Effects of somatotropin treatment on growth performance, carcass traits, and the endocrine system in finishing beef heifers

The objectives of this study were to investigate the effects of exogenous somatotropin on growth performance and carcass traits and on concentrations of somatotropin (ST), IGF-I, insulin, glucagon, free fatty acids (FFA), and glucose in plasma of finishing Simmental beef heifers. Three treatment groups, which included 12 heifers each, received a subcutaneous injection of a slowly released recombinant bovine somatotropin (rbST) at two doses (320 and 640 mg, respectively) or a placebo at 2-wk intervals. Treatments began at an average BW of 286 kg and continued until the animals were slaughtered at approximately 520 kg. Heifers had ad libitum access to corn silage and received 1.8 kg/d of concentrate. A before-feeding blood sample was collected from the jugular vein immediately before and 1 wk after each injection during treatment. Treatment tended to increase ADG and to reduce energy consumption per kilogram of gain, whereas feed and energy intake were not altered. Dressing percentage and conformation did not seem to be affected by treatment. Treatment led to a dose-dependent reduction of fat tissue in carcass as well as in kidney fat and to a simultaneous increase of lean tissue (P |is less than~ .05). Furthermore, treatment caused moderate increases of ST in both treated groups, whereas IGF-I concentrations increased markedly soon after the beginning of the treatment. Insulin and FFA concentrations were elevated in treated groups compared with controls. Glucagon concentrations were not altered by treatment. Effects on growth performance and carcass characteristics are discussed in view of metabolic and endocrine changes.