The THI Scale at a Glance
The Cow as a Thermometer
Physiological response to heat stress by THI level
No Stress
🌿 Calm · Grazing
Mild Stress
💦 Restless · Early signs
Moderate Stress
😵 Panting · Drooling
Severe Stress
💧 Standing · Seeking water
Emergency
🚨 Apathy · Collapse risk
🧮 THI Calculator
Calculate your herd's heat stress level in seconds
🌡️ THI Calculator
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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.
Mehr erfahren →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 %.
📚 The Science Behind THI
Peer-reviewed research underpins every recommendation we make
Milk Production Losses
Research consistently shows that dairy cows begin to produce less milk at a THI of 68. At a THI of 78, losses of 4–8 kg of milk per cow per day are documented — and significantly more in high-yielding animals.
📄 West, J.W. (2003). Journal of Dairy Science, 86(6), 2131–2144.
Reproductive Performance
Heat stress severely impairs follicular development, oocyte quality, and early embryo survival. Conception rates can drop by 20–30% when THI values consistently exceed 72, particularly during summer months.
📄 Hansen, P.J. (2009). Animal Reproduction Science, 113(1–4), 51–60.
Dry Matter Intake & Metabolism
At elevated THI levels, cows reduce dry matter intake by up to 10–12%, leading to negative energy balance, increased risk of ketosis, and greater body condition loss in the fresh cow period.
📄 Bernabucci, U. et al. (2010). Journal of Dairy Science, 93(9), 4234–4244.
Immune Function
Heat stress suppresses the immune response in dairy cows, increasing susceptibility to mastitis, lameness, and other infectious diseases — especially during the periparturient period when immune function is already compromised.
📄 Lacetera, N. et al. (2006). Journal of Dairy Science, 89(5), 1784–1791.
💡 Practical Strategies to Combat Heat Stress
Proven approaches for cooling, water supply, and nutritional management
❄️Cooling & Housing
- ›Install fans in resting and holding areas — at minimum every 3 meters along the feed alley — to maintain continuous airflow across cows.
- ›High-pressure misting systems (evaporative cooling) combined with fans can lower air temperature by 4–8°C and significantly reduce THI inside the barn.
- ›Maximize natural ventilation with open ridge vents and sidewall openings; white or reflective roof coatings can reduce radiant heat gain by up to 30%.
💧Water Supply
- ›Water intake can reach 150–200 liters per cow per day during heat stress — ensuring adequate trough capacity and water flow rate is critical.
- ›Provide at least one water trough per 15–20 cows and clean troughs daily, as cows actively avoid dirty or stale water, especially under heat stress.
- ›Position water sources near the milking parlor exit and throughout resting areas to minimize walking distances during peak heat periods.
🌾Nutritional Management
- ›Shift feeding to cooler parts of the day — early morning and late evening — when cows are more active and willing to eat.
- ›Increase energy density in the ration by adding bypass fat (rumen-protected fat) to offset the reduction in dry matter intake during heat stress.
- ›Supplement sodium bicarbonate (150–200 g/head/day) as a rumen buffer to counteract subacute ruminal acidosis caused by altered feeding patterns.