CHAPTER 38The Subfertile Stallion
Suboptimal pregnancy rates remain a serious problem within the horse industry. When performing breeding soundness examinations on stallions intended for stud service, more than one third of prospective breeding stallions fail the evaluation, most commonly due to ejaculation of low numbers of normal, motile sperm.1,2 Semen evaluations performed on stallions with lower than expected fertility often reveal similar findings. Unfortunately, the cause of ejaculation of low numbers of normal, motile sperm by stallions usually remains undetermined and, thus, is often medically untreatable.3–5 Alteration in breeding management of stallions ejaculating low numbers of normal, motile sperm is therefore usually directed primarily at limiting the number of matings required of the stallion (i.e., number of mares booked to the stallion and number of matings per estrous cycle) to ensure that an optimal number of normal, motile sperm are used for each breeding.4,6 Other methods of breeding management can also be tried in an attempt to improve fertility. Certainly there are many other factors that can contribute to lower than expected fertility in stallions in natural service mating programs. This chapter will discuss the economic impact of reduced stallion fertility, use of record analyses to assess the degree of subfertility, and some of the more commonly used management techniques used for stallions with suboptimal fertility in natural service mating programs.
ECONOMIC IMPACT OF LOWERED FERTILITY
The following example is used to further illustrate the magnitude of losses that can occur with just a 20% difference in pregnancy rate. If a stallion bred a book of 100 mares (using only 1 cover per estrus) and achieved a 60% pregnancy rate per cycle over a total of 3 estrous cycles, the stallion would achieve a 93% seasonal pregnancy rate requiring a total of 156 covers. By contrast, if only a 40% pregnancy rate per cycle was achieved, the stallion would achieve a 78% seasonal pregnancy rate requiring a total of 196 covers (Box 38-1). The lower fertility would culminate in an extra 40 estrous cycles of breeding and their associated board and veterinary expenses over the course of the breeding season yet produce 15 fewer foals (assuming all pregnancies result in live foals), as well as an additional 15 years of nonproductive maintenance expense (15 additional barren mares). If the boarding fee at the breeding farm is $26 per day, the 40 extra estrous cycles of breeding (21 days/cycle) result in an extra cost of $21,840 compared to the cost for the higher level of fertility. If veterinary fees average $250 for examinations and treatments per cycle, the 40 extra estrous cycles of breeding result in increased veterinary fees of $10,000 over that for the higher level of fertility. The cost of the lower fertility in this example totals $31,840 and still does not include lost income from 15 stud fees, maintenance expense for 15 extra barren mares for 1 year, or lost income from 15 foals that will not be produced. While expenses, fees, and sale prices vary, this example serves to stress the importance of maximizing reproductive efficiency of the stallion.
| Fertility Achieved per Cycle and per Season with Theoretical 40% PR/cycle* | |
| Lower Theoretical Fertility | |
| 100 mares bred 1st cycle × 40% PR/cycle = 40 mares pregnant on 1st cycle of breeding | 100 covers |
| 60 mares bred 2nd cycle × 40% PR/cycle = 24 mares pregnant on 2nd cycle of breeding | 60 covers |
| 36 mares bred 3rd cycle × 40% PR/cycle = 14 mares pregnant on 3rd cycle of breeding | 36 covers |
| Total mares pregnant after 3 cycles of breeding | 78 |
| No. barren mares | 22 |
| Total no. covers for season | 196 |
| Higher Theoretical Fertility | |
| 100 mares bred 1st cycle × 60% PR/cycle = 60 mares pregnant on 1st cycle | 100 covers |
| 40 mares bred 2nd cycle × 60% PR/cycle = 24 mares pregnant on 2nd cycle | 40 covers |
| 16 mares bred 3rd cycle × 60% PR/cycle = 9 mares pregnant on 3rd cycle | 16 covers |
| Total mares pregnant after 3 cycles of breeding | 93 |
| No. barren mares | 7 |
| Total no. covers for season | 156 |
Assuming only 1 cover is required per estrus, and all pregnancies result in production of viable foals, the lower level of fertility would result in 40 extra covers throughout the season, yet produce 15 fewer foals (i.e., 15 more barren mares).
* No. mares bred per cycle × theoretical PR/cycle = no. mares pregnant.
SOME FACTORS THAT AFFECT STALLION FERTILITY
Investigation of suboptimal fertility in a stallion should be directed toward identifying and correcting contributing factors. In some cases, treatment of a disease condition (e.g., infection, ejaculatory dysfunction, etc.) may improve the stallion’s fertility.7 In other cases, no treatment is indicated for the stallion, yet quality of the mare book will preclude significant improvement in fertility. More commonly, recommendations for altering breeding management practices can be made to improve the stallion’s fertility. The following discussion is provided to aid the veterinarian in developing a breeding management plan that could maximize the fertility of a stallion with lower than expected pregnancy rates.
Historical Considerations
To assess potential changes in reproductive management in an effort to enhance fertility of a given stallion, it is important to first obtain a meaningful history. An assessment of past breeding performance (assuming the stallion has been bred previously) is a good place to start, since previous fertility is one of the best predictors of future fertility.8 The number of seasons the stallion has been in service and whether/when any changes in fertility occurred provide useful historical information. Congenital problems that result in poor fertility are often evident from the first season the stallion is used for breeding, while acquired problems (trauma, deterioration in physical condition, occurrence of disease[s], or management changes) often result in a decline in fertility when compared to previous seasons or months within a given season. Age-related decline in a stallion’s fertility is often gradual in onset, although the difference in pregnancy rates achieved from one year to the next can be dramatic if it is due to progressive testicular degeneration.9
Owners or managers of stallions may only provide impressions based on a few salient observations (e.g., a low seasonal pregnancy rate), which are usually insufficient to support meaningful conclusions about fertility. It is therefore incumbent on the veterinarian to thoroughly evaluate breeding records to characterize the fertility problem. If breeding records do not exist, meaningful evaluation can seldom be performed. Poorly organized records can make the task arduous or even impossible. Even the more modern computerized record systems will require some reorganization and summarizing of salient fertility endpoints to ensure a given stallion’s fertility is adequately characterized.10
Do not hesitate to request records from previous years to substantiate whether a given stallion’s fertility has changed or a decline in fertility occurred as a result of deteriorating quality of the mare book. It is common for first-year stallions to have a demanding book, with many high-quality mares. After the first or second season, and until/unless the stallion’s offspring prove themselves, it becomes more difficult to fill his book. Thus, as the stallion’s popularity declines, his book tends to accumulate fewer young, fertile mares and an increasing proportion of mares with lower fertility (Table 38-1). This changing quality of the mares in his book will many times explain a decline in fertility.10
Table 38-1 Influence of Declining Quality of Mare Book on Fertility Achieved by a Thoroughbred Stallion in 2 Consecutive Years
The number of mares bred by the stallion in each season is another important consideration in assessing potential causes of lowered fertility. Pickett et al6 estimated that a minimum of 40 mares must be bred in order to adequately evaluate the stallion’s fertility. Many stallions do not breed a sufficient number of mares to adequately test their inherent fertility.11 The veterinarian should be cautious in concluding that a stallion that breeds a limited number of mares does in fact have lowered fertility.
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