Genetic parameters of ultrasound and computer tomography scan traits in Austrian meat sheep

Lamb meat is the main product in the Austrian sheep sector. Performance testing for growth and carcass traits with ultrasound (US) or computer tomography (CT) scanning has been obligatory for herd book sheep of meat focused breeds for some years. So far the selection of breeding stock is only based on the phenotypic performance of the individual, but a routine genetic evaluation system is planned for the future. Therefore this study aimed to estimate genetic parameters for performance traits and to compare both methods for performance testing. Heritabilities and genetic correlations were estimated for live weight (LW) and average daily gain (ADG), for US measured eye muscle (longissimus dorsi) depth (EMD) and back fat depth (FD), for three muscling scores and for six CT measured frame and carcass traits in 13,634 Austrian meat sheep. An across-breed analysis was carried out using performance records of Merinolandschaf, Suffolk, Texel, German Blackheaded Meatsheep and Jura sheep which were routinely tested for meat performance between 2000 and 2010. Genetic parameters were estimated with bivariate mixed animal models including both direct and maternal genetic effects and permanent environmental effects of the dam as well as fixed effects. Estimated direct heritabilities were low for the muscling scores (0.05-0.11), low to moderate for the growth traits LW (0.07) and ADG (0.16), and moderate for US scan traits EMD (0.28) and FD (0.29). CT traits were more heritable for fat (0.36 and 0.40), but not for muscle (0.24). Positive genetic correlations (0.46-0.84) were found between US and CT traits and thus it can be concluded that US is an effective alternative to CT as it has been used to date in Austria. The results further showed that correcting for live weight leads to negative genetic correlations of frame with carcass and growth traits. Large-frame animals tend to grow faster but have relatively less muscle and fat on them, because (at constant live weight) their growth is not as complete yet. The results of this study provide valuable information for progressing the selection system for Austrian meat sheep from phenotypic selection to genetic evaluation.