The first sex-selected filly was produced by surgical insemination with a limited number of flow-sorted X-sperm. This effort was followed by nonsurgical artificial insemination of mares with flow-sorted sperm. Reasonable conception rates with sorted stallion sperm have been achieved only if the timing of insemination was optimized relative to ovulation induced with hCG or GnRH. Although stallion sperm do not sort as efficiently as those from bulls, semen from some stallions can be sorted at rates greater than 2000 sperm/sec producing nearly 5 x 106 live sperm/hr of each sex. Thus, with low-dose insemination, several doses of sexed sperm can be produced with a sorter each day.
A hysteroscopic insemination technique, which arose from clinical examination of the mare's reproductive tract using the videoendoscope, has been used to inseminate mares with many fewer sperm than used with conventional equine artificial insemination. Very low numbers of sperm are placed directly onto the oviductal papillus at the uterotubal junction with the videoendoscope. Application of this technique to deposit small numbers of sex-sorted sperm deep into the reproductive tract appears to make predetermination of sex a practical possibility in horses. Recently, 38% of mares hysteroscopically inseminated with 5 x 106 fresh, sex-sorted motile sperm became pregnant compared to 40% when mares were inseminated with 5 million nonsorted motile sperm.
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