TEMPERATURE HUMIDITY INDEX

What is Temperature Humidity Index?• Temperature Humidity Index (THI) is a measure that has been used

since the early 1990s. It accounts for the combined effects of environmental temperature and relative humidity, and is a useful and easy way to assess the risk of heat stress.• When the THI exceeds 72, cows are likely to begin experiencing heat

stress and their incalf rates will be affected.• When the THI exceeds 78, cows milk production is seriously affected.• When the 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.

Formula• 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• Even mild heat stress causes physiological changes in the cow that

may have an impact on your bottom line.• The temperature can be mild but if humidity is high, cows start feeling

the heat.