By Nicola Tyler BSc (Hons), Nutrition Director, TopSpec Equine

Some minerals are also called electrolytes. This is because when dissolved in body fluids they carry spare ions which give them an electrical charge. Electrolytes use this charge to help control the water balance in a horse, trying to maintain a sufficiently hydrated status at all times, whilst avoiding dehydration or excessive hydration. In practice the dominant concern with horses is to avoid dehydration. This is because dehydration can be life-threatening and is frequently performance-limiting, with colic and ‘Tying-Up’ the most well-known consequences.

A horse has only to become mildly dehydrated in order to lose performance, sometimes reported as a lack of stamina and slow recovery rates. The horse’s homeostatic mechanisms are very impressive but at about 4% loss in body mass performance is reduced by up to 10%. Electrolytes also play an important role in muscle and nerve function and, through their role in hydration, both temperature regulation and the transport of nutrients and waste products throughout the horse’s body.

The most significant electrolytes are sodium, potassium and chloride, with calcium and magnesium playing a smaller role. While phosphate acts as an electrolyte, there should always be sufficient in feed to meet requirements.

A horse is more than 65% water, and even at rest a 500kg horse needs approximately 20-25 litres per day. This volume of water requirement can easily double in hot weather. The water can come from forage and feed as well as liquid water.

Electrolytes, mainly salt (sodium chloride) are needed to maintain full hydrated status. The main way salt is eliminated from the horse’s body is through sweat, and the other electrolytes are also lost through this route. In particular the loss of sodium through sweat can easily exceed the intake of sodium through the diet.

If the losses of electrolytes are very high a disruption in the balance of electrical charge both inside and outside of a muscle cell can upset normal contraction and relaxation processes. For example, if too much calcium and magnesium are lost in sweat the main nerve to the diaphragm sends an impulse in synchronisation with the heart, this is commonly referred to as ‘thumps’ and can be seen occasionally in dehydrated endurance horses at vet gates.

The amount of sweat lost by a horse is affected by the intensity and duration of exercise, his fitness, and the prevailing humidity. Equine sweat contains significant amounts of electrolytes, at a slightly higher concentration than in blood. Each litre of sweat contains about 3.5g of sodium, 5.6g of chloride and 1.2g of potassium, plus traces of calcium, magnesium and phosphate.

In medium work and moderate environmental conditions a 500kg horse can lose about 5 litres of sweat per hour, containing about 17g sodium, 28g chloride and 6g potassium. Even if he sweats considerably less than this, he will probably need either salt or a commercial electrolyte preparation adding to his feed. If he is receiving ad-lib forage (which is rich in potassium) salt may well be sufficient, however if he is offered, or eats, less than 2% of his bodyweight as dry weight of forage daily (this would be about 11kg hay ‘as fed’ for a 500kg horse) then electrolytes should be added his diet.

Sodium stimulates horses to drink so if sodium levels in the blood drop below the desired level a horse can lose his desire to drink, a situation sometimes seen at vet gates, towards the end of endurance events. The way to avoid low sodium levels in the blood is to ensure that through the total diet, a salt-lick, added salt and added electrolytes, the sodium input is slightly greater than the losses of sodium, mainly through sweat.

The need for additional electrolytes in the diet varies widely.

At one extreme a horse at rest on good pasture will need no more than the presence of a salt-lick 24/7 to ensure adequate intake. However, a horse sweating heavily will need more than the salt provided by his forage and 3-4kg of concentrate feed per day. He may well need access to a salt-lick 24/7 plus added salt in his feed, plus an added commercial electrolyte preparation for at least 48 hours following heavy sweating. A racehorse being fed 7-8kg of hard feed a day may not need any commercial electrolyte preparations in training, and certainly will not immediately before a race, but would benefit from them for 48-72 hours after racing, especially if diuretics and/or forage restriction have been used.

Commercial electrolyte preparations always provide less sodium than plain salt; this is because they contain other electrolytes and also a product to enhance palatability, usually sugar. Therefore commercial electrolyte preparations should always be fed on top of any salt added to the feed, not instead of the salt. A rare exception to this is for those few horses that dislike the taste of salt, then commercial electrolyte preparations become valuable.

It is always best to consult an experienced nutritionist to determine when and how much salt or commercial electrolyte preparation should be added to your horse’s feed but as a guide a 500kg horse in light-medium work, sweating lightly, could safely have 1 level tablespoon salt added to his feed twice daily.

The amount of salt and/or commercial electrolyte preparations you need to add clearly depends upon what you are feeding and how much your horse sweats but remember that a horse cannot store electrolytes for when they are needed so added salt and commercial electrolyte preparations can only top-up deficient levels in the horse’s fluids. This means that commercial electrolyte preparations, in particular, tend to be needed after sweating has taken place, not before. Sweating is not always obvious and judicial use of electrolytes following travelling, when discreet or ‘silent’ sweating may have taken place, should be beneficial.

Care must be taken not to add to feed or syringe-in commercial electrolyte preparations in advance of sweating, this will cause horses that do not drink enough water to effectively become dehydrated, reducing performance and risking ‘Tying-Up’ and even more severe effects. This is seen too often when horses are competing away from home.

When choosing a commercial electrolyte preparation make sure that either salt or chloride is the highest ingredient. Sugar (glucose/dextrose) should not be the first ingredient listed as it means that the product may not supply sufficient electrolytes to meet your horse’s needs. Good products will contain high levels of sodium, chloride and potassium, with additional low levels of calcium and magnesium.

Feeding instructions on the commercial electrolyte preparation label should clearly indicate varying levels of preparation to be added according to the horse’s level of sweating.

When feeding salt it is useful to know that: –

1 level teaspoon holds 5g salt
1 level dessertspoon holds 10g salt
1 level tablespoon holds 15g salt

Key management points

Make sure that your horse has 24/7 access to a salt lick.

If your horse is fed less than 3kg of compound feed/TopSpec blends daily, and sweats at all, make sure that you add salt to his feed (in proportion to 1 level tablespoon twice daily/500kg bodyweight). This is because he may not consume enough, if any, of the salt lick.

If he sweats more than lightly, is on very little hard feed and/or restricted forage intake, ring an experienced equine nutritionist for advice on how much salt and/or commercial electrolyte preparation to use.

Remember that products such as oats, flaked barley, flaked maize, sugar beet pulp, alfalfa pellets, grass cubes and various mixed chops contain no added salt.