Many mares have been under lights for 60-90 days and their owners are eager to start breeding them. They are so eager that that want to put semen in them as soon as there is a decent sized follicle. They’ll order semen as soon as they see it or hear about it. Unfortunately, this is setting the mare up for failure, if follicle size is the only guideline.
In order for mares to get in foal, they not only need to have a follicle, they need to be in heat. They also need to have an open cervix, unless it has been damaged. They need to have no bacterial infection. They need to have no fecal aspiration. AND they need to have a follicle that ovulates.
We hear over and over again, during breeding season, that mares didn’t respond to ovulatory agents or that the semen had caused “an allergic reaction”. Modern extenders do not cause allergic reactions. If a mare reacts to being bread, there is an underlying problem.
Follicle ovulation is not dependent on size; it is dependent on texture. The follicle(s), cervix, and uterus must be in synch for the mare to be fertile. So breeding early in the heat, when the uterus and cervix are not ready, causes problems, usually with delayed uterine clearance and other issues.
Breeding early in the season, before the uterus is ready, can cause major problems. Older mares may be transitional until late spring, even when using artificial lighting. They may appear to tease continually or may have slight changes but never really come in to heat or go all the way out. Follicles may grow and regress and not ovulate.
We have found that waiting for the older or transitional mare to have a normal ovulation prior to breeding the first time ensures that she is not being bred prematurely. Sure, you may have missed a viable heat, but that is easy to tell with hindsight. We’d rather miss a heat that put a semen into a mare that doesn’t ovulate for days or weeks, sometimes creating problems that last all breeding season. Breeding mares, and horses in general, requires patience.