The history of artificial insemination (AI) begins with a story about some Arabs stealing stallion semen from a rival tribe and bringing it home for breeding in a sponge. When AI began its modern development in Russia during the early 20th century byIvanov, the main object was not cattle but horses as documented by Milovanov. In the late 1940s, Christopher Polge, working with chicken and the economically more important bull semen, pioneered development of frozen-thawed semen technology. This, however, was done by unknowingly using a bottle containing glycerol and allowing the semen to remain in glycerolated extender for 18 h ("glycerol equilibration time"). Thus, frozen semen technology was developed without much knowledge of the physiology of spermatozoa or the events leading to fertilization. Sperm motility was then, and largely remains, the main criterion upon which success or failure of the freezing procedure is evaluated in the laboratory. Sperm motility, however, proved to be of very littlevalue in predicting the success of freezing swine semen. Polge had already shown in 1956 that the glycerol concentration in liquid semen was inversely related to its fertility. Glycerol concentrations of less than 2% were necessary to achieve good fertility also after freezing of boar semen, although maximum sperm motility was seen at a concentration of 7% glycerol.
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