Herbicide Effects on Industrial Hemp (Cannabis sativa) Biomass and Weed Suppression.

Field studies were established in 2015 at Bowling Green and Lexington, KY to evaluate industrial hemp (Cannabis sativa) tolerance to various herbicides. Hemp was planted into conventionally tilled soils in mid to late June at a seeding rate of 39 kg/ha in Bowling Green and 22 kg/ha in Lexington. Five herbicide active ingredients were applied preemergence (PRE) the day of planting and six postemergence (POST) treatments were applied to 30 cm hemp with a CO₂-backpack sprayer delivering 140.31 liters/hectare. Plots were 3.05 m wide by 6.1 m long and were sprayed with a 2.13 m boom sprayer leaving a 0.46 m visual check on either side of the sprayed area. A weed free check and a non-treated control were included and all treatments were replicated four times in a randomized complete block design.

                Hemp phytotoxicity was evaluated at 14 days after treatment for both PREs and POSTs. Hemp above-ground biomass and weed above-ground biomass were also evaluated. PRE herbicides did not injure hemp as much as POST herbicides, especially at the Bowling Green location. Mesotrione was the most injurious PRE evaluated (> 90%) while bromoxynil and MSMA applications resulted in low phytotoxicity (< 15%). Above-ground biomass was higher in the PREs with the exceptions of bromoxynil and MSMA. Weed above-ground biomass was higher in the POSTs with the exception of mesotrione. Results identified possible herbicides to include in a future IPM weed control program for industrial hemp.