Wool is almost entirely composed of protein with very high levels of cysteine (Cys) and serine (Ser) compared with other body tissues. Growth of wool requires more protein relative to energy, and draws amino acids, in particular methionine (Met) and Cys, disproportionately from the body pool. This creates a relative imbalance of amino acids available for synthesis of other body tissues. On the other hand, the exogenous supply of amino acids from the diet and microorganisms in the rumen do not usually provide sufficient Met and Cys to support maximum wool growth rates. A combination of these factors results in a lower utilization efficiency of amino acids for wool growth compared with the efficiencies for body-mass growth and reproduction (AFRC Technical Committee on Responses to Nutrients, 1993). This chapter reviews the utilization efficiency of dietary protein and amino acids for wool growth and describes the partitioning of amino acids to the skin and wool. Specific attention is given to Met catabolism, its relations to wool growth, and the effects of supplementation on wool growth and protein metabolism in general. Some possibilities to improve the utilization efficiency of amino acids are also discussed.
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