The objective ofthis trial was to estimate the upper critical levels of the temperature-humidityindex (TUI) measuring morning and afternoon rectal temperature (RT), respiratoryrate (RF), and hair coat surface temperature (ST) of ½, ¾ and7/8Holstein-Zebu (HZ) dairy cows during two consecutive years (two summers andtwo winters) in Coronel Pacheco, MG, Brazil. Correlation and multiple analysiswere determined using data obtained from 15 dairy crossbreed cows/season; fivefrom each genetic group (GG). Results showed that RF was more reliable thanRT as an indicator of heat stress based on both correlation and regression analysis.Estimated upper critical values of the TUI were 80, 77, and 75 using RT and79, 77, and 76 using RF for ½HZ, ¾HZ, and 7/8HZ dairy cows, respectively.When ST was used the estimated upper critical values of the TUI were very similaramong the three GG averaging 79. The ½HZ dairy cows were more heat tolerantthan those in the 7/8HZ GG while the ¾HZ were intermediate.
