The estrous cycle in mammals encompasses hormonal changes associated with the development of a Graafian follicle(s), ovulation, and CL formation. In domestic mammals (e.g., pig, cow, sheep, and goats) day 0 of the estrous cycle is defined as the first day the female is sexually receptive. The cycle consists of a follicular phase and a luteal phase (Fig. 1). The follicular phase is short, occurring several days before estrus, and is dominated by increasing concentrations of estradiol-17p being produced by the rapidly growing ovarian follicles. The luteal phase is longer and is dominated by progesterone secreted by the CL. The ovulatory cycle of poultry has only a follicular phase and is terminated by ovulation of the largest preovulatory follicle, which is oviposited (laid) about 24 hours later.
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