Avoiding dehydration

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You are here: Home / Articles / Horse care

Ruth Bishop

20 June, 2005

The horse is highly adapted to lose heat through sweat, and does so more efficiently than any other animal. In the performance horse, heat is dissipated through not only sweat but also by increased respiration rate. Both cause the horse to lose water.

Horses also lose valuable electrolytes, responsible for the correct function of nerves and muscles, through sweat. Rehydration after exercise, especially if a horse is competing again the next day, is essential to maintain performance.

Equine sweat is hypertonic — it contains more salts than body fluid — which means that a sweating horse loses more electrolytes than water. Typically, a working horse will lose 7-8 litres in an exercise bout, although the maximum sweat rate measured for horses is 15 litres per hour. The rate does vary depending on temperature, sunshine, level of fitness and work.

Offering water alone to a dehydrated horse does not rehydrate it. Instead, the water dilutes the body fluids surrounding the tissues, which in turn "turns off" the thirst mechanism. The best rehydration therapies include the use of electrolyte preparations, either in feed or water, to stimulate drinking.

The main electrolytes are calcium, phosphorus, magnesium, sodium and potassium. Most horses receive almost all of their daily electrolytes from forage. The fibre in the hindgut acts as a reservoir of water and electrolytes. It is likely that it is the type of fibre (high-digestible is better) rather than the amount of fibre that is important. Higher intakes of stalky or seed-type (low-digestible) hay or haylage require the horse to expend more effort to carry the extra weight around.

Electrolytes cannot be stored by the horse. Mega-dosing in the weeks ahead of the competition is of no use, but sensible use in the days before will stimulate water intake so that the horse enters the competition fully hydrated.

Top tips

One litre of sweat is estimated to dissipate heat resulting from 1–2min of extreme work or about 5min of sub-maximal work, summer or winter
Always add salt to the diet, either directly in the feed (typically 2oz per day) or by offering free access to a salt lick
In horses in hard exercise, a mixture of salt and lite salt (potassium chloride) ensures the supply of three important electrolytes is maintained
On the day of competition or a bout of exercise, the final offer of electrolytes should be made with water, at least 4hr before

This feed forum feature was first published in Horse & Hound (19 May, '05)