A COMPARISON OF GROWTH AND CARCASS TRAITS IN SCOTTISH BLACKFACE LAMBSSIRED BY GENETICALLY LEAN OR FAT RAMS

An experimental programme was initiated in 1990 to investigate the con sequences of selection for reduced fatness in hill sheep and to estima te genetic parameters for carcass traits in the same class of animal. Thirty-two progeny groups of lambs were born as a result of mating Sco ttish Blackface rams to ewes of the same breed on two Scottish Agricul tural College hill farms in 1991 and 1992. Sires were from two diverge nt selection lines for subcutaneous back fat depth and were selected u nder ad libitum feeding conditions on an index combining live weight a nd ultrasonic fat depth at 20 weeks of age. The index was designed to alter body composition but not body weight. Weaned male progeny (reare d extensively) were brought together from both farms in August of each year; reared on grass and finished for slaughter on swedes at a targe t condition score of 3. Measurements taken at weaning were: live weigh t, average ultrasonic measurements of average subcutaneous far depth a nd muscle depth. At slaughter, baits measured were: pre-slaughter live weight, pre-slaughter condition score, age at slaughter, cold carcass weight, killing-out proportion, Meat and Livestock Commission (MLC) f at class and MLC conformation class. Side and shoulder dissections wer e made on proportionally 0.2 and 0.8 of lambs respectively and lean ti ssue, subcutaneous fat, intermuscular fat and bone were measured. Comp arisons were made between the two genetic lines and genetic parameters were estimated from data adjusted to three different 'end-points': co nstant dissected subcutaneous fat weight, constant age at slaughter an d constant cold carcass weight. Moderate to low heritability estimates were obtained for most traits: pre-slaughter live weight = 0.36, cold carcass weight = 0.39, fat class = 0.13, conformation class = 0.09, l ean weight = 0.27, bone weight = 0.36 (constant subcutaneous fatness), intermuscular fat = 0.10, subcutaneous fat = 0.20 (constant cold carc ass weight). Correlations between ultrasonic measurements at weaning, and slaughter and dissected carcass components were moderate to weak. However, there is sufficient genetic variation in the carcass traits t o warrant inclusion in breeding goals for hill sheep. The results also show that genetic differences obtained by selection under intensive r earing conditions for divergent fatness are also seen in progeny reare d under extensive (hill) conditions.