What Makes One Stallion More Fertile than Another?

By Debra Ottier

Iron Horse Equine

With the breeding season upon us, we should take time to direct our attention to the horse often forgotten about in the fertility equation, the stallion. Yet, it is the stallion that will make or break a breeding operation. If a mare has poor reproductive performance, while unfortunate, it does not cost the farm little in comparison to having poor reproductive performance from a stallion. Understanding the behavioral patterns and limitations of the stallion as well as recognize and meet his basic needs is necessary to understand why some stallions are not able to settle all the mares they breed. What can make one stallion more fertile than another one is management, although inherent fertility certainly plays a large role as well.

A poorly managed stallion will have lower per cycle pregnancy rates. This is the only accurate test to determine a stallions’ real fertility. For example, stallion A breeds 10 mares for one cycle and gets 8 pregnant. He has an 80% per cycle pregnancy rate. Stallion B breeds 10 mares for one cycle, and gets only 4 pregnant, then rebreeds the open mares and gets 4 more in foal, then he has 80% pregnancy rate, but only a 40% per cycle pregnancy rate. This means that the number of mares pregnant at the end of the breeding season is an inaccurate measure of the fertility of a stallion.

It is believed the stallion can be taken out of his paddock, brought to a mare, he will cover her and in 11 months, junior arrives. But, why is it that some stallions do not settle the mares they breed? The management of the stud and failure to properly prepare and monitor the stallion during the breeding season are key factors in determining success. The stallion delivers the sperm required to fertilize the egg of the mare to produce an offspring. In order to do so, he must have adequate libido to perform his duties of servicing and delivering of the sperm package, the sperm package must be of good quality and the stallion needs energy to carry out these duties.

Let’s start with libido. This is the desire to do the job. If the stallion is not interested in the job, it can be very difficult to convince him. This is the top-limiting factor for breeding operations. The desire to do the job can be impacted by poor management and the stallions’ behavior is often overlooked, as most see the stallion as a sperm-producing machine. Other stallions intimidate stallions, they like to see their mares and not be isolated, and one must be respectful of their limitations. Libido can be increased behaviorally, or through the use of hormones, however, the latter is not recommended, as by disrupting the normal hormonal balance of the stallion, you could ultimately create a larger problem with lowered sperm production.

A stallions’ sperm number will decrease the more he breeds in a day. Therefore, limit his breeding activity daily, using semen collection services to reduce the number of times he needs to breed. When a stallion ejaculates, his output is usually enough to inseminate 4-5 mares at one time. Although he may have the libido to perform, he may not have the sperm package to deliver.

Nutrition plays a large role in ensuring the stallion has the energy and nutrients available for the high demands of breeding, as well as for adequate sperm production. A higher plain of nutrition needs to be started early in the season, such that the proper nutrients are available for the stallion when demands are high. He should have a minimum body condition score of 6 at the start of the season and should never drop below this score. While mineral balance is important for the body on a daily basis, it is also essential for sperm production. A poorly fed stallion is often a poor producer.

Along with nutrition, other thing to consider is the stallion behavior. Stallions are often reprimanded for acting stallion like, for example calling to other horses during exercise, or even in the breeding shed. This can have an impact on the stallions psyche with the stallion often not performing when he is supposed to for fear of punishment. One needs to be very careful to let the stallion act like a stallion in the breeding area, even if he likes to bite the mare when teasing, as this may be what he needs to increase his libido.

Another factor that plays a role in a stallions’ fertility is lighting, especially for the young stallion. There are breeds that direct the breeding season to February to July in order to ensure foals are born as close to January 1st as possible. This is not the natural breeding season for horses, which is from April to August. By starting early, one needs to be sure that the stallion is able to produce sperm sufficient to settle mares in this period. To do so, lights can be used to kick start the season, as the natural breeding season is analogous to longer daylight days. Older stallions appear to be refractory to light stimulus, and therefore, you do not see as large a drop in semen quality.

Age is also something one needs to consider as a factor for fertility. It has been noted that some start stallions breeding as early as 2 years of age. The stallion is not sexually mature at this age, is not producing enough testosterone to adequately produce sperm and has a higher nutrition requirement, as he is still growing and the demands of breeding could impact sperm production and libido. To determine if his qualities are perpetuated into his offspring, breed at this age, but keep in mind his limitations and manage him accordingly by reducing the number of times he is bred.

Even though we attempt to manipulate the stallions’ life and breeding opportunity, ultimately, it is up to Mother Nature whether the stallion is able to perpetuate his genetics into the offspring, and sometimes no matter what we do, we cannot alter the inherent genetic capability of the stallion. Herein lies the question of selection, and what qualities we wish to breed for, and what will it cost in terms of reducing the fertility of horses in general. But that is another story…