Whether it's summer or winter, your horse's major source of electrolytes/minerals is his basic diet. For example, the daily potassium requirement of a 1,000-lb. horse doing intense work is about 40 grams per day. Most hays contain a minimum of 1% potassium, so 10 lbs. of hay a day will meet or exceed the potassium needs of a horse at work and 5 lbs. of hay will keep a horse at mainetance (1 lb. of hay provides 4.5 grams of potassium).
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.
Salt: the Major Concern
At baseline, the horse needs to take in approximately 1oz. of salt a day to stay optimally hydrated. Sodium is the major mineral controlling how much water is in his body. Because it's 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. This loss of water in the tissues is what makes a dehydrated horse's skin stay tented up away from his body if you pinch it.
Horses that have not had access to salt can maintain their circulating blood volume well, but they're always somewhat dehydrated. If they never get stressed or exercised they'll probably be OK, but they quickly get into trouble with overheating, heat stress and serious electrolyte abnormalities if temperatures climb or they're worked.
The major error that people make when using electrolyte supplements is to ignore the horse's basic salt requirement and think the electrolyte supplement is all their horse needs. This simply is not the case. Most supplements contain far too little sodium to even begin to meet the baseline requirements. Horses still need salt.
Another common mistake is to add them to the horse's drinking water without also providing plain water. Some horses don't like the taste of electrolyte products or have mouth sores/ulcers/abrasions that are irritated by the electrolyte-spiked water. Horses with stomach ulcers may avoid electrolytes, too. The horse will also stop drinking supplemented waters once their sodium hunger has been filled. The result of any of these things can be that the horse doesn't drink enough plain water.
Easy Electrolytes
The first step in making sure your horse has an adequate intake of electrolytes is to feed him a mineral-adequate diet with 10 lbs. of hay/day.
The next step is to provide free-choice salt or add salt directly to feeds. If you provide salt free-choice, monitor how much the horse actually eats. Loose salt, either in granular or fine form (e.g., table salt with/without iodine), will usually be consumed more readily than salt in licks or bricks.
Make sure that the horse consumes at least 1 oz. of salt per day in cool weather, when inactive. That's a pound of salt every 16 days. With hard work (sweating) and warm or hot weather, the horse's salt needs will increase to 3 to 4 oz./day for an average-size horse.
