The length of the estrous cycle in dairy cattle is 18 to 24 days, with an average of approximately 21 days. As the nonpregnant cow nears the end of one estrous cycle, the preovulatory follicle (which contains the ovum, or egg), grows larger and produces increasingly more estradiol. Estradiol (a steroid ho-mone belonging to the estrogen family) plays an important role in reproduction, as it acts on the brain to induce behavioral estrus (heat), and sets into motion the preovulatory surge of luteinizing hormone (LH) from the anterior pituitary gland, which begins the process of ovulation.
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