This study from the Department of Animal Sciences, University of Arizona quantified heat stress and its influence on dairy cow metabolism and performance. It identified the temperature humidity index (THI) for lactating dairy cows producing more than 77 pounds (35 kg) of milk per day. THI represents the combined effects of air temperature and humidity related to heat stress. Read the study to know how heat affects production where milk loss begins, and see what the numbers say about keeping cows cool.