During hostile weather conditions or in the tropics with insufficient environmental cooling at night cattle will accumulate heat that they cannot disperse.
Temperature Humidity Index
Temperature alone is not a good way of measuring heat stress. Various indexes have been developed which take into account such factors as ambient temperature, relative humidity and evaporation rate. These are known as THI (Temperature Humidity Index).
Heat Stress is not only related to ambient temperature but also associated with humidity and air movement. When the humidity increases the animals evapo-transpiration is reduced and the animal cannot cool itself. This inability to cool itself increases the core body temperature and greatly depresses feed intake.
Extensive Grazing and heat load
One disadvantage of the temperature-humidity index (THI) is that it may not predict the true extent of heat stress in extensive grazing systems because it does not account for accumulated heat load.
Another shortfall of THI is that it does not account for solar radiation and wind speed which can affect heatload of cattle.
Zone of Comfort
The "Zone of Comfort” of Bos Taurus cattle range between a range of 4ºC -24ºC (39° F- 75° F). Within this temperature range animals are most efficient.
The “Zone of Comfort” of Bos Indicus e.g. Zebu, is 10°C (50°F) to 27°C (81°F).
Each genotype has a different and characteristic “zone of comfort” Science has established that under heat stress conditions, Bos Indicus breeds and their crosses have better heat regulatory capacity than Bos Taurus breeds, due to differences in metabolic rate, food and water consumption, sweating rate, and coat characteristics and colour.
Invisible signs of heat stress
Stockman’s Heat Stress Check List
Intensively reared beef cattle-feed lots