Abstract
The Australian lamb industry uses Australian Sheep Breeding Values to select for reduced subcutaneous fat depth (PFAT) and increased lean meat yield. Selection for reduced PFAT increases oin muscle weight, with muscularity associated with reduced muscle aerobicity. As isocitrate dehydrogenase activity (ICDH) is a good indicator of oxidative metabolism, we hypothesised that selection for reduced PFAT would decrease ICDH. ICDH was measured within the loin muscle of 13971ambs and data was analysed using a linear mixed effects model (SAS) with fixed effects for site, kill group within site, sex, birth type-rear type, age of dam, sire type and dam breed within sire type, and random terms for sire and dam. Within this model, covariates such as PFAT, intramuscular fat percentage (IMF) and weight of short loin fat and muscle were included individually to assess their phenotypic association with ICDH. Aligning with our hypothesis, selection for reduced PFAT decreased ICDH by 0.52 µmol/min/g tissue over the 4 unit PFAT range. However, neither short loin muscle nor fat weight demonstrated strong associations with ICDH. This contrasts with the premise of our initial hypothesis that selection for negative PFAT would decrease ICDH via its impact on whole body muscularity and the associated effect on muscle aerobicity. Alternatively, ICDH was strongly associated with IMF, with a 4% decrease in IMF aligning with a 0.84 µmol/min/g tissue reduction in ICDH. Selection for negative PFAT strongly reduces IMF, and when both covariates were used concurrently within the ICDH model, PFAT was not significant. This may imply that the impact of PFAT on ICDH is delivered via its negative impact on IMF, and appears to be independent of whole body adiposity or muscularity.