Hemp stalks contain two main types of fibre: bast or long fibres found in the bark (skin) and hurd (shive) or short fibres located in the core of the stem. (Figure i)
Figure i - Bast (skin or bark) and Hurd (core) fibres in a hemp stem
Depending on variety (or usage type) and planting density, bast fibres typically account for 20-30 % of the stalk.
Figure ii - Bast fibre content in stems of selected European cultivars
There are two types of bast fibres:
- Primary bast fibres - make up 70-90% of the bast. They are characterized by long length (up to 50 mm), high cellulose (50-70%) and low lignin (about 7%) content. These fibres are the most valuable part of the stalk.
- Secondary bast fibres - comprise the remaining 10-30% of the bast fibres. They are shorter (approximately 2 mm), more lignified and hence of lower value for some applications (i.e. textile). Low stand density of hemp crop favours development of this type of bast fibre.
Hurd is the short fibre (about 0.5 mm) found in the inner woody core of hemp stalks. Hurd accounts for 70-80% of the stalk and typically contains 20-30% lignin. For millennia, hemp was grown for bast fibre, primarily for textile applications, while hurd was considered a waste by-product of bast production.
In addition to genotypic differences, bast fibre content in the stems can be modified by seeding density. Higher bast fibre content is found in heavy seeded fields (40-60 kg/ha) that form a dense canopy of slender, unbranched plants(Figure iii), producing high yields of superior quality bast fibre.
Figure iii - Fibre type hemp cv. Silesia seeded at rate of 40 kg/ha
Fibre type varieties seeded at a low rate (i.e. 20 kg/ha) frequently generate tall, extensively branching plants(Figure iv)with thick stems, often exceeding one inch in diameter at the base(Figure v). Such plants are characterized by low ratio of bast to hurd fibre(Figure vi)and low quality of bast fibre (high proportion of short bast), both in the mainstem and in the multiple secondary branches.
Figure iv Fibre type cv. Silesia seeded at rate 15 kg/ha
Figure v – Stems of cv. Silesia seeded at 50 kg/ha (left) and 15 kg/ha (right)
Figure vi – Comparison of bast to hurd ratio in stems of hemp seeded at high
Owed to their different physical properties and chemical composition, bast and hurd fibres are suitable for a wide range of industrial applications including bio-composites, construction materials, textiles, insulation, bedding, paper production, ropes and twines and many others.
Until recently, major breeding efforts of the fibre usage type varieties were focused on increasing bast fibre content of the stem, mainly for textile industries. Revival of hemp production during the last decade resulted in developing applications for hurd as well, with hempcrete currently being one of the chief materials driving hemp for fibre expansion.