June 2025 THI Summary – What Did Cows Experience?

June proved to be a challenging and variable month for UK dairy herds in terms of heat stress. After a comfortable start, in-shed conditions deteriorated rapidly and consistently—particularly for farms in the south and Midlands. This data has been taken from the THI-live.com site.

Monthly Overview

• Weeks 1–2:
  Conditions started cool and comfortable, with most sheds staying below THI 65 and daily highs rarely exceeding 25°C. However, by Week 2, herds in the south were already facing 6 out of 7 days with THI above 65—pushing cows into mild heat stress territory.
• Weeks 3–4:
  A full-blown heatwave followed. All mainland UK regions experienced at least one day with a 24hr average THI above 70. Southern sheds saw THI highs of 83–84. This level of sustained heat caused widespread stress symptoms, including elevated respiration rates, reduced lying times, and increased water intake. Prolonged conditions like these can lead to visible performance drops.

Regional Differences

• South of England:
  THI remained above 65 almost every day from June 10th onward. Some sheds recorded internal temperatures above 34°C.
• Wales & Midlands:
  Repeated daily highs of THI >68 pushed cows beyond thresholds for yield decline.
• Scotland & Northern England:
  Generally cooler but not exempt. Short periods of stress occurred, with isolated days above THI 67.

Heat Stress Impacts

• Fertility Risks:
  Cows inseminated from early to mid-June—especially in the south and Midlands—are at increased risk of reduced conception rates, with effects expected to emerge in the coming weeks.
• Productivity:
  Prolonged time spent above THI 67 (over 60% of hours across two weeks in the south) likely reduced feed intake and lying time, suppressing yields.
• In-Shed Extremes:
  A peak THI of 84 was recorded in Wales (33.9°C at 61% RH), underscoring the importance of shed design and ventilation—it’s not just about outside temperatures.

Download our Heat Stress Brochure for more tips on spotting the signs of Heat Stress.

Behaviours You May Have Seen

• Increased respiratory rates (above 60 bpm)
• More cows standing for longer periods, often perching on beds
• Bunching, especially in darker or more ventilated areas

Don’t Forget the Dry Cows

Dry cows—particularly in late gestation—are highly susceptible to heat stress, and the long-term impacts go well beyond the current lactation.

Research shows that heat stress in the final weeks of pregnancy can:

• Lower milk yields in the next lactation
• Reduce calf birth weight, growth, and survival
• Impair mammary gland development in unborn heifers, affecting their future milk production

Prioritising dry cow comfort is critical to protect both current performance and future generations.

What can you do to reduce Heat Stress Risk?

Think About the Housed Environment

• Natural ventilation – Open ridge outlets and sidewalls help remove heat and humidity.
• Shade and uniform lighting – Limit hotspots from roof lights and reduce dark zones to prevent bunching.
• Convective cooling – Increase airflow directly over cows’ backs; positive-pressure tubes can deliver ~4m/s over each individual cubicle.
• Monitoring – Use in-shed sensors to understand how your environment is performing (see heat map example below).

South Gloucestershire farm: sustained high THI levels recorded over several consecutive hours:

Concerned about Heat Stress affecting your herd? Take a look at our Advisory Service and get in touch – marketsupport@galebreaker.com