The gestation rate was used to evaluate the reproductive performance of dairy cows and heifers confined in a free stall barn during the winter and summer seasons of 1993, 1994 and 1995. The temperature (T) and relative humidity (RH) were monitored daily during the summer (December, January, February and March) and winter (June, July and August). The rectal temperature (RT) and respiration rate (RR) were measured weekly at 9AM and 3 and 9PM, in each season, in 50% of the animals randomly selected and sorted in four groups based on their milk production. The averages of T and the temperature and humidity index (THI) were higher in the summer (25.6ºC e 75.8) than in the winter (19.0ºC e 65.3), while the RH were the same in both seasons (80.0%). The averages of RT and RR were always higher in the summer and at 3PM when compared to 9AM RT and RR (39.47ºC and 64.76 mov/min vs 38.89ºC and 44.21 mov/min). Gestation rate for milking cows was smaller in summer than in the winter. Conversely 85.4% of heifers became pregnant during the summer and 78.5% during the winter season.
Dairy cattle; fertility; heat stress