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In fibre hemp (Cannabis sativa L.) grown for the production of high‒quality textile yarns the presence of secondary fibres is unwanted. These fibres are too short for spinning and their presence hampers the production of fine and homogeneous yarns from the primary or long fibres. Primary fibres are present along the stem from bottom to top and hemp for fibres is traditionally harvested around the time of flowering, when the cell walls of these fibres are sufficiently thickened with cellulose to be extracted. In literature indications are found that the height up to which secondary fibres are present, moves upwards along the stem during the growing season, and that this process accelerates around flowering. To optimise the length of the stem part with primary fibres, but without secondary fibres, the background of secondary fibre development should be elucidated. It can be hypothesised that either flowering or the increasing plant size accelerates the formation of secondary fibres. To investigate this, an indoor experiment was conducted in greenhouses with mobile covers in which the day–length sensitivity of hemp was used to create size ranges of flowering and non–flowering plants for a single cultivar, Futura 75. Secondary fibre formation was recorded using microscopic techniques. The height up to which secondary fibres were present, depended on plant weight. The higher secondary fibre front in flowering plants was most likely caused by the higher weight of these plants as compared with non–flowering plants of the same height. As seed carrying inflorescences contribute to plant weight, dual use of fibre hemp for seed and high–quality textile fibres is not an option. Results from a field experiment confirmed the correlation between plant size and the height of the secondary fibre front. Therefore, to optimise the length of the stem part with primary fibres, but without secondary fibres above stubble height, for Futura 75 a relatively short crop of around 1.3–1.4 m should be harvested before flowering. This ideal crop height is likely to differ between varieties.
Westerhuis et al. (Fri,) studied this question.