The broadleaf auxinic herbicide clopyralid was applied to three varieties of corn (Pioneer 36B08, Pioneer 3730, and Pioneer 3559) to determine whether it was phytotoxic to this crop. The effects of clopyralid on the growth and development of corn were compared with those induced by the auxinic herbicides dicamba, 2,4-D, picloram, and fluroxypyr. When compared with the other auxinic herbicides, clopyralid, applied as a foliar spray at the three- and six-leaf stages of development, caused the least damage to all three varieties of corn. Among the auxinic herbicides tested, fluroxypyr and dicamba caused severe damage to the three varieties, whereas picloram and 2,4-D had significant detrimental effects on the growth and development of varieties 36B08 and 3730. Similar results were also obtained when corn seeds were germinated in petri dishes containing increasing concentrations of the auxinic herbicides. In addition to correlating these growth and development effects with auxinic herbicide–induced physiological changes in corn, we compared the effects of clopyralid and dicamba on proton efflux from isolated protoplasts of the three varieties. Results of these biophysical studies are consistent with those from our growth and developmental studies and confirm that clopyralid is least effective in eliciting a response in corn when compared with dicamba. We conclude that clopyralid does not cause the deleterious effects seen with other auxinic herbicides when sprayed under optimal environmental conditions, i.e., high humidity and temperature.

Nomenclature: Clopyralid; 2,4-D; dicamba; fluroxypyr; picloram; corn, Zea mays L. ‘Pioneer 36B08’, ‘Pioneer 3730’, ‘Pioneer 3559’.