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Temperament impacts skeletal muscle mitochondria in Brahman heifers, but this relationship has not been investigated in Angus steers. We hypothesized mitochondrial measures would be greater in temperamental than calm steers, in the trapezius (TRAP) than the longissimus thoracis (LT) muscle, and would increase from 13 to 18 mo of age. Samples from calm (n = 13), intermediate (n = 12), and temperamental (n = 13) Angus steers were evaluated for integrative (per mg tissue) mitochondrial oxidative phosphorylation (P) and electron transfer (E) capacities by high-resolution respirometry. Data were analyzed using linear models with repeated measures (time) and fixed effects of time, muscle, temperament, and all interactions. Leak, P with complex I (PCI), maximal coupled P (PCI+II), maximal noncoupled E (ECI+II), and E with complex II (ECII) decreased from 13- to 18-mo-old (P &amp;lt; 0.0001). Integrative PCI+II, ECI+II, and ECII were greater in the TRAP than LT (P0.005). Leak respiration was greatest in intermediate steers at 13 mo of age (P&amp;lt; 0.0001) but was unaffected by temperament at 18-mo-old. Additionally, ECI+II was greatest in temperamental steers (P0.05) and tended to be greater in intermediate than calm steers (P = 0.1) at 13-mo-old. Overall, PCI, PCI+II, and ECII were greater in temperamental than calm steers (P0.05). ECII was also greater (P = 0.03) and PCI and PCI+II tended to be greater (P0.08) in intermediate than calm steers. The contribution of leak (FCRLeak) and PCI (FCRPCI) to total ECI+II decreased (P &amp;lt; 0.0001) while the contribution of PCI+II (FCRPCI+II) and ECII (FCRECII) to E increased (P0.006) from 13- to 18-mo-old. FCRLeak and FCRPCI were greater (P0.01) and FCRPCI+II tended to be greater (P = 0.08) in the LT than the TRAP. FCRLeak was lowest in temperamental (P0.03) while FCRPCI+II was lowest in calm steers (P0.05). Temperament clearly impacts skeletal muscle mitochondrial capacities and efficiency of energy production in Angus steers, which may be related to product quality at harvest.

25 Temperament and Age Impact Skeletal Muscle Mitochondrial Capacities in Angus Steers