When Does Heat Stress Start in Dairy Cows? THI Thresholds Explained

The summer of 2022 saw temperatures in England exceed 40°C for the first time on record. For dairy farmers in the South and Midlands, that was a turning point: heat stress, long considered a problem for Mediterranean or American operations, had firmly arrived on British and Irish farms. With climate projections pointing to more frequent high-heat events, the question is no longer if heat stress will affect your herd – but when it starts and how to catch it early.

Understanding the THI

The Temperature Humidity Index (THI) combines air temperature and relative humidity into a single number that reflects what a cow actually experiences thermally. The formula is: THI = (1.8 × T + 32) – [(0.55 – 0.0055 × RH) × (1.8 × T – 26)].

Unlike humans, cattle cannot cool themselves effectively through sweating. Their primary thermoregulation mechanism – panting – becomes increasingly inefficient when air is already humid. This is why the THI, which accounts for humidity, is far more accurate than temperature alone.

At 25°C and 80% humidity – conditions common during British heatwaves – THI reaches approximately 74.5, placing cows in moderate heat stress.

The Five Stress Levels

• THI below 65: No stress – cows are comfortable
• THI 65–71: Mild stress – respiratory rate increases, milk yield may drop slightly
• THI 72–77: Moderate stress – reduced feed intake, milk loss of 2–4 kg/cow/day
• THI 78–83: Severe stress – immune suppression and fertility damage
• THI 84+: Danger zone – risk to life with prolonged exposure

The Modern Threshold: 65, Not 72

The commonly cited threshold of THI 72 dates from research conducted in the 1970s on cows producing 15–18 litres per day. Today's Holstein-Friesian herds in Britain produce 8,000–11,000 litres per lactation. This extraordinary metabolic output means cows generate far more internal heat.

Research by Herbut (2023) and West (2003) shows that high-producing cows experience measurable stress from THI 65 onwards – seven full index points earlier than traditional guidelines suggest. For producers managing average yields above 30 litres per cow per day, THI 65 is the actionable threshold for intervention.

What Makes UK and Irish Conditions Unique

British and Irish barns are traditionally designed for cold, wet winters – not heat. Many older cubicle buildings have limited natural ventilation when still, hot conditions prevail. This means indoor THI can be 3–6 points higher than outdoor readings, even on days that feel moderate to humans.

Furthermore, the UK and Ireland frequently experience high humidity alongside moderate temperatures. A day at 22°C with 85% relative humidity – typical for a wet British summer or an Irish July – yields a THI of approximately 69, already in the mild stress zone. Many farmers would not recognise this as a heat risk.

Visible Signs of Heat Stress

• Elevated respiratory rate (more than 60 breaths per minute)
• Cows clustering around water troughs
• Increased time standing rather than lying (cows lose more heat when standing)
• Saliva dripping, open-mouth breathing in severe cases
• Reduced feed intake during midday hours
• Milk yield drop 2–3 days after the heat event (lagged effect)

Monitoring THI on Your Farm

The most reliable approach is a digital thermo-hygrometer placed in the lying area – not outside. For a live GPS-based reading from your exact location, the THI Calculator at thi-cows.com provides both a current value and a 7-day forecast, so you can plan cooling interventions in advance and protect your herd before yield drops.

Calculate heat stress now

Free THI Calculator for your farm — GPS-based, with a 7-day forecast.