35 cooler than the horse’ s body, and the air humidity must be low to allow moisture to evaporate from the sweat.
When the air temperature rises to 32 ° C or higher, the gradient that allows heat to move from the skin to the air disappears. Heat can no longer be transferred into the air, no matter how much blood reaches the skin. At this point, the horse must rely almost exclusively on sweating, but sweating can only help if the moisture evaporates. High humidity prevents evaporation, creating a perilous situation where the horse continues to generate heat internally while losing its ability to cool itself externally. The result is rapid and potentially lifethreatening heat load.
DEHYDRATION: WHY WATER ALONE ISN ' T ENOUGH
When a horse sweats, it loses far more than water. Sweat contains sodium, chloride, potassium, calcium and magnesium, minerals that are essential for nerve function, muscle contraction, and the body’ s thirst response. As these electrolytes become depleted, the horse may actually lose the desire to drink, even when dangerously dehydrated.
A practical way to reduce internal heat production is through dietary management. A high‐oil diet( 250 – 750 ml linseed oil per day) can decrease the heat generated during digestion and reduce heat produced by the muscles during exercise. This reduces the heat load the body needs to dissipate, helping slow the onset of fatigue and supporting the horse’ s ability to cope in hot weather.
WHICH HORSES ARE MORE AT RISK
Not all horses regulate heat equally. A horse ' s fitness level plays a major role; an unfit horse produces more heat for the same effort, and its cardiovascular system is less able to support cooling. Younger and older horses, those carrying excess weight, and those with thick coats may struggle even in moderate conditions. Travel adds an additional burden, with horses losing two to three litres of fluid per hour during long journeys. Once they arrive, they may require 24 to 48 hours simply to restore their fluid balance before they can safely work.
Humidity, rough terrain, poorly ventilated floats or stables, and intense warm-ups can all contribute to accelerated overheating. Riders may unintentionally make things harder by working their horses during the hottest part of the day or by underestimating how quickly a horse can reach its thermal limit. Horses transported from cooler climates face an additional problem: they may need two to three weeks to acclimate properly to hot, humid conditions, and until they adapt, their tolerance is significantly reduced.
Dehydration affects performance long before it becomes a crisis. A fluid loss of just 2-3 % can reduce a horse’ s capacity for work by 10 %. More severe imbalances may lead to conditions like thumps( synchronous diaphragmatic flutter), where the diaphragm spasms in time with the heartbeat, or puffs, where panting becomes uncontrolled. Both issues arise from disruptions in the availability of calcium and potassium. Without restoring electrolytes, not just water, these problems can worsen rapidly.
For horses working regularly in hot weather, daily electrolyte supplementation is essential. Horses consuming mostly hay usually receive ample potassium but may be deficient in sodium, chloride, and sometimes calcium or magnesium. Providing salt twice a day and adjusting the quantity during hotter periods can help support normal hydration
2026 AQHA MARCH / APRIL ISSUE