Heat Stress (THI) & Before Milk Drops
Protects dairy cows
Enter air temperature and relative humidity to get the temperature-humidity index, the dairy-cattle stress band — from none to emergency — and clear advice on shade, fans and water.
Conditions
Add fans/ventilation, feed in cooler hours, plenty of water.
Next: follow the band's advice (add fans/ventilation, feed in cooler hours, plenty of water) and watch your high-yielding cows first — they feel the heat soonest.
THI = (1.8T+32) − (0.55 − 0.0055·RH)(1.8T − 26); thresholds shown are for dairy cattle and shift with breed, yield and acclimation.
Heat stress (THI) — key facts
- THI
- (1.8T+32) − (0.55 − 0.0055·RH)(1.8T − 26)
- None
- below 68
- Mild / moderate
- 68–71 / 72–79
- Severe / emergency
- 80–89 / 90+
- Intake & milk fall
- above ~72
- Serious risk
- above ~80
- Cool with
- shade, fans, foggers, water
- Privacy
- Runs in your browser; nothing uploaded
Heat plus humidity is what hurts the herd
Cattle don't read the thermometer — they feel the combination of heat and humidity, because both decide how easily they can shed body heat. A cow cools by panting and sweating, and that only works when the air can take up the moisture; on a warm humid day it can't, heat builds, and she eats less and milks less. The temperature-humidity index rolls temperature and humidity into one number with clear thresholds, so a vague "it's hot" becomes a band you can act on.
This tool returns the THI value, the dairy-cattle stress band from none to emergency, and practical cooling advice from your temperature and humidity. Above about THI 72 cows lose intake and milk; above about 80 it's serious. Use it to trigger shade, fans, foggers and extra water, to feed in cooler hours, and to watch high-yielders first. Pair it with the Livestock Water Requirement, Dry Matter Intake and Fat-Corrected Milk tools to manage the herd through hot weather.
One clear number
Heat and humidity rolled into the THI value.
Know the band
None, mild, moderate, severe or emergency.
Act in time
Trigger shade, fans, foggers and water early.
Protect yield
Cool before intake and milk start to slip.
Frequently Asked Questions
What is the temperature-humidity index (THI)?+
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.
How is THI calculated?+
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.
What do the THI bands mean for dairy cattle?+
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.
At what THI do cows start losing milk?+
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.
How should I cool heat-stressed cattle?+
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.
Which animals are most at risk?+
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.
Why does humidity matter as much as temperature?+
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.
Do the thresholds apply to all breeds and yields?+
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.
How often should I check THI?+
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.
Is the THI value exact?+
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.