CARCASS CHARACTERISTICS AND OPTIMAL SLAUGHTER WEIGHTS IN 4 BREEDS OF SHEEP

Wether lambs from four breeds, Rambouillet (n = 38), Targhee (46), Col umbia (38), and Polypay (38), were used to estimate an optimal slaught er weight for lean lamb production. Wethers, weaned at an average of 1 05 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 weig hts ranging from 39 to 70 kg. Fat, protein, moisture, and ash percenta ges were determined on fat and lean from 7-rib racks. Optimal slaughte r 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 l ambs were less physiologically mature than the other breeds, having le ss (P < .05) kidney and pelvic fat and a lower rack fat percentage. Ta rghee, Rambouillet, and Polypay wethers were similar in carcass fat an d protein composition at a constant slaughter weight. Optimal slaughte r weights for certified lean lamb carcasses of Targhee, Rambouillet, a nd Polypay wethers were 45 to 47 kg, resulting in a Choice quality gra de 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 yiel d grade. The derived optimal slaughter weights are limited to the popu lations sampled and may not be specific to other populations. Results indicate that breed differences in physiological maturity are reflecte d in carcass characteristics and that such differences should be consi dered in sorting lambs on slaughter weight.