All dairy producers are aware that their cows are subject to heat stress on warm summer days. Experts estimate heat stress causes $5-6 billion in lost milk production and decreased animal performance in the U.S. each year. 

Dairy cows are more sensitive to heat stress than many other mammals because of the high metabolic heat production due to fermentation in the rumen. Dairy cows experiencing heat stress show higher rectal temperatures, increased respiration rates, decreased feed intake, decreased milk production (volume and components), and other health problems (e.g., decreased reproductive efficiency). 

Very importantly, as dairy cows produce increasing amounts of milk the heat production from the rumen increases and makes them even more susceptible to the harmful effects of heat stress.