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 respo nse to bST administration were examined in two separate trials. In tri al 1, 40 cows were used in a 2 x 2 factorial, completely randomized de sign to determine the effects of bST and fat supplementation. The stud y 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 w ere 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 x 2 factor ial arrangement in which water or casein was infused into the abomasum of cows fed for ad libitum intake or at 80% of their requirements. Di ets and infusions were initiated simultaneously and continued for 11 d . All cows were given bST during the last 5 d. Infusion of water or ca sein did not alter the milk production response to bST, but restricted feeding reduced the bST response (3.2 vs. 7.2 kg/d). Concentrations o f IGF-I in plasma were increased by bST administration, and the increa se was greatest for cows fed for ad libitum intake. The milk productio n response to bST was not increased by additional energy or protein of fered to cows fed well-balanced diets.