Hot weather gets many people reaching for an electrolyte supplement for their horse, but often they have little understanding about electrolytes or what they do. This is scary because, used improperly, electrolytes can make the risk of dehydration or electrolyte-related performance problems worse for your horse.
Electrolytes are nothing more than minerals dissolved in the horse's blood stream. The horse must take in electrolytes/minerals year round to replace those lost in urine, saliva, bile, tears, intestinal tract secretions. Electrolytes are also lost in sweat, but in most cases the sweat losses are only part of the horse's total daily needs.
The major electrolytes in blood are sodium and chloride, which together make salt. Inside cells, potassium substitutes for sodium. Other important electrolytes (minerals in free/dissolved form) include calcium, magnesium, and phosphorus and the trace minerals zinc, iron, copper and manganese.
Whether it's summer or winter, your horse's major source of electrolytes/minerals is his diet. The daily potassium requirement of a 1,000-lb. horse doing intense work is about 40 grams per day, but most hays contain a minimum of 1% potassium, meaning just 10 lbs. of hay a day will meet or exceed the potassium needs of a horse at work (1 lb. of hay provides 4.5 grams of potassium).
Potassium is included in large amounts in all electrolyte supplements, but the fact of the matter is the diet already contains plenty. Of all the important electrolytes/minerals, the only ones that aren't present in adequate amounts in the diet are sodium and chloride-that's plain old salt.
At baseline, the horse needs to take in approximately 1 oz. of salt a day to stay hydrated. Sodium is the major mineral controlling how much water is in the horse's body. Because it is in such short supply in their diets, horses have evolved to have a strong hunger for salt, and their bodies will also save sodium at the expense of losing other minerals if they have to.
When sodium is in short supply, horses adjust by secreting less sodium in the urine (substituting potassium instead), producing more concentrated urine, and "robbing" the tissues surrounding the cells of water to preserve the volume of their circulating blood.