|The lifespan of spermatozoa in the female reproductive tract determines the maximum interval from insemination to ovulation that will provide the maximum fertility.
There is evidence that frozen-thawed spermatozoa do not bind as well to the oviductal wall and are lost from the oviductal reservoir sooner than fresh spermatozoa. Based on this information it would seem logical that mares bred with frozen semen should be inseminated closer to ovulation.
Because of the belief that frozen semen has a shorter lifespan in the mare, most breeding protocols for frozen semen attempt to inseminate as close as possible to ovulation.
Fresh semen remains viable in the oviduct for several days. Based on many studies, insemination with frozen semen should be performed within 24 hours before ovulation or within 6 hours after ovulation. Barbacinni (European Select Breeder Services Â– Italy) reported on 1,040 cycles inseminated with frozen semen within 6 hours before (363 cycles) or after (677 cycles) ovulation. Per-cycle pregnancy rates were similar for pre (41%) and post (41,3%) ovulation.
For many stallions, the number of insemination doses per mare is limited and veterinarians are asked to get mares pregnant with as few doses as possible.
This forces the veterinarian to examine the mare every 6 hours and to inseminate the mare with a single dose after the mare has ovulated.
Although the pregnancy rate is the same as that obtained with insemination before ovulation, the cost to the owner and the workload for the veterinarian are significantly higher.
Observations made in the Haras Nationaux in France indicate that two or more inseminations per cycle result in higher pregnancy rates as opposed to mares that were inseminated only once.
This has been observed for fresh semen (daily insemination until ovulation is detected) but also for frozen semen.
This observation suggests that when adequate number of doses are available, mares should be inseminated twice, if possible, with the last insemination occurring within 24 hours before ovulation. In this context, Barbacinni reported on the insemination with frozen semen 24 hours and 40 hours after hCG administration (ovulation occurs on average 36 hours after hCG).
Pregnancy rates with the timed insemination scheme were similar (76%) as those obtained with a single insemination after ovulation (71%).
Similar observations were made by the Colorado group. The advantage of the fixed time insemination scheme is the reduced workload, once-a-day examination versus 4 times-a-day examinations at 6-hour intervals.
Source: P. F. Daels (2003): New technics of artificial insemination in the mare. In: Proceedings of the 20de Studiedag van de / 20Ã¨me JournÃ©e d`Ã©tude de la Belgian Equine Practitioners Society - BEPS; www.ivis.org
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