Whole-body protein metabolism and energy expenditure in sheep selected for divergent wool production when fed above or below maintenance

Abstract

Rates of whole-body protein turnover and energy expenditure were measured in two groups of wethers differing in estimated breeding values (EBVs) for wool growth, but with similar EBVs for fibre diameter and liveweight (LW). The sheep were offered a pelleted diet at 1.2 times their metabolisable energy (ME) requirement for maintenance (1.2 M) followed by either 0.8 M or 1.8 M for 5 weeks. In the 5th week, whole-body protein metabolism was estimated by using intravenous injection of ¹⁵N-glycine (g N/day) and whole-body energy expenditure rate (EE) was predicted by the CO₂ entry rate technique using intravenous injection of NaH¹³CO₃. The higher N intake (8.7 v. 20.4 g N/day, P < 0.001) was associated with a higher whole-body protein flux (22.1 v. 34.2 g N/day, P < 0.001), and a higher whole-body protein synthesis rate (17.0 v. 25.5 g N/day, P < 0.001) and protein degradation rate (15.9 v. 20.7 g N/day, P < 0.001). Irrespective of feeding levels, sheep with high-fleece EBVs (F+) synthesised and degraded more body protein N (g N/day) than sheep with low-fleece EBVs (F–), and F+ sheep also retained more ingested protein N (P < 0.05) in wool and body tissue than F– sheep, but the significant differences due to genotypes disappeared when whole-body protein flux, synthesis and degradation were expressed as g N/kg LW^0.75.day (metabolic weight). Estimates of EE were lower when the sheep were offered 0.8 M than when offered 1.8 M (5.85 v. 7.68 MJ/day, P < 0.001) and were higher in F+ than in F– sheep (6.95 v. 6.58 MJ/day; P < 0.05), but F+ sheep had a significantly lower (P < 0.05) EE (MJ/kg LW^0.75.day) than F– sheep. F+ animals also retained more energy in wool and wool-free body tissue than F– animals (P < 0.05). The present study indicates that genetic selection for wool growth has resulted in increased efficiency of dietary protein and energy use for wool production and body-tissue growth in these sheep. Furthermore, there is no ‘trade off’ between deposition of nutrients in the body and wool in sheep with high EBVs for wool growth.