Hi,
I know for sure that goats are supposed to have Kelp Meal available to them free choice. If they eat alot of it they are lacking in something. Sometimes they might not want any, sometimes they want to eat more.
As for horses, I know some people that feed them Kelp Meal. I've started adding a bit to the evening grain (a tablespoon) but I really don't know if this is too much, too little, if they should be having it free choice like goats, not sure.
Anyway, glad your horses aren't any worse for wear.