Breeding Soundness Evaluations

Maintaining the overall vitality of breeding stallions entails a multifaceted approach. Approximately 60 days before the onset of the breeding season (i.e., commencing December 15), stallions are exposed to an artificial photoperiod consisting of 16 hours of continuous light followed by 8 hours of darkness. Increased light exposure may be useful for priming the hypothalamic-pituitary-testicular axis and stimulating testicular function in preparation for the beginning of breeding season (February 15).

Breeding soundness evaluation (BSE) consists of a physical examination, testicular palpation, testicular ultrasonography (including measurement of testicular size and assessment of testicular echotexture), and serial semen collections and evaluations. It is typically performed before the beginning of the breeding season, but an additional BSE may be necessary if a stallion has a drop in reproductive performance during the breeding season. Semen samples are collected by use of an artificial vagina (AV). Some stallions may not serve an AV, in which case they are allowed to breed a mare in estrus, and a dismount sample is collected and evaluated. Other means of semen collection include aspiration of semen from the vagina of a recently bred mare, use of a condom, and chemical ejaculation. Blood work is commonly performed on each stallion as part of the BSE and typically consists of a complete blood count (CBC), fibrinogen, biochemistry profile, and testing for equine infectious anemia. In addition to screening for underlying disease, these tests are useful for establishing a baseline of normal values should an illness arise later.

Maiden stallions are those that are beginning a career as a breeding stallion and typically have recently been retired from athletic competition. As such, there are both physical and mental adjustments that occur during the transition from performance to breeding. In most cases, the transition period is smooth and uneventful. However, some maiden stallions struggle with this process and experience, for example, an initial drop in body condition, change in demeanor, or foot problems. Fortunately, these issues can be easily mitigated by careful observation and early detection of problems. Concerning the mental adjustments that occur, the most noteworthy change is that a stallion is allowed to express normal stallion behavior, albeit within reason. Stallions must remain respectful to prevent aggression toward human handlers and other horses. It is important to realize that each stallion is an individual, and each new maiden stallion must be treated accordingly.

On arrival at the breeding farm, each maiden stallion undergoes a physical examination, and its medical history is thoroughly reviewed. If necessary, treatment and rehabilitation programs are developed and periodically monitored by the veterinarian. The stallion is vaccinated and dewormed according to the farm’s protocol and, if necessary, screened for infectious diseases such as equine viral arteritis (EVA), equine infectious anemia, and contagious equine metritis (CEM). These diseases are further discussed in a later section of this chapter (see Infectious Causes of Reproductive Tract Disease in Stallions).

After a period of adjustment, maiden stallions are introduced to mating by test breeding mares in estrus. This allows for conditioning of the stallion to the farm’s breeding procedure and evaluation of the stallion’s breeding behavior and semen characteristics. Both patience and repetition are required during the test breeding process because mating is a new experience for these stallions. Some stallions may require only a few breeding sessions before they are trained, whereas others need several attempts before they show interest in breeding. Dismount samples are routinely evaluated for the presence of live, motile, and morphologically normal spermatozoa. A stallion is considered trained after the horse willingly breeds a mare in an efficient and proper manner.

Monitoring and Managing Breeding Performance

When the breeding season begins, reproductive performance is closely monitored. The primary means is by periodic evaluation of individual stallion breeding records. The author uses commercial horse farm management system software,¹ which enables assessment of numerous reproductive parameters, including conception rates per cycle, session, and day; number of mares bred per day, week, and month; and overall pregnancy rates. Other means of monitoring stallions’ reproductive efficiency are through the use of manual records and graphs as described by Umphenour and colleagues (2011).

The breeding behavior and libido of individual stallions is also carefully observed. Changes could signify the onset of physical or psychological problems, such as musculoskeletal disease, reproductive tract abnormalities, systemic illness, and response to overbreeding. Orthopedic issues, such as stifle or hock arthritis, can adversely affect stallions’ willingness to breed. In some instances, preemptive measures can be taken to manage arthritic conditions, such as judicious use of systemic and local antiinflammatory agents, limiting the size of the stallion’s book of mares, and properly restraining the mare to prevent sudden and forceful movements during mating.

During the past decade, reinforcement breeding has increased in popularity in commercial live-cover breeding operations. This technique entails collection of the drippings from the stallion as he is dismounting the mare, mixing the semen with a suitable extender, and depositing the sample in the uterus of the mare that was just bred by that stallion. Reports have suggested that reinforcement breeding may have several benefits (Blanchard et al, 2006). These benefits include an increase in the total number of spermatozoa available for conception, an increase in spermatozoa longevity within the mare’s reproductive tract, and reduction in the severity of postbreeding endometritis. In return, improvement may be obtained with regard to per cycle and per season pregnancy rates, and it can help decrease the number of return covers. Indications for use of reinforcement breeding include breeding a stallion with historically low sperm numbers or low number of morphologically normal spermatozoa, managing a mare that has failed to conceive on one or more previous cycles, breeding a mare with an anatomic defect that impedes deposition of semen in the uterus, and instances in which the stallion fails to completely ejaculate following intromission.

Infectious Causes of Reproductive Tract Disease in Stallions

In high-scale breeding operations—especially in live cover situations—venereal disease is an important consideration. Aerobic cultures of the prepuce, fossa glandis, and urethra are routinely scheduled for each stallion to monitor for potential pathogens, such as β-hemolytic Streptococcus spp, Escherichia coli, Pseudomonas spp, and Klebsiella spp. Many of these bacteria can be normal or commensal flora of the stallion’s penis, so results should be interpreted with regard to purity and quantity of microbial growth and in relation to the stallion’s reproductive performance. The author prefers to collect cultures after every 10 mares bred by a given stallion, but the frequency of collection is at the discretion of the clinician and varies from farm to farm. If a pure and heavy growth of a potentially pathogenic bacterium is obtained and associated with poor conception rates, further diagnostic testing is necessary to identify the severity of the problem and determine the appropriate treatment. Careful physical examination of the external genitalia, semen evaluation, ultrasonography and endoscopy of the internal reproductive tract, and postbreeding uterine cultures of mares bred by a particular stallion are all methods that can be undertaken to identify the source and severity of the infection.

Contagious equine metritis is a venereal disease caused by the bacterium Taylorella equigenitalis. Clinical signs of CEM are more commonly seen in mares (e.g., severe vaginitis, cervicitis, and metritis), compared with stallions. Stallions are generally considered to be asymptomatic carriers but can spread the organism through the act of mating. Measures to prevent and control the spread of T equigenitalis include testing and treating all imported stallions and broodmares and undertaking active surveillance of the disease through a state-mandated program. An example of such a program is that outlined by the Kentucky Department of Agriculture (http://www.kyagr.com/statevet/equine/CEM.htm). The reader is encouraged to consult his or her local agricultural regulatory body for requirements regarding testing and monitoring for CEM.

Equine viral arteritis is caused by infection with the equine arteritis virus (EAV). A thorough review of EVA is provided (Holyoak et al, 2008) in the Suggested Readings. In breeding operations, the most important source of infection is contact with EAV-infected semen from virus-shedding stallions. Not all stallions exposed to EAV will become shedders, but those that do will harbor the virus in the reproductive tract and thus infect mares exposed to their semen. Other modes of transmission of EVA include inhalation of respiratory secretions, contact with infected fomites, and in utero infection. Clinical manifestations of EVA mimic those of other equine viral diseases and include fever, respiratory disease, vasculitis, and abortion. Abortion storms can occur following a herd outbreak of EVA, especially in a naive population of mares. Given the risk for disease transmission and the financial impact EVA can have on breeding operations, some states have a mandatory screening and vaccination program for breeding stallions. For example, the Kentucky Department of Agriculture requires that all Thoroughbred breeding stallions be vaccinated against EAV before each breeding season (http://www.kyagr.com/statevet/equine/EVA.htm). In addition, maiden stallions or stallions with an unknown history of vaccination must be screened for EAV antibodies before vaccination. Doing so helps ensure that the vaccine—and not a natural infection—was the cause for seroconversion. Seropositive stallions with no history of vaccination are required to undergo further testing, such as virus isolation from semen samples, to determine whether they are shedding EAV. Because testing and vaccination requirements for EAV may vary with locales, the reader should contact his or her state veterinarian’s office for further guidance.

Coital exanthema, caused by equine herpesvirus-3 (EHV-3), commonly causes superficial lesions on the skin of the stallion’s penis (glans and shaft) and prepuce, as well as on the mare’s vulva. These lesions can be painful and may be a cause of reluctance to breed. Diagnosis is made on the basis of clinical characteristics and lesion location, as well as by performing a polymerase chain reaction assay on samples obtained from active lesions. Equine herpesvirus-3 can be spread venereally or iatrogenically through contact with infected fomites. There is no commercially available vaccine for EHV-3, so the best means of prevention is immediate cessation of breeding. The length of sexual rest needed can vary with the stallion, but a 2- to 4-week quarantine period is often adequate to allow the lesions to heal and reduce future infectivity. Preemptive measures of avoiding virus transmission include careful evaluation of the external genitalia of the mare and stallion and use of proper breeding hygiene. Although coital exanthema is generally considered to be a self-limited disease, a topical antimicrobial agent (e.g., silver sulfadiazine cream or nitrofurazone ointment) can be applied to the lesions to prevent secondary bacterial infections. Information regarding the use of antiviral drugs for treatment of EHV-3 lesions is scarce. Topical acyclovir in the treatment of coital exanthema has been reported, but no mention was made with regard to the frequency or duration of administration. Abreva (docosanol 10% cream)² is an over-the-counter medication available for treatment of mucocutaneous herpetic lesions in humans. Anecdotally, this medication can be applied topically two to four times daily for 7 days in horses with EHV-3 lesions. The length of infectivity and whether EHV-3 is reactivated from latency are not known at present. Most stallions and mares can resume breeding after the lesions have adequately healed. It is common for lesions to leave permanent areas of depigmented epithelium.

Mares are seasonally polyestrous long-day breeders. When exposed to a stallion, they will naturally display signs of estrus or “heat” coincident with the follicular phase of the estrous cycle. Common signs of estrus include assuming lordosis posture, urination, and eversion of the clitoris. Examples of estrus detection include individual teasing, group teasing, observation in pasture, and simulation of breeding. A detailed account of estrus detection on a Thoroughbred breeding farm is provided (McCue et al, 2011). Mares are teased early in the morning with a teaser stallion, which interacts with mares individually in their stalls for approximately 15 to 30 seconds. At least two people assist with teasing to allow for careful evaluation of the mares’ behavior. Restraint may be necessary for those that are shy, have a foal by their side, or have a previous history of poor estrus behavior.

The importance of a good estrus detection program is threefold. First, it identifies mares that are in behavioral estrus and ready to be bred. These mares will be placed on the veterinary list to be evaluated that same day to determine the optimal time for mating. Second, it detects mares that are not in heat. Lack of receptivity is a valuable observation because it may indicate that a mare has recently ovulated (e.g., if she was in heat the day before), suggest that the mare is anestrous or in diestrus, or be used as a secondary sign of pregnancy if the mare was previously bred. Lastly, daily teasing can be used as an adjunct for identifying mares with abnormal estrous cycles (i.e., those that have irregular estrus behavior patterns), mares that are no longer pregnant, and mares that have experienced premature luteolysis secondary to endometritis. These mares will also be placed on the veterinary list for further evaluation to identify abnormalities.

Not all mares readily display signs of heat, despite having other characteristics indicating they are suitable for mating, such as cervical softening, uterine edema, and presence of a dominant ovarian follicle. For this reason, it is important to maintain good reproductive records to enable review of previous breeding performance. Mares with a history of poor estrus behavior can be teased more thoroughly, observed for a longer period of time, or be restrained with a twitch or lip chain to better detect their suitability for breeding. Also, some mares may not show characteristic signs of estrus and instead simply show a change in their demeanor or willingness to interact with a teaser stallion. It is for these reasons that the veterinarian’s familiarity with individual mares is invaluable in determining the optimal time for breeding.

The frequency of teasing depends on the status of the mares. Most foaling mares are teased beginning 25 to 28 days after foaling unless a foal-heat breeding is desired, in which case they are teased beginning 6 to 8 days after parturition. It is important to note that several Thoroughbred breeding sheds do not allow mares to be bred on foal heat before May 1. Barren and maiden mares are teased daily beginning in late January.

Simulation breeding or test jumping is commonly performed on maiden mares before a live cover. It is carried out in a safe area, such as a grass lawn or breeding shed. The mare is properly restrained, and a teaser stallion, fitted with a leather apron to prevent intromission, is allowed to mount the mare to simulate the act of mating. Test jumping is important because it conditions the maiden mare to the process of a live cover and allows the broodmare and stallion managers to gauge the mare’s breeding behavior. Successive jumps are performed until the mare stands quietly for mounting.

Synchronization of estrus is an important consideration in the breeding of Thoroughbreds. This results in part from the universal birth date of January 1 for Thoroughbreds registered in North America. Barren and maiden mares are exposed to an artificial photoperiod consisting of 16 hours of light and 8 hours of dark beginning on December 1. Most mares begin cyclic activity within 60 days of the initiation of light therapy. Other means of estrus synchronization include administration of oral altrenogest (Regumate³; 0.044 to 0.088 mg/kg, PO, every 24 hours) or a combination of progesterone (150 mg, IM, every 24 hours) and estradiol-17β (10 mg, IM, every 24 hours) in oil (P&E⁴—50 mg progesterone and 3.33 mg estradiol-17β per 1 mL; total dose of 3 mL, IM, every 24 hours) for 10 consecutive days. Mares are examined on the last day of P&E treatment, and a luteolytic dose of prostaglandin-F_2α is administered on that day. Successive evaluations are performed at the discretion of the veterinarian, and mares are bred on detection of a dominant follicle. An ovulatory agent (discussed later) is regularly administered in conjunction with breeding. Mares frequently are ready for breeding 5 to 9 days after the last dose of P&E.

In most live-cover situations, the goal is to breed a mare while she is in estrus with a dominant follicle and have her ovulate within 24 to 48 hours of breeding. Synchronization of ovulation is most commonly accomplished by use of an ovulatory agent. The two most commonly used ovulatory agents are the gonadotropin-releasing hormone agonist, deslorelin acetate (SucroMate⁵; 1 mL [1.8 mg/mL], IM), and human chorionic gonadotropin (Chorulon⁶; 1500 to 3300 IU, IV or IM). Timing of administration is at the discretion of the clinician, but these agents are most commonly administered the day before breeding or on the day of breeding. The interval to ovulation has been reported to be shorter for mares receiving human chorionic gonadotropin (35.9 h ± 2.8 hours), compared with deslorelin acetate (40.7 ± 3.2 hours). In most live-cover situations, this variation is of little consequence, and the clinician uses the agent that he or she is most familiar with. It is important to document the agent and quantity used so that doses remain consistent and can be determined from the mare’s record.

When a mare fails to ovulate in a timely manner (e.g., 48 to 72 hours after the initial breeding), a second breeding or “double” will be requested for that cycle. Second covers can be difficult to obtain, especially in stallions with large mare books. Nevertheless, they can be requested and are periodically granted. The clinician should not rely on doubles, but instead should strive to limit the number of breedings necessary to one cover per cycle.