All animals, including dairy cows, have four possible routes of exchanging heat energy with their environment. These are conduction, convection, radiation and evaporation. The first three are referred to as sensible routes of heat loss, and all require a thermal gradient. In other words, the cow must be hotter than the environment for heat to flow out of the cow into the surrounding environment.

In his fifth edition of Dairy Cattle and Milk Production published in 1939, Dr. Clarence Eckles reported that the “ratio between the human population and the number of cows in the United States had remained essentially the same since 1850.” At that time, there were 36 million dairy cows in the U.S. producing an average of 8,700 pounds of milk per lactation.

The optimal dry period length between lactations in dairy cows has been debated since the early 1800s. During this time, some English farmers believed a two-month dry period was optimal, while others believed a two-week dry period was adequate.

With an annual cost of more than $900 million in lost production, heat stress takes a serious toll on U.S. dairy production. Despite large capital investments in cooling equipment, milk production and reproductive performance drastically decline on many dairies during periods of heat stress.