Stallions That Accumulate

Proper management can help stallions with “sperm accumulation syndrome” still have a successful horse-breeding career.

A 3-year-old stallion was referred to the clinic after an initial semen evaluation at home suggested that there were no motile spermatozoa in the ejaculate. It was the first time the horse had ever been collected. The horse was in excellent body condition. The total scrotal width was measured at 9.3 cm, well above the minimum of 8.0 cm for a breeding stallion. A semen collection attempt was performed at 10 a.m.

A volume of 16 mls of semen, containing 774 million spermatozoa per ml was collected, for a total of 12.4 billion spermatozoa in the ejaculate. None of the sperm were motile, and only 32 percent of the sperm had normal morphology, with a high percentage of detached heads. A second semen collection was performed a few hours later. A volume of 15 mls, containing 274 million sperm per ml was obtained, with no motile sperm. Additional semen samples were collected once or twice daily over the next two weeks. After the epididymal sperm reserves were depleted and new sperm were entering the epididymides, spermotozoal motility began to improve. The percentage of motile sperm in the ejaculate peaked at more than 50 percent by Day 11. The stallion was then rested (i.e. no semen collections) for three consecutive days. A subsequent collection the fourth day revealed a sperm motility of only 5 percent.

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Sperm are produced in each testis and passed into the adjacent structure called the epididimis. Sperm mature and gain fertilizing capacity as they transit the epididymis and are stored there until ejaculation occurs. Epididymal sperm not removed by ejaculation are thought to be intermittently discharged into the urethra and voided in the urine. The epididymis of a sexually rested stallion should contain a moderate to high number of sperm, a majority of which should be motile. Even though it is common practice on some horse-breeding farms to collect a stallion once or twice at the beginning of the breeding season to “clean him out,” a normal stallion should still have a sufficient number of motile sperm in his ejaculate to get a mare pregnant even if he hasn’t been collected or bred in months. However, some stallions accumulate extremely high numbers of sperm in their epididymides, and often a high percentage of these sperm are dead. The rate of sperm production by the testes in affected stallions is generally normal, but epididymal function is apparently not. In many cases, sperm die within a few days after entering the tail of the epididymis. Stallions affected by “sperm accumulation syndrome” can be identified during a breeding soundness evaluation. Semen collected from an affected stallion after a week or more of sexual rest will contain very high numbers of sperm (often several hundred million sperm per ml) and a majority of the sperm will be dead. Pregnancy rates for affected stallions are very low.

Want to know more about breeding? Check out AQHA’s Equine Breeding Techniques report. It’s full of useful information about mare care, breeding tips and more.

Diagnosis is made by evaluation of sperm parameters in semen collected once daily for approximately seven days. In affected stallions, sperm numbers in the first collection will be very high and sperm motility will be low. The number of sperm per ejaculate typically begins to decrease and the percentage of live, motile sperm in the ejaculate begins to progressively increase after two to four days of daily collection. In most cases, the total number of sperm in the ejaculate and the percentage of motile sperm will stabilize as long as daily sperm collection continues. Affected stallions may have sperm motility of 40 to 60 percent or more when collected on a regular basis and sperm motility of less than 10 percent after just a few days of sexual rest. The solution for a stallion with this syndrome is routine collection or mating (i.e. daily or at least every other day) throughout the breeding season. In addition, collection of multiple ejaculates over several days is needed at the beginning of the breeding season to remove dead sperm that have accumulated in the epididymides during the winter months. Affected stallions properly managed can have a long and productive breeding career.