Long-Term Evaluation of a Prolonged-Release Formulation of N-Methionyl Bovine Somatotropin in Lactating Dairy Cows

Eighty Holstein cows (first, second, or third lactation) were used to evaluate the efficacy of a prolonged-release formulation for sometribove (n-methionyl bovine somatotropin). Cows were fed ad libitum a complete mixed diet and milked twice daily. Cows were allocated randomly by parity to two treatment groups receiving 500 mg sometribove in a prolonged release formulation or excipient at 14-d intervals starting at 60 ± 3 d postpartum and continuing for 36 wk. Treatment with sometribove increased FCM yield to a similar extent (11.4%, +3.1 kg/d) in primiparous and multiparous animals. Milk content of lactose, fat, ash, and Ca was not affected, but protein and phosphorous were slightly greater (<5%) in milk from sometribove-treated cows. Within a 14-d injection interval, animals treated with sometribove displayed a cyclic pattern in milk yield, but a similar pattern was not evident in feed intake. Protein and fat percentages in milk were higher the 2nd wk of the injection cycle (6 and 4%, respectively) and this difference appears unrelated to nutrient balance. Over the treatment period (252 d), sometribove-treated animals increased voluntary feed intake to a sufficient extent so that body weight gains, net energy balance, and body condition scores did not differ between treatment groups. Gross lactational efficiency (milk per unit of feed) was improved in sometribove-treated cows, and no adverse health effects were observed. Results demonstrate that bovine somatotropin administered by a prolonged-release formulation is effective in improving milk yield and productive efficiency.