The efficiency of utilization of dietary protein for ruminant livestock can be quantified using factorial analysis (Agricultural Research Council, 1984). The net efficiency for wool production is defined as the ratio of wool protein increment to the absorbed amino acids partitioned to wool growth. Absorbed amino acids for wool growth are calculated by subtracting the maintenance and weight gain requirement from the total absorption in non-reproductive animals (Standing Committee on Agriculture, 1990). Alternatively, the gross efficiency, defined as wool protein over the total absorbed amino acids, may be used.
The gross efficiency of dietary protein for wool growth is very low. Hogan et al. (1979) calculated an efficiency of 0.10-0.15 from a few experiments with adult Merino sheep. As a result, 0.13 is accepted as the average gross efficiency for wool growth (Standing Committee on Agriculture, 1990). Considering clean wool contains about 11% moisture (Adams et al., 2000), the efficiency for the wool protein ranges from 0.09-0.13, a value confirmed by an experimental estimate of 0.12 in mature Merinos (Mata and Masters, 1999).
The net efficiency of dietary protein for wool growth in Merino sheep has been estimated to be 0.20-0.25 (Standing Committee on Agriculture, 1990), substantially lower than the efficiencies of 0.59, 0.85 and 0.68 for weight gain, pregnancy and lactation (AFRC Technical Committee on Responses to Nutrients, 1993). Where the net efficiency has been measured, an increase in wool growth is usually coupled with a gain in body weight (Black et al., 1973; Masters et al., 1998; Mata et al., 1999). During weight loss, body protein could be partially used for wool growth, so both the net and gross efficiencies will be high as wool growth continues at the expense of body protein.
Protein efficiency for fibre production varies substantially between fibre producing species such as sheep, goats and rabbits (Table 17.2). There are major differences in fibre yield relative to body weight. Merino sheep grow 6-13 g day-1 clean wool and the annual wool production is 0.06-0.08 of live weight (40-65 kg, Hogan et al., 1979). Fibre growth rate in Angora goats is 15-18 g day-1 and the annual yield is about 0.18 of live weight (about 33 kg, Sahlu et al., 1999). The total wool production of Angora rabbits per annum varies between 1.2 and 1.4 kg or about 0.3-0.4 of live weight of 3.5-4 kg (Liu et al, 1992). The metabolic and regulatory mechanisms which determine such significant differences in capacity of the skin to grow wool fibre between species have not been defined.
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