I'd say for milder cracks, a farrier should be able to help. They'll also be able to determine whether a vet is needed. If you spot deep cracks, if your horse seems distressed or in pain, or if they seem to be unable to put weight on the hoof with the crack, just go direct to your vet. If you're a beginner horse owner and a little unsure, it's best to call your vet for advice, even if just for a little reassurance. You can also check out our guide to hoof care and common ailments for more advice.