Energy efficiency in industrial hemp stalk production depending on cultivar and flower harvest

Industrial hemp ( Cannabis sativa L.) is increasingly recognized as a promising energy crop; however, its energy balance depends on cultivar selection and the adopted production technology. This study quantified energy inputs, energy output and energy-efficiency indicators for the production of stalks and vegetative biomass as affected by cultivar, year/growing season and flower harvest. A field experiment was conducted in north-eastern Poland on a lessive soil using seven cultivars (Białobrzeskie, Henola, Santhica 27, Santhica 70, Earlina 8 fc, Kompolti, Finola), grown with or without manual flower harvesting at flowering. Total energy input (diesel fuel, machinery, materials and labour) averaged approximately 13.54–13.84 GJ ha⁻¹ and was dominated by material inputs (mainly mineral fertilisers; 58.3%), followed by diesel fuel (30.8%). Cultivar significantly affected yield and all energy indicators, whereas flower harvest had no significant effect on any parameter. Year/growing season significantly differentiated most of the assessed parameters. The highest fresh stalk yield was obtained for Kompolti (26.75 Mg ha⁻¹); however, the best overall energy output was achieved by Białobrzeskie, which produced 131.93 GJ ha⁻¹ from stalks and 151.36 GJ ha⁻¹ from total vegetative biomass (excluding seeds), with corresponding energy ratios of 9.23 and 10.31. Including threshing residues (vegetative biomass) increased energy output and improved energy gain and energy ratio compared with the stalk-only assessment. Interannual variability was pronounced: in 2024, energy output and energy ratio were higher than in 2025, highlighting the importance of weather conditions nested within the growing season and cultivar choice for optimising energy systems based on industrial hemp.