CONSTRAINTS TO DRY-MATTER PRODUCTION IN FIBER HEMP (CANNABIS-SATIVA L)

Fibre hemp (Cannabis sativa L.) may be an alternative to wood as a raw material for pulp and paper production. Research was done to assess the potential productivity of fibre hemp and to identify constraints to that productivity. Growth analyses were done on hemp crops in three consecutive years, using several cultivars and seeding rates. In 1987 the crops suffered severely from fungal diseases; stem dry matter yields were 9.4 t ha(-1). In 1988 and 1989 fungicides were applied and stem dry matter yields were respectively 11.9 t ha(-1) and 13.6 t ha(-1). The number of living plants m(-2) ranged from 86 to 823 at emergence, depending on treatment, and from 38 to 102 at final harvest. Increased seeding rates led to earlier canopy closure and higher initial biomass production, but more plants died during the growing season and the stem yield at final harvest was not affected by seeding rate. Average radiation-use efficiency (RUE; above-ground accumulated dry matter divided by intercepted photosynthetically active radiation) for the entire growing season under favourable growing conditions was 1.9 g MJ(-1), which is low compared to other C-3 crops. Leaf photosynthesis rates at saturating light intensities, however, were high: 30 kg CO(2)ha(-1) h(-1). The light extinction coefficient of hemp canopies was high (0.96), and may have reduced canopy photosynthesis rate and RUE. The high lignin content of the stems, and the high fat and protein content in the seed, also reduced RUE. Furthermore, dry matter invested in the tap root and in shed dead leaves was ignored. The RUE was also underestimated because an appreciable proportion of the biomass of plants that died during the growing season could not be collected. More research is needed to optimize cultural practices in fibre hemp.