Khama Kelman

The Australian lamb industry uses breeding values to select for progeny with increased post-weaning weight at 150 days (PWWT). Accurate weight prediction is essential to provide age estimates for lambs to reach target weights, to underpin breeding values, and for assessing the influence of growth rate on factors such as intramuscular fat and myoglobin concentration of lamb muscle. As growth curves can be biased when predicting weights at the edge of the available weight data, the key aim was to develop a population based random regression model to predict lamb PWWT. This fit was compared to an individual based Brody curve fit with comparable results confirming the rigour of the model. The PWWT results were then used to assess the impact of factors such as lamb birth-type rear-type and sire PWWT breeding value on lamb weight. Multiple births were hypothesised to limit the progeny of high PWWT sires from reaching their full weight due to nutritional restriction pre-weaning. Weight data totalling 164,797 observations was collected from 17,525 lambs across eight sites and five years of the Sheep Cooperative Research Centre Information Nucleus Flock. A Bayesian linear mixed model was fitted to the live weight data with fixed effects for site, year of birth, gender, birth type-rear type, age of dam, sire type, darn breed within sire type, sire PWWT as a covariate and random terms for sire, dam by drop and individual. Lamb PWWT was then analysed in a linear mixed model of similar structure. In line with our hypothesis, the weight of singles, twins and triplets at 150 days increased by 9.43, 6.67 and 3.68 kg across the 23 kg PWWT range (P<0.05) confirming that multiple births limit the full expression of weight potential.

Increased growth rate and carcass lean meat yield % are key profit drivers for the lamb industry, however redistributing lean tissue to more highly priced parts of the carcase will also increase its value. Faster growing lambs are known to be leaner and less mature at slaughter. Therefore we hypothesised that selection for growth using the Australian Sheep Breeding Value (ASBV) for greater post weaning weight (PWWT) would increase whole carcase lean weight, when animals are compared at the same carcase weight. Lamb carcases (n=1,218) from the Sheep CRC Information Nucleus were scanned in sections (fore, saddle, and hind) using Computed Tomography (CT) to determine fat lean and bone weights. Data was analysed using the log-linearised allometric equation logy = log a + b.logx. Fixed effects were site-year, sex sire type, birth-type rear-type and kill group within site-year, with random terms sire and dam by year. For the same carcass weight PWWT caused no composition differences, except in female lambs which had 3.3% more carcase lean (P<0.01) across the 25 unis PWWT range. Alternatively for the same fat, lean or bone weight these tissues were all proportionately heavier in the saddle region of the high PWWT lambs by 3%, 7%, and 16% across the PWWT range. Aligning with our hypothesis, PWWT was associated with increased total carcase lean, although only in females. Unexpectedly, PWWT caused a redistribution of carcass tissues to the saddle region, particularly for bone and lean, implying an altered conformation in these high growth lambs. Conflicting with the premise of our hypothesis, these effects appear to be independent of maturity as there was no whole body increase in bone weight. Furthermore loin muscle myoglobin concentration in the high PWWT lambs was increased (by 0.03±0.018 mg/g tissue) rather than decreased as would be expected in a less mature animal, In conclusion, PWWT redistributes carcase weight to the saddle region of lambs.

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.