Reproductive Examination of the Stallion: Evaluation of Potential Breeding Soundness

CHAPTER 2 Reproductive Examination of the Stallion: Evaluation of Potential Breeding Soundness

The stallion breeding soundness evaluation (BSE) is intended to estimate a stallion’s reproductive potential, which is the current ability of a stallion to impregnate mares, resulting in the birth of a normal foal. This estimation is based on criteria provided by the Society for Theriogenology that list reproductive characteristics, including breeding history, physical examination of the whole stallion, physical examination of the reproductive tract, breeding behavior, determination of bacterial growth in association with the reproductive organs, and semen quality measurements. It is recommended that the manual published by the Society for Theriogenology be used as a reference for performing this evaluation.

Following the description and listing of the foregoing characteristics, the clinician categorizes the stallion into one of three classifications: Satisfactory, Questionable, and Unsatisfactory. If a stallion is considered deficient in two or more of these categories (total scrotal width, progressively motility, percent morphologically normal sperm), he is considered either a Questionable or Unsatisfactory prospective breeder. The difference between these two categories is a matter of degree. A stallion considered Questionable would fall slightly below the cutoffs for Satisfactory, and an Unsatisfactory stallion would be very low in two or more categories. Although this may seem like a subjective judgment, in practice the distinction is usually quite clear.

Historically, a Satisfactory classification is intended to recognize a stallion’s ability to impregnate a full book of mares (45 by natural cover or 125 mares by artificial breeding) in an efficient manner (75% of the book in two or fewer normal estrous cycles) using fresh semen deposited in the uterus by natural cover or artificial insemination. These numbers were not based on biologic or intrinsic stallion fertility, but rather on a legal standard (i.e., the number of shareholders) set by the syndicate members. Since the publication of the Society manual, conditions under which stallions are bred have changed, including an increase in the mare book as well as how mares are bred (cooled-stored and frozen semen). The Thoroughbred industry, which uses natural (live) cover exclusively, has increased the maximum number of mares that stallions will breed to over 100, with some stallions approaching 200 mares. The major change in the Standardbred and Quarterhorse industries has been the widespread application of cooled-shipped semen to the point that some stallions breed almost their entire book using this technology, with some stallions breeding 150 to 200 mares.

Since the increase in the number of mares in some stallion books it has become clear that stallions have the biologic potential to breed larger mare books at a level of fertility that is similar to the smaller books described in the Manual. The Thoroughbreds have been using these larger mare books for approximately 10 years. It is the author’s opinion that these larger books have not altered the overall fertility and that stallions that would have been Satisfactory prospective breeders for 45 mares are just as capable of handling larger mare books with similar pregnancy rates.

The intent of the classification system is to provide a framework around which the clinician can describe, evaluate, and interpret the breeding potential of a stallion. This approach is distinct from the bull BSE, in which a classification less than Satisfactory in many cases results in the elimination of the bull from the breeding population. The primary criteria used are based on sperm quality (motility and morphology), total sperm numbers, and testicular health (testes’ size and efficiency of sperm production). Any attempt to define in absolute terms (i.e., using cutoff values), based solely on sperm values and testicular size, a process as complex as fertility will be subject to criticism. However, without guidelines and a framework there is essentially no process by which the results can be interpreted. The guidelines set by the Society for Theriogenology provide a reliable, conservative (i.e., some “fertile” horses may be Questionable, but it is less likely that a Satisfactory stallion will be “subfertile”) estimate of a stallion’s fertility. For instance, criticism may arise when a young stallion, recently retired from training, is classified as Questionable or Unsatisfactory, but subsequently “does fine in the breeding shed.” Once the stresses of racing are removed and the stallion is allowed to “let down,” they can show dramatic improvement in their reproductive parameters. In this case, although the BSE may be criticized for “not getting it right,” the criteria applied to the day of the evaluation determined that at that point in time the stallion may be limited in his reproductive ability. While the chances of improvement should be discussed with the owner, the final classification must be made based on the findings the day of the evaluation. In addition, the classification allows the clinician an opportunity to describe management options to the breeder that will maximize the stallion’s reproductive potential. As the number of mares bred to stallions increases, the role of management and the input that the clinician can provide will play a greater role.

It is important that the clinician performing the BSE provide more than a simple classification of a stallion, especially in cases when a stallion does not fit into the Satisfactory category. In this case, the stallion owner may become particularly concerned about the reduced classification status of the stallion and assume that the stallion is infertile, subfertile, or that the “condition” is irreversible. It is therefore critical that the clinician be able to communicate, in written form, the significance and interpretation of the findings recorded on the BSE form. It should be recognized that the results of the BSE and subsequent classification of a stallion can have important economic implications for a stallion’s breeding future; therefore, the routine evaluation should be performed as completely as possible and thoroughly interpreted to the stallion owner. If a complete evaluation cannot be accomplished, as occurs in many cases, the limitations of the clinician’s evaluation should be stated clearly to the owner.


A general as well as reproductive history of the stallion should be recorded as part of the BSE. The general history should include any illnesses and medications administered as well as recent use of the stallion such as in racing or performance. The reproductive history should include a thorough review of the breeding records if the stallion has a breeding history. These records provide an objective record of a stallion’s reproductive past, and values such as seasonal pregnancy rate, cycles per pregnancy, and the type of mares in the book (maiden, foaling, barren) can be determined. In general, a stallion’s reproductive history falls in one of three categories:

Group 3 stallions, along with group 1, are perhaps the most challenging, because the veterinarian must make recommendations with respect to the future management of the stallion. Few stallions are intrinsically sterile, but many experience some form of reduced fertility. Therefore, the evaluation should focus on identifying the specific causes of the reduced fertility such that subsequent recommendations can be based on managing those causes. It is also important to identify the source(s) of reduced fertility so that inappropriate therapies are not instituted that may have no effect or worsen the subfertile condition.

Although stallions are classified following completion of the evaluation as a means of simplifying the process for the client, the veterinarian should be prepared to thoroughly explain why that stallion may fall in a particular category and the significance of that particular category. To the layperson, a categorization less than Satisfactory may be interpreted to mean that the stallion is subfertile or sterile or that the “condition” is permanent. In the case of the young stallion described earlier, the lower classification may be temporary and after a period of time the stallion may show improvement. Therefore, re-examination of a stallion may be recommended.

Sep 3, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Reproductive Examination of the Stallion: Evaluation of Potential Breeding Soundness
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