Changes in carcass traits during growth in lambs of two contrasting breeds, measured using computer tomography

Male and female Texel (Tex, n = 130) and Scottish Blackface (SBF, n = 103) lambs were grazed together on low-ground pastures from birth to slaughter. Lambs were scanned using computer tomography (CT) on a maximum of four occasions during the growth period: at approximately 8, 11 and 16 weeks of age and pre-slaughter, which occurred in five batches over a 12-week period. The aim was to investigate the use of in vivo CT scanning over the growth period (from ca. 15 to 45 kg live weight, corresponding to an age of ca. 40 to 200d) to study changes in carcass traits and their relationships during growth, in two contrasting breeds. Relationships among CT traits and between CT traits and live weight were investigated within breed, to assess changes with growth. Increased live weight was associated with increases in total fat proportion, fat:muscle ratio, muscle:bone ratio, loin muscle area and proportion of fat in the chest region, in both breeds. Two-dimensional muscle shape (depth:width ratio) in the hind leg (HL S ) and the loin (LM S ), and proportion of muscle in the loin also increased with live weight in Tex lambs. Live weight was negatively associated with proportion of fat in the leg in both breeds, and with HL S , LM S and CT fat density in SBF lambs. Similar trends between CT traits and total fat proportion suggest that associations hold for increasing maturity, as well as size. The results suggest that slaughtering SBF lambs at lighter weights will avoid them becoming over-fat, maintain overall muscle proportion at a higher level and lead to rounder muscle shapes in the hind leg and loin. However, Tex lambs can be slaughtered at heavier weights without becoming over-fat, and this will result in a higher proportion of muscle being deposited in the economically important loin area, and the muscles in the loin and hind leg being rounder in cross-section. CT scanning lambs during growth allows accurate composition and muscle shape measurements on large numbers of lambs at different stages of development and can help understand the developmental patterns of different economically important sheep breeds. This knowledge can aid in management decisions to improve carcass quality at slaughter. Further work is now needed to examine the genetic background to the relationships between traits presented in this study, and their changes with growth, to allow the incorporation of such CT traits into selection programmes for different sheep breeds.