Thresholds for animal heat stress are important for distinguishing between thermoneutral and thermally challenging environmental conditions. It is a well recognized biometeorological problem that heat stress has a negative impact on productive performance and ultimately survival of agricultural animals. Accurate estimates of heat stress thresholds are useful for evaluating heat dissipation capabilities, growth rate, feed efficiency and for comparing management techniques for overcoming the adverse effects of heat stress. For individual animals, the heat stress threshold can be used to characterize sensitivity to hot conditions and possibly identify sentinel animals to represent a group of animals.

The current operative definition of the heat stress threshold focuses on the relationship between body temperature, Tb and air temperature, Ta. Changes over time are ignored. Many controlled laboratory studies have been conducted to compare the Tb-Ta relationship for cattle in thermoneutral and thermally challenging conditions. In some of these controlled studies, steers were thermally challenged by hot cyclic air temperatures (32±7 C) after exposure to thermoneutral cyclic conditions (18±7 C). Recently, data from these studies were used to find an appropriate model for estimating the heat stress threshold. As expected, there was a dramatic difference in pattern of the Tb-Ta relationship between thermoneutral and heat stressing conditions. During the thermoneutral period, the Tb-Ta relationship was represented by variations about a plateau, while the Tb-Ta relationship during the heat-challenging conditions showed a dramatic increase in Tb per unit of Ta. A notable and frequently overlooked feature in the Tb-Ta relationship is that the pattern of increase suggests the response can be represented by an ellipsoid. This ellipsoid shows hysteresis is present when the animals are thermally challenged, and indicates a lag between body temperature and ambient temperature. It is this lag effect that must be considered in defining the heat stress threshold. For a specified Ta, there are two levels for Tb; one for the thermal challenging phase of the cyclic air temperature, and another during the recovery phase of the daily cycle. The question is how to refine the definition of heat stress threshold to incorporate this hysteresis effect. This report will discuss plausible definitions based on a statistical model for the data.