For a long time, heat stress was something UK farmers read about happening elsewhere. That has changed. Warmer, more humid summers mean it is now a genuine concern on British farms, and it costs more than most people realise.
The frustrating part is that heat stress is largely preventable. The animals that suffer most are often your most productive ones, and the losses show up weeks later as lower yields and poorer fertility, long after the hot spell has passed.
What Heat Stress Actually Is
Cattle do not sweat efficiently. They shed heat mainly through breathing and through the skin, and when the air is both warm and humid, that becomes hard work. A dairy cow producing 30 litres a day generates a surprising amount of internal heat just from digestion and milk production, so she starts to struggle before you feel uncomfortable yourself.
The usual measure is the Temperature Humidity Index (THI), which combines air temperature and humidity into a single number. Dairy cattle can begin to show mild heat stress at a THI of around 68, which in UK terms is roughly 22°C with moderate humidity. That is a normal summer afternoon, not a heatwave.
Spotting It Early
By the time an animal is visibly distressed, the productivity loss has already happened. The earlier signs are worth knowing:
- Faster breathing - The first thing you will notice. Watch the flanks rather than counting breaths.
- Crowding and seeking shade - Animals bunching under trees or against buildings, often standing rather than lying.
- Reduced intake - Cattle eat less in the heat, which is part of why yields drop.
- More time standing - Standing exposes more surface area to the air. Lots of standing in the middle of the day is a warning sign.
- Drooling and an open mouth - These are later signs. If you see them, the animal is already in trouble.
The animals at highest risk are high-yielding dairy cows, heavily pregnant females, and any animal that is already ill or lame.
What You Can Do
Most of the effective measures are practical and low-tech:
Water First
A heat-stressed cow can drink well over 100 litres a day. Make sure there is enough trough space, the flow rate keeps up, and the water is clean and accessible. This is the single most important thing, and the easiest to get wrong when a group is bigger than the troughs were sized for.
Shade and Airflow
Natural shade, shade cloth, or simply opening up a building to improve airflow all help. In housing, the aim is to keep air moving over the animals. Stale, still air in a shed on a warm day is worse than being outside.
Work Animals in the Cool
Move, gather, and transport stock early in the morning rather than the afternoon. Handling adds heat at the worst possible time, so a 6am job beats a 2pm one in July.
Adjust Feeding
Feed the largest portion in the cooler evening hours, and talk to your nutritionist about the ration. Digestion itself produces heat, so when and what you feed makes a measurable difference.
Where Monitoring Helps
The hard part of heat stress is that conditions change quickly and the early signs are easy to miss when you are busy. This is where keeping an eye on the numbers pays off.
Local weather and heat stress alerts mean you know a difficult day is coming rather than reacting once cows are already panting. Combined with threshold-based health alerts on individual animals, you can catch the cow whose temperature or activity has shifted before she goes off her feet.
The point is not to replace stockmanship. It is to give you a head start, so the water troughs are checked and the shade is open before the worst of the day rather than after.
The Cost of Doing Nothing
Heat stress does not usually kill animals in the UK, which is exactly why it gets overlooked. The cost is less obvious: a dip in milk yield that takes weeks to recover, conception rates that fall through the summer and leave you with a breeding gap the following spring, and more cases of mastitis and lameness.
Add those up across a herd and a summer, and prevention looks cheap.
The Bottom Line
UK summers are warm enough now that heat stress is a normal part of the farming year, not a freak event. The animals that pay the price are your best performers, and the losses are easy to miss because they arrive late.
Get the basics right, water, shade, airflow, and timing, and keep an eye on the conditions and the individuals most at risk. A little planning on the hot days protects yield and fertility long after the weather has cooled.
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Questions about heat stress monitoring on your farm? Drop us a line at hello@agrianta.com and we'll do our best to help.