Enhancing Stallion Fertility

Science and good management are keys to keeping stallions at top breeding performance. 

Another equine breeding season is only a short time away, with many breeding sheds opening their doors on Feb. 1. It is a time of excitement and anticipation for horse owners, tempered by concern that the mare will become pregnant and carry a healthy foal to term.

However, the concern in the breeding shed involves more than just the mare and her reproductive health. The health of the stallion, especially as it relates to fertility, figures strongly into the equation. If the stallion is infertile, or if he has low fertility (he's subfertile), valuable time and dollars can be lost in a futile attempt to impregnate a mare.

While there is still much to learn concerning stallion fertility, researchers have made great strides in recent years, with more breakthroughs likely in the near future.

Research has done, and will continue to do, much to unlock reproductive secrets, but some basics also must be followed if a stallion is to become a successful breeder. One of the basics involves maintaining meticulous breeding records, augmented by a breeding soundness examination prior to breeding season.

Breeding Performance

Specialists in the field of equine reproduction tell us that it is highly important to determine a stallion's reproductive efficiency before the breeding season begins. In order do this on an ongoing basis, researchers say, it is important to keep thorough breeding records that can pinpoint any changes in the stallion's performance, and to conduct a breeding soundness examination on an annual basis.

Dickson Varner, DVM, Dipl. ACT (a specialist in reproduction), of Texas A&M University, reported that a number of commercial stallions were brought to that institution this past fall to be examined for reproductive health and semen quality in advance of the breeding season. It was a repeat of other years when commercial stallions also were examined prior to the breeding season.

Varner says timing of these tests is important, so that steps can be taken to correct problems that might surface during the breeding soundness exam, which includes an evaluation of semen.

There is a common thread among a number of the stallions that are examined at Texas A&M for fertility problems, he says: reduced sperm numbers and reduced sperm quality.

One of the newer and more successful approaches taken to solve problems stemming from low-quality semen involves use of density-gradient centrifugation, where defective sperm are removed, says Varner. Following this "filtering" procedure, the sperm recovery rate might be 15-60%, Varner says, but this population of recovered sperm is generally viable and of high quality, so fewer of them are required to achieve a good pregnancy rate following artificial insemination. This technique cannot be applied to Thoroughbreds because of a Jockey Club rule that does not allow semen manipulation and artificial insemination of mares.

Varner said the density-gradient centrifugation approach was used on semen from three commercial stallions in 2007, and all three had successful breeding seasons.

Sperm quality problems can increase when artificial insemination with cooled or frozen semen is involved. The problem stems in part from the fatty acids found in equine sperm. Bull sperm contains high amounts of omega-3 fatty acids that enable them to withstand the rigors involved in freezing. Horses, on the other hand, have sperm that is high in omega-6 fatty acids, which hinders sperm ability to be cooled and frozen, and is low in omega-3 fatty acids. The most important omega-3 fatty acid is docosahexaenoic acid (DHA). An omega-6 fatty acid found in semen is docosapentaenoic acid (DPA).

Ed Squires, PhD, of Colorado State University, puts it into perspective this way: "In semen, the fatty acid profile of stallions is similar to that of boars. Studies in boars have shown that a high DHA to DPA ratio in semen results in enhanced fertility, whereas higher levels of DPA relative to DHA result in reduced fertility.

"Men that have reduced fertility have also been shown to have lower levels of DHA in seminal plasma," Squires notes. "The ratio of phospholipids (fats containing phosphorous) to cholesterol in the sperm, and the ratio of unsaturated to saturated fatty acids, determines the ability of sperm to handle the rigors of cooling and freezing. Those species that have high cholesterol to phospholipid ratio have sperm that are very resistant to cold shock and thawing.

"Humans, rabbits, and roosters produce sperm that are very resistant to cold shock and their sperm freezes very well," he continues. "Sperm from boars and stallions have very low tolerance to cold shock, and, in general, their sperm freezes poorly. Sperm of bulls have high levels of DHA in the cell, where those of stallions have a high level of DPA. Increasing the ration of DHA to DPA in semen has been shown to increase fertilizing capacity and semen quality. Conversely, reducing the ratio of DHA to DPA was accompanied by a reduction in fertilizing capacity."

The researchers found that adding omega-3 fatty acids to a stallion's diet resulted in a more fluid condition of the sperm membrane, which, in turn, allowed sperm to handle the stress of cooling and freezing with potentially less damage.

Researchers at Texas A&M were the first to determine that omega-3 fatty acids could be helpful in improving the quality of either cool-stored or frozen stallion semen. Subsequently, similar research was conducted at Colorado State University and the University of Arizona. The results demonstrated an increase in semen quality. Today there are commercial supplements available designed to increase a subfertile stallion's DHA level.

What the Future Holds

Research into ways to enhance stallion fertility continues at both Texas A&M and Colorado State. At Texas A&M, for example, a grant from the Grayson-Jockey Club Research Foundation is aimed at developing "Newer Concepts of Stallion Fertility." A news release from the Foundation described the research this way: "Stallion fertility is governed by both environmental and genetic factors. The former have been studied extensively, but the genetic basis for reproductive failure in stallions has not been seriously examined. These investigators will examine the genetic structure of the Y chromosome, the site of a number of genes that govern stallion fertility, to correlate certain specific genes with stallion fertility problems. They will attempt to identify mutations and rearrangements in the genes that might be associated with problems. Ultimately, the goal is to identify problems in potential sires before they are retired to the breeding farm, so that maximal management techniques can be instituted."

The study is in its second year.

At Colorado State, research is under way to determine whether adding a significant amount of vitamin E to a stallion's diet will increase fertility. Vitamin E, Squires explains, is an important antioxidant that helps to maintain the good health of membrane lipids in sperm.

The first part of the study, Squires says, involves determining whether vitamin E will have a positive influence on stallion fertility, with the second part seeking to determine whether adding vitamin E to a supplement containing omega-3 fatty acids will make that supplement even more dynamic. The research began this past summer.

The Grayson-Jockey Club Research Foundation also provided a grant to researchers at the University of Florida for study of the "Normal and abnormal functions of specific proteins in stallion semen." The Foundation described the research thusly: "These researchers observed that the recent finding that a specific protein in seminal plasma protects viable sperm cells from being destroyed by uterine inflammatory cells is a landmark discovery that warrants further investigation. Identification of seminal plasma proteins that regulate the interaction between spermatozoa and the mare's reproductive tract enable understanding of basic semen biology and of biochemical causes of infertility and subfertility in stallions. The investigators propose to establish normal values for these proteins in fertile stallions and test subfertile stallions for deficiencies of the proteins. The long-term goal is to characterize the biological function of seminal plasma components that are of importance in fertility."

The study is in its first year.

The list of research projects goes on, but those mentioned above provide an example of what is being done to learn more about stallion fertility, or the lack of it, and to establish protocols for solving problems.

The Human Factor

It must be remembered that science alone can't be responsible for solving all breeding problems in stallions. Good management goes a long way in setting the stage for a successful breeding season.

John Steiner, DVM, Dipl. ACT, a reproduction specialist at the Hagyard Equine Medical Institute near Lexington, Ky., and Squires have taken a look at some of those basics. In an article Steiner wrote titled "Prepping Stallions For the Breeding Season," Steiner notes: "Look at the physical condition of the stallion. Is he overweight, or underweight? Is he getting out and getting exercise? It's good for a stallion to get out as much as possible not only for exercise, but for his overall well-being."

It has also been shown that exposure to mares even across a fenceline can enhance stallion fertility by stimulating natural environments where a stallion lives in a "band" or group of mares.

"Make sure his teeth are okay," Steiner continues. "He should be on a regular vaccination schedule. All of his vaccinations should be given 30 to 60 days before the breeding season starts. I prefer 60 days. That way, if he has a reaction to the vaccination and gets sick or has a fever, it won't affect his breeding ability. Don't vaccinate stallions in the middle of the breeding season because a high fever can affect sperm, which take about 60 days to mature in the stallion.

"Also, make sure the stallion has been on a good deworming program," he adds. "If you have an older stallion, keep an eye on him during the season. He might need some medication for his normal aches and pains.

"For any stallion I recommend a breeding soundness exam prior to the breeding season," continues Steiner. "This includes collecting the stallion and evaluating the semen. This gives you a baseline to evaluate the stallion, or can help you find problems early and allow you to manage the stallion differently. Then, if there is any problem during the breeding season, you have a baseline to go back to and compare to see where the problem lies."

When dealing with a young stallion that might be coming off the track or one that has been in training for performance, Steiner says, it is important to learn everything about the horse--from his personality to what types of medication he has been on. "There are no drugs that enhance fertility, but many can hurt fertility," says Steiner.

When discussing personality, Steiner poses some questions: "Is he aggressive? Is he timid? What are his vices? Then you work with him accordingly. Deal with the horse on an individual basis, and get him to the farm in plenty of time to work with him before the season begins.

"Have the same handler--an experienced stallion person--work with the stallion consistently," adds Steiner. "Don't reprimand him for acting like a stallion. Let him look around the breeding shed. Turn him loose in there, if possible, and let him mark his territory and be comfortable without other horses or people."

Squires, who authored the book Understanding the Stallion for Eclipse Press, discusses normal sexual behavior in the stallion. One of the most common causes of infertility in stallions, Squires says, is abnormal sexual behavior, and the most common cause of abnormal sexual behavior is mismanagement.

It is extremely important to train a stallion properly so that he develops normal sexual behavior in the breeding shed, Squires tells us.

Ideally, he adds, stallions should not be used for breeding until at least three years of age. This does not mean that they won't produce adequate amounts of sperm until reaching that milestone, he explains, but stallions bred at two years of age are much more apt to develop bad habits, such as mounting without an erection, failing to obtain an erection, and excessive biting of the mare.

Other areas of poor stallion management mentioned by Squires include overuse, particularly in the fall and winter when the stallion's libido is at its lowest; housing stallions in complete isolation; lack of exercise; and, most importantly, not preventing the mare from kicking during breeding.

Stallions that have been kicked, he says, are often those who mount the mare, enter her, and begin thrusting, but do not ejaculate.

Take-Home Message

Stallion management is a combination of making use of the latest technology that science has to offer, combined with a common-sense approach that involves good supervision of a stallion's care and activity and a thorough breeding soundness exam. Nothing can guarantee 100% success in the breeding shed, but the application of science, combined with appropriate management procedures, is a big step in that direction.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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