones will be required to protect non-target plants. The seeds are engineered to resist the herbicide, so farmers can spray the fields after the plants emerge and kill the weeds while leaving crops unharmed.