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.

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.

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.

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!

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.

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.

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.

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.

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.

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.