In a letter sent to McCarthy late last week, Democratic lawmakers from across the country said they were concerned about the health risks posed by Dow's Enlist Duo herbicide, which combines 2,4-D and glyphosate as a one-two punch to battle weeds that have evolved to become impervious to glyphosate alone. Dow has genetically modified corn and soybeans to make them immune to Enlist Duo so that farmers can spray entire fields with the herbicide, sparing the crops but killing the weeds.