A horses Vitamin and mineral requirements are different to that of a pig so I would be very cautious if you are considering doing this, as it is not always a wise thing to do.
A pig is an omnivore and a horse is a ruminant which also means that what one may be able to digest ie grass the other can not and vice versa.
It does as a general rule say on the bag, but I would maybe speak with the company who made this product, the supplier who supplies the product(or maybe a Vet?) just to be sure that you won't cause their species any problems by doing this.