The Temperature-Humidity Index (THI) and Your Holstein Herd: When Heat Stress Starts Costing You Money

One number for two weather readings

A hot dry afternoon and a warm humid one can feel very different to a cow. Dry heat lets her shed body heat through sweating and panting. Humid air blunts both. The Temperature-Humidity Index, or THI, folds air temperature and relative humidity into a single figure so you can compare conditions and decide when to act. University of Wisconsin dairy extension describes it as an approximation of the heat stress cattle experience, and gives an important warning: it does not account for solar radiation, so cows on pasture, in open lots or in outdoor hutches face more risk than the number suggests.

How to calculate it

One widely used version is the National Research Council formula, reproduced in a 2007 Journal of Dairy Science paper by Bohmanova, Misztal and Cole:

THI = (1.8 × T + 32) − (0.55 − 0.0055 × RH) × (1.8 × T − 26)

Here T is dry-bulb temperature in degrees Celsius and RH is relative humidity in percent. A few worked examples:

Air temperature	Relative humidity	THI (approx.)
20°C (68°F)	50%	65
25°C (77°F)	60%	73
30°C (86°F)	50%	78

Notice how quickly a pleasant-feeling afternoon crosses the commonly quoted thresholds. A 25°C day with moderate humidity already sits above 72. Several different THI equations exist, and they give different numbers for the same weather, so make sure the threshold you use was developed for the equation you use.

What "the threshold" really means

You will hear two numbers most often. Wisconsin extension notes that "the THI cutoff of 72, and more recently 68" was based on studies of when lactating cows show sharp drops in milk production. A very large U.S. analysis in the Journal of Dairy Science used 12.8 million test-day records from 923,026 Holstein cows and found that average adjusted yields started to fall at a five-day average THI of 69 for Holsteins and 72 for Jerseys. About 15 percent of the Holstein test-day records fell on or above that line, so a meaningful slice of an average year is already in the zone.

The Bohmanova paper adds a useful caution. Estimated thresholds varied by index and by location. For the NRC-based index they estimated about 72 in Athens, Georgia and 74 in Phoenix, Arizona, and across the seven indices they compared the thresholds ranged from 68 to 83. The lesson is not that the number is unreliable. It is that your threshold depends on how and where you measure.

Why cows suffer before the milk check shows it

By the time milk drops, the cow has already been paying for a while. Cattle of all ages can have welfare problems at lower THI values than the ones that cut milk, according to Wisconsin extension. Cows reduce feed intake, but intake alone does not explain the whole picture: one recent review of pair-feeding research concluded that lower dry matter intake accounts for only part of the milk loss, with the rest arising from effects of high body temperature on the udder, metabolism and reproduction. This is why heat stress costs show up in conception rates and health events weeks after the weather breaks.

Finding your own money line

Published thresholds are starting points. Your barn, your ventilation, your genetics and your cooling system decide where your cows actually start to lose money. Here is a straightforward way to find your number.

1. Log THI where the cows are. Wisconsin recommends estimating THI from the animals' immediate microclimate, such as the home pen or the parlor holding area, not from the weather station down the road. A cheap logger with temperature and humidity sensors in the freestall barn is enough.
2. Record daily and hourly highs. Cows respond to duration as well as peak, and the large U.S. study used a five-day average because heat loads accumulate.
3. Match to herd data. Line up daily THI against bulk-tank or parlor milk per cow, then against conception rates for cows bred during hot spells.
4. Look for the bend. Plot milk per cow against THI. The point where the line starts to fall is your working threshold. Do the same for conception.
5. Check the cows directly. Wisconsin extension says about 60 breaths per minute in lactating cows signals that more cooling is needed, and panting averages about 100 per minute. Count respirations in a sample of cows on hot afternoons.

Turning THI into dollars

The cost of heat is not one number. It is a sum, and every herd's sum is different. Build it from your own records:

• Lost milk: (average milk loss per cow per day on hot days) × (cows affected) × (hot days) × (milk price).
• Lost fertility: extra days open and extra services on cows bred in summer, priced at your own cost per day open.
• Health and culling: any rise in metabolic disease, lameness or early culls in the following weeks.
• Cooling cost: electricity, water and maintenance, which belong on the other side of the ledger.

Only the first two lines usually show up clearly in a herd's records, which is why many herds underestimate heat losses. Compare the total to the annual cost of better cooling and you have a defensible investment case. Our guides to heat stress and cooling and freestall cow comfort cover what to put in the barn.

Practical handling rules at high THI

Wisconsin extension suggests postponing handling tasks that can wait for cooler weather once THI reaches 72 or higher, and avoiding transport in moderate to severe conditions. That means moving vet work, regrouping and pregnancy checks to early morning where you can. A cow standing in a headlock in afternoon sun is adding stress to a system already at its limit.

The takeaway

Treat 68 to 72 as the range where you start paying attention, then replace it with your own number. Measure in the pen, not the weather report, and log it against milk and conception. For more tools and local resources, visit the Agriculture Directory, and for a lighter look at dairy life on hot nights, see farm.fm's bulk-tank alarm story. This site is independent and not affiliated with any breed association.