Heat Stress (THI) & Before Milk Drops

THI is a single number that combines air temperature and humidity to show how much heat load an animal feels. Cattle cool themselves by losing heat to the air, and humid air makes that far harder, so a warm humid day stresses a cow more than the same temperature in dry air. THI captures both in one value with established stress thresholds.

Using temperature in °C and relative humidity in %: THI = (1.8 × T + 32) − (0.55 − 0.0055 × RH) × (1.8 × T − 26). For example at 32 °C and 70% humidity, THI works out to about 82 — in the severe band. The tool applies this formula and places the result in the dairy-cattle stress bands for you.

For dairy cattle the common bands are: below 68 no stress, 68–71 mild, 72–79 moderate, 80–89 severe, and 90 or above an emergency. Above about 72 cows start losing feed intake and milk; above about 80 the effects become serious and animals are at real risk. The tool names the band and the level so you know how urgently to act.

Production and intake typically begin to slip from around THI 68–72, often earlier in high-yielding cows, and the losses grow as THI climbs. By the severe band (80+) intake, milk yield, fertility and rumen health are all affected. The point of the index is to flag the rise before it shows up as a tank drop weeks later.

Provide ample shade, move air with fans, and combine sprinklers or foggers with airflow to cool by evaporation; ensure plenty of cool, clean water within easy reach; and feed during the cooler parts of the day. Cooling should focus on the holding yard, feed line and resting areas where cows spend most time bunched together.

Watch high-yielding cows first — they generate the most metabolic heat and feel stress at lower THI than dry stock. Fresh and late-pregnant cows, dark-coated animals, and any that are sick or already low on water are also more vulnerable. Prioritise cooling and water for these groups when THI climbs into the moderate band and above.

Cattle shed heat partly by sweating and panting, which work by evaporation — and evaporation slows sharply when the air is already moist. So 30 °C at 80% humidity is far more dangerous than 30 °C at 30%. That's why THI weights humidity into the figure, and why misting only helps when paired with airflow to carry the moisture away.

The bands here are the widely used dairy-cattle thresholds, but they shift with breed, yield, acclimation and management. Bos indicus breeds and acclimated animals tolerate higher THI; high-yielding Holsteins feel stress sooner. Treat the bands as a guide and watch your own animals' panting, drooling, bunching and intake alongside the number.

Check it through the hottest part of the day during warm, humid spells, and remember overnight recovery matters too — if THI stays high at night, cows can't shed the day's heat load and stress compounds. During heatwaves, monitor temperature and humidity through the afternoon and have cooling running before the index reaches the severe band.

The formula is exact for the temperature and humidity you enter, but real heat load also depends on solar radiation, wind, housing, stocking density and the animal itself. Use the index and band as a decision trigger for shade, fans, water and feeding timing, and back it up by watching the cattle directly.