Breeding is an incredibly complicated subject, especially on the mare’s side of things. We could go into extensive details on hormones and the estrus cycle and this post would basically be a chapter of a book I started writing a long time ago 
But, many people are just simply looking for the answer to the question, “When is the best time to breed my mare?”
Well, if you want to get down to the simplest answer there is the best time to breed your mare is just before she ovulates. The egg will survive for about 12 hours in the fallopian tubes. Stallion semen, however, is good for about 48 hours in good conditions and can last up to 5 days. The idea here is to have semen present when the mare ovulates, that way you have semen just waiting for the egg to show up. It’s all really pretty common sense when you think about it.
If a mare is unable to be palpated to check for when she will ovulate, a good rule of thumb is to begin breeding the mare the second or third day of the estrus cycle and every other day until the mare goes out of heat.
If you can have the mare palpated then you will want to watch her follicle size and breed the mare just before she reaches the follicle size that she usually ovulates at. Obviously, if you have never bred the mare before and you don’t have any records on her follicle sizes and ovulation times then this can be difficult to determine but your veterinarian should be able to help you determine this.
In the pasture, a mare will allow a stallion to breed her for several days before she ovulates. Once she ovulates, she typically will not allow him to come near her. In nature, the stallion is basically making sure that he gets semen in her before she ovulates, just like you want to do when you breed your mare whether or not you can palpate her. The stallion doesn’t know what size her follicles are, he just knows that right now she’s nice and he can breed her. So, that’s the natural way of doing things and generally how pasture breeding works when the horses are left to figure things out on their own.
So, to put it simply the best time to breed your mare is just before she ovulates. How to tell when she is going to ovulate can be done by a veterinarian or just by watching the mare interact with the stallion naturally. If she is amiable and allows him to breed her, then let him. Once she starts to keep him away, then you are probably looking at a mare that has already ovulated and you will want to set up an appointment with your vet to determine whether or not she is pregnant in a few weeks.
2 Comments
October 5, 2009 at 3:18 am
With respect…has it ever ocurred to you that what people mean with this what time of the year is it better to breed, since there are several options? There are no options to what you explain…as far as I know, either she is in heat or she is not. Or am I missing something, here?
October 6, 2009 at 1:50 am
If only breeding was as simple as what you describe. Mares are seasonal breeders, meaning that they will only breed during the warm months. In addition, if you’re in the horse show industry or racing industry, then you want to breed your mares at the right time so that the foal is born when you want it born. So, you really don’t have that many options and mares can act like they are in heat when they really aren’t. The only way to make things work out the first time around is to ensure she is ovulating when you’re going to breed her. That and mares aren’t run with stallions like they used to be. Today the majority of the mares are artificially inseminated, so you obviously need to know when she is ovulating so that you can inseminate on the correct days.