Sire carcass breeding values affect body composition in lambs — 2. Effects on fat and bone weight and their distribution within the carcass as measured by computed tomography

• Breeding values for reduced post weaning c-site fat depth and increased eye muscle depth uniformly reduce carcass fat.
• Increased growth breeding values have minimal impact on proportions of carcass fat or bone.
• At equal carcass weight, Merino sired lambs have more bone than lambs sired by Maternal or Terminal sires.
• At equal carcass weight, Maternal sired lambs have more carcass fat than lambs sired by Merino or Terminal sires.

This study assessed the effect of paternal Australian Sheep Breeding Values for post weaning c-site eye muscle depth (PEMD) and fat depth (PFAT), and post weaning weight (PWWT) on the composition of lamb carcasses. Composition was measured using computed tomography scans of 1665 lambs which were progeny of 85 Maternal, 115 Merino and 155 Terminal sires. Reducing sire PFAT decreased carcass fat weight by 4.8% and increased carcass bone by 1.3% per unit of PFAT (range 5.1 mm). Increasing sire PEMD reduced carcass fat weight by 3.8% in Maternal and 2% in Terminal sired lambs per unit of PEMD (range 4.3 and 7.8 mm), with no impact on bone. Increasing sire PWWT reduced carcass fat weight, but only at some experimental locations. Differences in composition varied between sire types with Maternal sired lambs having the most fat and Merino sired lambs the greatest bone weight. Genetic effects on fatness were greater than the environmental or production factor effects, with the converse true of bone.