Effect of recombinant bovine somatotropin application intervals on Girolando cows milk production and composition

The aim of this study was to evaluate the effect of recombinant bovine somatotropin (rBST) application intervals on chemical composition of milk from Girolando cows with productivity below 20 L/milk/day and animals with productivity above 20.1 liters/milk/day. The study included 30 Girolando cows with production ranging from 13 to 28 L/milk/day. Cows were submitted to two milkings: 06:00 am and 04:00 pm. Milk samples were obtained at the end of the first milking. With the aid of individual meters, 40 mL of milk were collected in bottles containing preservative bronopol. On the day of collections, the individual milk production of each animal was notes. The experiment was performed on a 3x2 factorial design. Factor A corresponded to rBST application intervals divided into three levels (intervals) of rBST application every 10, 12 and 14 days, respectively. Factor B corresponded to the production levels of animals, divided into two levels: animals with production above 20.1 L/milk/day and animals with production less than 20 L/milk/day. About 500 mg rBST were subcutaneously applied in previously disinfected ischiopubic-rectal fossa. On the first day, all animals received rBST, the applications were  respected for planned application range for each group of animals. The Application interval of 14 days was better for animals with productivity below 20 L/milk/day and for animals with higher production, the best rBST application interval was 10 and 12 days. Application interval of 10 days resulted in higher fat content in milk from animals with lowear productivity, and fat and protein contents were higher for animals with lower productivity in all application intervals.    Key words: Growth hormone, production increment. milk quality.