The Estrous 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 producing a guideline for veterinarians responsible for the reproductive management of Friesian mares.

Data was gathered from three different veterinary clinics on 687 cycles (breeding seasons of 2009, 2010, 1022) of 340 mares (ages 3-21 years old) and included the inter-ovulatory interval (IOI) (number of days between ovulation in two consecutive estrous cycles), # of inseminations, the diameter of the follicle, the 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. The results were as follows:

• IOI – 24.46 +/- 0.32 days

• The mean number of inseminations per cycle was 1.97 +/- 0.06

• Follicle size 0-24 hours before ovulation was 4.99 +/- 0.06 cm

• Uterine edema peaked two days prior to ovulation and then decreased each day til ovulation

• The number of 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 from that seen in other common horse breeds. While the differences in 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).