Thermal Indices and Heat stress Assessment in Livestock: Way Forward

Abstract

All the thermal indices in livestock are based on relationship between certain climate and production parameters establishing an empirical relationship between them. The majority of the published reports indicate the use of temperature and humidity in the equations that was developed to suit the local environmental and animal needs. Temperature humidity index (THI) is a widely accepted thermal index in livestock and an ideal indicator of heat stress. It mainly depends on the weather parameters like temperature and humidity. Several THI have been designed from a combination of dry bulb, wet bulb and humidity and it provides specific correlation between these parameters and physiological responses. Although, THI index was found to be of relative success in quantifying the stress response in livestock, still there were two major drawbacks associated with such indices. Firstly, THI does not take into account the wind velocity and solar radiation which are considered cardinal weather parameters which hugely influence the stress response in animals. Secondly, the THI does not take into account the accumulated heat load over the period of time when it crosses the threshold values. Like THI there are few other thermal indices which used some of the primary cardinal weather parameters in the developed equation. Equivalent temperature index (ETI) was developed with a goal of determining the impact of heat dissipation on milk production. It included air temperature, humidity and wind speed as the core variables for the cows above their comfort zone. There are several cold and heat stress models for cattle but the drawback for those models were the exclusion of solar radiation as one of the parameter. Another thermal index called integrated relative thermal-comfort index (IRTCI) associates the interactions of the local environment and some animal characteristics. This index establishes that the interaction between climate parameters such as humidity, ambient temperature, radiation and production parameters like fur depth and production level strongly influences the mechanism of thermoregulation.