Q. Why is this a top resource article? "Research continues to focus on ways to alleviate heat stress through genetic advancement and knowledge around genes that are important in alleviating heat load, hormonal manipulation such as the use of human chorionic gonadotropin to improve pregnancy retention, feed additives that potentially reduce core body temperature and, lastly, advancements in cooling systems and areas of opportunities with cooling such as dry cows." —Todd Bilby, Merck Animal Health The Temperature-Humidity Index (THI) is a valuable measure that you know when cows are becoming heat-stressed and to what degree so you can use cooling methods appropriately. 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).

This is because data on the amount of thermal radiation (sunlight) received by the animal, wind speed and rainfall are not publicly available. However, records on temperature and humidity can be easily obtained from a meteorological station located nearby or from inexpensive personal weather devices.

A THI is a single value representing the combination of air temperature and humidity associated with the level of thermal stress. The THI was developed as a weather safety index to monitor and reduce heat stress-related losses. Utilizing both humidity and temperature is important compared with utilizing temperature alone to evaluate heat stress.

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.


As previously explained above, the Temperature-Humidity Index (THI) lets you know when your cows are becoming heat-stressed and to what degree so you can use appropriate cooling methods.

There are several THI indices that have been used in studies across varying climatic conditions due to the differences in sensitivity to ambient temperature and amount of moisture in the air among species.

For this article, I have focused on the THI calculation used to develop the more commonly used THI chart for cattle in the 1950s. It has recently been re-evaluated by scientists at the University of Arizona to develop new thresholds for heat stress on cattle.

The THI formula used for these studies is:

THI = Tdb – [0.55 – (0.55 x RH/100)] x (Tdb – 58)

This formula uses dry bulb temperature (Tdb, ºF) and the relative humidity (RH). The RH is divided by 100 to express the percentage in decimals.

THI formula

The Tdb is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. It is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature.

For example, if you take a day in Arizona when the dry bulb temperature is 80ºF and the humidity is 20 percent, your THI calculation would be:

80 – [0.55 – (0.55 x 20/100)] x (80 – 58)

This gives you a THI value of 70.32.

However, let’s take that same example and increase the humidity similar to a Florida environment of 90 percent. The THI calculation would be:

80 – [0.55 – (0.55 x 90/100)] x (80 – 58)

This gives you a THI value of 78.79.

This scenario gives you a rounded THI value of 70 in the Arizona example and 79 in the Florida example, with the differences being simply due to the increase in RH.

The THI is usually classified into level of heat stress. 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) and severe (90 THI or greater).

More recently, the THI has been re-evaluated at the University of Arizona on modern-day, high-producing Holstein dairy cows. 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 increasing with increasing THI values.

There is a THI chart that calculates THI based on increasing temperature and humidity. This chart is normally color-coded to represent the varying degrees of heat stress on cattle and can easily be used to assess the degree of heat stress animals are experiencing if you know the temperature and humidity.

Another option is to automatically calculate THI with heat stress apps that are available from iTunes. Dr. Don Spiers at the University of Missouri has developed a heat-stress app named Thermal Aid that calculates the THI based off of data from local weather stations.

Additionally, Purina Animal Nutrition LLC has an app named Cool Cow that calculates the THI based off of a thermometer-hygrometer that can be purchased and placed in your barn to obtain the temperature and humidity values. Or you can manually input the temperature and humidity.

Both of these apps can be used to determine the THI without having to do the hand calculation or having the printed THI chart. In addition, the apps will automatically classify the degree of heat stress based off of the THI value obtained. PD

Todd Bilby
  • Todd Bilby

  • Associate Director, Ruminant Technical Services
  • Merck Animal Health
  • Email Todd Bilby