Temperature-Humidity Index (THI) Table

A cow’s ideal temperature range (known as thermoneutral zone) is between -5 to 25°C. Heat Stress occurs when the cow is generating more heat than what she can lose through the environment.

Cows are susceptible to a combination of heat and humidity – measured by temperature humidity index (THI), which is why THI is often a better measure of potential heat stress risk than building temperature alone. As humidity increases, the lower the ambient temperature needs to be before a cow experiences heat stress.

The impact of humidity can be understood by simply comparing how hot you feel on a humid summer’s day contrasted to a dry summer’s day.

To understand the THI level in your building, you can measure the temperature and humidity using fairly low cost devices purchased on line, and look up the value using the table below. Alternatively more sophisticated measuring devices can work out the value for you.

The table highlights the different levels of Heat Stress severity, however research now suggests that sustained periods of time within the white THI region can impact fertility, and increase lameness long before impact on milk yield.

THI Table (Source: Burgos & Collier)