Effects of calving season on the voluntary waiting period and reproductive performance of Holstein cows in the tropical savannah

The effects of calving season [rainy (RS) and dry (DS)] on the voluntary waiting period (VWP) of 58 Holstein cows raised in the tropical savannah were investigated using data of temperature humidity index (THI), total antioxidant status (TAS), respiratory rate (RR), rectal temperature (RT), glucose, total cholesterol (TC), triglycerides (TG), velocity of uterine regression, and subsequent reproductive performance. Blood samples and clinical data were taken once every week, from calving until the sixth postpartum week. Reproductive data were collected until 180 days postpartum. THI differed between seasons (P < 0.05], as well as TAS (P < 0.001), RR (P < 0.001), RT (P < 0.01), glucose (P < 0.001), TC, and TG (P < 0.05), with higher values in RS. Although the velocity of uterine regression showed to be slower (P < 0.001) during RS, no differences were present regarding uterine health. Days open increased in RS (P < 0.001), but the number of services/conception was similar (P = 0.33). The results suggested cows under heat stress during the rainy season in the tropical savannah are more susceptible to a decline in the reproductive performance due to oxidative, metabolic, and uterine health problems.