Response of Nebraska Horseweed (Conyza canadensis) Populations to Dicamba

Abstract

Dicamba-resistant soybeans are being developed to provide an additional herbicide mechanism-of-action for postemergence weed control in soybean. Numerous broadleaf species, including horseweed, have evolved resistance to glyphosate. It is anticipated that dicamba will be used by farmers as a primary tool to manage these weeds. Studying and understanding variability in horseweed response to dicamba will aid in developing appropriate risk management strategies to extend the utility of the dicamba-resistance technology. Horseweed plants from ten Nebraska populations were treated with one of nine doses of dicamba in greenhouse experiments. At 28 days after treatment (DAT) visual injury estimations were made and plants were harvested to determine dry weight. There was a three-fold difference in the I90 (90% visual injury estimate) between the least (638 g ha-1) and most (205 g ha-1) susceptible populations. Two plants from five populations were observed for an additional three months. No plants treated at doses above 280 g ha-1 survived to set seeds. These results suggest that maintaining use doses of 560 g ha-1 or greater may fully control horseweed populations from Nebraska and minimize the risk of plants surviving to set seed, in addition to practicing other proven herbicide-resistance management strategies.