Understanding Breeding Soundness Exams for Stallions

Before breeding a stallion, there's one crucial step that should be performed: the breeding soundness examination. According to one veterinarian, this relatively simple evaluation can give stallion managers a good look at an animal's breeding potential before he even hits the breeding shed. Unfortunately, he added, this step is often overlooked. During a presentation at the 2012 Western Veterinary Conference, held Feb. 19-23 in Las Vegas, Nev., Walter Threlfall, DVM, MS, PhD, Dipl. ACT, a consulting theriogenologist (reproductive specialist) from Powell, Ohio, and professor emeritus in the Department of Veterinary Clinical Sciences at The Ohio State University's College of Veterinary Medicine, discussed what's involved in a stallion's breeding soundness examination and why it's an important step in managing breeding stallions.

When assessing a stallion for breeding purposes, Threlfall recommended starting with a detailed case history and general physical examination, followed by a thorough genitalia assessment and analysis of the horse's libido and semen.

History--Has the stallion been used for breeding? What is the intended use (natural cover or artificial insemination)? How many mares is the horse expected to breed? Are there any underlying medical issues? Threlfall said a thorough history will help the veterinarian evaluate a stallion's breeding suitability to the expected breeding program.

 

Physical Examination--While all stallions should undergo a thorough physical exam, horses that spend the majority of their time in a stall likely need a less intense evaluation as compared to stallions "turned out to the back 40 acres" for most of the year, Threlfall said. During a physical exam the veterinarian should pay special attention to the horse's hair and skin quality, his foot and leg conformation, and his eyes, as well as other body systems. Additionally, the horse should be thoroughly evaluated for genetic flaws that could be passed on to offspring, Threlfall said. For example, hereditary regional dermal asthenia (HERDA) and polysaccharide storage myopathy (PSSM) are genetic disorders that all potential breeding stallions should be tested for prior to mating.

Genitalia Examination--Veterinarians should also evaluate the stallion's reproductive anatomy during a breeding soundness examination. Threlfall explained that the penis must be able to retract fully, and there should be no abnormal swellings or enlargements of the genitalia. All aspects of the reproductive tract--including the penis, scrotum, and testicles--should be palpated for abnormalities, he added. Threlfall made special mention of cryptorchidism, noting that in his opinion two testicles must be present to consider the animal a stallion. While some veterinarians will allow cryptorchidism to pass a breeding soundness examination, Threlfall suggested stallions with only one descended testicle should not pass the exam.

He also noted that, especially during this portion of the exam, it's important to stay safe as some stallions will bite, kick, or strike when their reproductive tract is palpated. Twitch or tranquilize the horse if necessary to maintain safety.

Libido Evaluation--Next Threlfall said the horse should be collected, allowing the veterinarian an opportunity to evaluate the stallion's libido and examine him after breeding for potential lamenesses. At this point, semen should also be evaluated. Threlfall suggested the following procedure prior to collection:

  • Tease the stallion to prepare him for breeding;
  • Wash the stallion's penis with gentle soap and water, paying special attention to the diverticulum (Threlfall noted that while this step remains controversial, he has washed all stallions before breeding for over 30 years and has never noted any adverse effect); and
  • Rinse the penis thoroughly and dry with damp cotton.

Once the stallion has been collected, the veterinarian should evaluate his reproductive tract once more for abnormalities, Threlfall said. At this time the stallion will be most amenable to reproductive tract palpation, he added.

Semen Evaluation--This portion of the exam needs to be carried out with appropriate laboratory equipment, but thanks to advances in veterinary technology, semen evaluations can be performed on the farm. According to Threlfall, good quality sperm should have:

  • Good motility (Before extending, the sperm should be fast-moving and will often move in large circles, he said. Seventy percent progressively motile sperm is a good reference point [above that being good and below that being questionable], Threlfall said.)
  • Good morphology (structure; the midpiece is often slightly off to one side when connection to the head, hence the reason the sperm often move in large circles, he added.)
  • Good concentration (A standard veterinarians often work from is an average of 80 to 100 million per mL, but Threlfall noted that he has seen acceptable sperm concentrations range from 50 to over 950 million.)
  • Good volume (Threlfall said the reference range is 20 to 200 cc plus per ejaculate; however, the normal range is generally around 30 to 60 cc per ejaculate)
  • Consistent appearance (He noted there shouldn't be any "brown or grey color," representing the presence of dirt or smegma. Washing the penis tends to reduce the likelihood of this contaminating the semen, he said.)

Cultures will generally reveal some bacteria present in the ejaculate. The significance is in the source and the bacteria involved, he said.

Once the veterinarian has completed the breeding soundness examination, he or she will classify the stallion as satisfactory, questionable, or unsatisfactory for stud duty. In addition, he or she will suggest the number of mares that can likely be bred successfully by the stallion via either artificial insemination or live cover.

Threlfall noted that a stallion with a slightly lower "grade" in the breeding soundness exam (depending upon where the problem areas are) might still be very successful in breeding a small number of mares each year, while a stallion that scores high marks overall might be successful breeding hundreds of mares annually. To this end, Threlfall suggested the stallion manager seriously consider the situation before breeding a questionable or unsatisfactory stallion, especially if a large number of mares are to be breed.

Take-Home Message

Threlfall stressed that a thorough breeding soundness examination on stallions prior to breeding is a crucial part of the reproductive program. Identifying potential problems--such as genetic disorders, reproductive tract abnormalities, or semen quality issues--can help stallion managers set realistic goals for prospective breeding stock.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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