Is the temperature-humidity index the best indicator of heat stress in lactating dairy cows in a subtropical environment?

Several temperature-humidity indexes (THI) have been used to estimate the degree of thermal stress experienced by dairy cows. The present objectives were to develop equations using meteorological variables that predicted rectal temperature of lactating cows in a subtropical environment and compare the goodness of fit of these equations to those using 8 different THI. Rectal temperature was measured between 1500 and 1700 h in 1,280 lactating Holstein cows in north central Florida between August and December. Meteorological data recorded in the barn where cows were located included dry bulb temperature (Tdb), relative humidity (RH), dew point temperature, and wind speed. Wet bulb temperature was calculated. In the first series of analyses, regression analysis was used to model rectal temperature using the meteorological variables as well as THI. The r2 using Tdb (0.41) was slightly less than for models using all but one THI (r2 between 0.42 and 0.43). The r2 for equations using Tdb could be improved by adding RH (r2 = 0.43) or RH and RH2 (r2 = 0.44) to the model. In the second analysis, regression analysis was performed using forward selection, backward elimination, and stepwise selection procedures with the meteorological variables. All models gave a similar goodness of fit (r2 = 0.44). An analysis of variance with rectal temperature as a class variable was performed to determine the least squares means of meteorological measurements associated with hyperthermia. A Tdb of 29.7°C was associated with rectal temperature of 39°C, and a Tdb of 31.4°C was associated with rectal temperature of 39.5°C. In conclusion, Tdb is nearly as good a predictor of rectal temperatures of lactating Holsteins in a subtropical environment as THI. Estimates of values of meteorological variables associated with specific rectal temperatures should prove valuable in relating environmental conditions to the magnitude of hyperthermia experienced by heat-stressed cows.