Nitrogen fertilization and row width affect self-thinning and productivity of fibre hemp ( Cannabis sativa L.)

Abstract

In fibre hemp ( Cannabis sativa L.) a high plant density is desirable, but inter-plant competition may cause self-thinning, which reduces stem yield and quality. We investigated whether agronomic factors could reduce self-thinning in hemp. The effects of soil nitrogen level (80 and 200 kg ha −1), row width (12.5, 25 and 50 cm), type of sowing implement, and thinning method on self-thinning, growth, yield and quality of hemp were determined in field experiments in 1991 and 1992. Soil nitrogen level affected plant morphology before self-thinning occurred. Due to enhanced competition for light more plants died from self-thinning at 200 than at 80 kg N ha −1. In August, stem yield of living plants was similar at the two nitrogen levels, but 5% of the plants had died at 80 kg N ha −1 and 25% at 200 kg N ha −1. Although dry matter losses resulting from self-thinning were greater at 200 than at 80 kg N ha −1, crop growth rate was greater at 200 than at 80 kg N ha −1. Apparently, the crop growth at 80 kg N ha −1 was affected by a lack of nitrogen. At final harvest in September stem yield of living plants was 10.4 t ha −1 at 80 and 11.3 t ha −1 at 200 kg N ha −1, bark content in the stem was 35.6% at 80 and 34.0% at 200 kg N ha −1. The effect of row width on self-thinning was small relative to that of nitrogen level. More self-thinning took place at 50 cm row width than at 12.5 and 25 cm. During early growth and also in August stem yield was smaller when row width was larger; in September row width did not affect stem yield or quality. Type of sowing implement and thinning method did not affect self-thinning or stem yield.