Skin and Muscle protein metabolism in Merinos differing EBVs for wool growth

In the Australian sheep industry, Estimated Breeding Values (EBVs ) are being increasingly used to select Merino sheep for excellence in traits such as high clean fleece weight (CFW), low fibre diameter (FD) and high yearling live weight (YLW). It has been proposed that genetic differences in CFW may be related to skin protein metabolism and that it is sensitive to the level of nutrition (Williams and Morley 1994; Liu et al. 1998). The underlying physiological responses to EBV and plane of nutrition are not well understood. We selected 20 wethers from a commercial flock (18 month-old weighing about 33.3 kg) with similar EBV for FD and YLW, but 10 wethers had high EBV for CFW (F+), and 10 had low EBV for CFW (F-). We used a 2 x 2 experiment (F+ and F- wethers at two levels of intake, i.e. 0.8 and 1.8 x maintenance of commercial pellets) and determined CFW and protein synthesis rates (FSR) in their skin and muscle pools using the flooding dose technique (Rocha et al. 1993, i.e. i/v injection of deuterium-labelled [L-ring-D⁵] phenylalanine). The commercial pellets contained 13.3% crude protein, 20% crude fibre, 9.5 MJ/kg ME.