Dairy cattle experience heat stress when the Temperature Humidity Index reaches a
threshold of 68 to 72 depending on production level: the threshold is lower for higher
yielding cows. Heat stress reduces feed intake and milk production, increases loss of
body condition, is a major risk factor for acidosis and hits fertility hard. For example,
an increase in body temperature of about 0.50°C above the normal level of 38.5°C is
estimated to cause a decline in conception rate of 12.8% (Gwasdauskas et al., 1973).