Sheep Species

The objective of the current study was to evaluate the effect of bovine somatotropin (bST) on milk yield and composition of Holstein cows in early and very early lactations. Twenty-two lactating Holstein cows averaging 62 days in milk (DIM:25-115), 28.5 kg/d milk, a body condition score (BCS) of 2.87, and 648.2 kg of body weight (BW), were assigned to one of the four treatments in a completely randomized design with a treatment arrangement of factorial 2x2. Cows were fed with the regular diet during the fourteen days of pretreatment and the 56 days of treatment periods. The treatments were: 1). VE, cows in very early lactation (25-56 DIM) without bST; 2). VES, cows in very early lactation with bST; 3). E, cows in early lactation (67-115 DIM) without bST; and 4). ES, cows in early lactation with bST. Variables were adjusted for covariance effects using the data from the 28-day pretreatment period and analyzed by the General Linear model Procedure (GLM) of SAS (1990). Results showed that bST tended (P<0.09) to increase milk yield (34.8 vs. 36.4 kg/d) in cows between 25 to 115 DIM, however the increase was significant when bST was injected to cows in early lactation (67115 DIM). Neither the stage of lactation nor the injection of bST altered milk composition nor the rest of variables measured in the experiment.