Temperature-Humidity Index (THI) for Cattle

Heat gives no warning. Milk yield drops, breathing rate climbs and conception rate collapses well before the day feels unbearable to the person walking the alley in boots. That is why dairy and feedlot operations trigger cooling from an index rather than from a hunch: the reading taken inside the barn at ten in the morning decides whether sprinklers and fans come on today or whether the power bill gets a break. Consultants lean on the same figure to justify shade structures, and nutritionists reach for it to explain last week dip in dry matter intake.

The metric form published by the NRC in 1971 reads THI = 0.8 × T + (RH/100) × (T − 14.4) + 46.4, with T as dry-bulb temperature in degrees Celsius and RH as relative humidity in percent. The widely quoted Armstrong bands run: below 72 comfort, 72 to 78 alert, 79 to 88 danger, above 88 emergency. The 32 °C and 70% loaded on screen give 84.3, squarely inside danger. Notice how much humidity weighs: hold temperature at 32 °C and drop humidity to 40% and the index falls to 79.0; at 30% it reaches 77.3. Dry inland heat punishes a herd less than coastal heat at the same thermometer reading.

Two variables are all this index sees. Solar radiation and air speed sit outside it, so an animal standing in full sun on unshaded pasture suffers more than the number suggests, while a barn under forced ventilation suffers less; the black globe humidity index and radiant thermal load exist to cover that gap. The scale was calibrated on herds of the 1970s that produced far less milk, and later work finds yield losses in high genetic merit Holsteins starting around 68, which leaves the classic 72 trigger too relaxed for that profile. Zebu cattle shrug off bands that would flatten a Holstein. Measure where the animal stands, in the shade, not at the municipal weather station.