A Missouri farmer who prosecutors allege illegally applied the weedkiller dicamba on his cotton and soybean fields was indicted in federal court.

Bobby David Lowrey used dicamba on his fields in southeastern Missouri, prosecutors said, during the growing season in 2016, when Monsanto and other seed developers began selling soybeans and cotton that had been genetically modified to be resistant to the herbicide.

The genetically modified seeds were intended to be used together with new formulations of dicamba that...