Nitrogen fertilization and sex expression affect size variability of fibre hemp (Cannabis sativa L.).

Mechanical harvesting and industrial processing of fibre hemp (Cannabis sativa L.) require uniformity of stem length and weight. In 1991 and 1992 we carried out field experiments to investigate the effects of soil nitrogen level (80 and 200 kg haN) and row width (12.5, 25 and 50 cm) on the variability of weight and height in hemp plants. The crops were sampled 5 times between early June and early September. Row width did not affect size variability. At final harvest coefficients of variation (CV) of both weight and height were about 1.5 times higher at 200 than at 80 kg haN. Distributions of dry weight were positively skewed at all sampling dates except the first, with skewness larger at 200 than at 80 kg haN. Distributions of height were negatively skewed at all sampling dates except the first at 80 kg haN. At 200 kg haN they changed from negative skewness during the first part of the growing season to negative kurtosis in the second part of the growing season. More suppressed plants were present at 200 than at 80 kg haN. Contrary to most published results, we did not find a reduction of CV of weight nor of CV of height at the onset of self-thinning. Suppressed hemp plants can survive relatively well in the low-light environment under the canopy. Sexual dimorphism contributed to variability of height and weight, but the effects were smaller than those of nitrogen fertilization. The ratio of female to male plants was higher at 200 than at 80 kg haN, as a result of a shift in sex-ratio within the population of suppressed plants. As suppressed plants were much more slender than dominating plants, self-thinning eliminated the most slender individuals in a hemp crop. However, the presence of many more heavy individuals of low slenderness at 200 than at 80 kg ha N was probably the major cause of the difference in slenderness between 200 and 80 kg ha N.