Monsanto Co.’s newest variety of genetically modified soybeans is getting another tweak.

The biggest seed company will make its Xtend soybeans tolerant of a herbicide called glufosinate in the next two to three years, Chief Technology Officer Robb Fraley told reporters on a Jan. 4 conference call.

Xtend soybeans, which Monsanto says will be grown on 40 million acres in the U.S. this year, can already withstand two other weedkillers, dicamba and glyphosate.

The announcement matters because Xtend attracted...