This study was conducted to investigate relationships between mitochondrial respiratory chain complex activities, feed efficiency, and carcass traits in sheep. A group of Ghezel male lambs sired by a single ram were randomly allotted to individual pens. The lambs were fed ad libitum with a fattening diet containing 30% roughage (corn silage and alfalfa hay) and 70% concentrate for 70 d to individually phenotype each lamb for feed conversion ratio (FCR), adjusted FCR (aFCR), and residual feed intake (RFI). The lambs were then humanely killed and the liver, abdominal fat, pelvic fat, cardiac fat, warm carcass weight, and cold carcass weight, as well as the cross-sectional area of the LM and the fat depth over the 12th rib, were determined. A portion of LM was obtained to determine mitochondrial protein and respiratory chain complex activities (complexes I to V). Statistical analysis was carried out based on lambs exhibiting high and low RFI (n = 8), FCR (n = 8), or aFCR (n = 8) phenotypes. The lambs exhibiting the high-RFI phenotype consumed 110 g more feed daily (P < 0.05) than did the phenotype exhibiting low RFI, with no difference in ADG. Conversely, there was no difference in feed intake between the low- or high-FCR groups, but sheep exhibiting the lowFCR phenotype gained 70 g more (P < 0.05) per day compared with those exhibiting the high-FCR phenotype. It was determined that all 5 respiratory chain complex activities were greater (P < 0.05) in sheep exhibiting the low-RFI phenotype compared with those exhibiting the high-RFI phenotype, with significant (P < 0.001) negative correlation coefficients between RFI and respiratory chain complex activity. When efficiency was assessed using FCR, only activities of respiratory chain complexes III, IV, and V were less (P < 0.05) in the low-FCR phenotype compared with the highFCR phenotype, and there were no differences (P > 0.1) in respiratory chain complex activities between groups when FCR was adjusted for metabolic BW
