Khama Kelman

Genetic selection to increase lamb growth is advantageous in reducing the time it takes for lambs to reach market weight. However the effects of rapid growth on muscle fibre type, as indicated by oxidative capacity, is not clear. Muscle fibre type is intrinsically linked with attributes of meat quality, which are becoming increasing important to consumers. Therefore, changes to muscle fibre type through selection for growth may impact meat quality. Producers can impact lamb growth at any time during the growth period including the weaning (100 days) and post weaning time points (150 days). As not all lambs were weighed at these time points a growth curve was fitted to the live weight data of each individual animal to allow weight to be estimated at any time point during the growth period. Once estimates of weight and growth rate at the weaning and post weaning time points were obtained, the impact of growth on muscle fibre type and meat quality traits could be investigated. In the first experiment growth curves were fitted to live weight data for 18,185 lambs with weight data between birth and 300 days. Linear (cubic polynomial), nonlinear (Brody) and random effects models were fitted and the characteristics of the residual values examined to determine the best curve fit. The random effects model was hypothesised to have the lowest residual values as it is based on a population, rather than an individual fit, and is less biased when fitting growth curves to weight recordings from animals with different durations of data collection. However, the cubic polynomial fit had the lowest average residual values and the Brody fit had the highest. The difference between the residual values for the fit of each function was relatively small therefore the decision on which function to use was based on a variety of additional factors. As it is a population based fit, a random effects model accommodates missing weight recordings and different durations of data collection, which are common with industry data recording, without biasing the fit. A random effects model also allows environmental effects specific to the time of recording to be accounted for and can accommodate genetic differences in the shape of each animal's growth curve. Based on these characteristics the estimates of lamb weights and growth rates from the random effects model were used in this thesis. In the second experiment the impact of production factors on lamb weight at key time points in production were analysed. While several studies have published information on these effects the studies were generally small. Therefore, this analysis acted as a benchmarking for the magnitude of the effects, based on large data. The full expression of the post weaning weight breeding value is depressed with poor nutrition so an analysis was performed to determine the extent to which other forms of nutritional restriction, including lamb and dam factors such as birth type, dam breed and dam age, moderate the expression of the growth breeding value. As expected, factors such as multiple births, which act as a source of nutritional restriction, reduced the full expression of the growth breeding value and hindered lambs reaching expected weights. Muscle oxidative capacity is intrinsically linked to lamb growth and meat quality so in the third experiment the influence of genetic selection for growth on muscle oxidative capacity was examined. Oxidative capacity was indicated by the activity of the oxidative enzyme isocitrate dehydrogenase as well as the concentration of myoglobin in the muscle. As expected, genetic selection for growth reduced isocitrate dehydrogenase activity and myoglobin concentration. The reduction in oxidative capacity with increased growth was due to lambs tending toward a larger matures size and therefore being physiologically less mature at a given weight. The impact of lamb growth on oxidative capacity and meat quality has previously been estimated by the impact of the sire breeding value for growth on these traits or by using hot carcass weight as a proxy for whole of life growth. However the accuracy of these measures of growth is untested. The lamb weights and growth rates derived in the second experiment were used to determine the real impact of growth on oxidative capacity and meat quality. The impact of genetic selection on growth, or the use of hot carcass weight as a proxy for the impact of growth on meat traits, were both shown to be inadequate in quantifying the magnitude of the effect of growth on meat quality traits and indicating how the effect of growth varied with time. In the fourth experiment the impact of lamb growth on intramuscular fat, shear force and consumer overall liking was examined. Intramuscular fat was increased and shear force was reduced with lamb growth. Intramuscular fat and consumer overall liking have a positive association however contrary to expectations growth had no impact on overall liking. This is likely to be due to influences on overall liking other than intramuscular fat. In the final experiment the impact of lamb growth in isocitrate dehydrogenase activity, myoglobin concentration, iron and zinc concentration and meat browning were examined. As hypothesised, myoglobin, iron and zinc concentrations increased with lamb growth. Contrary to expectations isocitrate dehydrogenase activity and meat browning were reduced with lamb growth. Meat browning was thought to reduce due to its strong association with oxidative capacity and thus the oxidative indicator enzyme inverse isocitrate dehydrogenase. These experiments have contributed to the sheep industry at a highly practical and adoptable level showing why the full potential of genetic selection for growth may not be realised and demonstrating the impact of growth on meat eating quality. Currently, increased growth in lambs, particularly in the early stages between birth and weaning has positive effects on meat quality however lambs had few weights recorded between these times so work needs to be done to characterise these associations.