Here is something I wrote about feeding salt: https://www.thehorsesadvocate.com/feeding-salt-to-horses/

I prefer free-choice salt for horses even though salt intake in dehydrated horses has not been tested. However, when fed to dehydrated humans and mice, the increased salt will cause the conversion of glucose into fructose, subsequently inflaming the kidneys and islet cells of the pancreas resulting in hypertension (increasing blood pressure) and decreasing insulin production (increasing blood glucose). I understand that hypertension isn’t a problem in horses per se, but I do know that laminitis is an inflammatory condition of the hoof associated with blood circulation. I always want to reduce inflammation everywhere, including the blood vessels, so I don’t recommend adding salt to horses other than offering it as a lick they can access according to their needs.

Any mined source of salt is OK (Redmond, Himalayan). However, pure NaCl misses the other minerals available in mined sources, and “sweet licks” such as the red salt blocks with trace minerals have molasses and corn syrup added.

Thank you so much! This is very helpful.