Horses dissipate sweat very efficiently, so dependent on work level and humidity your horse may be losing sweat without that being obvious. Try this simple check. After work is their saddle cloth damp?
If yes, that's a good sign that you should be supplementing NAF Electro Salts or NAF Electro Liquid
Feeding following hard work, in hot weather or following heavy sweating to compensate for electrolyte loss
When horses work, or during hot weather, they regulate their body temperature by sweating. Sweating not only loses fluid as it evaporates, but essential body salts, particularly Sodium, Potassium, Calcium and magnesium. Failure to replace these body salts can result in fatigue
Electro Lytes is an easy to use liquid designed to replace the four essential body salts which may be lost through exercise, sweating or during hot weather.
Simply add to feed or dissolve in drinking water. For horses in heavy work or during periods of extreme heat, feed the more concentrated Electro Salts