Thermal balance model for cattle grazing winter range

Abstract

Beef cattle grazing semi-arid foothill range of the Northern Rockies during winter may be exposed to cold temperatures and high winds while grazing pastures with low nutritional value. Cattle can physiologically and behaviorally respond to the changing environment to lower metabolic requirements and reduce the effects of cold exposure. Requirements of grazing cattle may be overpredicted with models developed in controlled settings that do not account for energy conserving behaviors. We refined a simple thermal balance equation to model heat exchange of free-ranging cattle. We accounted for the complex interactions between animal behavior and the changing natural environment by applying the insulation characteristics of cattle's tissue and coat to, first, a simple geometric shape of an asymmetric ellipsoid, and second, to a three-dimensional computer model of a cow at different orientations to the sun and wind. A group of mature cows grazing native range were observed from dawn to dusk from 28 November 2003 to 21 January 2004. These observations were used to evaluate our model and as reference for analyzing cattle behavior in response to environmental variables. Correlation (r) between predicted and measured surface temperatures was 0.82, indicating the model successfully quantifies heat exchanges of cattle exposed to cold conditions in the field. We compared our model predictions with heat production measured in three studies, and with predictions based on the National Research Council beef cattle model. In all cases our model predictions were similar to those reported. Model simulations indicate behavior such as lying and orientation to the sun helped mitigate the effects of extreme weather, and that for many combinations of winter weather variables there is only a small increase in metabolic requirements due to cold exposure in mature beef cattle in a maintenance state. Our results indicate solar radiation contributes strongly to a cow's thermal balance. Thus, previous estimates that did not account for the irradiative environment may have overestimated metabolic requirements of acclimated cattle grazing winter range.