Correlated physiological responses to selection forcarcass lean content in sheep

Correlated responses in physiological traits in lines of Texel-Oxford sheep selected for high or lowcarcass lean content were examined. Serum samples were taken at the end of performance test, 20 weeks of age, from 66 rams when fed ad libitum and every 24 h when fasted for 56 h. The lean line had higher serum concentrations of β-hydroxybutyrate (BHB), non-esterified fatty acids (NEFA) and insulin-like growth factor- 1 (IGF-1) with lower concentrations of triglyceride (TRIG), creatinine (CREA) and UREA, but only the differences in UREA were statistically significant. There were substantial changes in serum concentrations of physiological traits in response to fasting as glucose and IGF-1 decreased while BHB, NEFA, TRIG, CREA and UREA increased. The correlated responses suggested that the lean line preferentially synthesises protein rather than deposits fat during normal feeding. When the animals are fasted, there may be relatively greater use of fat as an energy source in the lean line, rather than using products from protein catabolism as glucose precursors.The accuracy of selection was examined when physiological traits were incorporated into a selectionindex, which included the performance test traits: liveweight, ultrasonic backfat and muscle depths, to predict genetic merit for carcass lean content. UREA may be a useful predictor of genetic merit for lean meat production as it was correlated with estimated carcass lean content and there were substantial differences between the selection lines.