What’s the actual need? Is there a fortified feed in the picture?
Sweat is mostly salt, so adding 1tbsp per 500lb salt to the diet meets the minimum needs, and can be 2-3x’d for extra sweaty seasons
E-lytes are best for conditions around competition
Don’t get caught up on “designer” salts as being superior, or an actual electrolyte supplement, just because they contain a lot of additional minerals that plain white salt doesn’t have. Yes, sea salts contain a lot of extra minerals, many of which aren’t a requirement in the first place, some of which are really not something you want anyway (Cadmium?), but the good news there is that there’s such a teeny tiny amount of any of those that they might as well not be there.
That Celtic salt is " * No other ingredients except unrefined sea salt".
Here’s the GA

98% salt. 0.20% magnesium means in a typical 30gm serving, that’s 0.06gm of magnesium. The basic need of a horse is 10gm. There’s 0.02% K, which is 0.0006gm, and the basic need is 18gm
It’s salt, in a fancy and $$$ bag.