Methods of incorporating market signals and measures of carcass shape into genetic selection indices for terminal sire breeds of sheep

The price obtained for lamb carcass has a direct effect on the profitability of a sheep system and hence market requirements should be directly reflected in the objective of any breeding program for sheep. Despite their importance, market requirements are oflen assumed rather than known and market prices are not always used to form the objective. Moreover, although carcass shape affects commercial value, it has not been directly considered in current breeding programs. The aims of this study were (i) to develop methods of incorporating measures of carcass shape into selection programs (ii) to establish the requirements of the current and likely future markets for lamb and (iii) to develop methods for calculating economic values for selection objectives. Carcass shape is quantified commercially using a conformation score. However, the assessment is subjective and confounded with fatness and hence an objective assessment of muscularity is undoubtedly preferable. Five measures of muscularity (three for the M. longissimus thoracis et lumborum (LTL) muscle, one for the hind leg and one for the whole carcass) were derived from carcass dissection data for 100 Suffolk, 40 Texel and 20 Charollais lambs. Changes with growth, relationships with tissue composition and lean distribution, as well as the relationships between the different muscularity measures were investigated. Higher muscularity at a given live weight was generally associated with higher lean to bone ratio and carcass lean content. Associations with fat content were either non-significant or negative. Relationships with lean distribution were also non-significant. Correlations between muscularity measures in different regions were low in Suffolk and Charollais lambs but higher for the Texels. This suggested that more than one measure would be required, particularly for Suffolk and Charollais lambs, if the muscularity of a carcass was to be described effectively. The feasibility of developing live animal measurements of muscularity using measurements taken on X-ray Computer Tomography (CT) scans was investigated using data collected on the lambs described above. Measures for the LTL and the whole carcass were obtained using measurements of components similar to those used to derive the carcass measures. For the hind leg, indirect measures were developed using the ratio of two measurements taken on scans through the hind leg. Correlations between the carcass and corresponding CT measure ranged from around 0.6 for the hind leg and the whole carcass measure to around 0.35 for the LTL measures. The results indicated that the CT measures of muscularity derived provided good in vivo predictors of muscularity in the hind leg and through the whole carcass, but were comparatively less useful for predicting carcass LTL muscularity. Genetic parameters for each of the CT measures of muscularity were estimated using data for approximately 950 lambs from each of the three breeds. Heritability estimates for each of the CT measures of muscularity were moderate to high (0.210.57) and coefficients of variation were in the range of 5-10%. Genetic correlations with lean weight, fat weight, scan live weight and ultrasound measured fat depth were generally not significantly different from zero in each breed. Genetic correlations with ultrasound measured muscle depth were positive. Given these