Predictions of Stallion Fertility Derived From Semen Quality Evaluation
Thomas V Little
Gainesway Farm, Lexington Kentucky USA
Different strategies for predicting the potential fertility of stallions have evolved into a comprehensive checklist of tests or procedures referred to as the clinical fertility evaluation or breeding soundness evaluation (BSE).11,18 The conventional BSE consists of a medical and reproductive history, a general physical examination, a genital examination including bacteriologic cultures, an assessment of breeding behavior and a semen evaluation. In many instances, history, physical exam, genital exam and behavioral assessment reveal no deficiencies that would impact fertility. The prediction of potential fertility of the stallion in question is then derived almost entirely from the evaluation of semen quality.
Defining the relationship between stallion semen parameters and fertility has been the difficult objective of numerous studies. Investigators have successfully demonstrated associations between many specific semen parameters and fertility. These include sperm concentration,7,9,12 total sperm number,7,17 live sperm,7 normal morphology,3,8 abnormal morphology in entire ejaculates or dismount samples,3,8,16,17 total motility,9 progressive motility, 9 mean velocity, 9 and longevity under various conditions.15 Unfortunately, many of these correlations were only moderate in degree and in some instances, different studies examining some of the same parameters have yielded no correlation with fertility.7,12
Why is it difficult to demonstrate strong relationships between conventional semen parameters and fertility?
Failures to demonstrate a strong relationship may stem from difficulties in assessing semen quality or in determining fertility. Semen assessment errors may result from an inability to accurately characterize a semen parameter, from variation between ejaculates from the same stallion, from including semen parameters that actually have no impact on fertility, from neglecting parameters that do and from the existence of semen deficiencies that might arise or be expressed only after semen parameters have been measured. Fertility assessment may be hindered by examining stallions with too narrow a range of fertility, by breeding mares with redundant sperm numbers, by breeding an insufficient number of mares, by managing these mares poorly or by the inherent fertility of the mares bred
How well does a conventional breeding soundness evaluation predict fertility?
Demonstrating a significant correlation between specific semen parameters and fertility is not equivalent to predicting the fertility of a single stallion from one or more samples. That requires the formulation of a predictive equation, the prospective use of that equation on another set of samples and the demonstration that observed fertility results are consistent with the prediction.2
Consider the following table that categorizes the possible outcomes of a hypothetical breeding soundness evaluation and the subsequent fertility determination. For the purpose of discussion, consider the proportion of stallions that we would expect to fall in each cell.
Table 1. Possible Outcomes of a Breeding Soundness Evaluation and Subsequent Fertility
If our predictions are perfect, only one of two outcomes is possible for a given stallion. In practice, we accept some mistakes and we also defer some decisions until re-examinations or ancillary tests are performed.
Table 2. Possible Outcomes of a Breeding Soundness Evaluation and Subsequent Fertility
What percentage of stallions pass a conventional BSE and fail to meet fertility expectations? What percentage of stallions fail the BSE and are acceptably fertile? What percentage of evaluations produce ambiguous results? What percentage of marginal stallions are we willing to reject to ensure that all stallions that pass our examination perform satisfactorily? Is the following table a realistic goal?
Table 3. Pregnancy Rate Per Cycle Following BSE Examination
What is acceptable fertility with natural service, AI with fresh semen, AI with transported semen or AI with frozen-thawed semen? Can we distinguish a 40% pregnancy per cycle stallion from a 60% per cycle stallion? A 20% from 80%? Is our objective to rank fertility, to categorize fertility or to establish a minimum acceptable fertility standard based on intended use? Minimal requirements for fertile stallions have been suggested.4,6,7,10,11
What are the objectives of a breeding soundness examination?
The objectives of a breeding soundness examination have been stated in many different ways. Three recent examples are listed. How well do we meet these objectives?
To determine if a stallion can efficiently impregnate mares.13
To determine whether a stallion has the mental and physical faculties necessary to deliver semen containing viable spermatozoa and no infectious disease to the mare’s reproductive tract at the proper time to ensure establishment of pregnancy in a reasonable number of mares bred per season.5
To predict the potential fertility of a given stallion and to evaluate and determine with more accuracy whether a stallion’s semen is able to tolerate handling procedures such as cooling and freezing.15
Will ancillary tests improve predictive value of breeding soundness evaluations?
Sperm function assays, sperm chromatin structure assays and seminal plasma analysis have been developed as ancillary methods of semen evaluation. They can be classified by the functional compartment of the spermatozoon they were designed to test.15
Which ancillary tests have promise for predicting fertility? In what circumstances are they being applied? What advantage do they offer over traditional semen parameters? How will they be incorporated into a predictive equation for fertility? How important is it that they are functionally independent from one another? 1
Numerous conventional and ancillary tests of semen quality have been shown to correlate with fertility. Complete semen evaluation remains our most powerful laboratory method of evaluating the potential fertility of a stallion. Even when combined with other components of a breeding soundness examination, our ability to predict fertility is limited. We are probably most successful at identifying stallions with low fertility. Developing an equation that is predictive for fertility is complicated by uncertainty in both semen quality evaluation and fertility assessment.1
The ideal semen quality evaluation should take into consideration the heterogeneous nature of the sperm population within a single semen sample. It should quantify multiple, independent attributes essential for fertility from a large number of individual sperm.2 Results should be expressed as the proportion of sperm that possess sufficient quantities of all these essential attributes.2 The challenge will be to identify all of the essential attributes, to determine a minimum effective amount for each attribute and to develop techniques and instrumentation that measure these attributes in individual spermatozoa.
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