The THI, weather condition evaluation criterion
The THI (Temperature Humidity Index) weather condition evaluation criterion is a measure of thermal comfort with ease of use in animal husbandry.
Under high ambient temperatures and humid conditions, the physiological ability to dissipate this additional heat is more limited than with dry air (Beale et al., 2018) due to limited evaporation and associated thermolysis. Thus, indices used to assess the climatic environment typically include a combination of temperature, and humidity.
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The Temperature and Humidity Index (THI) is generally considered to be the most reliable indicator of stress for animals. This index combines both temperature and humidity. With the same temperature, as the relative humidity increases, so does the THI.
Other parameters that are also found in other indicators of weather conditions are:
- Radiation (action of the sun) is a concept that concerns only animals raised outdoors.
- Convection (heat transport) mainly concerns ruminants. Indeed, ruminants are capable of sweating (this faculty is absent or very reduced in pigs and poultry). The evaporation of this sweat will cool the animal. The air velocity brings then an important factor by accelerating the evaporation. This is why the CCPA Group uses the corrected THI, which integrates the air speed for a more accurate approach for ruminants. For poultries and pigs, respiration is the major means of thermolysis. Under high THI conditions, the proportion of body heat removal by convection decreases.
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Other criteria exist for a finer evaluation but are too complex or require specific devices that are difficult to use in field conditions.