Defining upper critical temperatures for an effective climate control to reduce heat stress

Abstract

The definition of homeotherm animal responses to temperature has traditionally based on metabolic and thermal physiology studies. The thermoneutral zone is defined as the range of ambient temperature within which metabolic rate is at a minimum, and within which temperature regulation is achieved by non-evaporative physical processes alone. As temperature raises, animals the upper critical temperature is reached, which is the ambient temperature above which thermoregulatory evaporative heat loss processes of a resting thermoregulating animal are recruited. These temperatures depend on a variety of factors including animal species and breed, age, adaptation or animal status, among others. These are influenced by other environmental conditions such as relative humidity and wind velocity. Determining upper critical temperatures seems essential to establish effective climate control strategies. However, evidence of research during the last decades shows that animal responses to heat stress are varied, as briefly reviewed in this article. These responses and include both short and long term responses depending on the magnitude and duration of the stress. Furthermore, as temperature raises a wide and complex range of behavioural, physiological and productive responses are triggered. Thus, the effects of heat stress may relate to animal welfare, health, productivity and sustainability concerns, and seem difficult to assess with critical temperatures only. Currently, major interest is devoted to develop indicators for heat stress, particularly early indicators. These indicators may vary among animal species, but are the basis for a better environmental control. In this sense, the advances of precision livestock farming are a sound basis for heat stress control. Therefore, apart from thermal indicators, animal-based information may supply very relevant information to decide about heat stress abatement strategies. This technology is more and more available for farmers, and very relevant developments are currently developed and will also be developed in the future. In summary, an information-based decision system related to animal indicators may be effective for a practical climate control in livestock farms.