UK Summer 2025 THI & Dairy Heat Stress Overview

The 2025 summer monitoring season has revealed just how sustained and regionally varied heat stress pressure has become for UK dairy herds. Through weekly in-shed THI tracking from May to September, patterns emerged that underscore both the direct and delayed impacts of elevated heat and humidity on cow comfort, fertility, and health.

May:

• Surprise early peaks in southern England and Wales (28–30°C, THI 74+) signalled the beginning of a season-long pattern.
• Although 24hr averages mostly remained in the 58–63 ‘comfort’ zone, early heatwaves brought THI 65+ moments — the threshold where fertility begins to decline.
• Behavioural indicators such as increased standing, bunching and water- seeking behaviour began emerging early.

June:

• June turned sharply hotter mid-month. All UK regions experienced THI >70, with southern sheds peaking at THI 84.
• In-shed highs topped 34°C, with sustained hours above THI 67 leading to respiration increases, reduced lying time and probable yield suppression.
• Fertility risks were highest for cows inseminated during June 10–25, particularly in the South and Midlands.

July:

• Widespread high humidity and temperature pushed THIs to 84 in multiple areas, especially the Midlands and Wales.
• 6–7 days per week with THI ≥65 became the norm, with standing time increasing, conception rated dropping and yield likely impacted.
• An increase in sole bruising and ulcer appearing in September/ October are likely due to this sustained heat and increased standing.

August:

• THI ≥65 persisted on 90%+ of days in southern sheds; Wales followed closely.
• Mid-month saw THI peaks of 85, with high humidity amplifying the stress.
• Despite some night-time relief, cows were under pressure for most of the 24hr cycle, again significantly affecting fertility and behavioural stability.

September:

Early September (1st–14th)

• THI dropped back below 65 for most of GB; only the South saw occasional THI 65–67 days.
• Most cows were comfortable, but previous heat events from August continued to impact conception rates and hoof health due to lagging physiological effects.

Mid–Late September (15th–30th)

• Intermittent midweek spikes (THI 75 in the South), but short-lived and didn’t lead to standing stress.
• Final week showed full return to cool conditions, with 24hr THIs in the high 50s.
• However, ongoing PD testing and hoof trimming now began to reveal consequences of earlier stress — with sole bruising, ulcers, and lower fertility still surfacing due to July–August exposure.

Key Seasonal Insights

• Longest exposure to THI ≥67 occurred between June 9 to July 20 — these weeks are the likely culprit for autumn fertility drops and hoof problems.
• Southern and western regions were hit hardest, but even Scotland saw enough hours above THI 67 to stress unacclimated cows.
• THI not only impacted cows during heat, but also primed long-term effects, including delayed lameness and fertility suppression.

How Can We Prepare for Next Year?

To protect cow comfort and performance during future heatwaves, it’s vital to manage the physical environment to reduce heat load and improve the cow’s ability to dissipate heat. Focus on three key areas:

1. Ventilation
2. Mechanical Air Movement
3. Solar Gain & Lighting

Ventilation vs. Air Movement: Know the Difference

While often grouped together, ventilation and moving air are not the same — and both are essential when managing heat stress:

Ventilation: Getting Fresh Air In

Ventilation is about bringing fresh air into the shed and removing stale, humid air — something that’s often under-prioritised on UK farms.
Ask yourself: Are you getting enough air exchanges through the shed in summer?

Here’s how to improve it:

• Exhaust (ridge outlet): Crucial for removing stale air. Test it using smoke — if smoke rises and passes under the ridge but isn’t drawn out, your ridge outlet isn’t working efficiently.
• Side inlets: These should be 2–4x the size of your ridge outlet. Uniform, open side inlets help pull in fresh air evenly and improve natural light balance, reducing bunching caused by dark spots.

More on improving natural ventilation can be found: Livestock Ventilation Systems | Galebreaker

Mechanical Air Movement

Once you have fresh air entering the building, the next step is moving that air across the cow to cool her.

This is where mechanical ventilation plays a role — creating air movement over the cow’s skin for convective cooling. The goal is to move heat away from the cow, not just circulate stale air.

Aim for:

• >2 m/s airflow directly over each cow’s back, especially at cubicles
• Ensure the air is fresh, cool and clean — not warm, dirty or stagnant

Benefits include:

• Reduced rectal temperatures
• Improved lying times
• Better milk yield

Managing Solar Gain and Lighting

Roof lights are a common source of solar gain, causing hot spots inside sheds. They can also lead to uneven lighting, which influences cow bunching behaviour.

• Hotter areas under roof lights = increasing shed temperatures
• Uneven lighting = cows bunch in darker areas, instinctively seeking shade as they would at pasture

Solutions:

• Cover roof lights or use materials that diffuse light to reduce both temperature spikes and “hot spots” of light/ dark
• Aim for even, uniform lighting to avoid bunching and encourage even distribution of cows throughout the shed

More information on heat stress and housing: Reducing Heat Stress in Livestock | Galebreaker

In this video, farmer Russell Morgan talks about the impact of heat stress on his dairy herd, and how Galebreaker products have been able to reduce the negative consequences.

If you’d like support in preparing your building for next summer, get in touch with Chloe Rodriguez on +44 7826 986944. She can arrange an advisory visit to carry out a full on-farm building assessment.