Increase in daylight results in decreased melatonin production Decreased melatonin allows hypothalamus to produce more GnRH GnRH in turn acts on the pituitary gland (anterior lobe) to release FSH & LH causing cyclical ovarian activity During the months of Nov-Jan mares are in the anestrous state. This is a period where no sexual activity takes place. Beginning in Feb, mares enter the spring transition or vernal transition phase. This is a transition from the anestrous state into estrous or breeding state. This phase lasts approx. 40-60 days and mares show cyclic ovarian activity but do not ovulate. After the first ovulation, mares will generally continue to have regular ovulatory cycles from April until October. This is the estrous state. In the fall there is an autumnal transition or fall transition from the breeding state into anestrous.
The Estrous State
Puberty: the age at which the reproductive organs become functional and the animal is considered fertile. The mean age in both mares and stallions is 18 months. It is not recommended that mares or stallions be bred until 3 years of age or older.
Estrous Cycle
This is the period from 1 ovulation to the subsequent next one. This period lasts between 20-22 days (avg. 21) depending on the individual mare. The estrous cycle can be split into 2 phases:
Estrus
Period of behavioural changes and sexual receptivity Lasts 5-7 days FSH stimulates initial follicular growth (day 1)
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