Increasing dietary protein supply for wool growth

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Black (1988) estimated that the ATP required by the follicles of a 40 kg sheep producing 20 g day-1 of clean wool represents only about

7% of the energy required for basal metabolism. However, 20 g of protein in the wool together with the sloughed root sheath cells is equivalent to about 150% of the maintenance requirement for protein (Black, 1988). Therefore, the level of absorbed amino acids has a major influence on the substrates available for protein synthesis and wool growth rate. Numerous experiments have demonstrated the effectiveness of dietary protein supply on increasing wool growth rate and this has been well reviewed for sheep (Black, 1988; Williams, 1995; Hynd, 2000) and for goats (Galbraith, 1998, 2000).

Feed proteins vary in their amino acid compositions and are also characterized by different rates of degradation in the rumen. This leads to considerable variation in the profiles of absorbed amino acids. Generally, feed protein containing a high level of sulphur-containing amino acids that is less degradable in the rumen would result in increased wool production. For example, canola (rapeseed) meal and lupin seed both contain similar and high levels of crude protein, but canola meal is less degraded in the rumen (AFRC Technical Committee on Responses to Nutrients, 1993) and contains more Met than lupin protein (20 vs. 4-10 g kg-1 protein; Hill, 1991). Merino lambs fed a diet containing canola meal grew 7-64% more wool than sheep fed a lupin seed diet (Masters et al., 1998; White et al., 2000), and the response depends on the level of intake and the proportion of canola meal in the diet. Similar positive responses to canola meal diets were observed in pregnant or lactat-ing Merino ewes (increased wool growth of 50-63%; Masters et al., 1996) and in adult dry ewes (12-20%; Masters and Mata, 1996). Reis and Colebrook (1972) compared casein, wheat gluten, Promine-D and zein for wool production and ranked their relative values as

100, 54, 40 and -11. However, fibre growth of cashmere goats fed a fishmeal diet was not improved compared to those fed a urea diet at similar N intakes (about 15 g CP kg-1; Galbraith, 2000). The fibre growth rate in cashmere goats was low, less than 40% of the mohair yield from Angora goats (Galbraith, 2000). The lack of response of fibre growth to dietary protein level was probably due to the nutrient demand for the maximum fibre growth already being met by the urea diet.

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