With the rise in popularity of Friesian horses here in the US, it becomes increasingly important to bring to the attention of breeders and veterinarians certain reproductive characteristics peculiar to the breed.
Managing Friesian Mares in their Native Environment.
"Our experience is that Friesians do everything slower, tend to have a longer oestrus and do develop much larger follicles. As with other breeds, it is difficult to set hard-and-fast rules, but we don't get many Friesians that ovulate follicles much less than 45mm (and rarely under 40mm). Their follicles can develop to 60 mm before ovulation - without anything abnormal going on.
The other thing that we commonly see is that the development and reduction of oedema is much slower and more pronounced-i.e.. I rarely inseminate Friesian mares with a lot of uterine oedema but rather wait until the oedema is going (they normally ovulate 1-2 days after the oedema starts decreasing). If the problem is shipment of semen with a distance of more than a day, then I would still wait for a 45mm follicle and very good uterine oedema and give hCG. If you are used to warmbloods/thoroughbreds, it is difficult to make yourself wait long enough with the Friesians before inseminating.
I advise most of our students that if they don't have any experience with Friesians they should delay insemination until they really don't dare wait any longer - and then wait one more day!"
Excerpts
Palpable and ultrasonographic signs of impending ovulation Transrectal palpation and ultrasonography are of course the commonest aids to determining when a mare should be inseminated. changes in uterine and cervical tone, uterine oedema and in follicle size, hardness and ultrasonic appearance can be used to help identify approaching ovulation. Certainly, uterine oedema is a good indicator that a mare is in heat, and usually reaches a peak approximately 2-3 days, and begins to wane around 24-48h, before ovulation.