Alleviation of the Long Term Effects of Heat Stress on Dairy Cattle


Micheal J. Brouk

Department of Animal Sciences and Industry, Kansas State University, 134 Call Hall, Manhattan, KS 66506-1600 Tel: 785-532-1207 Fax: 785-532-5681 email:


John F. Smith, Department of Animal Sciences and Industry, Kansas State University

Joseph P. Harner, III, Department of Agricultural and Biological Engineering, Kansas State University






Thermal stress in cattle results in major decreases in dairy production each summer. These decreases have been documented in many studies and reviews (Armstrong, 1994, Collier,, 1982, Ravagnolo,, 2000 and Ray, 1992). Igono and others (1992) proposed that the Temperature Humidity Index (THI) could be used to evaluate the thermal stress of the environment. This index combines relative humidity and temperature into a single value to estimate the potential environmental heat load. An environment is generally considered stressful for cattle when the THI exceeds 72. When THI is above this level, adverse affects are expected in cattle. Others have suggested (Hahn, et al., 1992) that feed intake of cattle will be reduced when temperatures exceed 75 F. Production losses can be minimized by proper heat abatement measures. However, the measures must also be cost effective and provide an economic return to the dairy operation.

Heat Exchange Mechanisms

Dairy cattle produce large amounts of heat from both ruminal fermentation and metabolic processes. As production increases, the total amount of heat produced increases. A high producing lactating cow will produce about 4,500 British Thermal Unit (BTU)/hr. In order to maintain body temperature within the normal range, cows must exchange this heat with the environment. Cattle exchange heat through the mechanisms of convection, conduction, evaporation and radiation. It is important to remember that heat exchange is a two-way street; the cow gives and receives heat energy from the environment depending upon the condition. For example, if the air temperature is lower than the body temperature of a cow standing in the sun, heat is transferred by conduction, convection and radiation to the environment while the environment is transferring heat to the cow by radiation from the sun. Protection from solar radiation by providing adequate shade is the first step in reducing heat stress in dairy cattle.

University research station scientists recognized summertime stress as a major dairy production problem in the 1940s. Extensive work at the Missouri experiment station sought to describe and define the heat loss mechanisms of dairy animals (Kibler and Brody, 1949, 1950, 1952 and 1954). One of the most significant findings was an accurate accounting of heat loss for dairy cattle by various mechanisms over a wide range of environmental temperature (Figure 1). At temperatures above 70 F, the heat loss is primarily due to moisture evaporation from the skin and lungs. As temperatures exceed 90 F, over 85% of the total heat dissipation is due to vaporization of water from the body surface and lungs. It is im