Monthly THI Summary – July 2025

July continued to challenge dairy herds across the UK, with elevated Temperature-Humidity Index (THI) values persisting over prolonged periods. These conditions, particularly prevalent in the Midlands, South of England, and Wales, have immediate and long-term implications for cow health and productivity, with direct consequences for farm economics.

Thermal Stress Patterns and Regional Variation

• THI thresholds ≥65 were recorded on 6–7 days per week in most regions except Scotland, where exposure was lower but notable peaks (up to 30.7°C and THI 79) occurred later in the month.
• Peak in-shed temperatures reached 33.9°C (THI 84) early July, with humidity levels frequently between 60–80%, significantly impairing cows’ latent heat dissipation via evaporative cooling.
• There was a clear regional difference, with herds in the South and Midlands experiencing longer periods and greater intensity of high THI levels — and therefore likely heat stress – compared to those in the North and Scotland.

How July’s Heat Has Affected Cows

• Fertility: Prolonged hot and humid conditions have knocked fertility. High THI affects how eggs develop, reduces their quality, and makes early embryo loss more likely. These fertility problems can last several weeks after the heat has passed, so July’s heat could impact pregnancy rates well into August.

• Milk Yield: Cows produced less milk when THI stayed above 68 for several days. This is because they eat less, use more energy just to keep cool, and their overall metabolism changes under heat stress.

• Lameness: When cows build up more heat than they can lose, they cope by spending more time standing instead of lying down to cool off — but this extra standing increases the risk of claw lesions and lameness (often seen in Autumn).

• Dry Cows: Heat stress during late gestation contributes to lower calf birth weights, premature calving, and impaired passive immunity transfer due to compromised colostrum quality and IgG absorption.

This graphic highlights the impact of different THI levels on dairy cow health and welfare

What Can Be Done?

1. Ventilation first:
   Make sure there’s enough air movement inside the shed, especially around cow-lying height. Open up shed sides where you can and ensure you have enough ridge outlets so hot air can escape, and fresh air can enter.
2. Boost with hybrid systems:
   Use fans or positive pressure tube ventilation to increase airflow. Aim for airspeeds over >2 m/s across each cow’s back to help cool them through convective cooling.
3. Reduce heat build-up:
   Paint or cover roof lights to reduce solar heat inside sheds. This also helps even out lighting reducing bunching behaviours, as cows like to bunch in darker areas (cows naturally perceive dark as shade).
4. Adjust the diet:
   Feeding cows energy-dense rations with added supplements can help them cope better with heat stress. It’s also beneficial to shift more feeding to cooler parts of the day, such as evenings, to encourage higher feed intake.
5. Management changes:
   Avoid inseminating cows during periods of high THI if possible, to help protect fertility.
6. Monitor the environment:
   If you don’t measure it, you won’t know it. Use sensors to track temperature and humidity inside sheds.

Have you noticed your herd bunching?

Cows bunching in darker areas of the shed may be caused by heat stress, biting flies, lack of ventilation or light avoidance. 

Below is a heat map showing the average hours per day spent in different THI ranges, based on data collected from multiple farms across the Southwest of England between May and July.

If you are worried about Heat Stress in your herd – get in touch with the Galebreaker Team on 01531 637900 or email marketsupport@galebreaker.com