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The Estrus Cycle of Friesian Mares

A researcher at Utrecht University in the Netherlands, as part of a Master’s Thesis, did a study “that was aimed at describing the characteristics of the estrous cycle in the Friesian mare and to produce a guideline for veterinarians responsible for reproductive management of mares from this breed”. Data was gathered from 3 different veterinary clinics on 687 cycles (breeding seasons of 2009, 2010, 1022) of 340 mares (ages 3-21 years old) and included the interovulatory interval (IOI) (number of days between ovulation in 2 consecutive estrous cycles), # of inseminations, diameter of follicle, softness of the follicle, uterine and cervical tone, uterine edema and free fluid in the uterus. Cycles in which hormones were given were analyzed separately, such that three groups of data were formed: cycles during which no hormones were used, cycles in which estrus was induced using PGF2α (ex. Lutalyse) and cycles in which ovulation was induced with hCG.

Results were as follows:

IOI – 24.46 +/- 0.32 days, mean # of inseminations/cycle was 1.97 +/- 0.06, follicle size 0-24 hours before ovulation was 4.99 +/- 0,06 cm, uterine edema peaked 2 days prior to ovulation then decreased each day til ovulation, # days between giving PGF2α and ovulation was 9.18 +/- 0.34, inducing ovulation with hCG resulted in a smaller diameter follicle prior to ovulation (max diameter 5.11 +/- 0.12 cm). In conclusion, “the estrous cycle of the Friesian mare is subtly different to that seen in other common horse breeds.”

Relevance: While the differences of the Friesian mare’s estrous cycle are subtle, this article is well worth reading for any Friesian owner/breeder and their veterinarian, especially if one is breeding with frozen semen. It can really help in fine-tuning the breeding of these Friesian mares. Nothing replaces the information that a mare’s breeding record can provide, but this article can certainly help to fill in the gaps.

Scientific article: Vliet, D. van “The oestrous cycle in Friesian mares.” (2014).

**You can find the complete article on the Foundation’s website under the Education/Links tab**

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