Evolved resistance to herbicides necessitates alternative weed control strategies. Allelopathic crops show promise as an alternative to exogenous herbicides and could be used to reduce establishment, growth, and reproduction of weeds. Individual cannabinoids and terpenes found in hemp (Cannabis sativa L.) show allelopathic potential, but allelopathic effects of plant residue have not been characterized. A laboratory assay found that crude, acetone-soluble hemp extracts diluted to 2.5 mg mL−1 reduced the germination of a bioindicator species. However, tested concentrations below 2.5 mg mL−1 were not more effective than the no-extract control treatment at reducing germination. A greenhouse study found that soil-incorporated hemp residue was not more effective than a maize (Zea mays L.) residue treatment comparison in reducing plant growth. However, a simulated chaff line experiment in the greenhouse showed that the equivalent of 378 g m−2 hemp residue on the soil surface can effectively reduce and delay the germination of waterhemp [Amaranthus tuberculatus (Moq.) Sauer] in comparison to bare-soil, or a maize residue treatment comparison. Together, these results show that incorporated hemp residue will likely not be an effective weed control practice. However, chaff lining hemp residue may be an effective practice for the control of certain weeds and warrants further research in a field setting.
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 [Amaranthus 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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