Genotype-specific nitrogen requirements overpower seeding density effects for irrigated fiber hemp in a high-elevation semi-arid climate

Industrial fiber hemp ( Cannabis sativa L.) is a resurging crop in the United States due to the passage of the Agriculture Improvement Act of 2014 and 2018. The interest in this crop stems from its ability to provide environmentally friendly raw materials for multiple industrial applications. This study evaluated the influence of cultivar, seeding rate, and nitrogen (N) rate on fiber hemp stem biomass, bast, and hurd fiber yields grown under irrigation. Treatments were two fiber hemp cultivars ('Altair' and 'Hliana'), three seeding rates (25, 35, and 45 kg pure live seed ha −1 ), and two N rates (80 and 160 kg N ha −1 ) in a 2 × 3 × 2 factorial in a randomized complete block design experiment with four replicates of each treatment combination. The proportion of bast fiber ranged from 27.6% to 32.3% and differed between cultivars, but not among seeding rates or between N rates. Stem biomass yield ranged from 3613 to 5152 kg ha −1 and was altered by the two-way interaction of cultivar × N rate ( P = 0.004). At the 160 kg N rate, Altair (5140 kg ha −1 ) produced a 28.5% greater stem yield than Hliana (3999 kg ha −1 ), but the cultivars produced similar yields at the 80 kg N ha −1 rate. Fiber hemp seeded at 45 kg ha −1 tended to have greater stem yield than when seeded at 25 kg ha −1 at the 160 kg N rate. The two-year average bast fiber yield was altered by a cultivar × N rate effect ( P = 0.019), and Altair produced a 31.3% greater bast fiber yield at the 160 kg N rate (1428 kg ha −1 ) than the 80 kg N rate (1088 kg ha −1 ). Based on the observed cultivar × nitrogen interaction, this study challenges the 'low-input' narrative often associated with industrial hemp. Our results demonstrate that in high-elevation, semi-arid environments, maximizing fiber yield depends on high-nitrogen luxury consumption (160 kg N ha⁻¹) in specific genotypes rather than increased plant populations.