LLEP Stud Services

THE NORMAL MARE REPRODUCTIVE CYCLE

The mare has what is known as a seasonally polyoestrus reproductive pattern. This means that she only cycles for part of the year but has multiple cycles within this period. The natural season for mares in the northern hemisphere is March to September. They start cycling in the spring in response to the increasing daylight and then will go through what is called a transitional period.

In this period, which may last from a few weeks to several months, the mare will have several waves of growing follicles where she will show the typical oestrus behaviour. Follicles are fluid filled structures on the ovary that contain the oocyte (egg). The oocyte is released from the follicle it grows large enough to rupture. This is called ovulation. During this transition period no large dominant follicle is produced and so ovulation does not take place. Thus mares may be served unnecessarily in this time. The ultrasound is useful here to visualise the ovaries and confirm ovulation before mating commences.

The oestrus cycle in mares is normally 21 days, but may vary from 18-24 days. Within this cycle there is two phases. Dioestrus is the progesterone dominated phase of the cycle which starts after ovulation of the dominant follicle and lasts for 14-15 days. Dioestrus is characterised by unwillingness to stand for the stallion (Striking, rearing, squealing, biting, kicking, tail swishing), a Corpus luteum (area where ovulation occurred) present on the ovary and a firm uterus with no oedema, and a closed Cervix.

Around day 16 the oestrus phase begins and lasts for 5-7 days completing the 21 day cycle. In this phase regression of the corpus luteum brings about a wave of follicular growth, followed by dominance of one (occasionally two) follicles which grows bigger than the rest and goes onto ovulate. Oestrus is characterised by willingness to be served by the stallion (frequent urination/squatting clitoral eversion (“winking”), Standing still and a Raised tai. On the ultrasound we will see a large Follicle 30-40 mm +, a flaccid uterus with oedema and relaxed and open cervix.