The EPA’s decision to allow a weedkiller on the market for two more years could strengthen the legal case of farmers suing Bayer AG who say the weedkiller damages crops, attorneys representing growers said.

The Environmental Protection Agency announced Oct. 31 that it has renewed the approval of herbicides containing dicamba, a chemical prone to drifting off fields and onto neighboring acres, where they can cause injury to soybeans, vegetables, trees, and other vegetation.

Normal EPA re-registration intervals are 15...