The cow reaches puberty at 10 12 months of age. Its estrous cycle is 21 days in length. The follicular phase of the estrous cycle is 3 5 days, during which time multiple follicles begin developing, but only one Graafian follicle will develop to become the ovulatory follicles. During the luteal phase duration of 17 18 days, predominated by progesterone, there are 2 3 waves of follicular growth. Only as progesterone is decreasing does one of these follicles proceed to the final stages of maturation. The other follicles become atretic. The cow displays estrus for 12 to 24 hours with ovulation occurring about 30 hours after the onset of estrus.
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