The major obstacle in standardizing techniques to evaluate stallion fertility is the huge variation in ejaculates from different stallions and even among ejaculates collected from the same stallion (1 ,2 ). Spermatozoa concentration, total volume, motility, percent live spermatozoa, and percent normal spermatozoa can range greatly (3 ), and any or all may have an effect on the ability of the ejaculate to fertilize. Cryopreservation of semen results in partial, irreversible membrane damage to the spermatozoa, reducing fertility rates even further. Using current industry protocols for cryopreservation of stallion semen, 50–75% of stallions do not produce ejaculates suitable for freezing (4 ). Pregnancy rates from natural or artificial insemination have shown little improvement in recent years and this has a significant impact on the economic feasibility of the horse breeding industry. Simple, accurate assessment of the viability of fresh and cryopreserved stallion spermatozoa prior to insemination could improve the reproduction rate in horses (5 ).
