Insemination of Frozen Semen
The earliest report of frozen semen was in the 1780's when Spallanzani reported that freezing stallion semen did not kill all sperm. It was not until 1948 when the accidental discovery that extender containing glycerol significantly increases the post thaw viability of sperm (Smith, Polge and Elliot, 1949; Nature 164:666) that procedures to freeze semen was successful. The first pregnancy from frozen stallion semen was reported in 1957 when epididymal sperm were frozen in a whole milk extender with 10% glycerol (1 of 7 mares became pregnant).
Frozen semen has not been used extensively in the United States although it has been successfully used in other countries (China). The reasons for limited use in the US has been the lowered fertility of many stallions and the restrictions imposed by breed registry associations. Recent advances in extender composition, freezing procedures and changes in association rules has fostered an increase in the use of frozen semen.
For successful cryopreservation:
- sperm must be mixed with appropriate extender (may depend on stallion)
- for some extenders, semen must be cooled slowly from body temperature to 5°C
- exposed to one or more cryoprotectant
- package in appropriate single dose container
- frozen by cooling rapidly from 5°C to below -100°C
- stored at -196°C under liquid nitrogen
- warmed rapidly from -196°C to 37°C
- place in mare soon after thawing
