Thirty-six crossbred Holstein first lactation pregnant heifers were used in a randomized block design to evaluate periods of time between applications of bovine somatotropin (BST) in a 500 mg dose, in the following treatments: A) control; B) BST each 28 days; C) BST each 21 days and D) BST each 14 days. Three blocks of females were used according to their production: 28.6 kg, 25.1 kg and 19.1 kg of milk/cow/day. The experimental period was 112 days, with daily control of milk production, milk sampling collections for fat analysis and somatic cells counting each 14 days, hearth girth measurements and body score condition evaluations made each 28 days. Fat corrected milk (FCM) was significantly higher (p<0.01) for animals treated each 14 days (24.3 kg/day), 21 days (23.1 kg/day) and 28 days (21.1 kg/day) than control (19.4 kg/day). Concerning milk production, BST each 14 days (26.5 kg/day), each 21 days (25.5 kg/day) and each 28 days (24.5 kg/day) were statistically higher (p<0.01) than control cows (21.6 kg/day). Fat milk production was higher (p<0.05) for BST each 14 days (0.916 kg/day), each 21 days (0.862kg/day) and each 28 days (0.753 kg/day) than for control treatment (0.717 kg/day). Linear regression considering periods among product applications was statistically significant for milk fat production (p<0.05) and milk fat content (p<0.05) both decreasing with the increases in application’s time. BST each 14 days increased FCM 25.5% (4.95 kg/cow/day) and fat production 28.1% (0.20 kg/cow/day) relatively to control. Inter-actions between treatments and blocks did not happened in this trial.
BST; Dairy cattle; Milk production; Cow
