What is the THI and why is it important for dairy cows?

The THI (Temperature Humidity Index) combines air temperature and relative humidity into a single value that reliably describes the thermal comfort of dairy cows. Unlike humans, cows can barely cool themselves by sweating. From THI 68 onward, silent heat stress begins with measurable effects on milk production, fertility, and the immune system.

At what THI value do cows experience heat stress?

From THI 68, high-producing dairy cows begin to suffer from heat stress. At THI 72, moderate stress begins. From THI 78, severe stress; above THI 84, acute risk to life. Modern high-producing cows (30+ litres/day) are sensitive from THI 65 due to higher metabolic heat production.

How do I calculate the THI manually?

THI = (1.8 × T + 32) − [(0.55 − 0.0055 × RH) × (1.8 × T − 26)]. T = temperature in °C, RH = relative humidity in %. At 28°C and 70% humidity, THI ≈ 78.4 – severe heat stress. Our free THI Calculator computes this automatically via GPS.

What symptoms does a cow show under heat stress?

Typical symptoms: breathing rate above 60/min (normal: 20–30), body temperature above 39.5°C, drooling, reduced feed intake, frequent seeking of water and shade, standing instead of lying. Breathing rate is the most reliable early indicator.

How much milk does a cow lose at THI 80?

At THI 78–80, a high-producing cow loses 4–8 kg of milk per day. For a herd of 100 cows over 30 heat stress days, this means a milk revenue loss of over €12,000–€24,000 per season – not counting downstream damage to fertility and animal health.

How much water does a cow need in the heat?

During severe heat stress (THI above 78), dairy cows need 150–160 litres of water per animal per day – nearly double the normal intake. Water must be cold (below 15°C) and always fresh. At least 10 cm of trough space per cow is needed.

Which breeds are most sensitive to heat?

Holstein-Friesian cows are the most heat-sensitive because their high milk production generates enormous body heat. Zebu crosses and tropical animals tolerate significantly higher THI values. Modern high-producing cows suffer from THI 65 – 3 points earlier than the classic threshold of 68.

How do I detect heat stress in the barn early?

Early indicators: breathing rate above 50/min, reduced rumination, animals standing instead of lying, increased water intake, groups crowding around fans or troughs. Real-time THI monitoring via GPS helps initiate cooling before milk production drops.

What helps immediately in acute heat stress?

Immediate measures: (1) fans at maximum power, (2) activate sprinklers – short intense spraying + 3 m/s airflow, (3) ensure fresh, cold water, (4) shift feeding to evenings and early mornings, (5) do not drive animals through narrow passages.

How does heat stress affect the fertility of dairy cows?

Heat stress reduces conception rates by up to 30%. Eggs are damaged during follicular maturation months before summer heat – fertility often remains impaired until autumn. Embryos die from a uterine temperature increase of just 1°C in the first 3–5 days after insemination.

Barn Cooling Strategies for Dairy Farms: Fans, Sprinklers and Ventilation

2026-06-05THI Cows Team

British and Irish cubicle housing was designed for a cooler, wetter climate. Wide Yorkshire boarding and open ridges work beautifully in a typical wet winter – but on a still, humid July afternoon, the same building can trap heat and drive indoor THI values several points above outdoor conditions. With UK summers trending hotter and humid heatwaves becoming more frequent, adapting your barn is no longer optional for high-output herds.

Here are the most effective strategies, ranked by practical impact.

1. Fans: The Most Cost-Effective Intervention

Fans reduce the felt temperature by accelerating evaporative cooling from the cow's body surface. The target is at least 2.0 m/s air speed at cow level in both the lying and feed areas. At this velocity, cows can dissipate heat effectively even at moderately elevated temperatures.

A 120-cm fan on a 3–4 metre drop cable covers approximately 8–10 cubicle spaces. For a 200-cow cubicle building, budget for 16–20 fans. Don't neglect the collecting yard – this is often the hottest point in the system and a source of significant stress during twice-daily milking. Fans in collecting yards yield immediate milk yield improvements on hot days.

For robotic milking systems, fit fans directly over the waiting area and robot box entrance. Cows that are uncomfortable will voluntarily reduce milking visits.

2. Sprinkler Systems: Evaporative Cooling at Scale

Wetting the cow's back triggers evaporative cooling – the same mechanism as sweating in humans, but applied externally. Short, intense cycles (30–60 seconds wet, 3–5 minutes fan drying) are far more effective than continuous misting.

Continuous misting increases relative humidity without meaningful evaporative benefit, which can actually worsen THI conditions inside the building. This is a common mistake in under-specified systems.

Sizing guide: 4–5 nozzles per metre of collecting yard width, triggered by thermostat at 22°C. For tie-stall buildings still found on some UK farms, overhead sprinklers on a timer above the stalls can provide meaningful relief.

3. Feed Management: Shift to Evening and Night

Feed intake triggers ruminal fermentation, which generates substantial internal heat. The metabolic peak occurs 4–6 hours after the main ration is consumed. By shifting 60–70% of the daily ration to after 6 pm, this heat load moves into the cooler overnight period.

Many UK farms still feed the main TMR delivery at 7–8 am, precisely before the hottest part of the day. A simple schedule change costs nothing and can noticeably reduce daytime stress indicators. Push-up frequency matters too: cows will not reach for distant feed in hot conditions, so push-up every 2 hours is recommended during sustained heat.

4. Water: Double the Provision

A heat-stressed cow can drink 150–160 litres per day, nearly double the normal requirement of around 80 litres. Provide at least 10 cm of trough space per cow, and keep water temperature below 20°C where possible.

Trough hygiene is critical in summer heat. Bacterial growth accelerates sharply above 25°C. Dirty water is refused by cows, compounding dehydration and dramatically amplifying heat stress effects. Clean troughs daily during hot spells.

5. Structural Adjustments for Existing Buildings

White or light-coloured roofing reduces heat absorption significantly compared to dark metal sheets. If reroofing is planned, this is a high-value upgrade. On existing buildings, removing or raising solid lower-wall cladding during summer months can dramatically improve airflow on still days.

For new builds, orient buildings east-west to maximise natural cross-ventilation and achieve ceiling heights above 4 metres so heat stratifies above animal level.

The Financial Case

A complete fan and sprinkler installation for 200 cows typically costs £10,000–£16,000 in the UK. A severe two-week heatwave (4 kg/cow/day loss at 38p/litre) across a 200-cow herd represents approximately £8,000 in lost milk income alone, plus fertility and health costs. The system pays for itself within two summer seasons.

Calculate heat stress now

Free THI Calculator for your farm — GPS-based, with a 7-day forecast.

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