Heat stress is not just a competition-horse problem. An average 600 kg paddocked horse can lose up to ten litres of sweat on a quiet summer afternoon without doing any work. This article explains what happens physiologically above 25 degrees, how to recognise dehydration before it escalates, and which supplements and routines genuinely make summer easier.
Why horses are vulnerable to heat
A horse has three times more body mass per unit of skin surface than a human. Heat dissipation is therefore slower, and sweating is almost the only escape route. Equine sweat is also three times more concentrated in salt than human sweat. One litre of sweat costs the horse roughly ten grams of sodium, four grams of potassium and one gram of calcium.
Above 25 degrees or in high humidity, sweating loses efficiency. The sweat no longer evaporates effectively and core temperature climbs. Above 41 degrees core, heat stress sets in: muscle cells underperform, the heart rate climbs, digestion slows and the risk of acidosis or colic rises sharply.
Symptoms of heat stress
- Sticky white sweat marks at low effort
- Elevated respiratory rate at rest (above 25 per minute)
- Slow skin tent test (skin pinch stays for more than two seconds)
- Dark urine in small amounts
- Loss of appetite and lethargy
- Muscle twitching in the flank or chest
- In severe form: no sweating despite high temperature (anhidrosis)
The skin tent test: five seconds, no equipment
Pinch a fold of skin on the shoulder or neck. In a hydrated horse, the fold drops back within a second. With mild dehydration, it stays one to two seconds. Above three seconds, dehydration is moderate to severe and vet advice is appropriate. Simple, free, applicable daily.
Hydration: not just water
A horse can have free water available and still dehydrate, because electrolytes are not replaced. Drinking pure water after electrolyte loss can even worsen hyponatremia: the blood is diluted, cells swell and the horse drinks less as the urge fades. The answer is isotonic water or free choice between water and an electrolyte mix.
Practical strategy: always offer fresh water in paddock and stable, ensure the trough is shaded, and supplement electrolytes daily above 22 degrees. Avoid ice-cold water immediately after exertion. Sun-warmed water often triggers a better drinking response than cold tap water.
Which electrolytes and supplements work
A balanced electrolyte supplement. Elektro+ delivers the correct ratio of sodium, potassium, calcium, magnesium and chloride matching equine sweat composition. It works both preventively (daily in heat) and curatively (after heavy work or transport).
General heat resilience. Tonic supports general resilience and digestion, two systems under pressure in summer. A horse with a calm gut digests better and dehydrates more slowly.
What does NOT help: plain table-salt buckets without potassium and magnesium. Pure sodium without the other minerals disturbs balance and can worsen dehydration. A proper electrolyte always has a multi-mineral profile.
Management that makes the difference
Work in the cool hours. Avoid training between 11 am and 4 pm in high summer. Early morning or late evening is significantly easier on the horse.
Shade and air circulation. A horse in shade with a breeze loses up to thirty percent less fluid than a horse in full sun without airflow. Design your paddock layout accordingly.
Cold-water hosing. Right after exertion, hosing the chest, neck and inside of the legs with cold water (10 to 15 degrees) drops core temperature faster than walking does. Follow with squeegeeing to keep evaporation going.
Forage as a water reservoir. Hay is a major water store: up to fifteen litres per day are absorbed via the hindgut. A horse with permanent access to good hay is structurally better hydrated than one on rationed forage.
When to call the vet
Call your vet immediately for:
- Body temperature above 40.5 degrees at rest
- No or very little urine output for 12 hours
- Collapse signs, staggering or refusal to move
- Anhidrosis: the horse no longer sweats despite heat and exertion
- Colic signs combined with dehydration
Conclusion
Heat stress is a daily risk in the European summer, not just at competitions. With a combination of smart management (shade, cool work hours, free water), a daily electrolyte routine and attention to general resilience, most horses pass through summer without problems. Read on in electrolytes after competition for the sport perspective, or in our knowledge base for other topics.