Miniature Horse Stallion Fertility

STALLIONS AND FERTILITY

When is a young stallion fertile? When can I start using my stallion to breed mares? How many mares can a young stallion breed in a season? How often can a young stallion be used in a given week? They are all good questions and bear discussion. First, however, it seems important to discuss some general points about stallion development.

CRYPTORCHIDISM and MATURITY

We have noticed a number of differences between the development of miniature horse stallions and full sized stallions. There have been numerous cases where veterinarians have evaluated yearling miniature colts and found that both testicles have not descended into the scrotum. As a result, the veterinarians have cautioned about infertility. Stallions whose testicles do not descend into the scrotum are given the name "cryptorchid". This syndrome is looked at unfavorably in stallions. With a testicle retained in the body cavity, fertility is considered to be reduced. Sperm must remain lower than normal body temperature in order to develop properly and remain viable. If a testicle is retained within the body of the horse, the sperm produced by that testicle are theoretically infertile therefore seriously reducing the fertility of the stallion. In full sized horses, the conventional wisdom appears to be if both testicles have not descended into the scrotum within the first year of life the horse will be a cryptorchid. This logic has also been applied to miniature horses. Unfortunately, it does not hold up in the vast majority of cases. It is our opinion that with miniature horses, it is not abnormal to see stallions whose testicles do not descend into the scrotum until they are three years of age. These "late bloomers" become very successful and fertile stallions and suffer no adverse effects from their testicles late decent into the scrotum. We have heard of very few true cryptorchid miniature horse stallions who's testicles totally fail to descend. It is not uncommon to find two year olds that do not show outward evidence of one or both testicles. By the time they are three, they are usually "intact" and fully functional as a breeding animal. The development of miniature horses is different than full sized horses. Some miniature horse stallions are mature as yearlings and can conceive pregnancies. Others do not reach maturity until they become three year olds. There are individual differences much the same as there are with the development and maturity of adolescent humans.

FERTILITY

There are a significant number of factors that influence the fertility of a stallion. Such things as descended testicles, testicular size, testicular consistency, sperm morphology (conformation in horse lingo), sperm concentration and motility (ability to "swim") as well as how often the stallion is used for breeding are a few of the more obvious factors influencing fertility. The quantitative measure of a stallion's fertility is a pretty difficult concept to get a person's mind around. In general fertility is a measure of the number of motile sperm present per ejaculation of a stallion. Through collecting semen from the stallion and doing some fairly precise counting of moving sperm under a microscope, the number of motile sperm per ejaculation is estimated and compared to norms for stallions. A normally fertile stallion produces in the neighborhood of 3,000,000,000 motile sperm per ejaculation. That is a bunch of sperm. The literature states that typical adult stallions produce approximately 6,000,000,000 sperm per day.

When a person takes into account the definition stated for fertility, the number of motile sperm per normal ejaculation and the number of sperm produced per day by the normal adult stallion, it doesn't take too much figuring to recognize that there are barely enough sperm produced per day for two ejaculations under perfect circumstances. When one factors in a percentage for motility, the fertility will drop below two ejaculations per day for optimum fertility. Young, developing stallions have a number of factors that reduce their general fertility from that of the adult. One of the limiting factors to sperm production is size of the stallions testicles. The testicles of young stallions have not fully developed. As a result, they are not yet able to produce sperm in the quantity per day that they will when they are a fully developed stallion. Less available sperm production equates to fewer ejaculations in order to maintain the normal fertility of 3,000,000,000 sperm per ejaculation.

The bottom line is that young stallions are not able to breed mares at the same frequency as adult stallions and maintain fertility. Normally young stallions have a huge portion of libido and are eager to breed mares. Mares can be covered, however the important factor of sperm/ejaculation is the overriding determiner of fertility. If the young stallion is used frequently, there will not be enough sperm available per ejaculation and his rate for settling mares will be sub-standard.

BREEDING POTENTIAL

There is substantial variation in opinion as to what is appropriate for breeding the young stallion. Authorities on the subject seem to vary in opinion. An article written by K.F. Dowsett in Current Therapy in Equine Medicine recommends that stallions below three years of age not be used for breeding at all. He and others state that there can be damage to the reproductive potential of stallions is they are utilized for breeding before they are fully mature. Larryann C. Willis in the text, The Horse Breeding Farm has developed a table of breeding frequency. Obviously the numbers of horses stated here are quoted for very large breeding farms. The most important point is to take note of the frequencies for the given stallions.

STALLION AGE	NUMBER OF MARES BRED / YEAR	FREQUENCY OF BREEDING
2 years	up to 12	2 to 3 services per week
3 years	up to 18	5 to 6 services per week
4 years	up to 28	once daily
Mature Stallion	up to 45	daily + 2 doubles per week*

* Willis explains the doubles as follows: "Some authorities recommend a limit of one breeding per day or six services per week. Others, however, believe it is safe to 'double' the stallion once or twice a week providing it is a 12 hour interval. Obviously doubling means covering two mares in the same day.

Willis continues by saying, "Although sperm is generally present in stallions 12 to 15 months old, at this age the horse is decidedly immature and should not be allowed to breed. A stallion of 24 months old usually is quite developed sexually. Sperm is present in sufficient volume and potency to allow limited breeding use of the horse..."

Please understand that this information being quoted is written based on conventions for full sized horses. To date, there has not been any published documentation that speaks directly to the fertility of miniatures. However, we have participated along with others in the Northwest United States in a groundbreaking study by Dr. Lisa Metcalf DVM, M.S. on this subject. At the time of this writing, results have not been published, however.

On our farm, we evaluate the maturity of each stallion individually. We have bred no more than 8 mares to a 2 year old stallion. Why? I'm not sure. It's probably because due to all of this reading, we are afraid that his "plumbing" will fall off. We do understand the valid reasons for spacing out breedings in order to let the young stallions sperm supply build up. It seems a waste of effort to breed the young stallion feverishly if one recognizes he is probably "shooting blanks" due to his immaturity and with developing sperm production rate. This situation is most likely to occur if the horse is repeatedly bred a number of times per day.

The unfortunate situation on breeding farms is that Mother Nature frequently does not cooperate. More than one mare comes into heat at the same time. This generally boils down to making value a judgment on the breeder's part. Do you over-use the stallion or pass up the heat cycle of one of the mares? If a farm ends up with a young stallion that is attracting a lot of attention, keep in mind that it is probably good business to be somewhat conservative about booking mares to him when he is young. If he is used to service many mares, the breeder can run the risk of having a healthy percentage of the mares come up open. Of course the stallion will, more than likely, get the bad rap of being a infertile. This is more than likely NOT the case. It is probably due to over use rather than some organic problem with the horse. It certainly is a lot better to produce a few foals the first year or two than to run the risk of setting the rumor mill afire about the perceived incompetence of your prized stallion.

Veterinarians can run a fertility check on stallions by collecting a sample of sperm. Those of you who are considering standing a young stallion, may consider having a fertility evaluation performed. At least, it would be useful to obtain guidance from a qualified veterinarian as to their recommendations for breeding the young stallion.