CARCASS CHARACTERISTICS AND OPTIMAL SLAUGHTER WEIGHTS IN 4 BREEDS OF SHEEP

Wether lambs from four breeds, Rambouillet (n = 38), Targhee (46), Columbia (38), and Polypay (38), were used to estimate an optimal slaughter weight for lean lamb production. Wethers, weaned at an average of 105 d of age and 38.5 kg, had ad libitum access to a finishing diet of 85% barley grain and 15% pelleted alfalfa and were slaughtered at weights ranging from 39 to 70 kg. Fat, protein, moisture, and ash percentages were determined on fat and lean from 7-rib racks. Optimal slaughter weight was estimated by using carcass backfat depth or extracted fat in the rack as dependent variables in linear equations. An acceptable lean lamb carcass was defined to have 2.5 to 6.5 mm of backfat depth and 26 to 40% rack chemical fat. On a weight-constant basis Columbia lambs were less physiologically mature than the other breeds, having less (P < .05) kidney and pelvic fat and a lower rack fat percentage. Targhee, Rambouillet, and Polypay wethers were similar in carcass fat and protein composition at a constant slaughter weight. Optimal slaughter weights for certified lean lamb carcasses of Targhee, Rambouillet, and Polypay wethers were 45 to 47 kg, resulting in a Choice quality grade and a 2.7 yield grade. The optimal weight for Columbia was heavier, between 45 and 55 kg, producing a Choice quality grade and a 2.4 yield grade. The derived optimal slaughter weights are limited to the populations sampled and may not be specific to other populations. Results indicate that breed differences in physiological maturity are reflected in carcass characteristics and that such differences should be considered in sorting lambs on slaughter weight.