Heat stress or hyperthermia occurs in dairy cows when the metabolic heat produced by the cow in combination with heat from the environment exceeds the cow's ability to loose heat to the environment. While all mammals can experience heat stress, the temperature threshold at which a lactating dairy cow experiences heat stress is much lower due to the internal heat production that is a byproduct of metabolism associated with milk production. Evaporative cooling is the primary method by which cows can lose heat to the environment. Because cows do not sweat excessively, heat abatement strategies using sprinklers and fans are common on many dairies.

Relative humidity in combination with ambient temperature, also called the temperature-humidity index (THI) determines heat stress because the efficiency of evaporative cooling decreases as relative humidity increases. When ambient temperature equals body temperature, evaporative cooling is the only route for heat loss to the environment. Generally, a THI in excess of 72 is a threshold for heat stress in dairy cattle. Because the THI experienced in Wisconsin over the past several weeks has exceeded historic norms, reproductive performance will be affected well into the fall months.

Heat stress affects reproductive performance in dairy cows by affecting both the oocyte Two to three days before ovulation, by decreasing the fertilization rate, and by increasing early embryonic loss within the first week or so after fertilization before pregnancy status can be established. The mechanism by which heat stress occurs is an increase in the body temperature of the cow. Normal body temperature in cows is 101.5 degrees Fahrenheit, but rectal temperatures can exceed 107 degrees in extreme hot weather.

In a study published by researchers in the Department of Dairy Science at UW-Madison in 2003, cows had a 41% greater risk of becoming pregnant during the fall of 2001 and a 71% greater risk of becoming pregnant during the winter of 2001/2002 compared with the summer of 2000. Overall, a 1°C increase in mean daily high temperature at TAI resulted in a 2-percentage-unit decrease in conception rate. During the summer of 2000, mean daily high temperatures did not exceed 85 degrees Fahrenheit – a far cry from the temperatures we have experienced in the summer of 2012. In another UW experiment published in 2010, fertilization rates in lactating cows were 83% in a thermoneutral environment but decreased to 37% under heat stressed conditions.

It is important to understand that the effects of heat stress that we have experienced thus far during the summer of 2012 will linger into the fall when the heat of the summer has long passed. For example, during the period from April to July of 1999 (a relatively hot summer for Wisconsin), conception rates at the UW Arlington Dairy research herd steadily decreased from over 40% to around 15%. Conception rates then steadily increased starting in August of 1999, but did not exceed 40% again until November. Although we cannot completely avoid the negative effects of heat stress on reproduction, use of heat abatement strategies and timed AI breeding protocols like Ovsynch can help work through the dog days of the summer of 2012.