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Breeding Soundness Examination of the Stallion

By: Dr. Sylvia Bedford-Guaus

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Physical Examination of the Reproductive Tract

A complete breeding soundness examination (BSE) should include a thorough general physical examination and a thorough examination of all the parts of the reproductive tract (penis, scrotum and accessory sex glands). A stallion with severe musculoskeletal or neurologic problems may have excellent semen quality and yet be incapable of mounting a mare and breeding, rendering him technically infertile.

The testicles are palpated for size and consistency. This serves to detect any signs of atrophy (decreased size due to disease or advanced age), hypoplasia (decreased size of congenital origin) or degeneration, in addition to the possible presence of masses or fluid within the scrotum that could compromise sperm production within the testicle.

Measurements of each individual testicle (height, length and width) and total scrotal width should also be recorded. Presently these measures are often taken by scanning the scrotum and each individual testicle with an ultrasound machine. Estimated sperm production efficiency can be calculated using a specific formula through the individual measurements obtained from each testicle. In general, a stallion should have a minimal total scrotal width of 8 cm to pass a BSE.

Ultrasonography can serve as an additional tool to evaluate the appearance of the testes or presence of fluid or masses (tumors or cancer) within the scrotum. This is also a very important tool for transrectal evaluation of the accessory sex glands (ampullae, seminal vesicles, prostate and bulbourethral glands). Optimally, ultrasound examination of the accessory sex glands should be performed in all stallions, but especially in those showing evidence of infection or blockage once semen collection has been attempted.

The penis can be evaluated during washing prior to semen collection. Inspection of the penis may reveal the presence of infection, tumors or scar tissue resulting from the placement of stallion rings to prevent masturbation. Masturbation is a normal behavior in all stallions that does not reduce semen production or performance in the breeding shed, and thus the use of devices to prevent such behavior is strongly discouraged and can be harmful to the stallion.

A BSE includes a minimum of three cultures taken from the dorsal urethral fossa (small orifice right above the urethral opening) and the urethra, before and after ejaculation. Additional cultures of the penis shaft, prepuce or semen can be taken if any problems are suspected, and depending on the preferences and experience of the evaluator.

Not all bacteria cultured during a BSE result in disease for the stallion or are transmissible to mares, and therefore the results of cultures must be interpreted with caution. It is common to obtain a positive culture of bacteria that are normal inhabitants of the penis or genital area and are not causing any problems. Furthermore, trying to eliminate these bacteria from the genitals of stallions with the use of topical disinfectants or antimicrobial drugs may actually cause irritation and enhance the growth of infective microorganisms.

We typically worry about potential infection when we obtain positive cultures of Pseudomonas spp. or Klebsiella spp. These types of bacteria have been associated with infections of the internal reproductive organs (accessory sex glands), penis and prepuce, yet their transmission and ability to cause endometritis (uterine infections) in bred mares has not been proven.

Observation of Libido and Mating Ability

Libido (sexual drive) and mating ability are important in order for a stallion to perform adequately during a long fully booked breeding season. The age and experience of the stallion should be taken into account when evaluating these parameters.

The breeding or semen collection process (time from entering to leaving the breeding shed) with an experienced stallion is usually accomplished within 3 to 5 minutes, without rushing. Inexperienced and/or immature stallions may take longer time to achieve an erection, mount, intromit and/or ejaculate.
Semen Collection and Evaluation

Semen from stallions is typically collected by the use of an artificial vagina. The most commonly used types of artificial vaginas used in the United States are the Missouri, Colorado State University, Polish and the Nishikawa or Japanese model. Each of them offers different advantages and disadvantages, and the selection of one over the other depends on individual as well as stallion preferences and experience.

In general, an artificial vagina consists of a tube (plastic, rubber or metal) with a water jacket. This jacket must be filled with hot water for a final temperature of 107.6 degrees to 118.4 degrees Fahrenheit (42 to 48 degrees Celsius), for adequate stimulation of the penis during collection. One should avoid contact of the artificial vagina with spermicidal substances like detergents or disinfectants, and the artificial vagina should always be clean and dry prior to use. Additionally, the inside of the artificial vagina should be lubricated with a non-spermicidal water-soluble sterile jelly prior to semen collection. To avoid damaging the sperm during processing procedures, precautions should also be taken for all devices (collection bottles, liners, microscope slides, insemination pipettes, syringes, pipettes) that will come into contact with the semen, and all should be kept in an incubator at body temperature (98.6 F or 37 C).

A BSE consists of the collection of a minimum of two ejaculates one hour apart. More ejaculates may be collected if deemed necessary during the evaluation procedure. When only two ejaculates are collected, the second ejaculate should provide approximately half the number of sperm from the first one. The number of sperm in this second ejaculate is considered a rough estimate of the potential daily sperm output of the stallion. The daily sperm output is the number of sperm that a stallion is capable of producing in a daily basis and will allow us to determine whether a stallion can sustain a full book of mares during the breeding season.

After collection, semen is taken to the laboratory and evaluated for gross appearance. The volume of semen, concentration of sperm in the sample, percentage of total and progressive sperm motility and the total number of sperm are recorded. Additionally, a drop of semen placed in formalin or stained is evaluated for sperm morphology under the microscope at high magnification. Morphological evaluation of sperm allows us to determine whether the sperm produced by the stallion have abnormal shapes such as large or small heads, bent or coiled tails or missing parts.

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