DIFFERENTIAL RESPONSE OF HEMP DOGBANE (APOCYNUM-CANNABINUM) TO CLOPYRALID, DOWCO-433, AND 2,4-D

Field studies were conducted to compare the short- and long-term control of hemp dogbane by POST applications of Dowco 433, clopyralid, and 2,4-D amine. Dowco 433 at 140 g ae ha-1 controlled 81 to 93% of weeds at 8 wk after treatment. Good control (79 to 89%) was also achieved with 560 g ae ha-1 2,4-D amine at 8 wk after treatment- Twelve months after treatment, control of hemp dogbane averaged 65 to 75% where Dowco 433 was applied at rates of 140 g ha-1 or higher. Similar results were obtained with 2,4-D amine at 560 g ha-1. Control with clopyralid was minimal. Weed height and dry weight were reduced with all treatments except clopyralid. The effect of early versus late application was also evaluated in two studies. In certain situations, control 8 wk after treatment was greater when application was made during the early reproductive growth stage as opposed to the vegetative growth stage.