An experiment was conducted to test 3 hypotheses regarding the variation in plasma parameters and their association with lamb growth and composition. The experiment tested whether sire genetics and available nutrition affected specific plasma traits, whether these traits were associated with growth and compositional characteristics, and whether data on plasma traits could improve the prediction of growth and carcass composition. Lambs (n = 103) produced by sires differing in estimated breeding values for growth, muscling, and fat were reared on LOW or HIGH planes of lifetime nutrition. Blood samples were collected immediately prior to weaning and before slaughter at 8 months of age. Concentrations of plasma creatinine (CRE), urea (PUN), insulin like growth factor I (IGF-I) were measured in all samples. Leptin was measured in samples from male lambs only (n = 56). HIGH nutrition increased preweaning and preslaughter IGF-I and leptin concentrations; nutritional effects on PUN and CRE were inconsistent. Lambs of the high muscle sire-type exhibited increased plasma IGF-I, reduced PUN, and an increased PUN : CRE ratio preweaning. Increased genetic potential of the sire for growth reduced PUN and increased PUN : CRE ratio. Plasma IGF-I correlated strongly with lamb growth rate (r² = 0.49). Interaction of nutrition and sire-type for preslaughter leptin mimicked the interaction occurring for carcass fat, providing an example of selection on genotype inducing a physiological (hormonal) change and resulting in a modified phenotype.

Stepwise regressions developed to predict carcass protein and fat mass from information available at weaning or at slaughter indicated that inclusion of plasma parameters in these models caused only small increases in the proportion of variance accounted for. The most significant benefit was an increase in the proportion of variance in carcass weight accounted for (from 65 to 73%) by including IGF-I and CRE as predictors compared with prediction from weaning weight alone. If slaughter data were available, then >90% of the variance in carcass fat and protein masses was explained by carcass weight alone and this was not increased by inclusion of any preslaughter plasma parameter in the model. It was concluded that while nutrition and the genetic capacity of the sire (especially for muscling) affect plasma constituents as well as lamb growth and carcass composition, the inclusion of the measured plasma traits in models to predict final carcass composition from weaning data or slaughter data is unlikely to improve the prediction sufficient to justify the additional measurement cost.