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The wool fibre consists almost entirely protein and wool keratin. To grow in one year, a fleece containing 3 kg protein the sheep would need to deposit a daily average of about 8 g protein or 1.3 g nitrogen.
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If this latter figure is compared with the 6.6 g nitrogen which a sheep of 50 kg might lose daily as endogenous nitrogen, it seems that in proportion to its requirement for maintenance, the sheep’s nitrogen requirement for wool growth is small.
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These figure however do not tell the whole story, since the efficiency with which absorbed amino acids are used for wool synthesis is likely to be much less than that with which they are used for maintenance.
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Keratin is characterised by its high content of the sulphur-containing amino acid, cystine, which although not an indispensable amino acid is synthesised from another indispensable amino acid, methionine.
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The efficiency with which food protein can be converted into wool is therefore likely to depend on their respective proportions of cystine and methionine.
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Keratin contains 100–200 g/kg of these acids, compared with the 20– 30 g/kg found in plant protein and in microbial proteins synthesised in the rumen and so the biological value of food protein for wool growth is likely to be not greater than 0.3.
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Wool growth reflects the general level of nutrition of the sheep. At sub-maintenance levels, when the sheep is losing weight, its wool continuous to grow, although slowly.
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As the plane of nutrition improves and the sheep gains in weight, so wool growth too increases.
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There appears to be a maximum rate of growth for wool, varying from sheep to sheep with a range as great as 5 to 40 g/day.
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The dependence of wool growth rate on the plane of nutrition (i.e. energy intake) of the sheep is due in part to the association between energy intake and the synthesis of microbial protein.
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The real determinant of wool growth rate is likely to be the quality of protein digested and absorbed in the small intestine of the sheep and it has been shown, for example, that a Merino must absorb 120 – 150 g protein/day to achieve its maximum rate of wool growth.
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In a ewe with a metabolisible energy intake of 12 MJ/day (i.e. twice its maintenance requirement), only 101 g microbial protein would be synthesised per day and only 101 x 0.8 x 0.85 = 69 g would be absorbed as amino acids.
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To achieve maximum growth of wool the sheep is therefore dependent on a good source of under-graded food protein.
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In practice this is likely to be supplied by the consumption of large quantities of protein in pasture herbage.
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This may be relatively highly degradable but can still supply much undegradable protein. For example, a ewe might consume 250 g protein/day, of which 0.3 (75 g) would be undegraded and 0.85 x 75 = 64 g would be absorbed in the small intestine.
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Nevertheless, wool growth in sheep is considerably increased by protein supplements protected from rumen degradation such as casein.
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As would be anticipated, the most effective of such supplements are those rich in the sulphur-containing amino acids.
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Wool quality is influenced by the nutrition of the sheep. High levels of nutrition increase the diameter of the fibres and it is significant that the finer wools come from the nutritionally less favourable areas of land.
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Periods of starvation may cause an abrupt reduction in wool growth; this leaves a week point in each fibre and is responsible for the fault in fleeces with the self-explanatory name of ‘break’.
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An early sign of copper deficiency in sheep is a loss of ‘crimp’ or waviness in wool; this is accompanied by a general deterioration in quality, the wool losing its elasticity and its affinity for dyes.
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