The management of stallions used in an artificial insemination program must be tailored to the individual stallion and to the facilities and personnel at the breeding farm. The stallion owner, farm manager, and veterinarian should develop a coordinated plan to optimize the stallion’s health for his athletic and breeding careers, to coordinate the housing and movement of all horses on the farm, to utilize farm personnel efficiently, to achieve a high level of fertility in an efficient manner, and to minimize risks to the stallion, personnel, and individual mares. A unique plan must be developed for a stallion being used in a breeding program but which is still actively competing in his athletic discipline. The health and breeding program must address the needs of the stallion based on his age, existing disease conditions such as orthopedic problems, location and number of mares to be bred, semen quality, and the innate fertility of the stallion.
General Health Considerations
The nutritional needs of the stallion used in an artificial insemination (AI) program are not unique. The goal of the feeding program should be to maintain the stallion at an ideal weight and fitness. Pasture, good quality grass or grass-alfalfa mixed hay, and water should be available at all times. Grain supplementation may be necessary to provide adequate vitamin and mineral consumption not supplied by hay. If the stallion is fed a balanced ration, it is unlikely that additional supplementation of the ration will increase fertility or daily sperm production. The stallion’s feeding program should be associated with an exercise program to keep the horse alert, athletic, and content.
A stallion needs daily exercise in a paddock or small pasture. If the stallion does not exercise freely during turnout time, it may be necessary to ride, drive, hand walk, or lunge the horse daily to maintain the stallion’s athleticism and body condition. A stallion needs exercise even during inclement weather. Many objectionable stallion behaviors are associated with a poorly implemented nutrition and exercise program.
Breeding stallions should be dewormed at regular intervals along with all the other horses on a farm. Fecal exams can be performed periodically to ensure the effectiveness of the parasite control program. To this author’s knowledge, reproductive performance of stallions has not been altered by regular use of commonly available de-worming compounds.
Yearly dental examinations should be performed to maintain normal mastication. Dental procedures should be carried out as needed. However, if tranquilization is required, promazine tranquilizers should not be used because of the risk of penile paralysis.
The farrier should evaluate the stallion’s feet at regular intervals of 6 to 8 weeks. Stallions receiving adequate turn out, frequently require minimal hoof trimming. Problems such as prior athletic injury, laminitis, hoof-wall cracks, flat soles, and underrun heels may require evaluation and treatment by the farrier and veterinarian to maintain hoof health and stallion longevity.
In general, stallions should be vaccinated against tetanus, eastern and western encephalomyelitis, and rabies. Many other vaccines are available and may be appropriate for use in individual stallions, based on age, horse farm population density, current farm disease status, and other factors. The vaccination program should be designed for the individual farm and stallion. Vaccinations against upper respiratory tract infections also should be considered for most active, breeding stallions. Maintaining immunity against influenza, rhinopneumonitis, and strangles may prevent infections with these viruses and bacteria and minimize the adverse effect of elevated body temperature on spermatogenesis and semen quality. Special effort should be made to use vaccines containing the most current serovars of influenza and rhinopneumonitis viruses. Other vaccines that should be considered based on disease prevalence at the farm include botulism and Potomac horse fever (Neorickettsia risticii).
Currently, a provisional license has been granted for the production of a vaccine against equine protozoal myeloencephalitis (Sarcocystis neurona) and West Nile virus encephalitis. Data are not available, at this time, to fully support the efficacy of these vaccines and potential adverse effects on breeding stallions. Consideration of the use of these vaccines in endemic regions of the country may be necessary.
Equine arteritis virus (EAV) can be spread from stallions to mares via respiratory tract secretions but, more commonly, through semen. Approximately 30% of stallions seropositive for EAV shed virus in their semen. Virus may be shed in the semen of an infected stallion for a short period of time or a lifetime. This virus can cause upper respiratory infections and abortion. All stallions used in an AI program should be tested serologically for EAV antibodies. A serologically negative stallion can be used to breed seropositive or seronegative mares without risk. If the stallion is seropositive for EAV antibodies, an aliquot of semen from one or more ejaculates should be submitted to the diagnostic laboratory for virus isolation. If the stallion is actively shedding virus in his semen, he should be used to inseminate only naturally exposed or vaccinated seropositive mares. Equine arteritis virus can infect mares inseminated with fresh, cooled, or frozen semen.
The use of the polymerase chain reaction (PCR) testing of semen for EAV is not accurate in determining the presence of virus in semen. If a stallion is determined to be serologically negative for EAV antibodies, vaccination against EAV should be strongly considered. Vaccination of the seronegative stallion prevents development of the carrier state if the stallion is exposed to field strain virus. However, vaccination apparently does not alter the carrier state once infection has been established. Vaccination of stallions against EAV should occur at least 30 days before the onset of the breeding season. The seropositive stallion that does not shed virus in his semen can be used safely in an AI program using fresh, cooled, or frozen semen. All stallions used for breeding should be tested annually for equine infectious anemia.
The economic value and importance of the stallion to a breeding program may be substantial. Many stallion owners elect to insure the stallion with mortality and/or fertility policies. These policies are not necessarily standardized but may include physical examination of the stallion and historical fertility data. A breeding soundness evaluation usually is not required.
Breeding Soundness Evaluation
A thorough breeding soundness evaluation should be performed on stallions entering an AI program. This evaluation should be done before purchase and before the onset of each breeding season. The purpose of the evaluation is to assess any physical limitations to breeding. The stallion’s willingness and manner of mounting an estrous mare or phantom should be assessed. Seminal quality should be determined. Specifically, the number of sperm ejaculated, percentage and type of sperm motility, morphologic analysis of sperm, bacteriologic and, possibly, viral status of the semen are determined. The longevity and type of sperm motility in semen extenders also should be determined. Any evidence of physical abnormalities or lesions of the external and internal genitalia should be noted.
The evaluation of semen quality may require the collection of numerous ejaculates of semen. The seminal quality of initial ejaculates from sexually rested stallions may not be representative of the stallion’s seminal quality while in routine use. The semen from sexually rested stallions frequently has markedly elevated sperm numbers, reduced sperm motility, poor longevity of sperm motility under shipped, cooled semen conditions, and an increased incidence of sperm morphologic abnormalities.
The semen quality of most sexually rested stallions stabilizes after three to six ejaculations over a period of 3 to 7 days. If the stallion’s semen quality has not stabilized, the practitioner may reach erroneous conclusions concerning the longevity of sperm motility in a shipped, cooled semen program, the acceptability of different semen extenders or antibiotics that are added to the extender, or the number of mares that may be bred using a single ejaculate.
The goals of the breeding soundness evaluation before the onset of the breeding season are to determine any limitations on the size of the stallion’s book; to identify any physical ailments that may have become apparent since the last breeding season; and to determine the suitability of the particular stallion for use in an on-farm or shipped, cooled semen breeding program. Selection of a good quality semen extender to maintain sperm motility for 24 to 72 hours or longer and control pathogenic bacteria in semen is made at this time. Semen quality and bacterial status of extended semen are evaluated periodically throughout the breeding season, in case adjustments are necessary. A final goal of the breeding soundness evaluation should be to establish the presence or absence of pathogens in equine semen, such as EAV, contagious equine metritis (CEM; Taylorella equigenitalis), Pseudomonas sp., Klebsiella sp., and Streptococcus zooepidemkus.