GENETIC AND PHENOTYPIC RELATIONSHIPS BETWEEN PHYSIOLOGICAL TRAITS ANDPERFORMANCE-TEST TRAITS IN SHEEP

Genetic and phenotypic relationships between physiological traits and performance test traits were estimated with data from lines of Texel-O xford sheep divergently selected for carcass lean content, for the ide ntification of physiological predictors of genetic merit for carcass l ean content. At the end of the performance test, blood samples were ta ken from 151 animals, the progeny of 31 sires, when fed and when faste d for 30 and 54 h. Heritability estimates for traits associated with p rotein metabolism, creatinine, urea and insulin-like growth factor-1 ( IGF-1), were higher with ad-libitum feeding than heritabilities of tra its associated with lipid and energy metabolism, beta-hydroxybutyrate, non-esterified fatty acids, triglyceride and glucose. Several physiol ogical traits, beta-hydroxybutyrate, creatinine and insulin-like growt h factor-1 (IGF-1), were moderately correlated with the performance te st traits of liveweight and muscle depth, but only urea and insulin-li ke growth factor-1 were significantly correlated with backfat depth. C orrelations between physiological traits and predicted carcass composi tion were estimated, as animals were required for breeding or were all ocated to another experiment. Based on phenotypic correlations, beta-h ydroxybutyrate and IGF-1 may be useful indicators of merit for predict ed carcass lean weight, with urea being an indicator of predicted carc ass lean content. Measurement of physiological traits when animals wer e fasted may not be required for the prediction of genetic merit using physiological traits, as genetic correlations between feeding and fas ting were high in absolute value for all physiological traits, except glucose.