Effect of somatotropin injection and dietary protein concentration on milk yield, and kinetics of hormones in dairy cows

Four lactating Holstein cows averaging 155 d postpartum were used in a cross-over design to examine the influence of somatotropin administration (33 mg/d for 28 d) and dietary CP (11 vs. 16% CP) concentration on milk yield and kinetics of glucagon, insulin, glucose, and somatotropin. Somatotropin and dietary CP were without influence on DM intake and digestibility. Milk yields were increased by both somatotropin administration and increased dietary CP. Increased dietary CP increased turnover of glucose. Injection of somatotropin increased entry rate, pool size, and basal concentrations of somatotropin but was without effect on kinetics of glucagon, insulin, or glucose. The absence of major changes in the kinetics of hormones and glucose suggests that other factors play a role in somatotropin enhancement of milk production. However, the observed changes are consistent with the homeorhetic influence of somatotropin on milk production. Adequate dietary CP is required to obtain maximal responses to somatotropin administration.