Breeding with Frozen Semen
Basic Information 
Synonym(s)
• Fresh cooled semen may also be referred to as cooled shipped or fresh chilled semen. Semen is collected from a stallion with an artificial vagina, filtered to remove debris and gel and extended with a commercial milk-based semen extender. Cooled semen is inseminated into a mare within 24 to 72 hours after collection.
• Frozen semen is collected from a stallion with an artificial vagina, filtered, and centrifuged, and a special freezing extender is added. Semen is then generally placed into 0.5 mL straws for freezing. The dose (typically 1 to 8 straws) depends on the post-thaw motility achieved. There is significant variability with regard to semen quality and fertility when breeding with frozen semen.
• Live cover, natural cover, or natural service refers to a stallion directly mating with a mare.
Overview and Goal(s)
• To place semen into the mare prior to or just after ovulation.
• With cooled semen, insemination may occur 24 to 48 hours before ovulation, with good results for most stallions.
• With frozen semen, insemination should occur within 6 to 8 hours of ovulation.
• If ovulation has occurred, best fertility rates are obtained within 6 to 8 hours, with decreasing success up to 12 to 18 hours postovulation.
Indications
• Fresh cooled semen is preferred to frozen semen for breeding, when available, because most sperm have decreased longevity when the semen is frozen. In addition, some mares may have more of an inflammatory uterine reaction to frozen semen (post–mating-induced endometritis). Fresh cooled semen is commonly used when mares are not located within convenient driving distance to a stallion for natural cover or the stallion is not available for live cover.
• Frozen semen may be used when the stallion is not available due to a busy show schedule, being overseas, or being deceased.
Contraindications
• Frozen semen may have a decreased success rate in older mares prone to post–mating-induced endometritis. However, with proper mare selection, good technique, and quality semen, similar pregnancy rates may be achieved between fresh chilled and frozen semen.
• Human chorionic gonadotropin (hCG) may be less effective in inducing ovulation in older broodmares who have received hCG multiple times.
• Stallions with poor sperm longevity (“poor shipper”) may not be candidates for either fresh cooled or frozen semen. A longevity test, looking at spermatozoal motility over time, can differentiate a poor shipper. Stallions may have decreased longevity with either fresh cooled and/or frozen semen. Occasionally, a stallion may not perform well with one method but may be better with the alternative method.
• An alternative breeding method in the case where both fresh cooled and frozen semen longevity are poor would be natural cover.
Equipment, Anesthesia
• Breeding pipettes, sterile sleeve, sterile lubricant, microscope, slides, cover slips, incubator (warming plate or warm water bottle to lay slides on).
Breeding pipettes that use an internal metal stylet (multiple-use insemination gun) allow direct insemination of 0.5 mL frozen semen straws and should be used with frozen semen to maximize deposition of the entire dose into the uterus.
• Sterile lubricant that is osmotically and pH-balanced should be used to avoid negative effects on spermatozoal quality.
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