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Scientific basis
- Zhang Q., Yang L. et al. (2025). Heat stress affects dairy cow performance via oxidative stress, HPA axis, gut microbiota, and multi-dimensional mitigation. Review · Oct 2025
- Ban B., Lee J. et al. (2026). Effects of a combined nutritional and cooling strategy on productivity and immune responses in heat-stressed Holstein dairy cows. J. Dairy Sci. · Jan 2026
- Ye C., Zhang Z., Yang Z. (2026). A new type of biomarker for heat stress: insights from immunology. Mastitis risk ×2–3 under heat stress. Feb 2026
The THI Calculator (Temperature Humidity Index) is the most important tool against milk losses caused by heat stress in dairy cows. When temperature and humidity combine, your cattle suffer silently – less feed, less milk, reduced fertility. Often for weeks before you notice it.
Heat stress in cattle begins at a THI of 68 and can reduce milk production by up to 8 kg per cow per day. With the THI scale, you recognise early when your barn climate becomes critical – and which measures against heat stress in dairy cows matter now.
The THI Scale: When Does Heat Stress Become Critical?
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🔬 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.
Learn more →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 %.
📚 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)
↓ Frequently asked questions about heat stress in dairy cows answered
💡 We read it for you — so you don't have to
Scientific updates and practical tips on heat stress management in dairy cattle.
- ›Thermoregulation & Blood Flow: To dissipate heat, blood vessels in the skin dilate, while internal organs (GI tract, mammary gland) suffer reduced perfusion, directly limiting nutrient delivery for milk synthesis (e.g. [Baratta et al., 2026](https://doi.org/10.3390/vetsci13070623)).
- ›Benefits of Cooling Dry Cows: Cows cooled during their 60-day dry period produce up to 4-5 kg more milk per day in the subsequent lactation and give birth to heavier, more viable calves (e.g. [Tao & Dahl, 2013](https://doi.org/10.3168/jds.2012-6278)).
- ›Intermittent Cooling in Holding Areas: Studies confirm that short, intense soaking combined with high air velocity (3 m/s) in the holding area before milking is the most effective way to lower core temperature (e.g. [Chen et al., 2016](https://doi.org/10.3168/jds.2015-10714)).
- ›Respiration Rate as Early Indicator: An increase in respiration rate above 60 breaths per minute correlates directly with the onset of heat stress. It is a more sensitive physiological indicator than ambient temperature alone (e.g. [Pinto et al., 2020](https://doi.org/10.1016/j.jtherbio.2020.102523)).
- ›Immune System Suppression: Chronic heat stress elevates inflammatory markers and reduces white blood cell activity, which has been shown to increase susceptibility to mastitis and hoof lesions (e.g. [Lacetera et al., 2006](https://doi.org/10.3168/jds.S0022-0302(06)72510-3)).
- ›Follicle & Oocyte Quality: Heat stress damages maturing oocytes months in advance. Consequently, fertility often remains severely impaired well into autumn, long after summer heat has subsided (e.g. [Roth et al., 2000](https://doi.org/10.1530/reprod/120.1.83)).
- ›Hormonal Dysregulation: Elevated core body temperatures disrupt the secretion of Luteinizing Hormone (LH) and progesterone. This results in weaker estrus expression (silent heat) and lower pregnancy rates (e.g. [De Rensis & Scaramuzzi, 2003](https://doi.org/10.1016/S0093-691X(03)00126-2)).
- ›Early Embryonic Loss: During the first 3-5 days post-insemination, the embryo is extremely sensitive to elevated uterine temperatures. A rise of just 1°C can cause embryonic death (e.g. [Ealy et al., 1993](https://doi.org/10.3168/jds.S0022-0302(93)77629-8)).
- ›Dry Period Impact: Recent studies demonstrate that heat stress during the dry period inhibits mammary gland development and drastically lowers subsequent pregnancy rates in the next cycle (e.g. [Fabris et al., 2019](https://doi.org/10.3168/jds.2018-15721)).
- ›Detection & Management Tools: Due to reduced physical activity under heat, traditional motion-based heat detection systems frequently fail. Progesterone testing or timed AI is scientifically recommended (e.g. [Schüller et al., 2014](https://doi.org/10.1016/j.theriogenology.2014.01.029)).
- ›Rumen Acidosis Risk (SARA): Heat stress reduces rumination and saliva production (loss of bicarbonate). Studies show that selective sorting destabilizes rumen fermentation. Rumen buffers are essential (e.g. [Eslamizad et al., 2020](https://doi.org/10.3168/jds.2020-18417)).
- ›Ration Density & Bypass Fats: As dry matter intake drops by 10-15%, nutritional studies demonstrate that increasing energy density with rumen-protected fats mitigates the negative energy balance (e.g. [Rabiee et al., 2012](https://doi.org/10.3168/jds.2011-4895)).
- ›Leaky Gut Syndrome under Heat: Recent research shows that reduced blood flow to the digestive tract damages the intestinal barrier. This triggers systemic inflammation and wastes energy needed for milk synthesis (e.g. [Baumgard & Rhoads, 2013](https://doi.org/10.1146/annurev-animal-031412-103644)).
- ›Electrolyte Shifts (DCAD): Through sweating and high respiration rates, cows lose high amounts of potassium and sodium. Adjusting the Dietary Cation-Anion Difference is critical for acid-base balance (e.g. [Sanchez et al., 1994](https://doi.org/10.3168/jds.S0022-0302(94)77150-2)).
- ›Feeding Frequency & Hygiene: Trials show that offering fresh feed during cooler early morning and late evening hours reduces feed heating and helps stabilize overall dry matter intake (e.g. [Ominski et al., 2002](https://doi.org/10.3168/jds.S0022-0302(02)74136-1)).
❓ FAQ
Frequently Asked Questions: Heat Stress in Dairy Cows & the THI Calculator
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. Therefore, a cow at 28°C and 80% humidity suffers significantly more than at 35°C in dry air. From THI 68 onward, silent heat stress begins with measurable effects on milk production, fertility, and the immune system. The THI is today the internationally recognised standard for heat stress assessment in dairy farming.
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 – milk production drops noticeably. From THI 78, severe stress is present; above THI 84, there is acute risk to life. Importantly, modern high-producing cows (30+ litres/day) are sensitive from THI 65 due to their significantly higher metabolic heat production. Humidity plays an often underestimated role.
How do I calculate the THI manually?
The THI formula is: THI = (1.8 × T + 32) − [(0.55 − 0.0055 × RH) × (1.8 × T − 26)]. T is the air temperature in °C, RH is relative humidity in %. Example: At 28°C and 70% humidity, THI ≈ 78.4 – already severe heat stress. Alternatively: Our free THI Calculator computes the current value automatically using your GPS location.
What symptoms does a cow show under heat stress?
Typical heat stress symptoms in cows: breathing rate above 60 breaths/min (normal: 20–30), body temperature above 39.5°C, drooling and hypersalivation, reduced feed intake, frequent seeking of water troughs and shade, standing instead of lying (better heat dissipation). In severe cases: coordination problems and circulatory failure. Breathing rate is the most reliable early indicator – check it daily!
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. Freshly calved cows in the first 60 days of lactation are especially at risk. Source: West, J.W. (2003), Journal of Dairy Science.
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 (approx. 80 litres). Water must be cold (below 15°C) and always fresh. At least 10 cm of trough space per cow is needed. Inadequate water supply immediately reduces feed intake – further intensifying heat stress.
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 (e.g. Brahman breeds) and tropical animals tolerate significantly higher THI values thanks to specialised thermoregulation. Brown Swiss and Simmental fall in between. Modern high-producing cows suffer from THI 65 – 3 points earlier than the classic threshold of 68 suggests. The higher the milk yield, the earlier heat stress sets in.
How do I detect heat stress in the barn early?
Early indicators of heat stress in the barn: breathing rate above 50/min (already alarming), reduced rumination activity, animals standing instead of lying, increased water intake, groups crowding around fans or water troughs. Technically, a real-time THI monitoring via GPS helps: when the THI exceeds the threshold of 65–68, cooling measures begin immediately – before milk production noticeably drops.
What helps immediately in acute heat stress?
For acute heat stress: (1) Fans at maximum power – airflow is the most effective cooling mechanism, (2) Activate sprinklers – short, intense spraying combined with high air velocity (3 m/s), (3) ensure fresh, cold water at all times, (4) shift feeding to evenings and early mornings, (5) do not drive animals through narrow passages – stress raises body temperature by an additional 1–2°C.
How does heat stress affect the fertility of dairy cows?
Heat stress reduces conception rates by up to 30%. Eggs are damaged during the follicular maturation phase months before summer heat – so fertility often remains impaired well into autumn. Elevated core temperatures disrupt the secretion of LH and progesterone, leading to weaker or silent oestrus. Embryos in the first 3–5 days after insemination die from a uterine temperature increase of just 1°C. Source: Hansen, P.J. (2009), Animal Reproduction Science.