It has been more than 40 years since Canadian investigators reported the birth of the first foal from the insemination of a mare using frozen stallion semen. Since then, the technology of semen freezing has evolved tremendously. On a yearly basis, thousands of mares are inseminated with frozen semen around the world. Although the technology for freezing stallion semen has improved dramatically, pregnancy rates of mares bred with frozen semen average around 50%. These results lead us to believe that research for optimizing the efficiency and fertility of frozen semen has come to a halt in the last 30 years. However, one has to look at averages with a certain degree of caution since these numbers sometimes can be misleading.

It is well accepted that although an average pregnancy rate per cycle with frozen semen is around 30-40% when properly used, there is a great deal of variation, and it is not uncommon to have per cycle pregnancy rates ranging between 0-70%. In addition to this variation, there is a lack of standardization in the information that is available regarding insemination freezing and thawing protocols, as well as semen handling procedures.

Factors Affecting Fertility Of Frozen Semen

In order to maximize the efficiency of the use of equine frozen semen and to establish a successful insemination program with frozen semen, one has to try and control the factors that determine the success. The three major factors that should be considered are 1) the stallion and the quality of his semen after thawing; 2) age and status, as well as reproductive history, of the mares; and 3) the reproductive management of the mares and the handling of the frozen semen