Stallion Breeding: Can He or Can't He?

Regardless of the number of mares being bred, it is important to monitor the stallion's reproductive success in order to maintain his reputation and give yourself time to address any problems he might have. Evaluating breeding records can enable you to determine if changes in breeding protocols might be needed if you would like to increase his book size or if you need to accommodate a heavy competition schedule during the breeding season.

If your stallion is only breeding a few mares, examining the records could indicate changes that could be made in breeding protocols to more efficiently use the time spent with outside mares. Conversely, the analysis might show things are going well and changes are not necessary. As the number of mares bred to the stallion increases, so does the importance of record evaluation, which can reveal changes in fertility that, if addressed early, can usually be managed successfully.

How Did He Do?

Many criteria are used to analyze breeding performance of a stallion. The more information available in the breeding records, the greater the likelihood of identifying the reason for a drop in pregnancy rates. Breeding records should include the number of mares bred by the stallion, status of the mares (foaling, barren, or maiden), age of the mares, number of cycles bred, number of covers per cycle, and results of pregnancy examinations. Records can be stored in systems that range from hand-written notebooks to complex computerized spreadsheets from which statistical analysis can be performed. There are a number of commercial software packages available for analyzing reproductive data.

Records are of no use unless they are analyzed. Pregnancy rate is the endpoint in a breeding program, so any analyses performed should identify if the rate is changing or remaining stagnant. Most breeders calculate the year-end pregnancy rate, which is the number of mares pregnant at the end of the breeding season divided by the total number of mares bred. Acceptable pregnancy rates vary, but most breeders and mare owners expect 85-90% of the mares bred to be pregnant at the end of the season (assuming the mares were reproductively sound).

Breeding efficiency is important because it identifies the time, energy, and expense needed to achieve a single pregnancy. Statistics that measure the efficiency of the breeding operation allow for optimal management of the stallion. Three parameters--cycles per pregnancy, first cycle pregnancy rate, and pregnancy rate per cycle--indicate the fertility of the stallion.

Cycles per pregnancy are the number of cycles a mare is bred before she becomes pregnant. The higher the number, the more effort involved in obtaining a pregnancy. A value of under two cycles per pregnancy is considered acceptable.

First cycle pregnancy rate is the percentage of mares bred which become pregnant on the first cycle of the season. This is a good measure of the stallion's fertility. Infertile mares which end up being bred multiple cycles will not have a greater impact on the result than a fertile mare as each mare is only counted once. An acceptable first cycle pregnancy rate is 65-70%.

Pregnancy rate per cycle is the pregnancy rate for all cycles combined. This is determined by dividing the number of pregnancies by the total number of cycles the mares are bred. An acceptable value is 60-65%. For example, if a stallion is breeding 25 mares, some mares will conceive on the first cycle that they are bred, some on the second, and so forth. All of these cycles are added together to obtain the total number of cycles bred. The number of mares pregnant is divided by the number of cycles. If the stallion breeding 25 mares has 21 pregnant mares and it took 46 cycles to reach that point, the stallion would have a 46% pregnancy rate per cycle.

Worth the Risk?

Sub-fertile stallions can have acceptable seasonal pregnancy rates, it just takes more cycles to reach that level. Also, the types of mares bred (foaling, barren, or maiden) can influence the apparent fertility of the stallion. Barren mares generally have lower fertility than other groups. Foaling mares could be expected to be in the high fertility category. Maiden mares can be highly fertile, and since they are usually bred early in the season, their chances of becoming pregnant are high. If a stallion is breeding primarily maiden and barren mares, he will be busy early in the breeding season. If the stallion is breeding a large number of foaling mares, he will be busier later in the breeding season. For example, a stallion with a 65% first cycle pregnancy rate will have fewer mares to breed on subsequent cycles than a stallion with a 35% first cycle pregnancy rate.

Foaling dates will be later in the year when mares are bred to a stallion with a low first cycle pregnancy rate. Also, mares bred late in the season eventually miss a year because they foal later and later each year.

Breeding Soundness Exam

If evaluation of breeding records indicates low pregnancy rates, a breeding soundness examination should be performed to identify the cause of the stallion's infertility. However, before you start pointing fingers at the stallion, look at his book of mares and management. Low pregnancy rates at the end of a breeding season could be due to overuse, having too many subfertile mares, or poor management.

The exam should be performed as soon as the problem is noticed and well in advance of the next breeding season so adjustments can be made before the stallion is committed to breeding a specific number of mares. A typical breeding soundness examination consists of obtaining a detailed reproductive and health history, performing a physical examination, and collecting at least two ejaculates one hour apart and evaluating the semen from those ejaculates. Motility (movement), morphology (structure), concentration, volume, total sperm numbers, and longevity of the sperm are evaluated.

External genitalia should be examined by palpation and/or with ultrasonography. The internal organs can be examined by transrectal palpation and ultrasonography. Measurements of the testes' length, width, and height should be done also.

Aerobic bacterial cultures of the stallion's semen, urethra, and penis are usually performed. Peripheral hormone levels might be evaluated in some horses. In cases where the results of the breeding soundness examination do not explain infertility questions, further testing such as sperm chromatin structure assay (SCSA), viability staining, acrosome reaction tests, and electron microscopy might be performed.


There are many causes of infertility in the stallion. One of the most common ones is testicular degeneration--a premature decrease in the production of normal sperm by the testes. It can occur in one or both testes and might be temporary or permanent depending upon the cause and severity of the problem. Causes of testicular degeneration include thermal (heat) injury, exogenous hormones (from outside sources) or other drugs, ingestion of toxic plants, and age-related changes. The cause of the degeneration is not known in many stallions.

In early stages of degeneration, the testis will become soft. The testicle might have shrunk, although the initial decrease might be so small that it goes unnoticed. The first indication of a problem might be a decrease in sperm numbers or an increase in abnormal sperm. This will be seen 40-60 days after the insult occurred. Libido and semen volume might be normal. The degeneration can be partly or completely reversible.

The most common known cause of testicular degeneration is thermal injury, which can occur due to trauma, fever, obesity, or failure of thermal regulation due to conformation. One change typically seen with degeneration due to thermal injury is a rise in the percentage of sperm with detached tails. This is due to the effect of the temperature increase on the sperm in the epididymis and will be seen within three to four days of the insult.

Horses affected by testicular degeneration might exhibit a gradual decline in semen quality, so annual monitoring of semen quality allows you to follow changes in seminal parameters.

Testicular hypoplasia is the incomplete development of the gonads. It can occur in one or both testicles and can be genetic or acquired. In a clinical study of 1,000 young stallions, researchers identified hypoplasia in 3% of the animals. Hypoplasia is not reversible and can occur to varying degrees of severity. To provide a prognosis, hypoplasia must be differentiated from testicular degeneration, which is difficult.

Cryptorchidism (the retention of one or both testicles in the abdomen or inguinal region) is thought to have a genetic component in its transmission, but this has not been proven. Some breeds do not allow registration or showing of cryptorchid stallions. Sperm production in a cryptorchid will be decreased as compared to a stallion with two testicles. These stallions can be significantly impacted by any insult to the descended testicle. For example, in a case of severe testicular trauma, a stallion with two testicles could potentially undergo unilateral castration (removal of one testis) and continue his breeding career while a cryptorchid stallion will be out of a job.

Drugs affect stallion fertility. The use of anabolic steroids in stallions has caused testicular degeneration, decreased sperm production and quality, and decreased testicular size. Many drugs are used on performance horses when they are young and the testicular tissue is extremely susceptible to alterations, so testicular changes can occur and not be noted until the stallion becomes a breeding prospect. The impact these drugs have on fertility is generally not considered when they are used because most individuals are more interested in the horse achieving maximal performance potential. Therefore, you must use caution when using any drugs on colts which could be used for breeding. (The National Library of Medicine maintains a website,, with toxologic information about many substances and drugs. One section of the site deals with reproductive toxicology.)

Spermiostasis or sperm accumulation can contribute to infertility. Some stallions accumulate large numbers of spermatozoa in the epididymis and deferent ducts. These sperm undergo degenerative changes resulting in increased abnormal sperm and decreased motility. Ejaculates from these horses are typically very high in concentration. Concentrations of over 1 billion sperm per milliliter are not unheard of (a concentration of 150-250 million sperm per milliliter is considered normal). In severe cases, the ducts become "plugged" and sperm are not released in association with ejaculation. Horses which are affected with spermiostasis need to be collected regularly to maintain maximal semen quality.

Bacterial infection of the accessory sex glands, epididymis, or testicles can have a negative impact on fertility. This is due to the effect of fever and local inflammation associated with infection. Additionally, stallions with bacterial infections can infect susceptible mares via breeding.

With shipped cooled semen, the goal is a minimum insemination dose of 500 million progressively motile sperm when received by the mare owner, although this can vary with individual stallions. The average dose of shipped cooled semen is expected to decrease 50% in progressively motile sperm after 24 hours of storage. We have received semen with no motility from "poor shipping" stallions. Unfortunately, many times the assumption is made that all stallions produce semen that is capable of being shipped.

Take-Home Message

There are many potential causes of infertility in the stallion. Maintaining complete records and analyzing the records during and after the breeding season can identify changes in fertility, and early identification potentially allows correction of the problem. If the problem cannot be corrected, then tailored management changes can be made to minimize the effects of the problem on the stallion's fertility. A regular breeding soundness examination allows evaluation of the stallion's contribution to the problem and the potential for treatment.


Year-end pregnancy rate 85-90%

Cycles per pregnancy

Less than 2.0

First cycle pregnancy rate


Pregnancy rate per cycle


Sperm concentration per milliliter

150-250 million

About the Author

Pete Sheerin, DVM, Dipl. ACT

Pete Sheerin, DVM, Dipl. ACT, is a practitioner at the Rood and Riddle Equine Hospital in Lexington, Ky.

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