Finally, the summer has made an appearance and brought us some warmer weather. Working horses sweat more as the temperature warms up, dissipating the heat produced by their exercising muscles. The muscles are not particularly efficient and as they work, they produce heat, which must be dissipated if the horse is not to internally overheat. Sweat helps to take the heat from the muscle into the air, as it evaporates. When the air is warmer, more sweat is produced because warmer air can take less heat.
Sweat consists of water, protein and electrolyte salts; mostly sodium, chloride and potassium, with a little calcium and magnesium. Horse sweat is more concentrated than human sweat, and horses don’t dilute their sweat over time as they exercise; instead they keep on losing large amounts of electrolyte salts. These salts must be replaced in the diet if the horse is to stay healthy. Consequences of not replacing these essential salts include dehydration, fatigue, overheating, muscle dysfunction and poor performance.
Most horse feeds do not supply enough sodium or chloride for working horses, and this includes, forages (hay, haylage and grass), compounds such as coarse mixes and nuts, and vitamin and mineral supplements. Potassium usually occurs in large amounts in forage, so extra supplementation is much less frequently required. The amount of electrolytes required by a horse depends entirely on how much they have sweated, so requirements can change day to day.
Free choice salt blocks do not necessarily supply enough sodium and chloride to working horses, because researchers have shown that some horses will not take in enough voluntarily. Instead, electrolyte salts should be added to the feed to ensure the horse is replenished sufficiently. All working horses need to be fed extra sodium and chloride and horses in endurance training who are working for extended periods of time should have potassium added as well.
Electrolytes are added to water to encourage rehydration in a dehydrated horse, who has lost a substantial amount of fluid via sweat. The solution should be isotonic i.e. at a similar concentration to body fluids and this is determined by the amount of water the salts are mixed in, not the proportion of salts in the electrolyte product. Electrolyte rehydration solutions are a distinct use to adding electrolytes to the feed, and the two should not be confused. You will replenish small amounts of salt via a rehydration solution, but never enough for a hard working horse in the summer months.
All horses that sweat during work (training and competition) or travelling should have electrolyte salts added to their daily diet. Hard working horses who are dehydrated after work (training or competition) should also be offered an oral rehydration solution made from an isotonic salt solution.