The dairy industry is rife with acronyms, slang and shorthand, and in our world of ventilation and bovine environmental products, one of those that reigns supreme is THI in dairy cows, or Temperature Humidity Index.

This is the first in a series of VES-Artex Unpacked Articles that will help you digest common dairy industry elements in a way that makes them first understandable, but more importantly: actionable.

In its most basic sense, Temperature Humidity Index, or THI, is the measure of the combined effect of environmental temperature and the relative humidity to help assess the potential risk of heat stress to your herd. There is technology like DairyBOS® that integrate with your fans, ventilation systems, and other sensors to monitor THI in real-time, but to calculate the data point manually, the formula is:

THI = Dry Bulb Temperature – [0.55 – (0.55 x Relative Humidity/100)] x (Dry Bulb Temperature – 58)

*Note: Dry Bulb Temperature is a fancy way of saying a thermometer exposed to air without exposure to radiant heat and moisture.

According to the University of Minnesota, a THI of less than 68 indicates no heat stress, with a THI of between 80-90 marking moderate to severe heat stress conditions and anything above that being a major red flag for severe heat stress.

Why THI Especially Matters for Cows

Unlike humans, when temperatures and humidity rise, cows don’t have the ability to naturally cool themselves through sweating. According to Dr. Mike Wolf, VES-Artex’s Consulting Veterinarian and an industry expert on dairy ventilation, cows sweat about the same amount as an average human, but also create much more heat and humidity through respiration, rapid metabolic rates, and the production of urine and feces in enclosed areas such as the milking parlor.

“It's a much larger animal that has a much bigger body mass relative to its surface area,” Wolf said. “And so sweating is not an efficient way for her to cool herself. And that's why we don't use the human heat index.”

Additionally, since THI is a double-levered measurement, it can be difficult to monitor if you don’t have the right tools, and even more difficult to manage and control. Temperature, for example, can be controlled by leveraging high-pressure fogging systems and cooling the air with appropriate ventilation, but with the fog, you’re also going to raise the humidity levels.

“As an industry, we do poorly monitoring THI,” Wolf said. “Since we can’t completely control THI, is really more about awareness and determining what factors we can adjust to get a better outcome.”

Control What You Can Control

Some things Wolf suggests dairy operations should consider, or questions they can ask, when it comes to controlling THI include:

• If you’re soaking cows, what are you doing to evaporate the moisture off? Unevaporated moisture adds humidity to the barn along with elements like urine, feces and the respiration of the cow.
• Pay attention to air exchanges. The more you replace the air in the barn or parlor with your ventilation system, the less humidity and gasses will accumulate in the enclosed area.
• Humidity is the force multiplier when it comes to THI. In hot, arid climates, THI isn’t likely to be as much as an issue in climates where humidity is a bigger factor, such as the Midwest or northeast United States.