An investigation into the differential response of hemp dogbane (Apocynum cannabinum L.) to two pyridine herbicides

Field and laboratory experiments were conducted to study the differential effects of fluroxypyr and clopyralid herbicides on hemp dogbane (Apocynum cannabinum L.). In field experiments, fluroxypyr provided excellent control of hemp dogbane while the response to clopyralid was very poor. Good season long control of hemp dogbane was obtained with postemergence application of fluroxypyr at rates as low as 140 g ai/ha. In certain situations, control four and eight weeks after application was superior when application was made during the early reproductive growth stage as opposed to the vegetative growth stage. Twelve months after treatment, fluroxypyr provided 65 to 75 percent control of regrowth. Clopyralid treatments at this time were not significantly different than controls.;Laboratory studies revealed that both herbicides are readily absorbed and translocated in hemp dogbane. Absorption of fluroxypyr occurred faster and to a greater extent than clopyralid. Of that absorbed, more clopyralid than fluroxypyr was translocated from the treated leaf and apical translocation of clopyralid was greater than fluroxypyr. Both herbicides exhibited similar patterns of basipetal movement. Basipetal translocation was slightly greater when herbicides were applied during the early reproductive growth stage relative to the vegetative growth stage. More fluroxypyr than clopyralid was metabolized. However, neither compound was rapidly metabolized and with both herbicides, nearly 50% or more of recovered...