THI - Temperature Humidity Index & Cows

January 25, 2017 Guest Blogger Comments

Taken from studies from the Australia Dairy Association. Temperature Humidity Index (THI) is a measure that accounts for the combined effects of environmental temperature and relative humidity on cattle / livestock to assess the risk of heat stress and prevent major effects.

When THI exceeds 72, cows are likely to begin experiencing heat stress and their incalf rates will be affected.
When THI exceeds 78, cows milk production is seriously affected.
When THI rises above 82, very significant losses in milk production are likely, cows show signs of severe stress and may ultimately die.

A number of important points should be made about the THI:

A THI of 72 may under-estimate heat load in high-yielding Holstein-Friesian cows – increasing milk yield increases cows' sensitivity to heat stress.
Recent research shows that increasing milk production from 35 to 45 litres/day reduces the threshold temperature for heat stress by 5°C.
THI does not account for solar radiation or air movement – those two factors, along with air temperature and relative humidity, determine the heat gained and lost between the cow and the environment.
THI does not enable you to measure the accumulation of heat load over time, e.g. after several days. Despite these limitations, THI is still a useful and easy way to assess and predict the risk of heat stress; however, it is wise to be conservative. If you have a herd of high-producing Holstein-Friesian, it is better to overestimate the risks of heat stress using a lower THI than get caught out.

THI is calculated from air temperature and relative humidity using the following equation:

THI = (Dry bulb temperature ^oC) + (0.36 x dew point temperature ^oC) + 41.2

As you can see in this chart, a THI of 78 occurs at: