From breeding to practise: Effects of sowing date and pruning system on flower yield and cannabinoid composition in four Slovenian feminized industrial hemp breeding lines

Four Slovenian feminized industrial hemp (Cannabis sativa L.) breeding lines (G8c, G10c, MERc, CHc), developed within the high‑CBD breeding program at the Biotechnical Faculty, University of Ljubljana, were evaluated in 2024 field trials at Ljubljana and Krško. The effects of sowing date (direct field seed sowing vs. seedlings transplantation to the field, both at density 8,333 plants per ha) and apical pruning (topping) on growth (plant height), flower yield, CBD, and Δ⁹ THC content were assessed. Early- season waterlogging in Ljubljana (heavy soil) caused severe negative effects on plant establishment and growth. Sowing date acted oppositely across sites: direct sowing outperformed transplanting in Krško (medium heavy soil), whereas transplanting increased flower yield in Ljubljana. Pruning reduced plant height but did not change flower yield or cannabinoid concentrations. CBD was primarily genotype dependent, with MERc the highest at both sites (5.02-6.66 %). THC ranged 0.12–0.21 % across treatments. Significant breeding line × sowing date interactions occurred for plant height and flower yield. This study provides the first applied evaluation of Slovenian feminized industrial hemp breeding lines and indicates priorities for subsequent breeding efforts. However, the results represent one year of field data .To draw robust conclusions, multi-year field trials will be required