Evaluating the vegetative and reproductive response of hemp (Cannabis sativa) to simulated off-target events of 2,4-D and dicamba

Introducing soybean cultivars resistant to 2,4-D and dicamba allowed for postemergence applications of these herbicides. These herbicides pose a high risk for off-target movement, and the potential influence on crops such as hemp is unknown. Two studies were conducted from 2020 through 2022 in controlled environments to evaluate hemp response to rates simulating off-target events of 2,4-D and dicamba. The objectives of this study were to: 1) determine the effects of herbicide (2,4-D and dicamba) and rate (1x to 1/100,000x labeled rate) on visible injury, height, and branching, and 2) determine the effect of 2,4-D rate (1x to 1/100,000x labeled rate) on visible injury, height, branching, and reproductive parameters. Herbicides were applied in the early vegetative stage, and evaluations took place 14 and 28 days after treatment (DAT) and at trial termination (42 DAT in the greenhouse trial and at harvest in the growth chamber trial). In the greenhouse trial, 2,4-D and dicamba at the 1x rate and the 1/10x rate caused 68, 78, and 20% injury 28 DAT, respectively. At the time of trial termination 42 DAT, plants treated with 1x rates of 2,4-D and dicamba or 1/10x dicamba were 19, 25, and 9 cm shorter than the nontreated control, respectively. At trial termination, simulated off-target rates of 2,4-D and dicamba did not influence branching or plant weight. In the growth chamber study, the 1x and 1/10x rates of 2,4-D caused 82% and 2% injury 28 DAT, respectively. Plant height, fresh weight, and cannabidiol (CBD) levels of plants treated with simulated off-target rates of 2,4-D were not different from the nontreated control. These studies suggest that hemp grown for CBD exposed to off-target rates of 2,4-D or dicamba in early vegetative stages may not have distinguishable effects 42 DAT or at harvest.