What Makes Some Stallion Semen Subpar?

Learn about 14 factors that can affect your stallion’s fertility.
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What Makes Some Stallion Semen Subpar?
Many factors can affect semen quality. If a stallion has a fertility problem, work with your veterinarian to determine and address the root issue. | Anne M. Eberhardt/The Horse

14 Factors that can affect your stallion’s fertility

Breeding stallions are sensitive guys. Anything from ambient temperature to body weight can affect their semen quality. In this article, we’ll describe those factors and 12 more that can impact fertility. Three veterinarians well-versed in stud health will help: Edward Squires, PhD, Dipl. ACT (Hon), adjunct professor in the University of Kentucky’s Department of Veterinary Sciences; Dickson Varner, DVM, MS, Dipl. ACT, former professor and Pin Oak Stud Chair of Stallion Reproductive Studies at Texas A&M University’s College of Veterinary Medicine and Biomedical Sciences; and Peter Sheerin, DVM, Dipl. ACT, of Rood & Riddle Equine Hospital, in Lexington, Kentucky.

1. Breed Differences

“There is a lot of anecdotal evidence that certain breeds of horses have more fertility problems, in both the mares and stallions,” says Squires. Draft horses and Friesians, for instance, tend to have poorer quality semen than most other breeds. Friesian semen, in particular, doesn’t cool or freeze as well, he says.

“There’s likely a genetic component to that, which may include inbreeding in a breed with low numbers,” says Varner. “The lesser-known breeds with less population dynamics may have some subfertile sires that transfer their reduced fertility to their get. Some of these breed variations are simply due to limited numbers in the breed and the need to propagate the breed by use of subfertile individuals.” 

2. Genetics

Every breed has certain lines known for having low fertility. “There is a line of Standardbred stallions, for instance, that have fertility problems,” says Sheerin. “You can say the same for Thoroughbreds and Quarter Horses; there are some family lines that have poor-quality semen.” 

There’s usually a genetic reason a stallion isn’t as fertile as others. “Stallions are a lot like men; we are not culled based on reproductive ability, as would be the case for most livestock,” says Varner. 

Rather, stallions are typically selected as sires based on their pedigree, competitive performance record, and ­conformation.

“Little regard is given to their reproductive potential,” says Varner. “If a stallion has a great performance record he is generally considered a good candidate for breeding. He may start his breeding career with reduced fertility and require veterinary assistance to maximize ­fertility.” 

In other domestic livestock species, such as cattle, pigs, goats, and sheep, fertility is a major consideration. Producers do not want to perpetuate animals with low fertility. “In cattle, freezability of semen is also very important, especially in dairy cattle,” Varner adds. “Dairy bulls have been screened for decades for freezability of semen, but that’s not the case with ­stallions.” 

As a result, stallion semen’s freezability is not great and can vary considerably between horses.

Testicular size is also genetic, says Squires, adding that size affects semen quantity more than it does quality.

3. Body Condition

Stallions with excessive body condition (a score of 7 or higher on the 1-9 Henneke Scale) have more fat in their scrotums, insulating the testicles, than those in moderate condition. 

“This interferes with proper thermoregulation, which interferes with semen quality and fertility,” says Sheerin. In other words, the testes and the sperm within stay too warm. By the same token, a starving horse or one in poor body condition will also have poor semen quality because he’s not receiving enough nutrition to support reproductive health.

4. Nutrition

Extreme dietary changes can affect sperm output and quality, says Squires. Fortunately, most owners he’s worked with feed their breeding stallions adequate, well-balanced diets.

“If a stallion is eating feed that is high in estrogen, however, this can have an adverse impact on semen, due to the hormonal feedback on the hypothalamic-­pituitary axis,” says Sheerin. “On the other hand, feeds that are high in omega-3 fatty acids (e.g., fish oils, flaxseed oils, etc.) can have a beneficial effect, aiding the longevity of semen in some cases.”

Squires adds that offering supplemental omega-3 fatty acids to stallions that have poor semen quality post-cooling or -freezing can help enhance that quality.

5. Trauma or Injury

Scrotum or penis damage can affect semen quality in a variety of ways, but it primarily interferes with thermoregulation, says Sheerin. “If the stallion is kicked and there’s a hematoma (a swelling or pocket of blood) in these areas, there will be swelling, and this also insulates the testicles,” he says. “If there is trauma to the spermatic cord or cremaster muscle and the stallion can’t raise or lower the testicles (for thermoregulation), this will also have an adverse effect” on semen quality.

6. Fever

A prolonged higher-than-normal temperature can affect sperm production and viability. If a stallion spikes a fever just before or during breeding season or when being collected, he might be infertile until new sperm completes its production cycle.

“It takes about 60 days for sperm development, so the stallion should be checked about 60 days after the fever incident to see if he is producing normal sperm again,” says Sheerin.

“Often people think of reduced semen quality and fertility being associated with localized trauma to the scrotum (causing inflammation and heat), but systemic infection or anything that causes a rise in body temperature can have a similar adverse effect,” Varner adds. “We encounter this fairly often; the horse was ill and had a fever for a few days and later developed a period of subfertility or infertility.”

Some horses might experience a mild fever a day or two after routine vaccination. “Even though it is temporary, short-lived fevers such as this will often result in reduced semen quality,” says Varner. Inoculate your stallions at least a month or two before breeding season starts.

What Makes Some Stallion Semen Subpar?
Annual changes typically don't affect stallions the way they do mares. | Photo: iStock

7. Hot Weather

A severe and prolonged summer heat wave can also impact fertility.

“In some states like Texas where heat and humidity are high, this environmental stress can induce a change in semen quality,” says Varner. “Some stallions, when exposed to hot weather conditions, start to develop what we call a hydrocele, which is serous fluid accumulation in the cavity that separates the testes from the scrotum. If there is a lot of fluid in there, it tends to insulate the testes and they cannot thermoregulate properly, and semen quality will deteriorate.”

The best testicle temperature for sperm to remain viable is a few degrees lower than body temperature.

8. Season

Annual changes typically don’t affect stallions the way they do mares. “We recently published an article that looked at the effects of dual hemisphere breeding on fertility of Thoroughbred stallions,” says Varner. “In this study we found no ill effects when stallions were used sequentially in both hemispheres (going back and forth between summer and winter), when examining 10 consecutive breeding seasons. In this instance stallions tend to be continually exposed to periods of increasing day length, with reduced exposure to short winter days.”

Sperm numbers, however, can decline when stallions are breeding during the shorter days of the year.

“With shorter days there is a decrease in testosterone, and sperm numbers go down, and behavior becomes more subdued, whereas with longer days in the spring, everything gets better for optimum reproduction,” says Squires.

Sheerin adds that “the semen won’t go from good to bad. It might go from excellent to good, but the stallion will still be fertile. This allows us to do things like collect and freeze semen in the fall, when the main breeding season (and demand for semen) is past.”

Thoroughbreds can only be bred by live cover, due to registry requirements, so for that breed managers and veterinarians must make do during the U.S. breeding season, which begins in February.

9. Medication

“We don’t know what kind of effect all medications might have on testicular function, but a person needs to realize that any medication given to a horse could potentially have a deleterious effect on semen quality,” says Varner. “If you check the literature, you’ll see that drugs like anti-ulcer medications can have a detrimental effect on testicular function.”

Each stallion also responds to each medication differently.

10. Hormones

Some breeders inject lackluster stallions with testosterone for a libido boost. “One or two injections are probably not going to hurt, but if you do this routinely there will be detrimental effects,” Squires says. “If a stallion is injected for many days with high levels of testosterone, this can cause a drop in semen quality and also decrease the size of testicles. This can dramatically affect semen quality and sperm numbers.”

Varner suggests monitoring blood testosterone levels closely in these situations to minimize side effects.

Some trainers medicate performance stallions with steroidal hormones called progestogens (e.g., Regumate or altrenogest) to reduce aggressive male behavior or to make stallions more manageable when showing.

“If given for a short period of time, like one month, studies we’ve conducted have indicated that in mature horses such drugs do not seem to have a detrimental effect on semen quality,” says Varner. “Other studies have demonstrated that if the same drug is given in the peripubertal period (when the young horse is reaching puberty), it could have a detrimental effect on semen quality, and the possibility exists that the effect could be long-lasting.

“We’ve examined stallions here that have been treated daily with oral altrenogest as performance horses over a three- or four-year period, beginning when they were as young as long yearlings or 2-year-olds in training, and they have reduced sperm quality and reduced testicular size,” he adds.

Dexamethasone and other corticosteroids can also be detrimental if given for long periods, says Squires. Owners planning to use a stallion for breeding purposes should avoid administering these drugs long-term.

11. Stress

Any prolonged stress—be it from hard work, showing, illness, housing, weather, etc.—can be detrimental to semen production and quality, says Sheerin.

“It often depends on the duration of the stress, so we need a good historical perspective when a stallion comes in for an examination and has reduced semen quality,” says Varner.

12. Age

In some breeds and disciplines, stallions might not enter the breeding pool until their performance careers are over—careers that might last until they are 20 or older.

“Then their fertility problems may be due to age-related testicular dysfunction,” says Varner. All males, including stallions, develop age-related testicular degeneration eventually.

“This condition can affect some stallions in their early teens, whereas it may not become apparent until early to late 20s in other stallions,” he says. “The condition is associated with reductions in testicular size, sperm output, and semen quality, and results in reduced fertility or even sterility in advanced cases.”

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“Typically, the older horse will have smaller testicles—and probably softer testicles—and high-volume ejaculates with low concentration of sperm,” says Squires. “That combination equates to a lower quality of semen.” Breeders will notice conception rates dropping.

13. Housing & Social Environment

Whether stallions interact with mares or only other male horses also impacts semen quality.

“Stallions that are housed together, such as in the same barn, have hormonal profiles very similar to bachelor stallions that have no interaction with mares,” says Sheerin. “Their sperm numbers are lower, testosterone levels are lower, etc. Stallions that can see or interact somewhat with mares will be more like a harem stallion and will have higher testosterone levels, better libido, and better semen quality.”

What Makes Some Stallion Semen Subpar?
During artificial insemination, improper collection techniques can impact sperm quality and motility, which is being analyzed here. | Photo: Stephanie L. Church/The Horse

14. Improper Collection

If the stallion is being collected for breeding via artificial insemination, a number of human mistakes can affect semen viability. These include:

Too much lubricant

“All lubricants are potentially detrimental to sperm, so you want to use the minimum amount necessary, and use only those lubricants that have the least toxic effects on sperm,” says Squires.

For instance, if the stallion mounts the breeding dummy and doesn’t ejaculate, and more lubricant is added to the artificial vagina (AV) before he mounts and does not ejaculate again, and the cycle continues, the amount of lubricant contaminating the ejaculate can be ­excessive.

“Some people use the whole tube of lubricant on their sleeve to lubricate the AV, but it only takes a small amount,” says Squires. “There are also some good nonspermicidal lubricants on the market today that can help with this problem.”


“If the AV is too warm—or too cold—when you collect a stallion, this may damage the sperm,” says Squires. “If you don’t drain the water (which determines the AV temperature) out of the AV quickly enough, this can also damage the sperm” as it warms.


“You need to wash the penis to make sure you get a good, clean collection,” says Squires. “Otherwise, this may have an impact on motility and sperm quality.”

Improper cleaning

If the AV or dummy has been cleaned with something that has spermicidal qualities, the semen may be ruined. “Some people spray the breeding dummy with disinfectant between semen collections,” says Varner. “Chlorhexidine can have an adverse effect on sperm if the stallion’s penis comes in contact with this disinfectant on the breeding dummy.”

Dilution with presperm fluid

Make sure you collect the proper portion of the ejaculate. “There are three components in the ejaculate,” says Squires. “One is presperm, one is sperm-rich, and one is a gel. The presperm comes first and sometimes there may be quite a bit of it. We typically like to hold the penis away from the AV for a moment and let that pre-sperm go on out. Then we put the AV on the horse and get the sperm-rich fraction.”

Also remember that the horse usually doesn’t ejaculate on the first mount. “He often takes multiple mounts,” says Squires. “If you don’t change the collection bottle each time, you get a lot of presperm in the bottle.”

Under natural circumstances the stallion transfers his semen directly into the protective confines of the mare’s reproductive tract. “When we intercept that semen in an AV, we need to realize that semen preservation (whether it’s to be frozen, cooled, or used within the hour) all begins with the collection process,” says Varner. “The semen needs to be handled correctly and needs to be in an environment where it is not exposed to light, potential toxicants, or cold shock.”

Take-Home Message

Many factors can affect semen quality. If a stallion has a fertility problem, work with your veterinarian to determine and address the root issue.

“Often the stallion owner wants to find a simple answer like a supplement or an injection, but there is no silver bullet to change poor semen quality into great semen quality,” says Sheerin.

What you can address are body condition, collection methods, housing and social environment, and stress levels. Other factors, such as genetic and breed differences, might be irreversible, and you must work with those limitations.


Written by:

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey’s Guide to Raising Horses and Storey’s Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at https://insidestorey.blogspot.com that comes out on Tuesdays.

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