| Summary: | This thesis reports on an investigation of the association between muscularity and meat quality in Scottish Blackface (SBF) and Texel (TEX) lambs, and the in vivo assessments of these traits using X-ray computed tomography (CT) with a view to their possible inclusion in breeding programmes. The objectives of this work were: (i) to develop comprehensive in vivo assessments of muscularity using spiral CT scans; (ii) examine the relationship of the new muscularity indices with carcass and eating quality; (iii) explore the associations among CT assessments of carcass composition, muscularity and muscle density, and (iv) investigate the possibility of limiting the antagonism between selection for reduced fatness and maintaining eating quality by introducing a CT predictor of intramuscular fat (IMF) as an additional selection criterion for the breeding programmes using CT. The calculation of muscularity indices requires the measurement of the muscle mass and skeletal dimension of the regions of interest. Priority was given to the hind leg (HL) and lumbar region (LR), where high priced cuts are located. The utilisation of new novel imaging technology called spiral CT scanning, which captures detailed information on any specific region, was explored. An algorithm to automatically segment the spiral CT scans (SCTS), and procedures to assess the real dimensions of skeletal structures, were developed. Compared to previous CT m uscularity measurements, the accuracy was much higher with the new index in the HL (correlations with equivalent indices bases on dissection of 0.89 vs 0.51). The accurate measurement of femur length by CT used in the calculation of the new HL index made an important contribution to the higher accuracy of the index. The improvement in accuracy was smaller for the LR (0.55 vs 0.44). The association of CT muscularity indices and carcass traits by dissection in both breeds showed that improved muscularity is not phenotypically correlated with detrimental effects on other carcass quality traits. The correlation coefficients, after adjusting for carcass weight, were positive with meat yield and low and negative, or close to zero, with fatness. This is particularly relevant for the terminal sire breeds, in which the economically important traits included in breeding programmes tend to be carcass composition traits. In the case of SBF, the CT muscularity indices provide an opportunity to improve carcass conformation, a trait included in current breeding objectives. Differences in muscularity of the HL and LR, assessed by the CT muscularity indices, and eating quality were investigated between sexes (ram vs ewe lambs), breeds (SBF vs TEX) and progeny of high- and low-muscularity sires (HM, LM). TEX lambs had 16% greater muscularity than SBF in both regions, whilst differences between sire groups were 4%. Ewe lambs had slightly higher values of muscularity for the F1L than rams but no difference was found for the LR. Meat from SBF lambs was more tender, and had stronger lamb flavour and higher overall liking scores than TEX meat. Sex had a weak influence with ram lambs having a stronger abnormal flavour and lower overall liking in the LR only. No significant differences in meat eating quality were found between HM- and LM-sired lambs, suggesting that improved muscularity would not have unfavourable effects on sensory traits. Genetic parameters for the CT muscularity indices, predictions of carcass muscle and fat weights and CT muscle density were estimated. The estimates of heritabilities of the CT muscularity indices showed they were at least moderately heritable (from 0.38 to 0.92) in both breeds. CT muscle density, measured in the LR, had a moderate to high heritability in both breeds, and strong negative genetic and phenotypic associations with IMF and carcass fat weight. Little association was found between muscularity indices and CT muscle density, implying that improved muscularity would not have a negative effect on CT muscle density. These results suggest, overall, that the effect of selection for improved muscularity of sheep is likely to be favourable for carcass quality and neutral with respect to meat eating quality. Because of the strong phenotypic and genetic associations with IMF, CT muscle density may be a promising selection tool to counteract possible negative effects of decreasing fatness on IMF and therefore eating quality. The inclusion of CT muscle density as a selection criterion allowed more favourable genetic responses in IMF, without further unfavourable increases in carcass fat weight or detrimental effects on leanness. Because increased economic values for IMF led to different expected gains in IMF and other traits in the breeding goal, the definition of the specific values depends on the desired gains in all traits by the industry. Positive returns for the industry from using CT muscle density at the second stage of selection can be obtained for all economic values included in this simulation. The economic benefits were maximised when the proportions of ram lambs CT scanned were 0.15.
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