Crop physiology of fibre hemp (Cannabis sativa L.)

Fibre hemp ( Cannabis sativa L.) may be an alternative to wood as a raw material for the production of paper pulp. The effects of enviromnental factors and cultural measures on the functioning, yield and quality of fibre hemp crops in the Netherlands were investigated.Until flowering (generally in August), the radiation use efficiency (RUE, above-ground dry matter accumulated per unit of photosynthetically active radiation intercepted) of hemp was 2.2 g MJ -1, after flowering it dropped to 1.1 g MJ -1, This reduction was mainly caused by senescence, resulting in a lower rate of canopy photosynthesis. When flowering was prevented by artificial daylength extension the RUE remained high throughout September, and stem dry matter yield was increased by 2.7 t ha -1. Breeding late-flowering hemp seems a promising stategy to improve the stem yield potential of hemp.Hemp grows at low temperatures, its base temperature is 1 °C for leaf appearance, and 2.5 °C for canopy establishment. Thermal time is a simple and accurate tool to describe leaf appearance and light interception in field-grown fibre hemp.Hemp is grown at high plant densities to improve stem quality and to increase the allocation of above-ground dry matter to the stem. However, in dense hemp crops self-thinning (densityinduced mortality) may occur, causing losses of dry matter. It was established that in fibre hemp optimum plant density is approximated by the highest density possible without inducing self-thinning. During self-thinning an increase in biomass is accompanied by a reduction in plant density, consequently, in fibre hemp optimum plant density is a function of yield.N fertilization affected self-thinning: at similar above-ground dry matter, a higher plant density was sustained at 80 than at 200 kg N ha -1. The reduced mortality at the low nitrogen level resulted from a shift from mainly competition for light, which enhances differences in plant size, to competition for nutrients, which does not give large plants a disproportionate advantage over small plants. Variability of both weight and height of hemp plants was higher at 200 than at 80 kg ha -1N, and more suppressed plants were present at 200 than at 80 kg ha -1N. Sexual dimorphism contributed to variability of height and weight, but the effects were smaller than those of nitrogen fertilization.It is concluded that fibre hemp may be an interesting 'new' crop for arable farmers in the Netherlands: hemp may be profitable, its stems potentially have a large non-food market, the crop requires little or no biocide and suppresses weeds and some major soil-borne diseases.