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.

Blood Flow Simulator

How much blood does the udder lose as heat stress rises?

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Heat Stress Level

Scientific threshold for heat stress: from THI 68

❄️ Perfect cow comfort! ❄️

50 (Very cool)68 (New Threshold!)90 (Extreme heat stress)

Real-Time Physiological Measurements

Total Blood Volume in Body

approx. 13.2 Gallons

The total blood inside an adult cow (1,650 lbs). We want to emphasize: this is the total blood in her body!

Udder Blood Flow (Normal)

6,600 Gallons / Day

Needed for 110 lbs daily yield. The blood circulates extremely fast (500 times daily through the udder)!

Lost Udder Blood Flow

−33 Gallons / Day

This circulating blood is missing for milk synthesis (Ratio: 500 gal of blood flow per 1 gal of milk).

What does this mean for you in the barn?

Your cow is in her absolute comfort zone. She uses only a fraction of her blood for cooling her skin. The rest of the blood flow pumps undisturbed through the udder and rumen. Since her entire blood volume must circulate through the udder 500 times daily to produce 110 lbs of milk, every single drop counts!