First of all, yes, this could be a problem, as to much salt intake can cause kidney damage and even failure. Does your horse get fed any grain? If so, look and see if there is an salt added in it, and if so, how much?
A horse with moderate work needs about 3 tablespoons of salt, but horses at rest with no work need about 1.5 tablespoons, and horses in heavy work about 5 tablespoons per day- based on a 1000 lb horse that only sweats when they are worked. This is all dependent on how much your horse sweats, so during summer if your horse sweats due to warm air, you may want to add another tablespoon of salt to your horse's diet.
How often do you work your horse? What is the environment like in summer and winter? How much does your horse weigh? You don't want to feed more than one tablespoon of salt per 200 lbs or 100 kg per day for a horse.
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