What is the THI value?

The Temperature-Humidity Index (THI) measures heat stress in dairy cows. It combines temperature and relative humidity. Heat stress begins at THI 68 (mild), becomes moderate at THI 72, and critical at THI 90.

When do cows suffer from heat stress?

Dairy cows show first signs of stress at a THI of 68: increased respiration rate, reduced feed intake, and declining milk production. Stress is severe above THI 79 and critical above THI 90.

How much milk is lost during heat stress?

At mild stress (THI 68–72): 0.5–1 kg/day; moderate stress (THI 72–79): 1–3 kg/day; severe stress (THI 79–89): up to 6 kg/day per cow.

How is THI calculated?

THI = (1.8 × Temperature°C + 32) − (0.55 − 0.0055 × Relative Humidity%) × (1.8 × Temperature°C − 26). Our free calculator determines THI automatically using real-time GPS weather data.

How can heat stress in cows be reduced?

Key measures: shift feeding to cooler hours, ensure water supply (up to 160 L/cow/day), add buffer substances to the ration, install fans and sprinklers in the barn.

🔬 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

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.

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.