Meeting amino acid requirements through dietary manipulation is usually not cost effective because of the high prices of protein feeds and of supplements of Met or Cys. Genetic selection of sheep or goats for a high wool growth rate appears to be the most effective way to improve feed efficiency and amino acid utilization.
Williams et al. (1972b) compared the wool growth rate and Cys metabolism in two Merino lines selected for high or low fleece weight (F+ vs. F-) and showed that the F+ sheep grew 16% more wool with relatively lower sulphur concentration (2.99% vs. 3.48%) than the F- sheep. At the same intake, the entry rate of Cys in the F+ sheep was lower, but the recovery rate of [35S]Cys in wool was higher (0.45 vs. 0.38) than in the F- sheep, indicating a relatively high efficiency of utilization of Cys for wool growth. In addition, when supplemented with Cys or Met, the increase in wool growth rate was markedly higher in the F+ sheep than the F— sheep (57% vs. 8%; Williams, 1976; Williams et al., 1972a,b). In another experiment, sheep selected for high clean fleece weights grew more wool than the low fleece weight sheep (0.069 vs. 0.045 g 100 cm 2 day"1) at the same plane of nutrition, and the wool growth rates were closely related to their skin fractional protein synthesis rate (18.2 vs. 14.4% day"1) and total protein synthesis in the skin (0.36 vs. 0.29 g 100 cm"2 day"1; Masters et al., 2000). The difference in wool growth was due to a higher proportion of active follicles and/or a higher efficiency of follicles (wool growth rate / follicle density). Similar trends were found in fibre-producing goats, where Angora goats grew 2.25 times more fibre than Cashmere goats over a period of 112 days, and also had a substantially greater response in fibre growth to a supplementation of rumen protected Met (62% vs. 30%; Souri et al., 1998a). Angora rabbits produce 1.2-1.4 kg clean wool per annum at their mature weight of 3.5-4 kg at a net efficiency of 0.43 (Liu et al., 1992), double that in Merino sheep. There is no doubt that genetic differences and genetic merit at a given level of nutrition and responses to varying nutrition are the major factors that determine utilization efficiency of amino acids for wool.
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