Growth, and fibre yield and quality of three industrial Hemp (Cannabis Sativa L.) cultivars and influenced by plant population density and planting date

Abstract

Hemp (Cannabis sativa L.) is a non-intoxicating plant with various industrial applications and is gaining interest in South Africa due to the lifting of restrictions on cultivation. However, there is a lack of recommended cultivars and appropriate management practices, such as planting dates, planting densities, irrigation and fertiliser applications, which pose significant challenges to hemp fibre productivity and quality. This lack of recommended practices may increase farmers' production costs and negatively affect the overall sustainability of hemp cultivation for high-fibre output. Therefore, the study's objective was to assess the effect of planting date and plant population density on fibre yield and quality of the three industrial hemp cultivars. Two field experiments were conducted during the 2024/2025 cropping season in Limpopo (Towoomba Research Station) and Gauteng (Agricultural Research Council, Roodeplaat), respectively. Experiment I was laid out in a split plot design, with planting dates (November 2024 (PD1) and December 2024 (PD2)) as main plots, and a factorial combination of three cultivars and two planting densities (33 (D1) and 66 plants/m² (D2)) as sub-plots, replicated three times. Experiment II, sown in February 2025, consisted of a factorial combination of two planting densities (33 and 66 plants/m2) and three cultivars (ARCCAN1 (C1), ARCCAN3 (C2), and C3) in a randomised complete block design with three replications. In Experiment I, data were collected at the final harvest from 10 plants. However, in Experiment II, each plot had three tagged plants for observation and data were collected 45 days after planting and every fortnight until final harvest. Plant height was measured with a 5 m ruler from the base to the apical bud. The stem diameter was measured with a Vernier calliper, and the internode length was measured with a ruler. All developing branches from the stem were counted to assess the number of side branches. Plant biomass, fibre yield, and quality were assessed at harvest maturity from the plants that were harvested from an area of 0.1 m2. Biomass samples were oven-dried for 48 hours at 65°C, after which the dry mass was measured. The hemp fibre and hurd were separated by decorticating wet stalks. The hurd was collected separately and dried in the oven at 65°C for 24 hours. Across both experimental sites, plant population density, planting date, and cultivar significantly influenced growth traits, biomass production, and fibre yield and quality. At Towoomba Research Station, planting date strongly influenced growth, biomass, and fibre traits, with the November planting (PD1) outperforming the December planting (PD2). Compared with PD2, PD1 produced higher stalk biomass (9.96 vs. 8.36 t ha⁻¹), hurd biomass (4.52 vs. 3.75 t ha⁻¹), bast fibre yield (2.13 vs. 1.67 t ha⁻¹), and fibre content (11.51 vs. 8.60%), which was associated with greater plant height, stem diameter, and internode length. Cultivar and plant population density combinations showed clear differentiation, with cultivar C3 producing taller plants and greater total aboveground, stalk, and hurd biomass, particularly at D1, where C3 at D1 recorded the highest total above ground biomass, stalk biomass, and hurd biomass (28.95, 8.16, and 7.27 t ha⁻¹, respectively). In contrast, bast fibre yield and fibre content were higher in cultivars C1 and C2, with the highest bast fibre yield in C1 at D1 (2.33 t ha⁻¹) and fibre content in C1 at D2 (14.56%), while cultivar C3 consistently recorded the lowest fibre content. The biochemical composition followed a similar trend, with higher hemicellulose and cellulose contents under PD1, and higher lignin content under PD2, suggesting that PD1 and cultivar C3 favoured biomass accumulation, whereas C1 and C2 favoured fibre quality. In Roodeplaat, plant population density and cultivar affected growth traits, biomass accumulation, and fibre yield and quality. Plants grown at lower density (D1) were consistently taller, producing 15.4%, 19.4%, and 17% greater height than those at higher density (D2) at 45, 59, and 73 days after planting, respectively. Stem diameter and side branch number were also greater under D1, confirming that reduced plant density promotes structural growth through lower intra-specific competition. Cultivar C3 consistently outperformed C1 and C2 in plant height, stem diameter, and branching, indicating superior vegetative vigour. Chlorophyll content differed significantly among cultivars at 45 and 59 days after planting, with C3 recording higher values than C1 and C2. Stalk biomass, total above-ground biomass, and bast fibre yield increased with higher plant population density, and C3 produced greater stalk and hurd biomass than C1 and C2. In contrast, fibre content in C3 (5.37%) was significantly lower than in C2 (9.20%) and statistically comparable to that in C1 (7.17%), highlighting a consistent trade-off between biomass production and fibre concentration across environments.