For a cow to achieve optimum performance, their ambient temperature should be between 5°C and 25°C.
There will be some variation, according to breed, weight and feeding levels, but this is a useful target temperature range to keep in mind. If cows are left exposed to temperatures in excess of 25°C for prolonged periods of time, heat stress occurs and cows will start cooling themselves down by panting, drooling and increasing internal blood flow.
All of these actions use energy and nutrients that would normally be utilised by the cow. The effects of heat stress can be seen by:
It is important to remember that cows already generate a high volume of heat, so it does not take a large increase in ambient temperature for cows to start suffering from heat stress. The temperature humidity index (THI) is a good indicator of potential heat stress problems.
The use of a hygro-thermometer or local temperature and humidity data will help you make heat stress decisions.