Effect of divergent selection for yearling growth rate on protein metabolism in hind-limb muscle and whole body of Angus cattle

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The impact of divergent selection for daily gain from birth to 1 year of age on protein metabolism in muscle and whole body was evaluated in 12-month-old Angus steers from the Trangie (NSW, Australia) selection experiment [Parnell, P.F., Herd, R.M., Perry, D., Bootle, B., 1994. The Trangie experiment-responses in growth rate, size, maternal ability, reproductive performance, carcass composition, feed requirements and herd profitability. Proc. Aust. Soc. Anim. Prod., 20: 17–20.]. The steers used from these lines differ in yearling weight by 40%. Eight steers from each of the high and low yearling growth rate lines were studied. Four steers from each line were fed to maintain weight and four fed at 1.6 times maintenance. In the whole body there were no significant between-line differences in flux of phenylalanine (Phe, a gross indicator of whole body protein synthesis). Mean values were, respectively, 0.99±0.10 and 1.07±0.11 μmol Phe/min/kg liveweight for high and low growth-lines. However, in hind-limb muscle, cattle selected for high growth rate had lower rates of protein synthesis and degradation than those selected for low growth rate. Low growth-line cattle were more sensitive to nutrition than high growth-line cattle, increasing their hind-limb muscle protein synthesis and oxygen consumption in response to increased feed intake to a greater extent than high growth-line steers (protein synthesis 215, 205, 424 and 264 nmol Phe/min/kg hind limb; oxygen uptake 110, 110, 141, 103 μmol O 2/min/kg hind limb for high growth steers fed 1.6 and 1 times maintenance and low growth steers fed 1.6 and 1 times maintenance, respectively). These results are consistent with more efficient use of feed for liveweight gain in high growth-line steers. The results suggest that low growth-line steers have inherently higher rates of protein synthesis in muscle than do high growth-line steers, and that this may contribute in part to higher oxygen consumption in muscle, particularly at higher levels of feed intake. Based on the increment in hind-limb oxygen uptake in low line cattle with increased feed intake, it can be estimated that approximately 70% of the between-line difference in feed efficiency occurs in muscle.