We also investigated the time interval required at an average THI of 68 before milk yield losses became significant. This data indicated that milk yield losses became significant after 17 hours of exposure to an average THI of 68 and equated to a 2.2 kg per day loss in milk yield. Thus, our data indicates that for dairy cows producing more than 35 kg/day need additional cooling when minimum THI is 65 or greater or when average THI is 68 for more than 17 hours per day.