Genetic parameters for carcass composition and muscularity in sheep measured by X-ray computer tomography, ultrasound and dissection

Live weight and ultrasound data were available for 18,747, 50,673 and 49,595 Charollais, Texel and Suffolk lambs, respectively, that were collected as part of sire referencing schemes (SRS) for each breed in the UK. Of these lambs, 922, 941 and 944, respectively, were scanned using X-ray computer tomography (CT). CT predictions of carcass lean and fat weights were derived along with six in vivo measures of muscularity. Heritability estimates for the in vivo measures of muscularity were in the region of 0.30-0.60. Estimates for the lean and fat weight predictions were around 0.40. Correlations between three CT muscularity measures (a loin, hind leg and whole carcass measure), three corresponding dissection muscularity measures, lean and fat weight predictions, live weight and ultrasound measures of muscle (UMD) and fat depths (UFD) were estimated for each breed. Correlations between the corresponding dissection and CT measures of muscularity were positive (0.41-0.58) for each breed. Correlations between the muscularity measures (CT and dissection) and the lean weight prediction, live weight and UMD were also positive but lower, and those with the fat weight prediction and UFD were low or negative. These parameters can now be used to help develop effective two-stage selection programmes for sheep, which make use of CT, and include improving both carcass composition and muscularity among the breeding objectives.