Monthly THI Summary – August

August continued to place UK dairy herds under sustained heat stress, with most regions experiencing prolonged periods where THI exceeded the critical threshold of 65. Daytime peaks often reached high 70s to mid-80s, particularly in southern regions and Wales. Nights provided some relief, with THI dropping below peak levels, but in many cases overnight cooling was insufficient to fully offset daytime heat stress. This meant cows spent significant portions of the 24-hour cycle under stress, which could have contributed to fertility challenges and production losses (especially in high performing herds).

THI Patterns

• Persistent exposure: Since mid-June, southern sheds have faced THI ≥65 on over 90% of days, the Midlands and Wales on 80%, and the North on 57%. Even Scotland exceeded a third of days.
• Mid-month extremes: Peak THI reached 85 in Wales (33.5°C @ 62% RH), while the South of England hit 35°C, though with lower humidity (THI 82).
• Humidity effect: Lower temperatures combined with high RH still left cows under stress, with many regions experiencing days where THI averaged 68 or more.
• Regional swings: Scotland and the North, usually less affected, saw sudden jumps with more than half of hours above THI 67 during hotter spells, testing cows less adapted to heat events.

Impacts on Cows

• Fertility: The most significant losses, with overlapping stress periods reducing conception and pregnancy rates.
• Milk yield: Days with THI ≥68 added production pressure. However, due to the let up at night, milk yields would likely have been sustained. Milk is often significantly affected when the daily average THI is sustained for 3+ days.
• Behavioural stress: Bunching at troughs, increased standing, reduced lying, and higher respiration rates were widely observed.
• Regional resilience: Southern herds faced sustained pressure, while abrupt swings in the North and Scotland created challenges.

Was August Concerning?

Yes. Conditions were consistently high enough to affect fertility across most of the UK, with sharp mid-month peaks adding further strain. High humidity compounded the challenge, confirming heat stress is a growing and significant concern for UK herds.

Practical Solutions for Farmers

• Ventilation improvements: Open side walls, unclutter ridges and reduce solar gain (e.g. cover roof lights).
• Mechanical cooling: providing air speeds >2m/s over will reduce heat load during peaks.
• Shade and water: Ensure uniform lighting indoors, shade outdoors and ample, clean trough space.
• Long-term planning: Design sheds to maximise natural airflow and supplement with mechanical when needed.  

Looking Ahead

As summer eases, watch for an increase in claw lesions this autumn, linked to longer standing times during heat stress.

Now is also the time to plan for next year: open side walls, maximise ridge ventilation, and consider mechanical systems to ensure sheds are prepared for future hot, humid summers.

For more information about THI take a look at our advice pages.