The objective of this present study was to evaluate the effects of heat stress during the last 3 wk of gestation on performance of multiparous Holstein cows (n = 20). Dry cows randomly assigned to treatments approximately 21d before expected calving date based on mature equivalent milk production and parity.Treatments were only imposed during the last 3 wk of gestation and included heat stress (HT; n = 10) or cooling (CL; n = 10). The stall areas for CL cows equipped with sprinklers and fans that turned on at 0700 to 1900 h. After parturient, all cows were housed in a barn with cooling devices. Air temperature and relative humidity of each pen were recorded every 15 min, and temperature- humidity index (THI) was calculated. The average temperature humidity index during the last 3 wk of gestation was similar and not different between treatments (69.66 and 69.82, respectively, for CL and HT).Heat stressed cows had higher rectal temperature (P? 0.0001), greater respiration rates (P? 0.0001) when compared with CL cows. Gestation length (P = 0.29) and dry period length (P = 0.3) was not different between treatments. Heat stress during the last 3 wk of gestation decreased DMI of the cows (P? 0.05) compared with cooling, but not with postpartum (P = 0.03). In addition, a tendency of treatments by time interaction was observed (P = 0.07). Relative to HT cows, CL cows had greater milk production (P 0.05). No treatments effects on milk fat, protein and SNF concentration (P = 0.62, P = 0.054, P = 0.053). Calf BW was 2.7 kg lighter for calves born from HT cows (P 0.05). Colostrum density of HT and CL cows was 1.05 and 1.06 g/cm 3 , respectively (P 0.01). The number of insemination for each conception for CL and HT cows was 1.9 and 2.2, respectively (P = 0.09). CL cows tended to have higher eating and chewing time (P = 0.13 and P = 0.06) and their ruminating time were higher than HT cows (P ? 0.03). In contrast, standing time of CL cows were lower than HT cows (P? 0.01). In general, results of this study indicate that heat stress abatement during late gestation behind of cows welfare, improves dairy cows performance n the subsequent lactation. Key words: dairy cows, heat stress, gestation period, temperature humidity index
