Evaluation of Stallion Sperm Morphologic Features
Charles C Love
Department of Physiology and Pharmacology, College of Veterinary Medicine 
Texas A & M University, College Station, Texas USA

The morphologic evaluation of sperm features is a common parameter used when a stallion is evaluated for breeding potential.  Some studies have described relationships between sperm morphologic features and fertility (Hendrikse 1966, Bielanski and Kaczmarski  1979,  Bielanski et al. 1982, Hurtgen and Johnson 1982, Jasko et al. 1990, Hellander et al. 1991, Love et al. 1998).  These studies are contrasted by others who have found no relationship (Kenney et al. 1971, Voss et al. 1981, Dowsett and Pattie 1982).  It is difficult to compare these studies because of the variability in the number of stallions, types of abnormalities evaluated, as well as the overall analytical approach.  One of the consistent patterns in most of these studies is that only single rather than multiple semen samples are evaluated for each stallion, thereby limiting the ability of the clinician to determine the variability in sperm quality that occurs over an entire breeding season.  Therefore, if the single sample evacuated for some reason is not representative of that stallion and is artificially lower than his actual value, that stallion would be considered to be potentially less fertile than he really is.  This is one possible reason for the confounding results seen in previous studies.

Recently (Love et al. 1998), we have taken a different approach when studying the effect of morphological features on fertility and have identified specific abnormalities that reduce fertility as well as the degree of drop in fertility that is associated with each incremental change in an abnormality.  This allows the clinician to evaluate the semen from an individual stallion and determine types and degree of normal and abnormal spermatozoa and relate those results to an expected level of fertility.

As with most assays of any type there are advantages and limitations when compared to other sperm function assays, some of which are listed and described below:

The advantage using sperm morphology include the following:

1.          Samples can be preserved immediately following semen collection and evaluated at a later time, therefore, unlike the evaluation of sperm motility this assay does not need to be performed immediately.

2.          Preservation is a simple process that can be performed adequately in a field situation by the clinician or a layperson. 

3.          Preserved samples can be transported or sent for expert evaluation or a consultation.

4.          Fertility information is available that allows the practitioner to render an opinion concerning the stallions sperm quality and the expected fertility of that stallion.

There are also limitations of this assay which include the following:

1.         Variation in the identification of certain sperm abnormalities

2.         Variation in the interpretation sperm abnormalities and how they impact fertility

As the horse population continues to grow nationally and internationally there is also an increase the sale and shipment of both stallions and semen.  There appears to be little continuity either within or between countries concerning the evaluation of the whole stallion for breeding soundness or the evaluation of sperm.  While there are several sperm parameters (total motility, progressive motility, morphology, longevity of sperm motility and total sperm numbers) that are routinely used to evaluate stallion sperm there is very little consensus as to how these results should be interpreted, especially with respect to a stallions future fertility.  Some evaluators use a cutoff value, above which a stallion is not considered for breeding.  One limitation of this approach is that the cutoff values do not have fertility outcomes to support such a strict assessment.  For example, if a value of 60% morphological normal sperm is used as a cutoff value does that mean those stallions, which fall below this value, are of limited breeding value?  If so, how much does their fertility differ?  Another approach is to evaluate stallions potential fertility based his ability to impregnate a given number of mares.  This approach takes into account a combination of factors including sperm quality (motility and morphology), resulting in an overall assessment of the stallion’s capability.  Both of these approaches are limited by the evaluator’s subjectivity in performing the assay, especially with respect to semen quality.  This subjectivity can be accounted for by differences in experience level, quality of equipment (microscope) used, clinical experience and interpretation of results.  The impact of this variation is that stallions may be incorrectly categorized especially if they are considered less fertile than they actually might be.  Therefore, if a protocol were established that would result in objective assessment of sperm morphologic features, it would allow for a more universal establishment of semen quality standards.

References

1.        Bielanski  W, Kaczmarski F  1979. Morphology of spermatozoa in semen from stallions of normal fertility.  Journal of Reproduction and Fertility, Suppl 27: 39-45.

2.        Bielanski W, Dudek E, Bittmar A, Kosiniak K 1982. Some characteristics of common abnormal forms of spermatozoa in highly fertile stallions. Journal of Reproduction and Fertility, Suppl 32: 21-26.

3.        Dowsett KF, Pattie WA 1982. Characteristics and fertility of stallion semen. Journal of Reproduction and Fertility, Suppl 32: 1-8.

4.        Hellander JC, Samper JC, Crabo BG  1991. Fertility of a stallion with low sperm motility and a high incidence of an unusual sperm tail defect. Veterinary Record 128: 449-451.

5.        Hendrikse W 1966. The semen of stallions of normal fertility. Animal Breeding Abstracts 34: 326.

6.        Hurtgen JP, Johnson LA 1982. Fertility of stallions with abnormalities of the sperm acrosome. Journal of Reproduction and Fertility, Suppl 32: 15-20.

7.        Jasko DJ, Lein DH, Foote RH 1990. Determination of the relationship between sperm morphologic classifications and fertility in stallions: 66 cases 1987-1988. Journal of the American Veterinary Medical Association 197: 389-394.

8.        Kenney RM et al. 1971. Stallion semen characteristics for predicting fertility. Proceedings of the American Association of Equine Practitioners 17: 53-67.

9.        Love et al. in press. Intra- and inter-stallion variation in sperm morphology and their relationship with fertility. Journal of Reproduction and Fertility, Suppl 56.

10.     Voss JL, Pickett BW, Squires EL  1981. Stallion spermatozoal morphology and motility and their relationship to fertility. Journal of the American Veterinary Medical Association 178: 287-289.