Recombinant bovine somatotropin stimulates short term increases in growth rate and insulin-like growth factor I (IGF-I) in chickens

Recombinant bovine somatotropin (rbGH) was administered by subcutaneous injection at daily doses of 0.5 or 2.5 mg/kg for a two week period in female broiler chicks between 4 and 6 weeks of age. Half of the chicks received dietary corticosterone at a 1 ppm level. Growth rate was significantly increased 6.1% and 6.9% following one week of treatment with 0.5 or 2.5 mg/kg rbGH respectively. Treatment with the same respective doses of rbGH in the presence of 1 ppm corticosterone, supplied to suppress any possible immune response elicited by the heterologous somatotropin, resulted in an 8.0% and 7.8% increase (P less than .05) in growth rate during the first week of treatment. The rbGH-associated increase in growth rate was accompanied by a significant increase in food intake, higher circulating levels of IGF-I, and lower plasma T4 concentrations, while plasma T3 levels were unchanged. All effects were attenuated during the second week of treatment, concomitant with the development of high antibody titer against rbGH regardless of dietary corticosterone administration. Carcass parameters relating to bone, muscle and fat were not different between rbGH-treated and control chickens at the end of the two week treatment period. Thus rbGH is capable of stimulating a short-term improvement in growth rate, which is related to increased feed consumption and is of limited duration.