Long- and short-term climatic patterns influence all livestock types: poultry, pigs, sheep, goats, dairy and beef cattle and others. Extreme environmental conditions, such as cold and hot conditions due to air temperature, thermal radiation, humidity and wind speed, cause stress and suboptimal animal performance, which lowers productivity. With beef cattle in the Northern Cape and North West in particular, but also in parts of KwaZulu-Natal, heat stress is known to be a major source of production losses.
THE TEMPERATURE-HUMIDITY INDEX
Globally, the level of heat stress is measured with different indices. In South Africa, the temperature-humidity indices of the South African Weather Services (SAWS) for different species of livestock are the broadly applicable formulas used to assess the impact of extreme weather on livestock. The index is a measure of apparent temperature, caused by the combined effects of air temperature and relative humidity on the subjective perception of heat and humid weather. Wind speed can be an aggravating factor. These handy tools can be used for risk management of heat stress and may prevent adverse effects.
The temperature-humidity index (THI), for beef cattle particularly, is calculated as follows:
THI = 0,8 + RH*(Ta – 14,4) + 46,4 where RH indicates relative humidity and Ta indicates dry-bulb temperature.
Although there are several categorisations of heat stress indices, SAWS has categorised the livestock weather safety index as ‘normal’ under 74, ‘alert’ between 74 and 79, ‘dangerous’ between 79 and 84, and ‘emergency’ when above 84 (see map).
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