A review of the biology linking fibre diameter with fleece weight, liveweight, and reproduction in Merino sheep

Abstract

There is increasing economic pressure on the Merino industry to grow finer wool and at the same time to turn off more sheep meat. Fleece weight generally decreases with finer fibre diameters, so theoretically, finer wool sheep should be able to partition more nutrients to reproduction and muscle growth. However, complex and often indirect mechanisms appear to prevent this in practice. This review summarises the available data on genetic and phenotypic relationships between wool fibre diameter and fleece weight, liveweight, fatness, and reproductive performance, and develops mechanistic hypotheses for these relationships that could be tested experimentally.

Selection for reduced fibre diameter may decrease liveweight, fatness, and reproductive performance. These characteristics are also affected by the availability of nutrients, and finer fibre diameter may affect nutrient metabolism through 2 adaptations that tend to maintain fleece weight: an increase in follicle density and/or an increase in relative fibre length. These act in different ways. Follicle density may affect final body size through linkages established during fetal development. Increased fibre growth rate requires an increase in protein synthesis rate in the skin, which in turn may affect whole-body protein turnover rate and the sensitivity of tissues to insulin. Other pathways potentially affected by fibre diameter include the control of feed intake, prenatal programming of insulin sensitivity, and a reduced responsiveness of tissues to nutrient supply. However, none of these mechanisms accounts for the reports of an increased proportion of dry ewes in fine wool sheep, particularly if run in environments that experience periods of low quality pasture.

Although associations between fibre diameter and growth rate, fatness, and reproduction rate are repeatable, there are not yet enough data for such associations to be realistically accounted for in breeding goals. More experimentation and a better knowledge of the causative relationships will facilitate development of finer wool sheep for environments where sheep turnoff normally provides a significant proportion of income from the sheep enterprise. Such mechanistic understanding will facilitate the development of molecular techniques for targetted selection, and the identification of more useful attributes for a quantitative genetics approach to improving simultaneously the profitability from both wool and meat.