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 wel l as on other experimental conditions. In the present experiment, grow th, carcass composition, and clinical chemistry responses to recombina ntly 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 assi gned 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 wa s increased by an average of 30% and feed efficiency was improved by a n average of 22% by ST treatment compared with control, and responses did not differ among ST. The IGF-I, insulin, and glucose concentration s were increased by 107, 700, and 53% compared with control, respectiv ely, and did not differ among ST treatment groups. Urea nitrogen respo nses to A-bST and oST were transiently greater than those to M-bST. Al though quality grade was not affected by treatment, an average .8-kg i ncrease in weight of retail cuts was calculated from yield grade. Carc asses of ST-treated lambs were calculated to have 1.3 kg more muscle a nd 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 treatme nt groups on a total weight basis. Thus, both bST variants and oST imp roved 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.