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.
Where THI is used
Research has identified THI values above which heat stress begins. These THI values vary between the different livestock species of interest and within the same species, they vary for the different classes of animals.
Normally THI is used in cattle breeding, both in dairy and in meat cows. Cows are indeed very delicate animals, which suffer a lot of heat stress with serious consequences on their productivity and on the quality of their final output. This happens because a large part of the energy deriving from food is used to maintain a constant body temperature and it is therefore not destined for other activities (milk production, growth, pregnancy, fattening, etc.).
How do I calculate THI?
Heat stress is caused by a combination of temperature, relative humidity, solar radiation, air movement, and precipitation. The majority of studies on heat stress in livestock focus on the two main environmental stressors: temperature and relative humidity (RH).
There are different formulas to calculate the THI, but one the simplest and most common is the following:
THI =0.8*T + RH*(T-14.4) + 46.4
where T = ambient or dry-bulb temperature in °C and RH=relative humidity expressed as a proportion i.e. 75% humidity is expressed as 0.75.
The water vapor concentration of the air is important since it can drastically reduce the ability of the animal to use evaporative heat loss through the skin and lungs.
Cattle can tolerate much higher temperatures at lower relative humidity because they are able to dissipate excessive heat more effectively by sweating.
However, during hot and especially humid conditions, the natural ability of cattle to dissipate heat is compromised due to the lowered ability to utilize evaporative cooling.
The THI chart
The initial studies conducted in the 1950s at the University of Missouri indicated a stress threshold of a 71 THI, so animals were experiencing heat stress at a THI of 72 and greater. The levels of stress were separated into:
- Mild (72 to 79 THI)
- Moderate (80 to 89 THI)
- Severe (90 THI or greater)
Today’s cows are much more susceptible to heat stress than the cows of the 1950s due to the increased milk production and feed intake.
The recent studies show that modern cows become heat-stressed starting at an average THI of 68 with the levels of stress increase with increasing THI values (Vitali,2009).
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.
Further factors to be taken into consideration
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 getting caught out.
The THI value provides a set of categories that indicate the heat stress of animals.
However, for its interpretation, it is necessary to know the predominant climate conditions in the area of study, as well as relative humidity and how it has been used to differentiate the discomfort category according to the type of heat: dry, typical of semiarid climates or moist heat.
The microclimatic changes that are taking place on our planet, global warming and pollution will have a significant impact on the stress tolerance of cows in the future.
We will not be surprised if the table will change again and adapt to the new standards.
Free tools to manage THI
Avoiding getting bored with formulas and getting lost in the calculations, today you can manage the THI simply by using our smartphone.
Free apps have been developed for both Android and iOS that can help breeders monitor the THI in their stalls during the hot season.
This is just one additional tool.
We know very well that in order to guarantee a state of well-being, the animals need something else. Proper ventilation is one of the most used solutions all over the world. We have discussed topic several times in our blog, you can deepen the subject by reading:
Cooling & Cross-Ventilation System: first installation in EU with exhaust fans for dairy cows
Heat stress in cows: dynamic ventilation during summer
Water atomizer in dairy farms: suggestions and advice
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