Tissue growth and development in crossbred lambs sired by ten breeds

The relative growth of carcass tissues and the partitioning of fat between depots was examined in a total of 1400 crossbred lambs from the Meat and Livestock Commission's (MLC) Ram Breed Evaluation. The evaluation was carried out over a five-year period in 10 commercial flocks of Scottish Blackface, Scotch Haltbred (Border Leicester × North Country Cheviot) and Mule (Bluefaced Leicester × Swaledale) ewes. An average of 43 sires from each of the following sire breeds was involved: Border Leicester, Dorset Down, Hampshire Down, Ile de France, North Country Cheviot, Oxford Down, Southdown, Suffolk, Texel and Wensleydale. Lambs of each cross were slaughtered over a range of 9 kg selected on the basis of estimated adult body size to cover the fatness range within which most lambs are slaughtered in Great Britain.Flocks were analysed as two groups: early flocks, from which most lambs were slaughtered before October, and late flocks. Results for early flocks are given in this Abstract. Results for late flocks were similar but they tended to fatten less quickly.Allometric growth coefficients for tissues on side (half carcass) weight were: lean, 0.77±0.015 (S.E.); total separable fat, 1.82±0.039; bone, 0.51±0.025. Perinephric and retroperitoneal fat, and subcutaneous fat, grew relatively faster than total separable fat: 1.16±0.028 and 1.16±0.011, respectively. Intermuscular fat grew relatively more slowly: 0.80±0.012. Sire breed differences were recorded (P<0.001) for the weight of each of the tissues at equal side weight. Texel crosses had the highest lean weight and Southdown crosses the lowest.At the same total lean weight, Texel crosses tended to have less lean in the loin and more in some of the other joints, especially the shoulder, than other crosses.