The Reproductive Cycle of the Mare

 

The reproductive or oestrus cycle in the mare is characterised by repeated periods of oestrus (or fertility) interrupted by periods of anoestrus (or infertility).  Horses are seasonally polyoestrus breeders.  This means that they have repeated oestrus cycles during a specific part of the year.  The mare’s oestrus cycle usually starts late in the spring, and lasts over the summer months into the early autumn. She will then stop cycling over the winter and early spring months; this is known as the period of anoestrus.  This is nature’s way of preventing a foal being born in the harsh winter months.
It is important to understand the mare’s oestrus cycle in order to maximise your chances of successfully breeding a mare.

The oestrus cycle is complicated and is regulated by the actions and interactions of a variety of different hormones in response to the changes in daylight hours as the seasons change.

The mare’s reproductive cycle normally last for an average of 21 days. She will be receptive to the stallion for approximately 5 days within this period and an egg will be released within 24 hours  prior to the end of this short time period.  This will be followed by 16 days of unreceptiveness if she is not bred and the egg that she has produced has not been fertilised. The cycle will then begin again.

Hormonal Control
Increased daylight hours in the spring and summer months are detected by the pineal gland. This gland is situated in the brain and secretes a substance called Melatonin.  High levels of Melatonin in the mare’s blood stream in the winter and early spring months inhibits the release of another hormone -  Gonadotrophin Releasing Hormone (GNrH), which prevents the mare from cycling.

As day length (also known as photoperiod) increases, the pineal gland reduces the amount of melatonin produced and in response GnRH is produced by the hypothalamus.  The hypothalamus is a small, but important part of the brain that controls the action of the Autonomic Nervous System as well as releasing hormones that control and regulate the secretion of other hormones from the pituitary gland. The pituitary gland is about the size of a pea and sits in a bony cavity at the base of the brain.  It produces a number of different hormones that all aid in maintaining homeostasis within the body.

 

Increased levels of GnRH stimulate the pituitary gland to produce Follicle Stimulating Hormone (FSH). FSH is released into the bloodstream and travels to the ovaries, where it stimulates them to produce eggs. Concentrations of FSH usually increase from about Day 15 of the mare’s cycle.  A number of follicles may be stimulated by the initial rise in FSH, but only one or two will develop to such a size (3cm +) that will react to the hormonal message to ovulate and subsequently produce an egg.

As the ovarian follicles develop, they secrete the hormone oestrogen. Oestrogen levels peak 24-48 hours before ovulation occurs and immediately returns to basal levels after ovulation.  Oestrogen causes the mare to exhibit the behavioural signs of oestrus. FSH levels and oestrogen levels rise at the same time, to ensure that maximum follicular development and the behavioural signs of oestrus are synchronised, thereby maximising the chance of successful breeding.

Increased oestrogen levels then begin to inhibit the production of FSH from the pituitary and stimulate the release of Leutinising Hormone (LH). LH is the hormonal trigger for ovulation to occur.  This burst of LH causes the follicle to mature and then to release an egg. LH reaches a peak just after ovulation and then returns to basal levels a few days after.

After the follicle has ruptured and released an egg, it fills with blood and clots to form the Corpus Luteum (CL). The tissue within the CL then produces another hormone – progesterone. Progesterone levels increase and reach a maximum around 4-5 days after ovulation and are maintained at this level until approximately days 15-16 of the mare’s cycle.

If the mare has not conceived, progesterone levels drop 3-4 days prior to her next ovulation. This drop is caused by the action of another hormone – prostaglandin. Prostaglandin is secreted by the mare’s uterus and levels rise 14-17 days after ovulation if the mare is not pregnant.  Prostaglandin causes the destruction of the CL, progesterone levels drop and the mare’s cycle starts over again. The ‘not-pregnant’ message is also thought to involve the release of another hormone – oxytocin.  Oxytocin is produced by the pituitary and travels to the uterus, where it enhances the release of prostaglandin. Oxytocin is also involved in instigating the birthing process as well as involved in lactation.

If the egg is fertilised when it enters the uterus, the mare recognises she is pregnant; the mechanism by which this occurs is not fully understood. The fertilised egg prevents the prostaglandin secretion from the uterus. The CL continues to secrete progesterone, which prevents the cycle from beginning again.

As the pregnancy continues the placenta forms, which starts to produce Pregnant Mare Serum Gonadotropin (PMSG). The combination of PMSG and progesterone act to maintain the pregnancy to term.