The following are important parts of the stallion's reproductive anatomy. The combined role of these structures is to produce and deliver billions of live sperm cells (called spermatozoa) into the mare at breeding.
Like the mare, the stallion is sensitive to day length which is first detected through the eye: sperm production and sex drive are highest during summer. The stallion also uses his eyes, ears, nose, taste, and touch during courtship, mounting, and ejaculation.
The scrotum is the pouch of skin in which the testes are carried. It hangs close to the body, between the rear legs. The testes must be kept outside the body, at a lower temperature, for sperm production.
The testis (plural testes) is the sperm producing factory of the stallion. Both are carried in the scrotum, though one may be slightly higher than the other. They should not be twisted or swollen. They should feel full, firm, resilient and of equal size. Sperm leaves the testis after it is produced and enters the epididymis. If the testes are injured or inflamed sperm production may be reduced or eliminated.
During development in the uterus the testis originates in the abdomen next to the kidney and must drop down into the scrotum. Failure to drop down may cause a testis to be retained in the abdomen, a condition known as cryptorchidism.
The epididymis is a long coiled tube where sperm matures. It lays along the top of the testis in the scrotum. It is divided into a head, body, and tail. The sperm passes from the head to the body to the tail as it matures. The tail of the epididymis becomes the storage depot for sperm, and may contain 50 billion sperm cells. Sperm leave the tail of the epididymis via the ductus deferens. Blockage of the epididymis or ductus deferens would prevent sperm from entering the urethra.
The ductus deferens is a long muscular tube which runs up from the tail of the epididymis into the abdomen of the stallion and into his urethra. The ductus deferens widens before it enters the urethra. The widened portion is called the ampulla, and may sometimes be blocked.
The urethra is the pipe leading from the bladder. It passes towards the tail, below the rectum and anus, like the mare, but then turns downward and passes under his belly and into the penis. It carries urine from the bladder and sperm from the ductus deferens.
The accessory sex glands (prostate, seminal vesicles and bulbourethral glands) line the urethra and discharge their contents into the urethra at ejaculation, to mix with the sperm.
The spermatic cord is a collection of all the tubes running between the stallion's abdomen and his testis. It includes the ductus deferens, and the testicular artery, vein and nerve.
The penis is a long muscular tube which begins at the pelvis and extends forward and downward. It is composed of erectile tissue which fills with blood during sexual arousal. The tip is called the glans penis, which enlarges greatly ejaculation. This enlargement is called "belling of the penis".
The prepuce is the fold of skin which envelops the penis. When the penis is retracted this fold forms two openings through which the penis must pass. Collections of dead cells, known as smegma, may accumulate here. When the penis is erect, the skin of the prepuce is extended to its full length allowing the prepuce to be examined or cleaned free of smegma.
The urethral fossa is a small trench running around the urethral opening in which smegma may acumulate. A large cavity connected to it, called the urethral sinus, or urethral diverticulum, also collects smegma. Smegma may be removed most easily when the stallion is aroused and the penis is fully erect. The penis should be washed with warm water and cotton. Soap is not necessary, and generally should be avoided.
The ischiocavernosus, bulbospongiosus, and urethralis muscles pulse rhythmically when the stallion ejaculates. This may be seen as tail flagging by the stallion while he is on the mare. A dreamy expression comes into his eyes. Some stallions may faint following ejaculation.
The following is a brief overview of sperm production.
The testis produces the hormone testosterone which gives the stallion his male characteristics, and is necessary for sperm production. It takes about two months for mature sperm to be produced. Most of the time is spent in the epididymis. Sperm are produced by meiotic division which alters their genetic make-up. They are therefore "foreign tissue". It is necessary to keep sperm away from the other tissues in the testis, otherwise the body will reject the sperm as it would a skin graft. The separation is achieved by the blood-testis barrier. If the testis is kicked my a mare, or becomes severely inflamed, the barrier may break down and the stallion may produce antibodies against his own sperm, resulting in infertility.
The testis must also be kept cooler than the rest of the body. Heat stress may lead to temporary infertility. If a stallion's sperm output suddenly declines it is worth reviewing what condition was the stallion in two months ago, which is when the damaged sperm was produced.
The following are the two most important factors controlling sperm output in healthy stallions:
Sperm production in stallions starts low and peaks at about 6 years. It remains high for most of adult life. Sperm production may decrease in older stallions (greater than 20 years old).
Sperm output in the stallion peaks in May or June and is lowest in midwinter.
Sperm production may be decreased by heat, injury or inflammation of the testis. The stress of showing, summer, transportation, or illness may also depress sperm output. Stallions given anabolic steroids (eg Equipoise or Winstrol) will have depressed semen production and may be rendered completely sterile. Anabolic steroids should not be given to animals intended for breeding.
A stallion should always be assessed for breeding soundness prior to purchase as a breeding animal, or if questions arise about his fertility. Guidelines for stallion breeding soundness have been established by the Society for Theriogenology. A stallion that passes the breeding soundness examination is expected to successfully breed 75% of a full book of mares. A full book is considered 40 mares by live cover or 120 mares by artificial insemination. Evaluation of the stallion involves the following:
The stallion should be identified, so that the evaluation may not be fraudulently applied to a different stallion. His previous record should be assessed in terms of how many mares were covered previously, and how many became pregnant. One should try to distinguish what proportion of his book were barren, maiden and foaling mares since this will influence his overall success.
The goal in semen evaluation is to measure the total number of healthy spermatozoa the stallion can produce. This requires measuring the volume and concentration of sperm in his semen to determine the total number of sperm, and then to estimate what fraction are healthy by examining both their movement (motility) and what the individual sperm cells look like (morphology). Because a stallion can store large amounts of sperm in his epididymis the evaluation requires that two ejaculates be collected an hour apart. The first ejaculation removes the stored semen. A complete analysis is performed on both ejaculates to make sure that the second ejaculate is "representative". To meet this criterion the second ejaculate should be about the same volume as the first, but with roughly half the number of cells. Motility may be slightly improved. The second ejaculate is the one on which the final assessment depends.
The following are the minimum numbers of morphologically normal, progressively motile sperm required in the second ejaculate in billions, in a given month.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1.2 | 1.7 | 1.8 | 1.8 | 2.0 | 2.2 | 1.8 | 1.7 | 1.2 | 1.2 | 1.2 | 1.0 |
The following is a typical calculation performed on the second ejaculate. This stallion would pass the breeding soundness evaluation based on these numbers, provided it appeared to be a representative sample when compared to the first ejaculate.
| Volume | X | concentration | X | motility | X | morphology | = | Total |
|---|---|---|---|---|---|---|---|---|
| 55 ml | X | 125 milion per ml | X | 75% (0.75) | X | 80% (0.8) | = | 4.1 billion |
If the stallion is to be used for artificial insemination it is frequently helpful to determine the longevity of his semen stored under refrigeration in different extenders for up to 48 hours. This gives an indication of how well the semen would tolerate shipment.
During semen collection and evaluation the stallion should be observed for normal sexual behavior. The ideal stallion will be eager and able to breed, but also well behaved.
The semen should be cultured once, and the urethra should be cultured before and after ejaculation for pathogenic bacteria. A blood sample and semen sample may also be submitted for Equine Viral Arteritis serology and culture.
The testes should be palpated for abnormalities and if possible examined by ultrasound. Their width may be measured by calipers, or the length, height and width measured by ultrasound.
It is valuable for a veterinarian to examine the internal organs of the stallion by rectal examination.
A stallion may be used in many different ways. Here is a brief description of the different kinds of breeding possible with a stallion.
A stallion is turned out with a group of mares for the breeding season. Mares may be examined for pregnancy to monitor progress. Venereal diseases may be transmitted under these conditions from mare to mare via the stallion.
The stallion is brought to the mare and breeding takes in the breeding shed by live cover. Most Thoroughbreds are bred this way. Semen for live cover may remain viable for up to 5 days in the mare, but 48-72 hours is a good rule of thumb.
The stallion is collected into an artificial vagina and the semen is collected, analyzed, extended and perhaps divided between several mares. Mares are bred immediately following semen collection. Like live cover, fresh semen viability in the mare is at least 48-72 hours with a healthy stallion.
The stallion is collected, the semen chilled and shipped by courier or overnight express to the mare, who may reside in a different part of the country. Viability of shipped cooled semen in the mare is about 24 hours. Semen is frequently shipped in the blue "Equitainer".
The stallion is collected, the sperm extended and frozen in liquid nitrogen. Frozen semen may be stored indefinitely, perhaps for several years or even longer than the stallion's life. Frozen semen may be shipped internationally. Longevity of frozen semen in the mare is very low, usually about 8 hours at the most. As a result, mares must be bred very close to ovulation. Frozen semen breeding is very labor intensive, and conception rates are low. However, the value of the foal frequently justifies the effort.