The impacts of weed interference on hemp (Cannabis sativa) yield are largely unstudied despite causing serious economic losses in most cropping systems. For high-cannabidiol (CBD) hemp, understanding the role of weed competition on CBD and tetrahydrocannabinol (THC) content may help promote profitability and regulatory compliance. Therefore, we tested the effects of varying waterhemp Amarantbus tuberculatus (zero, one, three, and five waterhemp plants per planting hole) and hemp (zero or one hemp plants per planting hole) planting densities on total hemp yield, chemical composition, and aboveground waterhemp biomass in plasticulture. There was no significant total biomass or stripped floral biomass yield loss resulting from waterhemp competition, although unexpectedly high variation in hemp phenotypes likely limited the ability to detect subtle differences between treatments. Furthermore, there was no significant effect of competition treatment on total CBD, total THC, or measured terpene composition. However, waterhemp biomass was reduced significantly by competition from hemp in comparison with hemp-free treatments. Suppression of waterhemp by hemp and lack of significant yield loss suggest that hemp can be highly competitive and grown successfully without herbicides in certain circumstances.
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