Thermostasis is the process by which warm-blooded animals keep their body temperature constant despite changes in environmental temperatures. Heat stress occurs when calves are incapable of dissipating enough heat to maintain their core body temperature below 38.5ºC. This increase in body temperature results from the combination of heat from the environment and that produced internally during nutrient metabolism.
In the northern hemisphere it is quite common for calves to be housed in polypropylene hutches with a small restricted outside area. During warm summer days calves seek the shade supplied by the hutch, however the heat inside of it will be concentrated. The calf then resorts to physiological mechanisms to lower its body temperature.
Under these conditions it is easy to verify an accelerated respiratory rate also accompanied by increases in rectal temperature, ear temperature, heart rate, and salivary cortisol (SC).
Ambient temperature and humidity determine the heat stress threshold
Not only is the absolute ambient temperature important but its combination with humidity in what is known as the temperature-humidity index (THI).
In adult cattle the upper critical value for this index is between 72 and 74, with some authors even considering up to 78 before respiratory rate and rectal temperature increase. In young calves however the optimum THI has not been determined, it is suspected however it should differ from a mature lactating cow since the heat increment resulting from forage fermentation in the rumen is very low, and the metabolic heat associated with milk production is non-existent.