The oestrous cycle in Friesian mares

This study aimed to describe the characteristics of the oestrous cycle in the Friesian mare, and to produce a guideline for veterinarians responsible for reproductive management of mares from this breed. The description of the oestrous cycle included the following parameters: the interovulatory interval (IOI), follicle diameter, follicle softness, uterine tone, cervical tone, endometrial oedema and free fluid. Also analysed were the number of artificial inseminations (AI) per cycle, any hormones administered, the age and reproductive status of the mare and the season in which the mare was monitored. The data were collected from three different clinics or stud farms in the Netherlands and included 687 cycles from 340 mares, aged between 3 and 21 years. Multiple cycles in one part of season were averaged into a single representative cycle for each individual mare. Cycles that were altered by the use of hormones were grouped and analysed separately. In non-manipulated oestrous cycles, an IOI of 24.46 ± 0.32 days was recorded. The IOI was not significantly affected by season (spring, summer), reproductive status (barren, maiden or lactating) or mare age. The maximum follicle diameter was 5.18 ± 0.04 cm at approximately 48 hours prior to ovulation. Growth rate of the pre-ovulatory follicle was 0.242 ± 0.08 cm per day until day -1 (day 0 = day of ovulation). Cessation of follicle growth was a good indicator of impending ovulation within 24 hours. Uterine oedema began to increase on day -8 and peaked on day -6. In conclusion, the reproductive cycle of Friesian mares is characterized by a relatively long IOI, large follicle diameter and early peak in uterine oedema compared to other horse breeds. The signs of an impending ovulation within 48 hours, and therefore for AI with fresh or chilled semen, were: a softening pre-ovulatory follicle of >4.0-4.5 cm with a thick follicular wall, allied to decreasing uterine oedema