A study was conducted to evaluate the effects of bST in crossbred dairy cows (Bos indicus x Bos taurus) raised in a semi-arid zone, in the Northeast of Brazil. Fifteen cows (second and third lactation; 42-155 days in milk) received five applications of bST, at 14-day intervals. Other 15 cows were used as controls. The animals were fed with diet consisted in elephant grass and concentrate. During the first application of bST, the milk production of the treated cows (12.6 kg/day) did not differ from the control animals (11.3 kg/day), but in the second and third applications, the difference between the groups was significant (2.0 and 2.3 kg). In the fourth application of bST, there was an increase in the difference between groups (3.0 kg) and in the fifth application, such difference was 3.4 kg (14.0 kg versus 10.6 kg/day). The bST treatment and stage of lactation interaction was not significant. Rectal and milk temperatures were higher (0.1 and 0.6 °C, respectively) in bST treated than in control animals, but differences were not significant. For the group of 30 cows, the daily milk yield was related to milk temperature (r = 0.38 to 0.69). Based on the temperature-humidity index, the cows were under light heat stress in the morning and moderate stress in the afternoon. Therefore, crossbred cows raised in the semi-arid area of Brazil showed responses to bST without apparent signs of severe heat stress.
Bos indicus; Brazil; heat stress; milk yield; semi-arid; somatotropin
