Frozen Semen Processing and Quality Control

The first pregnancy obtained by using frozen-thawed semen in the equine species dates back to 1957 (Barker and Gaudier 1957) and was obtained using epididymal semen. Nonetheless, for many years, this reproductive technology has achieved limited progress in the horse compared with other species (e.g. cattle). This was mainly due to the fact that for a very long period of time only a few horse registries allowed the use of frozen-thawed equine semen: hence, economic interests and resources allocated to research have has always been minimal, thereby obstructing advances in this area. Today, almost all the horse registries allow the use of this reproductive technique, thus encouraging researchers to focus on identifying better semen freezing techniques and simpler artificial insemination protocols. This has resulted in an improvement in fertility of cryopreserved equine semen, further enhancing the use of the technique by breeders. Moreover, a few papers published over the last few years (Loomis, 1999; Vidament, 2005; Loomis and Graham, 2008) clearly showed that the development of improved techniques for freezing equine semen allowed a greater percentage (85-90%) of stallions, than has traditionally been reported in the literature, to be capable of producing semen with commercial acceptable post-thaw quality.

Benefits of frozen semen include access to semen from stallions: 1) standing in other countries or continents and from competition stallions, 2) that become ill or get infectious diseases, injured or overbooked during the breeding season and 3) when the mare is at the optimum time for breeding. However, success with frozen semen requires that the practitioner be familiar with the techniques with properly thawing, evaluating and handling of frozen semen as well as the breeding strategies employed to maximise fertility (Loomis and Squires 2005).

Data published in the last 15 years clearly show that excellent pregnancy rates can be obtained by using equine frozen-thawed semen in a commercial setting (Metcalf 2007). The high fertility of cryopreserved stallion semen that is empirically observed in the field may be attributed to the more sophisticated freezing methods and simpler insemination protocols developed over the last few years, but may also because more practitioners are familiar with the techniques of properly thawing, evaluating and handling frozen semen. In addition, pregnancy rates have improved as semen with known superior fertility data is often used preferentially. Nevertheless, the majority of the frozen semen fertility data were collected under field conditions and not from controlled studies. Ideally, to determine more accurate fertility of equine frozen semen, experiments should be designed so that large numbers of mares are randomly assigned to be inseminated with frozen-thawed semen of different stallions and containing different numbers of progressively motile, morphologically normal spermatozoa. [...]