Hormonal reasons for the states of breeding in the mare
Daylight periods are perceived by light receptors in the eye which results in an effect on the pineal gland that releases a hormone known as Melatonin. When Melatonin levels decrease, as the result of increased daylight hours, this results in the onset of oestrus, ie the breeding season. There are several hormones that become active to result in the oestrus period:-

Increased Melatonin levels effect the hypothalamus part of the brain which in turn results in the increased release (from the hypothalamus) of GnRH (Gonadotrophin Releasing Hormone).

GnRH affects the anterior pituitary gland to induce the release of FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone).

FSH & LH have an effect on GnRH and gonadotrophins and vicer versa during the oestrus cycle in a rhythmic fashion that repeats approximately every 22 days until the sequence is either interrupted by pregnancy, change in season or pathological reasons.

At the same time as the hormones released from the brain are being produced the ovary is also producing hormones in a similar pattern.

The ovary goes through several stages in the 22 day cycle:-

i)Growth of follicles in the ovary results in the release of follicular oestrogen
ii)Release of ovum on stimulation by LH which results in the development of the Corpus Luteum (Ovary post ovulation) and a reduction in follicular oestrogen.
iii)Growth of Corpus Luteum resulting in increased progesterone production.
iv) In the event of no pregnancy the Corpus Luteum begins to decline resulting in the reduction of progesterone.
v) The cycle begins again with the growth of follicles and the release of oestrogen.

The Oestrus cycle is divided up into two distinct phases; oestrus and dioestrus:-

Oestrus - this is the period (5-7 days) where the mare exhibits signs of receptivity to the stallion. Usually this behaviour begins to wane about the time of ovulation although some mares may carry on for a further 1-2 days post ovulation. Ovulation coincides with the LH peak rise resulting in increased follicular oestrogen and ovulation of one ovum from the ovary.
Dioestrus - this is the period where the mare is not receptive to the stallion. Her progesterone levels are high and the corpus luteum is in evidence. This period usually last 14-16days.

The end of one oestrus phase and the beginning of another is marked by ovulation and is often used as a reference point for relating various events during the cycle.
Ovulation is taken as day 0.