Effect of Supplemental Dietary Fat or Protein on the Short-Term Milk Production Response to Bovine Somatotropin

Effects of supplemental energy or protein on the milk production response to bST administration were examined in two separate trials. In trial 1, 40 cows were used in a 2 × 2 factorial, completely randomized design to determine the effects of bST and fat supplementation. The study consisted of a 7-d pretreatment period and a 42-d treatment period. Fat was top-dressed at 3.0 Mcal/d of NE_L, and bST was administered. Supplemental fat had no effect on milk production, and NE_L intakes were unaffected. Administration of bST increased milk production by 7.1 kg/d, and the milk production response was unaffected by supplemental fat. In trial 2, 4 cows were used in four periods with a 2 × 2 factorial arrangement in which water or casein was infused into the abomasum of cows fed for ad libitum intake or at 80% of their requirements. Diets and infusions were initiated simultaneously and continued for 11 d. All cows were given bST during the last 5 d. Infusion of water or casein did not alter the milk production response to bST, but restricted feeding reduced the bST response (3.2 vs. 7.2 kg/d). Concentrations of IGF-I in plasma were increased by bST administration, and the increase was greatest for cows fed for ad libitum intake. The milk production response to bST was not increased by additional energy or protein offered to cows fed well-balanced diets.