Having a stallion on the premises adds another dimension to an equine operation whether it be on a farm where only a few mares are bred each year or one where that number is in the hundreds.


The master or controlling organs of a stallion’s reproductive system are the testes. Within their confines, spermatozoa are produced, along with testosterone, the primary male sex hormone.

Because of his aggressive nature a stallion must have his own housing and exercise area so that he can be segregated from other horses. The stallion also requires unique handling especially during breeding season; and this requires some expertise from a management point of view.

The reason for putting up with the workload as well as the expense is to breed the stallion to mares and raise foals either for private use or for the commercial market.

When that stallion sires foals which can compete and win or perhaps just provide us with the replacement riding horse of our dreams the effort is all worthwhile. However when we discover some months into the breeding season that the stallion is not getting mares in foal the time effort and expense are all wasted and a mental and emotional drain begins as well.

When we consider that foaling rates overall range somewhere in the neighborhood of 55-60% it becomes instantly obvious that simply covering a mare and getting a foal are not a sure thing. The problem is not always with the mare. Frequently it involves a stallion which is lacking in fertility. There are many reasons for sub-fertility and infertility in stallions. Some reasons are well-documented and others are shrouded in mystery. Researchers have cleared away many fertility mysteries through the years but a great many remain.

Stallion Physiology

For example light is a prime stimulus in re-awakening the mare’s reproductive system from dormancy to productive capacity. The same is true of a stallion. In addition some of the same hormones are involved in each system.

The stallion’s reproductive system includes two testes with their attached epididymides two deferent ducts an urethra a penis and accessory sex glands–bulbo-urethral prostate and vesicular.

The master or controlling organs of a stallion’s reproductive system are the testes. Within their confines spermatozoa are produced along with testosterone the primary male sex hormone. The testes normally are ovoid in shape and measure from 80 to 140 millimeters in length and 50 to 80 millimeters in width. They will weigh approximately 225 grams.

Testicular size is significant. Generally speaking large testes will produce more sperm than will small testes.

In the normal stallion the testes are housed within the scrotum. However in some cases the testes do not descend from the body cavity. This is referred to as cryptorchidism. If only one of the testes is retained within the abdominal cavity the stallion usually will be fertile. If both testes are retained the production of spermatozoa usually doesn’t occur and the stallion will be infertile.

Whatever the case retained testicles are considered a serious fault. The problem is also believed to be inherited and as a result many breed organizations have rules against registering a cryptorchid stallion. There are enough problems with subfertility from unknown causes. It isn’t wise to breed on known causes.

As is the case with mares the reawakening of a stallion’s reproductive system in the spring begins with the hypothalamus in the center of the brain and the pituitary the small gland located at the base of the brain.

Under the influence of increased light the hypothalamus releases gonadotropin releasing hormone (GnRH). It travels to the pituitary gland where it stimulates the secretion of two other hormones follicle stimulating hormone (FSH) and luteinizing hormone (LH). These two gonadotropins then travel to the testes where FSH exerts its effect on the Sertoli cells and LH on the Leydig cells.

Sertoli cells are involved primarily in the production of spermatozoa and Leydig cells secrete estrogen. Stallions produce a great deal of estrogen when reproductively active. They bind testosterone that key hormone that is involved with a stallion’s libido. The testes do not constitute the only place in the stallion’s system where testosterone is found but it is there that the concentrations are highest.

Proteins produced by the Sertoli cells bind testosterone and also are responsible for regulating the concentration. When testosterone levels are high production is slowed and when levels are low more is produced.

One known cause for subfertility involves the administration of anabolic steroids which results in the wrong signals being given regarding testosterone production. The stallion’s reproductive system recognizes the anabolic steroids as testosterone. When steroids are administered the system receives the same signal as when normal production of testosterone reaches a high level. As a result production is shut down or curtailed even though there actually is not an excess of testosterone present. This means that testicular size will diminish and sperm production will be drastically lowered.

It will take at least two months after steroid administration ceases before spermatozoa production will return to normal or at least near normal.

Last month we discussed how mares have an estrus (coming into heat) and an anestrus season (when they do not come into heat and do not ovulate). The periods of estrus coincide with long warm days and the periods of anestrus are associated with short cold days. With stallions it is much the same. When days are short the pineal gland within the brain releases melatonin which it is believed inhibits the hypothalamus from releasing GnRH. A failure to release GnRH has something of a domino effect inhibiting the secretion of LH and as a result the production of testosterone in the testes.

The limited supply of testosterone in the testes results in a quieting of the stallion’s reproductive system. Libido is reduced along with testicular size and the manufacture of spermatozoa. Once again there is a similarity with what occurs within a mare’s reproductive system when the ovaries diminish in size and eggs do not mature during anestrus.

As is the case with mares lights can be used to stimulate a stallion’s reproductive system. After first allowing the system to shut down as the result of lack of light it can be reawakened by placing the stallion under photostimulation for about 16 hours a day usually beginning sometime in December. However it has been found that a stallion will "shut down" earlier in the season after being under lights which could curtail his libido and fertility at the peak breeding period.


A mare does her part in the reproductive process by producing eggs to be fertilized. The stallion does his part by producing spermatozoa to fertilize them. The production of spermatozoa is called spermatogenesis. This is not a short simple process. Normally 57 days are required beginning with the germ cells located within the seminiferous epithelium of the testes and ending with the release of spermatozoa.

When at the 57-day point of maturity the spermatozoa are released within the seminiferous tubules and travel to the epididymis which is attached to the dorsal surface of the testis.

While being stored in the epididymis the spermatozoa continue to mature and it is there that they acquire the ability to swim and to fertilize an egg. The overall length of time involved from beginning to end of this process–reaching the point where spermatozoa are mature and capable of fertilization–involves a total of 65 days.

When we take this into consideration the matter of subfertility immediately becomes a bit more complicated. A stallion’s fertility can be compromised by something that happened to him two months or more earlier.

When the weather is cold muscles within the scrotum and spermatic cord contract causing the testes to be drawn closer to the body for warmth. When temperatures rise the reverse happens-the testes are lowered in an effort to prevent temperatures from rising to a point where spermatozoa are adversely affected.

Elevated temperatures for even short periods of time as the result of fever for example can have a negative effect on spermatogenesis. When we realize that the whole manufacturing process for spermatozoa takes 65 days this is significant. A major portion of the breeding season can be lost.

Spermatozoa as mentioned are stored within the epididymis until ejaculation with any overabundance being released and washed away in the urine.

A key component in the transferring of spermatozoa from the epididymis into the mare’s reproductive system is the penis. A stallion must have the ability to attain an erection in order to ejaculate either within the mare or if artificial insemination is involved into an AV (artificial vagina).

During ejaculation the spermatozoa are moved out of storage in the epididymis and along the way are mixed with secretions from the accessory sex glands. This entire mixture is what we refer to as semen. As muscular contractions send this mixture along its way it arranges itself into three parts or fractions. The first fraction is believed to be from the bulbo-urethral gland and is thought to help cleanse the urethra for the two fractions that follow. The second fraction contains about 45% of the total semen volume and more importantly carries with it about 75% of the spermatozoa. The final or third fraction is called the sperm-poor fraction and contains the gel portion. This portion normally is filtered out during the artificial insemination process.

As with mares the stallion’s reproductive system involves intricate synchronization with potential for problems existing all along the way. Any malfunction or disruption of proper timing can result in subfertility or infertility.

It becomes instantly obvious that if one has a stallion which is sub-fertile early detection and treatment are essential.

Two researchers in the Department of Large Animal Medicine at Texas A&M University dealt with early detection in five in-depth peer-reviewed papers published in Veterinary Medicine in 1996. The two are Terry L. Blanchard DVM MS Diplomate ACT and Dickson Varner DVM MS Diplomate ACT.

The treatises aimed at helping guide veterinarians through the evaluation process to determine a stallion’s fertility begin with a discussion of a basic examination and end with a discussion on predicting a stallion’s fertility.

A breeding soundness examination they recommend should be step number one in the detailed process of determining fertility.

"A properly performed breeding soundness examination" the two researchers write "will provide the basis for predicting a stallion’s fertility when bred to a reasonable number of fertile mares. Perhaps more important the examination can provide information that dictates management procedures to optimize the fertility of a potentially sub-fertile stallion."

The first step in a breeding soundness examination Blanchard and Varner advise should be a gathering of the stallion’s history. This includes everything from pedigree to deworming and exercising schedules.

"Explicit information about previous and current reproductive management can be informative" they emphasize "particularly if a recent change in management has been associated with a change in fertility status."

They list 11 areas for which specific information should be sought:

1. Number of mares bred and number pregnant.

2. Frequency of breeding. This should include the number of mares bred daily as well as in the previous week.

3. Number of services per cycle.

4. Pregnancy rate per cycle.

5. Cumulative pregnancy rate for season.

6. Pertinent information concerning mares including the number of maiden lactating and barren mares bred with respective pregnancy rates for each group.

7. Sexual behavior. Does the stallion fully intromit the penis into the vagina remain mounted until ejaculation occurs and show positive evidence of ejaculation (such as sperm present in dismount sample) during copulation?

(Information collected on stallions used in artificial insemination programs the two researchers say should be the same in items 1 through 6 but should also include the number of mares bred with each ejaculate as well as the frequency of each collection. Also determined should be the average volume concentration total sperm number sperm motility and sperm morphology in ejaculates collected. The final four areas of determination that follow are for stallions used in AI programs.)

8. Type and source of semen extender used. This can also include the water source and type and source of antibiotics added to the extender.

9. Semen handling and insemination protocol. This should include the type of artificial vagina used and its maintenance as well as the type of lubricant used.

10. Interval from semen collection to breeding and method of storage for semen held for later insemination.

11. Pregnancy rates for other stallions bred under the same management scheme using the same semen extender and handling methods.

The importance of the above steps becomes obvious in the reading. If the stallion is improperly utilized and cared for or in the case of AI programs semen quality is compromised through contamination or inappropriate handling the root cause of a subfertility or infertility problem could well rest with management rather than the stallion’s reproductive system.

The next step recommended by Blanchard and Varner is an assessment of the stallion’s general body condition:

"Pay particular attention to the eyes and limbs. While horses with eyesight problems can be used for breeding special housing and management considerations may be required for the safety of the stallion and handlers. Chronic hind limb lamenesses or back injuries that might at first appear to be mild can worsen with age or during an intensive breeding schedule. Poor teeth condition can adversely affect the stallion’s ability to maintain body condition particularly if the horse is pasture-bred.

"Also note any evidence of recent or current infectious disease because a rise in body temperature can adversely affect spermatogenesis. Poor sperm quality may be evident as early as five to seven days after the rise in body temperature but the most dramatic effects are likely to be seen about 30 days later.

"Additional laboratory tests including serologic testing for equine infectious anemia or equine viral arteritis a CBC (complete blood chemistry) a serum chemistry profile fecal egg counts and urinalysis are sometimes performed. In some cases blood typing is requested. Finally do not overlook evidence of potentially heritable conditions-cryptorchidism parrot mouth microphthalmia (abnormal smallness of one or both eyes) and wobbler syndrome."

These latter traits the researchers emphasize are ones that might be transmitted to offspring.

Next comes a thorough examination of the stallion’s genital area. This examination Blanchard and Varner emphasize should be thorough because "certain abnormalities can significantly affect a stallion’s potential fertility."

They also point out as has already been mentioned that testicular size is significant. "Testicular size correlates highly with daily sperm production so accurate testicular measurements help predict the number of mares a stallion can be successfully bred to."

Examination of internal genital organs they point out can be done through palpation via the rectum or with the use of ultrasound.

Once it has been determined that the stallion’s physical reproductive apparatus is in good order it is time to determine his mental state regarding breeding.

Blanchard and Varner put it this way: "Excellent semen quality in a breeding prospect is inconsequential unless the stallion has the desire and ability to deliver the semen to the mare’s reproductive tract or an artificial vagina."

The only way to check this out is to present the stallion to a mare in heat and observe the results. The stallion with good libido will show all the signs of sexual arousal complete with an erection.

If the initial breeding soundness examination results in a finding that the stallion has all of the proper equipment in good working order has libido and is not afflicted with any one of a number of diseases venereal and otherwise the next step in the Texas A&M recommended procedure is semen collection and evaluation.

To obtain a semen sample the stallion normally ejaculates into an artificial vagina. This might require some advance conditioning.

Once the sample is collected the way in which it is handled is very important report Blanchard and Varner:

"Immediately after its collection semen should be quickly transported to a laboratory while minimizing physical trauma exposure to light cold shock or excessive heat. To enhance its reliability the semen evaluation should be performed in a thorough methodical manner by an experienced person in an adequately equipped laboratory.

"Total sperm number calculated as the product of sperm concentration and semen volume is one of the more important measurements used in estimating a stallion’s fertility. Total sperm number in an ejaculate is subject to seasonal variation but is also affected by numerous other factors including frequency of ejaculation age testicular size size of extragonadal sperm reserves and various forms of reproductive disease. Total number of sperm in stallion ejaculates typically ranges from three billion to 20 billion.

"When the stallion is young and on a frequent breeding schedule sperm numbers are usually at the lower end of the range; sperm numbers are usually at the upper end of this range when the stallion is older and on an infrequent breeding schedule. Fertile stallions should be expected to have at least four billion total sperm in the ejaculate and two billion total sperm in the second ejaculate after one week of sexual rest."

Just having a large number of sperm present might not be enough. Motility also is a factor although no one is absolutely sure of the precise role it plays in fertility.

This is what Blanchard and Varner have to say in part on the subject:

"A positive relationship between sperm motility and fertilizing capacity has been demonstrated in many species although this correlation cannot be considered absolute. In a recent study of semen and sperm motion characteristics involving 90 stallion seasons the characteristics with the highest correlations to fertility were the percentages of motile and progressively motile sperm; yet only 20% of the variation in fertility among stallions studied was due to differences in sperm motility."

(Sperm "motility" is described as sperm exhibiting motility of any form while "progressive sperm motility" describes sperm that are moving in a rapid linear manner.)

In addition to checking for number and motility the structural detail of sperm is examined under powerful microscopes to determine if there are defects.

Blanchard and Varner recommend the following:

  • "At least 100 to 200 sperm should be evaluated for evidence of morphological defects. (Morphology is the science of the forms and structure of organisms.) Abnormalities in sperm morphology traditionally have been classified as primary secondary or tertiary.
  • "Primary morphological abnormalities are considered to be associated with a defect in spermatogenesis and are therefore of testicular origin.
  • "Secondary morphological abnormalities are created in the excurrent duct system.
  • "Tertiary morphological abnormalities develop in vitro as a result of improper semen collection or handling procedures.
  • "However the origin of some sperm morphological defects is unknown."

Just how significant the presence of defective sperm might be is debated. The Texas A&M researchers noted that in one study investigating effects of sperm morphological defects on fertility in 64 stallions it was found that the percentages of these deformities in the form of abnormal heads midpieces and proximal droplets significantly adversely affected fertility. In other words the higher the percentage of defects the lower the fertility.

The good news is that defective sperm do not seem to have an adverse effect on normal healthy sperm within the stallion’s reproductive system.

Scientific Help

Advances in science have placed additional laboratory weapons at the disposal of researchers in their quest to learn why some stallions are sub-fertile and others are infertile.

Here is a capsule look at some of the laboratory tests and what they might reveal as described in detail by Blanchard and Varner:

Karyotype AnalysisThis test occasionally will yield useful information regarding a genetic basis for subfertility and infertility. It involves an in-depth study of chromosomes.

Chemical Analysis of Seminal Plasma–This involves a close study of the seminal plasma that is included in the ejaculate. It has been found that high concentrations of seminal plasma adversely affect sperm motility during cooling and storage. That isn’t all. It is believed that some stallions produce seminal plasma that is toxic to spermatozoa.

Electron Microscopic Analysis of SpermAs the title implies this involves examining sperm under either a phase-contrast or light microscope at 1000 times magnification.

Sperm Chromatin Structure AssayThis test has been developed to evaluate the integrity of sperm chromatin. (Chromatin is described as being the more readily stainable portion of the cell nucleus.) This assay defines abnormal chromatin structure as a heat- or acid-induced susceptibility to DNA denaturation. (Denaturation is the destruction of the usual nature of a substance while in its normal location.) The greater the sperm DNA denaturation the greater the chances for subfertility or infertility.

Antisperm Antibody TestThis test is designed to search for antibodies that can cause the destruction of sperm.

The above tests wouldn’t be considered part of a normal breeding soundness examination but when called upon have been helpful to ferret out reasons for subfertility and infertility.

The same can be said for hormonal assays and testicular biopsy. Although normally not a part of a breeding soundness examination they are extremely valuable when subfertility is suspected.

"Reproductive malfunction due to an endocrinologic abnormality has not been firmly established except in stallions given testosterone anabolic steroids or Altrenogest all of which adversely affect spermatogenesis" report Blanchard and Varner. "However abnormal concentrations of reproductive hormones commonly point to testicular dysfunction in infertile and sub-fertile stallions.

"The goal of reproductive endocrine assessment in stallions is to detect which components in the hypothalamic-pituitary-gonadal system might be contributing to abnormal reproductive function."

Only when it is known which components have gone awry can remedial steps be taken to correct the malfunction.

Merely taking one blood sample and having its endocrine status evaluated isn’t adequate the two researchers state. "Assays of single blood samples may not reflect the endocrine status of a stallion as most hormones are erratically released into the bloodstream. Blood sample collection has been recommended at about 30-minute intervals for six to eight hours to obtain accurate measurements of mean hormone concentrations."

Once it is determined that abnormal hormone concentrations are present additional tests are required to identify the source of the problem. The problem for example might be with the pituitary gland which might not be capable of secreting FSH and LH in the correct quantities.

Another tool available in the fight against subfertility is the testicular biopsy. The biopsy allows the investigator to examine tissue for abnormalities or disease.

Following is the conclusion drawn by Blanchard and Varner on hormonal assays and testicular biopsy:

"The use of hormonal assays including stimulation tests to identify pituitary and testicular responsiveness will sometimes aid in clarifying cause of impaired testicular function in stallions. More commonly abnormal hormone concentrations in stallions reflect a testicular problem and may sometimes allow a prediction of severity of testicular damage. Testicular biopsy allows direct assessment of cells as well as testicular hormone concentrations and may prove beneficial for pinpointing the site of dysfunction in stallions with poor semen quality with further research."

All of the foregoing we can readily conclude should accomplish a couple of things for the stallion owner. First and foremost the breeding soundness examination and laboratory tests if needed should inform the owner whether the stallion has reproductive problems. Second this information can be parlayed into a treatment protocol.

Can He Get Mares In Foal?

The question that surfaces next is this: How do we make a determination as to whether a stallion is fertile sub-fertile or infertile?

We turn again to Blanchard and Varner:

"Stallions typically are classified as satisfactory questionable or unsatisfactory prospective breeders. A stallion with a satisfactory rating should be able to achieve a season pregnancy rate of 75%. A minimum 75% seasonal pregnancy rate assumes that the stallion breeds a standard number (book) of mares during a typical breeding season of 135 to 150 days. The horse breeding industry generally accepts the book size for novice stallions as the standard number for a breed. In the United States Thoroughbred stallions typically are booked to breed 40 to 45 mares by natural service while Standardbreds (or other breeds of stallions bred by artificial insemination) typically are booked to breed 120 to 135 mares.

"When making recommendations for a stallion’s book it is assumed that the stallion will be bred to mares of normal fertility under good management conditions. If the stallion’s book contains high percentages of older chronically barren mares the pregnancy rate per cycle and season will suffer and will not accurately represent the stallion’s innate fertility."

Defined by Blanchard and Varner as an unsatisfactory prospective breeder would be a stallion that: 1) does not have two normal scrotal testes; 2) harbors a venereal pathogen or ejaculates semen containing blood purulent material, or urine; 3) exhibits abnormal mating behavior or ejaculatory dysfunction; or 4) is severely deficient in two or more categories of semen analysis.

Stallions which are borderline in two or more of the above categories should be classified as questionable breeding prospects.

Neither of these two classifications–questionable and unsatisfactory–implies that a stallion is sterile the two researchers emphasize.

They recommend that stallions which fail a breeding soundness examination be examined at intervals-usually 60 days. This is particularly true for stallions classified as questionable. Young stallions which are recently retired from a performance career for example might improve greatly in testicular size and sperm output as they adjust to new surroundings and mature sexually.

Why Is He Sub-fertile?

What is the prime reason for subfertility?

"In one study of 1044 stallions" report Blanchard and Varner" 36% failed breeding soundness examinations. One common cause of failure is ejaculation of a low number of progressively motile sperm."

And what can the owner or breeding manager do when such is the case? Their answer is as follows:

"In such horses satisfactory pregnancy rates often can be achieved by 1) limiting the book size to fewer mares and 2) intensifying mare management (by conducting more frequent examinations and giving hormones to control ovulation time). The goal is to breed each mare one time 24 to 48 hours before ovulation. Reducing the number of services required to breed the same number of mares increases the number of sperm per ejaculate which will optimize pregnancy rates.

"If a stallion is going to stand at stud despite failing a breeding soundness examination we recommend that semen be collected once a day until daily sperm output is established. Knowing the number of normal motile sperm ejaculated daily enables the booking of an appropriate number of mares for such stallions."

While good management most assuredly can increase the success rate for a sub-fertile stallion it does not produce answers as to exactly why the subfertility or infertility problem occurred in the first place. Hormonal assays can tell us that the stallion has too much or too little of a specific hormone but what caused the dysfunction that resulted in the imbalance initially?

Janet F. Roser PhD Associate Professor Department of Animal Science University of California Davis sheds some light on the subject while at the same time acknowledging that mysteries remain in a paper titled "Idiopathic subfertility/infertility in stallions: New Endocrine Methods of Diagnosis."

In her introduction Roser says this: "There are a number of conditions associated with poor fertility such as mismanagement anabolic steroid treatment infection fever tumors injury disease behavior or endocrine disorders such as hypogonado-tropic hypogonadism (low concentrations of gonadotropin releasing hormone or GnRH). However for stallions with poor sperm quality and endocrine disorders not associated (with) hypogonadotropic hypogonadism the exact nature of their condition and the hormonal factors involved have not been elucidated.

"A significant percentage of idiopathic (of unknown cause) sub-fertile/infertile stallions have been diagnosed with primary testicular degeneration marked by progressive softening and/or reduction of the testes endocrine imbalance and poor seminal parameters. These stallions make up a heterogeneous population of poor breeding performers with unknown pathophysiology. Recent evidence indicates that local regulatory factors such as inhibin (a non-steroid testicular factor believed to be a peptide and thought to inhibit pituitary production of follicle stimulating hormone or FSH) and insulin-like growth factor 1 or IGF-1 (panacrine/autocrine factors) are important modulators in steroidogenesis (the