Correlated responses in physiological traits to divergent selection on components of efficient lean growth identified physiological traits for use as predictors of genetic merit and provided a biological explanation for the between-selection line differences in protein and lipid deposition. Responses (differences between high (H) and low (L) selection lines) in protein metabolism during ad-libitum feeding were associated with divergent selection for daily food intake (DFI) (reduced serum creatinine concentration (H = 1·30 v. L = 1·56, s.e.d. 0·08 mg/dl)) and for lean growth rate on an ad-libitum feeding regime (LGA) (increased serum urea (H = 48 v. L = 36, s.e.d. 4 mg/dl) and creatinine (H = 1·74 v. L = 1·45 mg/dl)) concentrations, but not with selection for lean growth on a restricted feeding regime (LGS). Following 24-h fasting, responses in lipid metabolism, in the form of higher serum non-esterified fatty acid concentrations, were detected with divergent selection for both LGA (399 v. 248, s.e.d. 66 µmol/l) and LGS (H = 361 v. L = 107 µmol/l). The high LGS line appeared to ‘preserve’ protein to a greater extent than the high LGA line and similarly there was greater maintenance of lipid depots by the low LGS line compared with the low LGA line. A tentative ranking of the two pairs of lean growth selection lines on the basis of ‘importance’ of protein deposition would be high LGS, high LGA, low LGA and low LGS.
Coheritabilities for serum creatinine concentration with predicted lysine balance and lysine required for protein deposition (-0·17 and 0·17, s.e. 0·08) indicated that serum creatinine concentration may usefully be included in breeding value prediction for lysine requirement and protein deposition to increase the accuracy of predicted genetic merit.
Fasting did not increase the coheritabilities for serum creatinine concentration, so inclusion of serum creatinine concentration in a selection criterion for dietary lysine requirement or protein deposition does not require withdrawal of food before blood sampling animals.