Variations in the strength of wool fibres - A review

Abstract

This review outlines the factors that may influence the strength of wool fibres and the associated changes in structure and protein composition that have been observed in weakened fibres. The strength of a wool staple is dependent on the intrinsic strength of the fibres that it contains and the total cross-sectional area of fibre being tested. The minimum fibre diameter and the rate of change of diameter along a staple are important determinants of strength. Different sheep kept under similar conditions show a large range of staple strengths. Estimates of heritability for staple strength are sufficiently high (0.17 to 0.49 in Merinos; 0.20 to 0.58 in Romneys) to prompt the establishment of selection programmes in both breeds. A variety of physiological and environmental factors influence the strength of wool fibres. Nutrient supply exerts a major influence via effects on fibre diameter. In addition, there are specific effects of some amino acids (methionine and lysine), trace elements (copper and zinc) and vitamins (folic acid). Seasonal effects are important in breeds which exhibit a large annual rhythm of wool growth, e.g. Romneys, but not in Merinos. Pregnancy and lactation influence fibre strength through competition for essential nutrients but hormonal factors may also be involved. Fibre strength may also be influenced by stress involving excessive secretion of glucocorticoids and by various parasites and diseases which can influence nutrient supply and cause stress. No clear association has been established between the strength of wool fibres and the proportions of the constituent proteins. The content of high-tyrosine proteins in the matrix of weak fibres is frequently, but not invariably, reduced. Likewise, fibre strength has been associated with the proportions of components of the high-sulfur proteins in some studies, but not in others. Thus in Romneys, but not Merinos, tender (weak) wool contained a higher proportion of orthocortex than sound wool, and hence contained less ultra-high-sulfur proteins. Weak fibres produced by specific nutritional treatments in adult sheep and lambs show a loss of cuticle scale pattern and malformed or degraded fibres.