Influence of dietary intake and genetic capacity for wool growth on the composition of mid-trunk skin of Merino sheep

Abstract

Some nitrogenous constituents in the skin were examined in sheep from two flocks with different genetic capacities for wool growth and in sheep consuming different diets. In the first comparison, in which the sheep consumed a maintenance (1 x M) ration, the average water, lipid and nitrogen contents of skin were similar for the two flocks. The collagen content of skin from sheep with low wool growth (Fl-) was greater than that in skin of high producers (I?]+)-0.40 v. 0.36 g g-1 dry defatted skin (P < 0.05). The F1+ sheep also maintained lower concentrations of non-peptide cystine in plasma (51 v. 68 8mol L-l: P < 0.05) and in skin (1.1 v. 1.9 8mol g-l dry defatted skin: P < 0.05). In a second experiment, sheep bred from the two flocks (Fl+ and F1-), consumed rations providing either 0.8 x M or 2 x M (the latter diet containing 12.5% formaldehyde treated casein). Trephine-sampled skin from the high producing sheep and from the sheep with greater intake had lower concentrations of collagen per unit mass of skin (P < 0.05), but similar collagen per unit area of skin. Both diet and flock influenced the weight of skin sampled by trephine. The distribution ratios of non-peptide a-amino nitrogen and of cystine between skin and plasma were significantly greater in the F1+ derived sheep: 4.35 v. 3.1 L plasma kg-1 skin for cystine. In a third comparison, skin was sampled from sheep of an unselected flock (n = 50) after each received one of four dietary treatments ranging from 0.7 x M to 2.6 x M (ad lib.) for 60 days. As the dietary allowance increased, the weight of skin per unit area of skin increased, the quantities of total protein and of saponifiable lipid per unit mass of dry skin remained constant, but the content of hydroxyproline significantly decreased from 0.20 to 0.13 mmol g-l dry skin (P < 0.05). The quantities of non-collagenous protein solubilized by first sodium chloride (0.5 mol L-l) and then tartaric acid (0.1 mol L-l) per unit mass of collagen increased significantly with increased intake of feed.