Assessment of Breeding Records

Critical analysis of breeding records will aid in determining the specific sources of a fertility problem such as mare-related factors, management factors, intrinsic stallion factors, or a combination of these. In all commercial programs, each of these factors contributes to the perceived fertility of a given stallion. When properly analyzed, the breeding records offer a wealth of information regarding changes in management that could improve breeding performance. For instance, some stallions have improved fertility when bred more frequently, whereas other stallions’ pregnancy rates decrease when breeding frequency is increased. Close examination of breeding records can demarcate these differences (Table 155-1). For example, Stallion 1 had an apparent improvement in fertility as breeding frequency increased from one to three times daily, and pregnancy rate also tended to increase from Session 1 to Session 3 on days when three mares were covered. Conversely, pregnancy rate declined precipitously when the stallion was allowed 1 or 2 days of sexual rest between covers. Stallions like this are commonly encountered when breeding records are evaluated. Such stallions typically have large testes and tend to develop stagnant stores of sperm in the extragonadal ducts when not breeding frequently. Sperm numbers in ejaculates are generally not the limiting factor for establishment of pregnancies. Management of this type of stallion should aim to maximize reproductive efficiency and can include covering of noncommercial mares to avoid periods of sexual abstinence that will negatively affect breeding performance. This breeding strategy becomes especially important in the days and weeks before the onset of the commercial breeding season because these stallions are generally quite susceptible to a condition termed occluded ampullae after extended periods of sexual rest. In contrast, the records for Stallion 2 demonstrate that fertility of some stallions is lowered as breeding frequency increases. As is evident with this stallion, pregnancy rates are often improved in the last covering session of the day when such stallions cover multiple mares in a day. It is possible that mares in the last covering session of the day were being covered nearer to the time of ovulation. Stallions with this breeding scenario may be managed most effectively by limiting the number of mares that are covered per day so that the stallion can ejaculate the threshold number of normal sperm necessary to achieve an acceptable pregnancy rate. Alternatively, mares might be covered closer to the time of predicted ovulation in an attempt to reduce the need for extended sperm longevity in the mare’s reproductive tract.

Reinforcement Breeding

This breeding approach is becoming more widely used in natural cover programs; however, the technique is not authorized by certain governances. As such, the technique should only be applied after authorization by the governing body of any breed registry.

The technique involves collection of semen from the penis immediately on dismount of a stallion from a mare after an ejaculatory mating. The protocol recommended by the author is to maintain the dismount semen sample at body temperature, strain it through a filter to remove extraneous debris, and then mix the filtered semen with a small volume (5 to 10 mL) of warmed good-quality semen extender. The extended semen is then loaded into an all-plastic syringe, and the filled syringe is affixed to a standard insemination pipette. Covered mares are immediately placed in stocks, and the perineal area is prepared for insemination of the extended dismount semen sample. As the pipette and hand advance through the vagina, any fluids grossly free of urine or blood are also aspirated into the pipette, and the contents are discharged into the uterine body.

Data from five commercial stallions whose semen was sometimes used for reinforcement breeding of mares during a single season were summarized (Table 155-2). The stallions all resided at the same breeding facility. Three of the five stallions (60%) had an improvement in fertility when reinforcement breeding was used, compared with traditional breeding. One stallion, Stallion A, tended to dismount mares prematurely, and dismount semen samples from this stallion were known to contain up to 5 billion sperm. As such, the rationale for reinforcement breeding was apparent. Stallions B and C did not dismount prematurely during ejaculation, so the reason for any beneficial effects of reinforcement breeding is speculative. Improved longevity of sperm viability may be a possibility because metabolic substrate (glucose) was available in the semen extender. It is also possible that simple deposition into the uterus of sperm that would otherwise be lost for breeding purposes may have resulted in colonization of the oviducts by the increased sperm numbers. Others have reported a similar improvement in fertility for a high proportion of Thoroughbred stallions after reinforcement breeding.