Sorry to double post but I wanted to add, they do have a hormone you can give them to keep them from going into season. I don't remember what its called but I'm sure if you talk to your vet about it he will know.
Its very expensive though.
There are herbal supplements you can give too that are not too expensive and I think have good results. They are mostly to keep the mares mood more constant during the breeding season.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France