Nature strives to keep species in balance with their environment. Mice, for example, suffer death rates so high that few survive their first year, but their woeful attrition rate is offset by the capacity of each female mouse to produce several litters of 10 or more offspring annually. Conversely, horses have low attrition rates, so nature equips them with a physical makeup that limits them to, at best, a single pregnancy per year, ideally producing a single foal.
Working with this basic capacity, veterinary medicine has had two effects on equine fertility. First, advances in breeding technology have improved breeding efficiency, increased the breeding capacities of stallions and enhanced the breeding success of both stallions and mares with impaired fertility. Among major horse breeds, live-foal rates per cycle have averaged 60 to 65 percent for more than 25 years, an impressive statistic considering that, over the same period, veterinary advances have enabled popular stallions to service much larger books of mares. "The fact that we are still able to achieve the same live-foal rates," as those of 25 years ago, says Tom Bowman, DVM, "means that we really have made a lot of advances."
But science also has had a second, less-positive impact on equine breeding: perpetuating fertility problems. In the wild, horses with inherited reproductive problems, such as a tendency toward twinning, fail to consistently reproduce, and, thus, the problems become self-limiting. As a result, wild herds enjoy consistently high fertility. But among domesticated horses, medical advances enable reproductively abnormal animals to breed and the foals to survive and reproduce themselves, fostering a growing population of poor reproducers. "Man's intervention," says Patricia Sertich, VMD, assistant professor and clinical educator in theriogenology at the University of Pennsylvania, "has allowed males and females to breed who may not have been successful otherwise, so we have in effect selected for a population that may be less breeding sound."
So what degree of success can you expect? Obviously, it depends on a host of individual and management factors, but reproductive experts say that average breeders can aim high.
For the 80 to 90 percent of horses who are reproductively normal, ideal management can yield nearly 100 percent fertility, although normal horses sometimes fail to reproduce for undetectable reasons. Even if your mare has an abnormality, you may still get quite a few foals from her with veterinary help. The bottom line: "I'm sure that every owner goes into the breeding season assuming that all the mares will be in foal by the end of the year, every year," says Bowman. "I think it is more reasonable to assume that if you can end up with 75 percent of your broodmare band every year producing live foals, that's excellent. If you have 80 or 85 percent producing live foals, that's way above expectations."