Modeling dairy cow thermoregulation during warm and hot environmental conditions 1: Model development

A steady-state heat transfer model developed by McGovern and Bruce (2000) and modified by Berman (2005) was modified more to model thermoregulatory responses of lactating Holstein cows to heat stress. The new model describes metabolic heat generation and heat exchange to the surrounding environment. Heat exchanges included respiratory latent and sensible heat exchange, evaporative heat loss via the skin, solar radiation heat gain, and convective and long-wave radiative heat exchanges. In McGovern and Bruce (2000) four mechanisms cows have to mitigate heat stress effects were implemented in a step-wise manner. Reported cow responses to heat stress indicate that mitigation mechanisms such as increased respiration and sweat rate, and body temperature overlap rather than activate in sequence. The model was modified to allow overlap in animal responses to given environmental conditions. Tissue insulation and body temperature were related to respiration rate. Sweat rate was related to skin temperature. Metabolic heat production was related to daily milk production and respiratory cooling was based on data for Holstein cows. Model inputs include cow size, daily milk production, dry-bulb and dew-point temperatures and air velocity. These variables impact which mechanisms and the levels a cow needs to employ to achieve thermal balance. Model results compared favorably to information reported in the literature. The modified model can be used to determine what thermal conditions (ex. temperature, humidity, solar gain and air velocity) a cow can be in and maintain thermal balance for given physiological responses (i.e., respiration rate, sweat rate and body temperature).