THI Heat-Stress & What the Heat Is Costing Your Milk Tank
Reads temperature
Heat plus humidity is a single number — the THI — and above 72 it pulls milk out of the tank. Enter the weather and herd to get the milk loss in kg and dollars, the fertility hit, and whether cooling pays for itself.
Today's weather & herd
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System cost ≈ $28,000 for 100 cows; saves $9,360 over the 90-day season.
Next: run fans and sprinklers over the feed bunk and holding pen during the 12 hottest hours; even partial cooling recovers part of the $144/day you are losing.
THI = (1.8T+32) − (0.55 − 0.0055·RH)(1.8T − 26), NRC/Armstrong. Milk loss ≈ slope × (THI − 72) scaled by yield level and hours of stress (St-Pierre 2003). Planning estimate — actual loss varies with airflow, water access and acclimation.
THI heat stress — key facts
- THI formula
- (1.8T+32) − (0.55 − 0.0055·RH)(1.8T − 26)
- Stress threshold
- THI ≈ 72
- Loss slope
- ≈ 0.12–0.36 kg/cow per THI unit
- Steeper for
- high & elite yielders
- Conception hit
- up to 55 points in emergency heat
- Shade relief
- ≈ 3 THI units
- Sprinklers relief
- ≈ 8 THI units
- Tunnel barn relief
- ≈ 11 THI units
- Sources
- NRC/Armstrong, St-Pierre (2003)
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THI heat-stress zones
These zones are the NRC/Armstrong dairy-cattle thresholds the calculator classifies your THI into. The loss multiplier steepens as cows move into hotter zones, and the conception-rate drop is the expected hit to fertility within each band.
| Zone | THI band | Loss multiplier | Conception drop (pts) | What you see |
|---|---|---|---|---|
| No stress (comfort) | < 72 | ×0.0 | 0 | Thermoneutral — no production penalty. |
| Mild stress | 72–79 | ×1.0 | 10 | Cows seek shade; intake and milk begin to dip. |
| Moderate stress | 79–89 | ×1.6 | 25 | Visible panting, drooling; clear milk loss. |
| Severe stress | 89–99 | ×2.3 | 40 | Open-mouth panting; large loss, fertility hit. |
| Emergency / danger | ≥ 99 | ×3.0 | 55 | Life-threatening; deaths possible without action. |
Production levels and cooling options
| Production level | Milk (kg/day) | Loss slope |
|---|---|---|
| low | 15 | 0.12 |
| medium | 25 | 0.20 |
| high | 35 | 0.28 |
| elite | 45 | 0.36 |
| Cooling system | THI relief | Cost ($/cow) |
|---|---|---|
| No cooling | 0 | 0 |
| Shade only | 3 | 60 |
| Shade + fans | 5 | 150 |
| Fans + sprinklers (soak) | 8 | 280 |
| Tunnel / cross-vent barn | 11 | 650 |
Loss slope is kg milk lost per cow per THI unit above 72; THI relief is the units each system removes. Sources: Armstrong (1994), St-Pierre et al. (2003).
Heat plus humidity is one number, and it has a price
A dairy cow sheds metabolic heat by evaporation, and humid air blunts that escape route — so heat stress is never about temperature alone. The Temperature-Humidity Index folds both into a single figure, and above a THI of about 72 cows eat less, stand more, pant, and pour less milk into the tank. The hotter and more humid the day, and the more she produces, the steeper the loss. On top of the milk, conception rate falls — a cost that surfaces months later as repeat services and a stretched calving interval.
This tool turns that biology into money. It reads the THI and heat-stress zone from your weather, predicts the milk loss per cow and per herd, prices it by the day and the season, flags the fertility hit, and tests whether shade, fans, sprinklers or a tunnel barn pay for themselves. Use it to decide whether to invest in cooling and how urgently. Pair it with the Heat-Stress THI, Cattle Cooling Water and Fat-Corrected Milk tools for a complete summer-management picture.
How to use the calculator
- 1Enter the weather. Give today's air temperature in °C and relative humidity in percent — both feed the THI formula.
- 2Describe the herd. Set the production level, the number of cows and your milk price per kg.
- 3Set the exposure. Enter the hours per day cows spend above THI 72 and the length of your heat-stress season.
- 4Choose a cooling option. Pick the system to evaluate — shade, shade plus fans, fans plus sprinklers, or a tunnel barn.
- 5Read the verdict and act. See the milk loss, the daily and seasonal cost, the conception drop and the payback in days, then decide.
Frequently Asked Questions
What is the THI formula this tool uses?+
It uses the NRC (1971) / Armstrong (1994) Temperature-Humidity Index: THI = (1.8 × T + 32) − (0.55 − 0.0055 × RH) × (1.8 × T − 26), where T is air temperature in °C and RH is relative humidity in percent. This is the same form used by the US National Weather Service and most dairy heat-stress services, so a THI you read here matches the published thresholds exactly.
At what THI do dairy cows start losing milk?+
Mild heat stress begins at a THI of about 72. Below that cows are thermoneutral and there is no production penalty. From 72 to 79 cows seek shade and intake dips; from 79 to 89 the loss is clear with visible panting; above 89 it is severe, and above 99 it is an emergency where deaths are possible. The tool classifies your THI into one of these five zones automatically.
How is the milk loss calculated?+
Loss rises with how far the THI sits above the 72 threshold. The tool multiplies that excess by a per-THI slope that depends on production level — roughly 0.12 kg/cow/THI for low yielders up to 0.36 for elite cows (St-Pierre 2003) — then by a zone multiplier (steeper in the hotter zones) and by the fraction of the day spent above threshold. Multiply by herd size and milk price for the daily and seasonal cost.
Why do high-yielding cows lose more milk to heat?+
A cow's heat load is largely the heat of her own metabolism, and a 45 kg/day cow generates far more metabolic heat than a 15 kg/day cow. With more heat to shed and a smaller surface-area-to-output ratio, high and elite producers cross into stress sooner and lose more milk per THI unit. That is why the tool's loss slope steepens with production level.
Does heat stress affect fertility as well as milk?+
Yes, and often more lastingly. Conception rate falls roughly 10 percentage points in mild stress, 25 in moderate, 40 in severe and over 50 in emergency conditions (Schüller 2014). The tool flags the expected drop for your THI zone. Expect more repeat services, lower summer conception and a longer calving interval — a cost that shows up months after the hot spell.
How does the cooling payback work?+
Each cooling option removes a set number of THI units — shade about 3, shade plus fans 5, fans plus sprinklers 8, a tunnel barn up to 11. The tool re-runs the milk-loss math at the cooled THI, finds the milk recovered per day, multiplies by your milk price, and divides the system cost per cow by that daily saving to give a payback in days. Anything inside one heat season usually pays.
Which cooling system gives the best return?+
For most herds, fans plus sprinklers over the feed bunk and holding pen is the sweet spot: it removes about 8 THI units at a moderate cost and pays back fast in a hot climate. Shade alone is the cheapest first step and removes the solar load. A tunnel or cross-vent barn gives near-full relief but only pays where the heat season is long and milk price is high. Compare them directly in the tool.
What does 'hours above THI 72' mean and why does it matter?+
Heat stress is not constant — a day might peak at THI 85 in the afternoon but sit in comfort overnight. The hours-above-threshold input scales the daily loss to the fraction of the day cows are actually stressed (out of 24). A long, humid spell that stays hot at night does far more damage than a brief afternoon peak, and the tool reflects that.
Why does humidity matter so much?+
Cows shed heat largely by evaporation — sweating and panting — and high humidity blunts that escape route. The THI formula weights humidity for exactly this reason: 32 °C at 30% RH is uncomfortable, but 32 °C at 80% RH is dangerous because the cow cannot evaporate heat fast enough. Two days at the same temperature can fall in different stress zones purely on humidity.
How many days is a typical heat-stress season?+
It varies hugely by location — a temperate dairy might see 60–90 stressful days, a tropical or subtropical one 120 or more. The tool lets you set the season length so the seasonal cost and the cooling payback reflect your climate. Use weather records for the number of days the THI usually exceeds 72 in your area.
Are these milk-loss figures exact?+
They are well-founded planning estimates built on published THI thresholds and loss slopes, but real loss depends on airflow, water access, acclimation, parlour design and the individual herd. Treat the numbers as a basis for deciding whether to invest in cooling and how urgently, then confirm against your own bulk-tank records across a hot spell.
Should I invest in cooling for my herd?+
Compare the payback in days against your heat-stress season length. If a system pays back well inside one season — common for fans and sprinklers in a hot climate with a high-yielding herd — it is almost always worth it, because it then banks the recovered milk every year and protects fertility too. If the payback runs past several seasons, start with the cheaper shade step and re-evaluate.