Chapter 5: Thermal Indices and Their Applications for Livestock Environments

Heat exchanges with the environment are crucial processes for maintaining homeothermy by humans and other animals. These exchanges involve heat production, conservation, and dissipation, and are dependent on both biological and physical factors. The complexity of these exchanges has led to many attempts to represent the environmental aspects by surrogate thermal indices as a basis for assessing the biological effect and consequent impact of the thermal environment. Resultant index values represent effects produced by the heat exchange process. For humans, comfort assessment is primary; for animals, assessing performance, health, and well-being have been foremost. Emphasis in this chapter is on thermal indices used in animal studies and their applications, with a view toward strategic and tactical decisions for rational environmental management. Illustrative examples are included, as are considerations for future efforts. Historically, most efforts to develop thermal indices have been for human applications, and involve both psychological and physiological aspects. Jendritzky et al. (2002) states that more than 100 such indices have been developed over the past 150 years, usually based on two thermal parameters to represent the complex heat exchanges between the human body and the thermal environment. Ideally, “...each index value will always result in a unique thermophysiological effect, regardless of the combination of the input meteorological input values” (Jendritzky et al., 2002). This utopian goal is unattainable in reality for thermal indices representing livestock environments because of the complex interactions of the environment and animal factors,