Temperature Humidity Index

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:

• 31ºC and 40 % relative humidity; or
• 27ºC and 80% relative humidity