Cows will have estrous cycles at regular intervals until they conceive. Estrus is controlled by a complex interaction of several hormones. Heat (heat = estrus = a time when the female is receptive to the bull) is stimulated by the hormone estrogen which is secreted by a large follicle on the ovary. Estrogen also stimulates the rupture of the follicle and release of the egg. After the follicle ruptures, a corpus luteum is formed. The corpus luteum produces a hormone called progesterone which prevents estrus. If the cow does not conceive, the corpus luteum regresses, progesterone concentrations decline, and the female comes into estrus again. Estrous cycles occur about every twenty-one days.

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