Comparison of performance, clinical chemistry, and carcass characteristics of finishing lambs treated with recombinant ovine or bovine somatotropins.

The variation in growth and carcass composition responses of lambs to somatotropin (ST) treatment may depend on the source of ST used as well as on other experimental conditions. In the present experiment, growth, carcass composition, and clinical chemistry responses to recombinantly produced ovine ST (oST) and two bovine ST (N-methionyl-bST[M-bST] and N-alanyl-bST[A-bST] were compared. Lambs weighing 42 kg were assigned to treatment groups of control (no injection) or 4 mg/d of M-bST, A-bST, or oST administered by s.c. injection for 6 wk. Growth rate was increased by an average of 30% and feed efficiency was improved by an average of 22% by ST treatment compared with control, and responses did not differ among ST. The IGF-I, insulin, and glucose concentrations were increased by 107, 700, and 53% compared with control, respectively, and did not differ among ST treatment groups. Urea nitrogen responses to A-bST and oST were transiently greater than those to M-bST. Although quality grade was not affected by treatment, an average .8-kg increase in weight of retail cuts was calculated from yield grade. Carcasses of ST-treated lambs were calculated to have 1.3 kg more muscle and 1.9 kg less fat. Although fat and muscle were affected more by oST than by M-bST on a percentage basis, they did not differ among treatment groups on a total weight basis. Thus, both bST variants and oST improved growth performance and carcass leanness. Decreased responses of some carcass variables to M-bST treatment may have been related to the presence of antibodies that were indicated by an increased number of positive responders in a relative bST binding assay.