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.

Heat Stress in Dairy Cows: When Does It Get Critical?

2026-06-09THI Cows Team

Summer heatwaves increasingly challenge dairy farms worldwide, leading to noticeable drops in livestock productivity. When temperatures rise, dairy cows suffer from heat stress, which directly impacts their welfare and milk yield. But when exactly does the thermal load become critical for the animals, and how can farmers react in time? The Temperature-Humidity Index (THI) offers an objective scientific measure to identify heat stress early and implement targeted mitigation strategies.

What is THI?

The THI (Temperature-Humidity Index) is an internationally recognized metric used to assess the barn climate and the thermal comfort of livestock. Unlike ambient temperature alone, the THI value accounts for both temperature and relative humidity (Claim #15). This is critical because high humidity significantly limits a cow’s ability to dissipate heat through sweating and respiration.

Scientifically, this relationship is defined by the classic formula established by the national research body (NRC, 1971) (Claim #14):

$$THI = (0.8 \times T) + \frac{RH}{100} \times (T - 14.4) + 46.4$$

Where $T$ represents the air temperature in degrees Celsius and $RH$ is the relative humidity in percent. Under this formula, it is clear that heat stress can occur at lower temperatures if the relative humidity is high. To avoid calculating this value manually for your barn, you can use our free online THI Calculator, which computes the value for your location in real-time.

The 4 Warning Stages of Heat Stress

The welfare and productivity of dairy cows can be classified into four warning stages based on the THI value. As the index increases, physiological stress and production losses rise exponentially.

THI 68–71: Mild Heat Stress

At THI 68, mild heat stress begins, and cows show the first physiological adaptation responses. Typical symptoms include an increased respiratory rate and mild restlessness within the herd (Liu et al., 2023) (Claim #1). Although the stress appears moderate from the outside, the cow’s body is already actively working to prevent overheating.

THI 72–77: Moderate Heat Stress

When the THI rises to the 72–77 range, cows enter the moderate heat stress stage. At this level, increasing the respiration rate is no longer sufficient to regulate core body temperature. Consequently, feed intake drops because digesting feed generates additional metabolic heat. This reduced appetite leads directly to a drop in milk yield of 10% to 20% (Chen et al., 2024) (Claim #2).

THI 78–87: Severe Heat Stress

At THI 78, the herd suffers from severe heat stress. Cows struggle to dissipate excess heat, leading to massive production losses. The daily milk loss increases drastically and can result in up to 4 kg of milk loss per cow per day (USDA Heat Stress Report, 2025) (Claim #3). During this phase, the cows' immune system is compromised, increasing their susceptibility to diseases such as mastitis and hoof issues.

THI > 88: Life-Threatening Heat Stress

A THI value above 88 represents an extreme danger and indicates life-threatening heat stress for dairy cattle. At these extreme levels, cows are at high risk of heat stroke and circulatory failure. To prevent permanent damage or fatalities, immediate intervention is required (Anderson et al., 2023) (Claim #4).

How Much Milk Are You Really Losing?

The drop in milk production is directly linked to the THI warning stage. The longer the animals are exposed to critical values, the more severe the losses. The following table provides an overview of the daily milk loss per cow across the different THI ranges:

THI Range	Stress Level	Estimated Milk Loss (per cow/day)	Scientific Source
THI 68–71	Mild Heat Stress	0 – 2 kg	Chen et al., 2024 (Claim #5)
THI 72–77	Moderate Heat Stress	2 – 5 kg	Liu et al., 2023 (Claim #6)
THI 78–87	Severe Heat Stress	5 – 8 kg	USDA, 2025 (Claim #7)
THI > 88	Life-Threatening Heat Stress	> 8 kg	Anderson et al., 2023 (Claim #8)

These figures demonstrate that heat stress is not a temporary nuisance but a severe economic threat that quietly undermines dairy farm profitability.

What Does Heat Stress Cost You?

The financial impact of heat stress on dairy farms is substantial and often underestimated. In addition to direct losses in milk revenue, farms face costs from reduced fertility and increased veterinary treatments.

A calculation example illustrates the economic damage: a 100-cow farm loses approximately €6,000–13,500 per season during 30 heat stress days (USDA, 2024) (Claim #10). This major loss shows that investing in preventive cooling measures and precise monitoring systems is not only a matter of animal welfare but also a highly profitable decision that pays off quickly.

First Measures – What Helps Immediately?

Combating heat stress effectively requires a combination of immediate actions and technical barn solutions. The most effective methods include:

Shade and Sufficient Water (Basic Prevention): Providing adequate shade and access to clean, cool water are the basis of any heat stress prevention strategy (USDA, 2025) (Claim #12). Cows have a significantly higher water requirement during hot periods, so water troughs and flow rates must be scaled accordingly.
Fans and Sprinklers (Active Cooling): Installing mechanical fans combined with low-pressure sprinkler systems is one of the most efficient active measures. This combination facilitates evaporative cooling on the cow’s skin, effectively lowering the thermal load in the barn by 5 to 8 THI points (Anderson et al., 2023) (Claim #11).
Transition to THI Monitoring: To operate these cooling systems efficiently and save energy, continuous monitoring of the barn climate is essential. Only by tracking the specific THI of your location can you activate fans and sprinklers exactly when needed.

Calculate THI Now

Don't wait for your cows to show severe stress symptoms and for milk production to drop. Use our free online tool to determine the current heat stress status for your exact location.

➔ Calculate the current THI value for your cows

For continuous monitoring of your farm with automatic reports and a precise 7-day forecast, you can use our advanced dashboard features. Try it risk-free: ➔ Try BOS-THI 14 days free

Frequently Asked Questions

At which THI is it dangerous? It becomes dangerous for dairy cows at THI 78, where severe heat stress and high milk losses set in (USDA, 2025). A value above 88 is immediately life-threatening and requires immediate intervention (Anderson et al., 2023).

How much milk do I lose? Milk loss varies by stress level: under mild stress (THI 68–71), the loss is 0 to 2 kg per day (Chen et al., 2024). Under moderate stress (THI 72–77), it rises to 2 to 5 kg per day (Liu et al., 2023), and under severe stress (THI 78–87), it reaches 5 to 8 kg per cow per day (USDA, 2025).

What helps immediately? The baseline requirement is ensuring sufficient shade and clean, cold water (USDA, 2025). Active cooling using a combination of fans and sprinklers can lower the THI value in the barn by 5 to 8 points (Anderson et al., 2023).

Calculate heat stress now

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