Chapter 26 EVALUATION OF REPRODUCTIVE EFFICIENCY

Evaluation of the reprodufffctive efficiency (RE) in the horse breeding industry is an important aspect of routine mare and stallion management. Although many breeding enterprises have systems that record day-to-day breeding activities, there are no accounting systems that monitor and summarize the day-to-day breeding activities so that management has current information regarding breeding success and failure. Since reproductive outcome directly affects financial outcome, however, reproductive outcome is often not determined until the breeding season is over; therefore, it is too late to implement changes in management strategy. In addition, clinicians should make evaluation of reproductive efficiency a routine aspect of the stallion breeding evaluation.

Historically, RE is restricted to the determination of seasonal pregnancy rate and cycle/pregnancy, two measures that reflect a stallion’s average fertility, usually after the conclusion of the breeding season. However, limiting the evaluation of reproductive success to these two values alone assumes that the only source of reduced fertility is confined to the stallion while ignoring the effects of mare type and management. There are additional measures of RE that can pinpoint specific areas of reduced fertility in a breeding establishment that is often not related to a primary stallion limitation.

What is reproductive efficiency? For the purposes of this chapter RE is defined as the thorough evaluation of all available stallion, mare, and management information. Stallion factors include those endpoints associated with the breeding soundness evaluation such as semen quality, testes health, and the physical condition of the stallion. These factors have been previously described and outlined in the stallion manual published by The Society for Theriogenology.¹ Measurable mare factors include the reproductive status of the mare (i.e., maiden, barren, or foaling), mare age at the time of breeding, foaling date, and interestrous interval. Factors controlled by management include the size of a stallion’s mare book, composition of the book (e.g., number of maiden, foaling, and barren mares), and intensity of mare management leading up to and through breeding (i.e., how is ovulation determined, how often are mares bred when in heat).

The goal of breeding record evaluation is to describe and define non-sperm factors and the role they play in understanding and diagnosing the primary cause(s) of reduced fertility in the stallion and mare. Although subjective, a working estimate is that sperm quality accounts for 20%–40% of the variability in the fertility of the average stallion. This assumes that the stallion does not have a dramatic sperm-limiting factor such as extremely poor semen quality or very low sperm numbers. This hypothesis suggests, therefore, that 60%–80% of a stallion’s fertility can be explained by non-sperm factors, broadly categorized as management and mare factors.²

Historically, the overall (average) fertility of a stallion can be described by determining the seasonal pregnancy rate and cycles per pregnancy.¹

Seasonal pregnancy rate (SPR) is represented as a percentage and is based on the number of mares diagnosed as pregnant at a particular point in time divided by the total number of mares that have been bred to that stallion up to that point in time. This value is usually determined following the breeding season, but it can be evaluated at any time during the season. SPR is an important economic endpoint but is not a sensitive indicator of a stallion’s fertility because it does not reflect the total number of cycles that a mare is bred to achieve the pregnancy (i.e., efficiency). As an example, a stallion can achieve a relatively low pregnancy rate per cycle yet end the season with a seasonal pregnancy rate similar to a stallion achieving a relatively high pregnancy rate per cycle; the only difference is the stallion with a relatively low pregnancy rate per cycle must breed the mares in his book more times (i.e., less efficiently) during the season to reach the same seasonal pregnancy rate. Therefore, although SPR is important economically and is the one most familiar to the horse breeder, it is not a sensitive endpoint when trying to describe and identify the source of reduced fertility.

Several factors are important when determining SPR. The time of year (e.g., during the breeding season, shortly after the breeding season, or many months after the breeding season) when SPR is determined will influence the value. As the point in time approaches the next breeding season, the value will more closely approximate the foaling rate. How often a mare was bred during the season (the number of estrous cycles) will also affect SPR. The more opportunity a mare has to become pregnant, the more likely she is to get pregnant. Mares added to the book at the end of the season or mares removed from the book and passed to another stallion may be bred only once, thereby artificially inflating the mare book size and resulting in the appearance of a lower SPR. Therefore, when a stallion is presented with a low SPR, determination of adequate mare exposure is important. In addition, the inclusion or exclusion of mares that exhibit early embryonic death (EED) will also modulate SPR. Most embryonic loss is probably mare rather than stallion related. Therefore, to describe a stallion’s inherent fertility, all diagnosed pregnancies (which indicate that the stallion was able to accomplish fertilization in those mares) should be included in calculating the SPR. Yet, including EED as a pregnancy when figuring seasonal pregnancy rate is of no economic relevance (i.e., no foal results and no stud fees will be transferred), and an inflated end of season economic picture will be assumed.

Cycles per pregnancy (pregnancy rate/cycle, or C/P) is a more sensitive indication of a stallion’s fertility because it measures how efficient a stallion is in establishing pregnancies. This value is determined by counting all estrous cycles from all mares that a stallion has bred and dividing it by the total number of pregnancies. The total number of cycles can be determined by counting all dates bred excluding multiple matings in the same estrous period (i.e., doubles). The assumption is made that all mares were in normal estrus at the time of breeding and were bred near the time of ovulation. This assumption is not always correct, especially when artificial insemination (AI) is used, since mares can be bred regardless of whether they are in standing estrus.

SPR and C/P are the two most common endpoints initially determined when evaluating stallion fertility. They are, however, seasonal averages that measure overall fertility of not just the stallion, but the mares and management as well. It is possible, however, to be more specific when one is interested in identifying the specific source of a reduced SPR or elevated C/P. Although cutoff/threshold values are not always appropriate, a SPR of 80% and C/P of 2.0 can be used as working reference points, below and above which one might consider to investigate sources of reduced fertility.

There are several broad categories, as outlined in the following text, that record analysis should evaluate as well as the endpoints that can be used to evaluate those categories.

INFLUENCE OF MARE TYPE ON PREGNANCY OUTCOME

Mare type refers to the reproductive status of the mare coming into the breeding season and is commonly divided into maiden, barren, and foaling mares. These groupings are important because of differences in inherent fertility and opportunity to be bred.

Barren

This group contains non-pregnant mares coming into the breeding season of interest. Mares in this group will usually have lower fertility than the other groups. There are several reasons why a mare might be barren, including:

Not bred the previous season. This can occur because the owner simply decided not to breed the mare, or the mare may have foaled late in the previous season and did not have ample opportunity to become pregnant. Mares barren because of these reasons have normal fertility.

Subfertile. These mares have intrinsic fertility problems that contributed to failure to become pregnant the previous breeding season, and they are commonly older than the rest of the mare population. In most cases, the subfertile mares will account for the majority of the mares classified as barren.

Mares that have aborted. Mares that have aborted during the previous breeding season. Some managers and computer programs use the term slipped instead of the term aborted.

Foaling

These are mares that produced a foal in the current breeding season and will be rebred during the same season. One should expect high fertility in this group of mares, as they recently conceived and carried a foal to term. This group may exhibit reduced fertility if a predominant proportion of the mares foaled late in the breeding season, thus having only one estrous cycle available for breeding. Fertility in this group may also be reduced if some event (injury/illness to the stallion) prematurely shortens the breeding season.

Maiden

These are mares that have never been bred. Mares in this group are generally young; however, occasional maiden mares are older because of owner election not to breed when young (usually because of a continuing performance career). Older maiden mares are generally less fertile than young maiden mares. Mares that are recently retired from performance careers may not be cycling regularly when they first become available for mating (typically in February) and thus may require more time and subsequently more breedings to become pregnant. However, since maiden mares are generally available for breeding early in the season, their chances of eventually becoming pregnant are high.

Evaluation of Mare Type

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2. Reproductive Endocrinology of the Stallion
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4. Anatomy and Physical Examination of the Stallion

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