Monsanto has crops resistant to glyphosate. Bayer is selling cotton and soybeans resistant to glufosinate, another weedkiller. Monsanto’s newest corn is tolerant of both glyphosate and glufosinate, and the company is developing crops resistant to dicamba, an older pesticide. Syngenta is developing soybeans tolerant of its Callisto product. And Dow Chemical is developing corn and soybeans resistant to 2,4-D, a component of Agent Orange, the defoliant used in the Vietnam War.