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.

🔬 The Temperature-Humidity Index Explained

What is the THI?

The Temperature-Humidity Index (THI) combines ambient temperature and relative humidity into a single value that reliably describes the thermal comfort and heat stress experienced by dairy cows. From a THI of 68 onward, high-producing dairy cows begin to suffer from heat stress — with measurable effects on milk production, fertility, and immune function.

THI Formula

THI = (1.8 × T + 32) − [(0.55 − 0.0055 × RH) × (1.8 × T − 26)]

Where T = temperature in °C and RH = relative humidity in %.

Example values:

Cool + humid

20°C · 80%

66.9

THI

Warm + medium

28°C · 60%

THI

Hot + humid

35°C · 70%

88.9

THI

Hot + dry

38°C · 20%

81.7

THI

History of THI

THI was originally developed in the 1950s to assess human comfort and was subsequently adapted for livestock by Bianca (1962) and later refined by Berman et al. for dairy cattle. Today it is the internationally recognized standard for heat stress assessment in dairy farming worldwide.

Today scientists worldwide use the same index — and the evidence is clear:

What the Research Says

Heat Stress in Dairy Cows: What the Science Says

Heat stress & milk production

Heat stress not only reduces milk yield but also massively alters milk components. Recent studies show: Around 50% of milk loss is not caused by reduced feed intake, but by the enormous glucose consumption of the immune system fighting endotoxins (see Leaky Gut).

📄 e.g. Fontoura / McFadden (2022–2024)

Heat stress & reproduction

Even starting at a THI of 68, the conception rate drops drastically. Heat stress directly interferes with hormone balance, reduces oocyte quality months before estrus, and frequently leads to early embryonic loss in the first 16 days of pregnancy.

📄 e.g. Ferraz et al. (2021)

Heat stress & Leaky Gut syndrome

To cool the body, the cow diverts extreme amounts of blood from the gastrointestinal tract to the skin. The consequence: The gut suffers from hypoxia, and the intestinal lining becomes permeable ("Leaky Gut"). Bacterial toxins (LPS) enter the bloodstream and cause systemic inflammation.

📄 e.g. Koch et al. (2020) / McFadden (2024)

Heat stress & dry matter intake

Feeding behavior changes significantly in the heat: The cow eats less, sorts feed more, and reduces rumination. This reduced dry matter intake exacerbates the negative energy balance, decreases saliva flow, and significantly increases the risk of rumen acidosis (SARA).

📄 Various recent JDS reviews (2022–2024)

Heat stress & immune system

The cow's survival priority shifts to cooling, massively suppressing the immune system. The liver is overloaded by the altered metabolism. Because the cow stands longer to dissipate heat, the pressure on the hooves increases. Result: Drastically more mastitis and hoof diseases.

📄 e.g. Immunometabolic Reviews (2023–2024)

Heat Stress & Economic Losses

Economic losses do not arise from milk reduction alone. Higher veterinary expenses for mastitis treatments, claw care, and extended calving intervals heavily weigh on profitability. Every day with relative heat stress (THI > 68) costs an estimated €4.10 per cow in lost revenue and consequential costs.

📄 e.g. St-Pierre et al. (2003) / Key et al. (2014)