Thermal Stress on Dairy Animals and Management Strategies for its Amelioration

There is a range of thermal conditions within which animals are able to maintain a relatively stable body temperature by means of behavioural and physiological means. When environmental temperatures move out of the thermoneutral zone (or comfort zone) dairy animals begin to experience either heat stress or cold stress. Either stress requires the animals to increase the amount of energy used to maintain the body temperature and there is less energy available for productive processes. Thermoneutral zone is the range of environmental temperatures where normal body temperature is maintained and heat production is at the basal level. The ranges of thermoneutral zone are from lower critical temperature (LCT) to upper critical temperature (UCT). LCT is the environmental temperature at which an animal needs to increase metabolic heat production to maintain body temperature. UCT is the environmental temperature at which the animal increases heat production as a consequence of a rise in body temperature resulting for inadequate evaporative heat loss.

Temperature-humidity index (THI) could be used as an indicator for assessing the heat stress levels on animals. THI is determined by equation from the relative humidity and the air temperature and is calculated using the following formula (Kadzere et al., 2002)

THI=0.72 (W+D) +40.6

Where W – wet bulb temperature (ºC)
D – dry bulb temperature (ºC)

The principle of THI is that as the relative humidity at any temperature increases, it becomes progressively more difficult for the animal to cool itself. THI values of 70 or less are considered comfortable, 75 – 78 stressful, values greater than 78 cause extreme stress.