Beat the Heat Index & Size the Fans and Soakers
Sizes fans
Enter temperature and humidity to get the Temperature-Humidity Index, then size the fans and soakers needed to pull cows below the milk-loss threshold (THI 68 for high producers). See fan count, soaker nozzles, kW, water use, THI reduction and the milk saved — on the NRC THI formula.
Set the heat load
Next: even with this kit the cows sit at THI 71.9 — still 3.9 above the 68 threshold. Add more air speed (basket/high-volume fans), increase soaker frequency, or provide shade and night cooling; consider moving milking to cooler hours.
THI = (1.8·T+32) − (0.55 − 0.0055·RH)(1.8·T+32 − 58) (NRC 1971). Thresholds: Zimbelman et al. 2009 — THI 68 onset of milk loss in high producers. Cooling modelled as effective-THI reduction from air speed (≈1.7 THI per m/s), evaporative soaking and shade per extension heat-abatement data. Fan/soaker coverage are planning figures — verify against your fan's rated throw.
Cattle THI cooling — key facts
- THI formula
- (1.8T+32) − (0.55−0.0055·RH)(1.8T+32−58)
- High-producer threshold
- THI 68 (milk loss onset)
- Milk loss slope
- ≈ 0.25 kg/day per THI > threshold
- Fan coverage
- ≈ 21 m² per 48-in panel fan
- Air-speed cooling
- ≈ 1.7 effective-THI per m/s
- Soaker spacing
- ≈ 1 nozzle per 0.6 m feed line
- Evaporative cooling
- removes most heat in dry heat
- Basis
- NRC 1971/2001 · Zimbelman 2009
Temperature-Humidity Index reference grid
THI computed with the NRC (1971) formula. Cells are shaded by stress band: green = no stress (<68), amber = mild (68–72), orange = moderate (72–80), red = severe (≥80). Notice how rising humidity pushes the same temperature into a higher band — humidity blocks the cow’s evaporative cooling.
| Temp ＼ RH | 30% | 50% | 70% | 90% |
|---|---|---|---|---|
| 26 °C | 70.8 | 73.1 | 75.4 | 77.7 |
| 28 °C | 73 | 75.7 | 78.4 | 81.1 |
| 30 °C | 75.2 | 78.3 | 81.4 | 84.5 |
| 32 °C | 77.4 | 80.9 | 84.4 | 87.9 |
| 34 °C | 79.6 | 83.5 | 87.4 | 91.3 |
| 36 °C | 81.9 | 86.1 | 90.4 | 94.7 |
| 38 °C | 84.1 | 88.7 | 93.4 | 98.1 |
| 40 °C | 86.3 | 91.4 | 96.4 | 101.5 |
Source: NRC (1971) THI formula; thresholds Zimbelman et al. (2009) & extension heat-stress charts.
Heat-stress bands and group thresholds
| THI | Band | What it means |
|---|---|---|
| <68 | No stress | Comfortable; no abatement needed. |
| ≥68 | Mild stress | Milk loss begins in high producers; start cooling. |
| ≥72 | Moderate stress | Intake & milk drop; cooling required. |
| ≥80 | Severe stress | Major losses; full fans + soakers, shade. |
| ≥90 | Danger | Emergency — risk of death; maximise all cooling. |
| Cow group | THI threshold | Loss kg/d·THI |
|---|---|---|
| High-producing (≥35 kg/d) | 68 | 0.25 |
| Mid-lactation | 70 | 0.16 |
| Dry / far-off | 72 | 0.05 |
Don’t just measure heat stress — size the kit to remove it
Heat stress is the single biggest summer drag on milk yield, and the Temperature-Humidity Index is how it is measured: it folds temperature and humidity into one number, because a humid 32 °C punishes a cow far more than a dry 35 °C. For a modern high-producing cow, milk yield starts falling at a THI of about 68 — and from there she loses roughly a quarter-kilogram of milk a day for every THI unit above the threshold. The fix is cooling, but the question is always how much.
This tool answers that. It computes the THI from your weather, then sizes the fans and soakers needed to pull the cow’s effective THI back below the threshold — reporting the fan count, soaker nozzles, kilowatts, water use, the THI reduction achieved and the milk saved per day in dollars. Pair it with the Cattle Cooling Water and Silage Loss tools to plan the whole summer feeding-and-cooling programme.
Size the system
Fan count and soaker nozzles for your barn area and feed line.
Value the milk
See the milk and dollars saved versus no cooling, per day.
See what each kit adds
Toggle fans, soakers and shade — watch the THI drop.
Plan the supply
Total kW and litres/hour to size power and water.
How to size cattle cooling from THI
- 1. Enter the weather. Put in the air temperature in °C and the relative humidity in percent.
- 2. Pick the cow group. Choose high-producing, mid-lactation or dry to set the milk-loss THI threshold.
- 3. Enter the barn. Add the freestall/holding area to cool, the feed-line length and the herd size.
- 4. Choose the kit. Toggle fans, soakers and shade to see how much effective THI each removes.
- 5. Read the sizing. See fans needed, soaker nozzles, kW, water use and the milk saved per day.
Frequently Asked Questions
What is the THI threshold for milk loss in dairy cows?+
For high-producing cows, measurable milk loss begins at a Temperature-Humidity Index (THI) of about 68 — lower than the long-quoted 72, because modern high-yielding cows generate more metabolic heat. The tool uses THI 68 for high producers, 70 for mid-lactation and 72 for dry cows, and sizes cooling to hold the herd below the chosen threshold.
How is THI calculated from temperature and humidity?+
The tool uses the NRC (1971) formula: THI = (1.8·T + 32) − (0.55 − 0.0055·RH)·(1.8·T + 32 − 58), where T is the dry-bulb temperature in °C and RH is relative humidity in percent. For example, 30 °C at 50% RH gives a THI of about 78 — moderate heat stress. Higher humidity raises THI sharply because it blocks evaporative cooling from the cow.
How many fans do I need to cool my cows?+
The tool divides the area you want to cool by the coverage of a 48-inch (1.2 m) panel circulation fan — about 21 m² each at a useful cow-level air speed of roughly 2 m/s. A 420 m² freestall area therefore needs about 20 fans. It also reports the total fan power in kilowatts so you can plan electrical supply and running cost.
How many soaker nozzles do I need?+
Feed-line soakers are spaced to wet the cows along the bunk — roughly one nozzle every 0.6 m of feed line. A 60 m feed line needs about 99 nozzles. The tool reports the count and the water use in litres per hour while running, so you can size the supply line and the pump.
Do fans or soakers remove more heat?+
On a hot, dry day the soaker-plus-fan evaporative cycle removes far more heat than fans alone. Air movement helps the cow shed heat by convection — roughly 1.7 effective-THI units per metre per second of air speed — but wetting the cow's hide and then evaporating that water with fan air removes about twice as much apparent heat. That is why high-heat systems combine soakers on the feed line with fans, not just fans.
What does 'effective THI reduction' mean — does cooling change the actual THI?+
Cooling does not change the air's measured THI; it changes the heat load the cow actually experiences. The tool models fans, soakers and shade as reductions in the cow's effective (apparent) THI — air speed, evaporation off a wet hide and removing radiant sun load all lower the cow's heat burden. Sizing to push effective THI below the threshold is what eliminates the milk loss.
How much milk does heat stress cost?+
Above the threshold a high-producing cow loses roughly 0.25 kg of milk per day for each THI unit of stress, so a cow sitting at THI 78 against a 68 threshold loses about 2.5 kg/day. Across a 100-cow herd that is 250 kg of milk a day — the tool turns it into a dollar figure at your milk price, which is usually many times the running cost of the cooling.
Does humidity matter as much as temperature?+
Yes. THI weights both, and at high humidity the cow cannot dump heat by evaporating moisture from her lungs and skin, so the same air temperature feels much hotter. That is why 32 °C at 80% RH is more dangerous than 35 °C at 30% RH — and why soaker (evaporative) cooling works brilliantly in dry heat but less so in humid heat, where moving more air becomes the priority.
What if the cooling still leaves the cows above the threshold?+
The tool flags the residual THI above the threshold. To close it, add air speed with more or larger fans, increase soaker frequency (more wet-evaporate cycles), provide shade to cut radiant load, and use night cooling and cooler-hour milking. In extreme humid heat there is a ceiling on what evaporative cooling can achieve, so air movement and shade carry more of the load.
How often should soakers run?+
Soakers run on a cycle — typically wetting the cow for 1–3 minutes, then off for several minutes to let the water evaporate (and the fans drive that evaporation). The goal is to wet the hide thoroughly, not to keep it continuously wet, which wastes water and can chill or create mud. The tool's water-use figure assumes a few on-cycles per hour.
Where do the THI thresholds and cooling figures come from?+
The THI formula is NRC (1971), adopted in the NRC Nutrient Requirements of Dairy Cattle (2001). The 68-onset threshold for high producers follows Zimbelman et al. (2009) and current extension heat-stress charts. Fan coverage, air-speed cooling, soaker spacing and evaporative effect are drawn from university heat-abatement engineering data (Univ. of Arizona, Kansas State, Univ. of Wisconsin).
Is this for dairy cows only, or beef and buffalo too?+
The THI formula applies to all cattle, and the cooling engineering is general — so it is useful for beef feedlots, holding pens and buffalo too. The milk-loss output is dairy-specific (it values lost milk), but the THI, the cooling reduction and the equipment sizing are valid for any group of cattle you need to keep below a heat-stress threshold.
How accurate is the equipment sizing?+
It is a sound planning estimate. Real fan throw depends on the specific fan, mounting height and spacing; soaker performance depends on nozzle type, pressure and cycle; and effective cooling depends on airflow reaching cow level. Use the result to scope a system and order equipment, then commission and measure cow-level air speed and surface wetting on site, adjusting fan spacing and soaker timing to the actual barn.