A model of thermal acclimation in cattle.

A model of temperature effects on cattle daily feed intake has been developed that takes into account (1) time course of thermal acclimation and (2) behavioral responses to short-term thermal stress (STTS). The key difference between this model and the classical intake-temperature model is its consideration of the acclimation state of the animal. Time course of physiological acclimation is represented by a running average temperature. The STTS is defined as the difference between the current acclimated temperature [Tacci(L)] and current daily mean temperature (Ti). Ruminants were postulated as decreasing activity in response to STTS. An empirical relationship between daily feed intake (I) and environmental temperature was derived as: I = 100% - b'[Ti - Tacci(L)]2. Length of the acclimation period, L, and value of the behavioral response coefficient, b', were determined for grazing time of free-roaming cattle and feed intake of five breeds of feedlot cattle. Cattle breeds displayed apparent differences in L but no significant differences in b'. Feeding situation (feedlot vs free-roaming) had a significant effect on b' in cattle, but not on L. Because of explicit treatment of acclimation and stress, two-dimensional representation of thermal environment may be a more meaningful expression of effective environmental temperature in fluctuating environments than mean daily temperature alone. This model may have value in the interpretation of laboratory studies, as well as field studies, because the time frame of experiments will influence the results obtained, depending upon the acclimation state of the animal.