Skin characteristics and skin protein composition of Merinos differing in estimated breeding values for wool growth and fed at above and below maintenance

Abstract

Twenty castrated male sheep were selected from a commercial flock, 10 with high estimated breeding values (EBVs) (F+) and 10 with low EBVs for wool production (F–). Five of each EBV treatment were offered feed at 0.8M [0.8 times their metabolisable energy (ME) requirement for maintenance] and the other 5 were offered feed at 1.8M for 6 weeks. On the midside area of skin, the F+ group had a greater rate of clean wool growth (g/100cm2.day) and daily growth in fibre length (mm/day) than the F– group at both feeding levels (P<0.05). Fibre diameter (FD) was finer for F+ than F– sheep at both feeding levels (P<0.001). As the feeding level increased, FD increased (P<0.001), but the ratio of fibre length growth to mean FD remained constant. The secondary follicle density in F+ sheep decreased with an increase in the feeding level (P<0.01), whereas the secondary follicle density of F– sheep remained unchanged (P>0.05). The ratio of secondary to primary follicles (S/P) was 21% higher (P<0.01) in skin from F+ sheep than in skin from F– sheep, but this difference was not related to the feeding level. Skin thickness at the midside was thinner (P<0.05) when sheep were underfed, but F+ sheep had a thicker (P<0.05) skin than F– sheep. The trephine (diameter: 1.5 cm; area: 1.766 cm2)-sampled skin weight was heavier for F+ than for F– (0.339 v. 0.294 g, P<0.05) but did not differ (P>0.05) with the level of intake. The weight of the dermis layer was greater in F+ than F– sheep (0.231 v. 0.190 g per trephine, P<0.05) and F+ sheep also tended to be more responsive to feeding level than F– sheep (interaction: P = 0.06). True protein content per 100 g wet skin did not differ between F+ and F– sheep or between feeding levels, whereas the protein content per 100 cm2 skin was significantly (P<0.05) higher in F+ sheep. The concentration of collagen in skin protein (g/100 g) was lower in F+ sheep than F– sheep at both feeding levels (P<0.05). Of the amino acids evaluated, significant differences occurred between F+ and F– sheep for the methionine (Met) concentration in plasma (P<0.05) and in the free amino acid pool in the skin (P = 0.06). The distribution ratio of skin Met concentration to plasma Met concentration was significantly lower in the F+ sheep than F– sheep (0.77 v.1.18 kg skin/ L plasma). Results presented here indicate that skin characteristics such as skin thickness, follicle density, S/P ratio, skin weight, dermis weight, true protein or collagen content were a consequence of genetics and nutritional variation in wool growth rate, fibre length and fibre diameter. Selection for high EBVs for wool growth has resulted in animals which grow more wool at the 2 levels of feed intake provided, supported by their higher S/P ratios, higher skin and protein masses, more responsive dermides and higher efficiency of Met usage in skin tissue.