Pre-Emergent Herbicide Rate Effects on Phytotoxicity and Weed Suppression in Industrial Hemp.

Kentucky has been experimenting with industrial hemp since 2014.  The major cause for crop failure has been high weed pressure.  In May of 2016, a study was initiated to evaluate the safety of pre-emergent herbicides of varying rates on industrial hemp.  Four pre-emergent herbicides were utilized with three rates.  A weedy and weed-free check were added to the treatments and the experimental design was a randomized complete block with four replications.  The study was established in two locations: Bowling Green and Lexington, KY.  Herbicides and rates were as follows:  pendimethalin (0.64, 0.96, and 1.28 kg/ha), metolachlor (1.06, 1.39, and 1.78 kg/ha), fomesafen (0.13, 0.26, and 0.34 kg/ha), and clomazone (0.21, 0.55, and 0.84 kg/ha).  ‘Finola’ was planted in the Bowling Green location where as ‘Felina’ was planted in Lexington at 22.4 kg/ha.  Herbicides were applied 1 day after planting.  Phytotoxicity was rated on a scale from 0 (no injury) to 100 (complete control of industrial hemp) three weeks after germination.  Clomazone resulted in the highest crop injury and was significantly different from other herbicide treatments and the control.  Pendimethalin resulted in the least amount of phytotoxicity and was similar to the control.  Metolachlor and fomesafen were less injurious at lower rates.  Higher rates of these herbicides became more injurious to the crop warranting further investigation.  In conclusion, pendimethalin may be a suitable pre-emergent herbicide for industrial hemp in the future.   Clomazone, most likely, is not a viable option.