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[First paragraphs]: Description of the thermal environment and the livestock response can be complex, and has been the subject of extensive research for over fiver decades inspired in part by a joint report sponsored by ASAE (now ASABE) and ASHRAE. This 1959 report presented the "State of the Art" of the thermal environmental requirements of poultry (Stewart and Hinkle, 1959), dairy cattle (Yeck, 1959), beef cattle (Nelson, 1959), swine (Bond, 1959) and sheep (Kelly, 1959). Even though the report was comprehensive, data were noted as being incomplete for understanding the biophysical interactions between the animal and its thermal environment as required for effective management and engineering design. Heat loss for poultry was primarily based on basal (fasted) conditions, for example, and the role of the skin and hair in heat dissipation from cattle was inadequate. Comprehensive studies have been conducted in the intervening 50 years to evaluate the effects of nutrition, acclimation or conditioning, dynamic changes in the environment, physiological state, and social interactions on livestock productivity responses to the thermal environment: temperature, humidity, radiation, and air velocity.