The estrous cycle in cows is defined as the number of days from one period of sexual receptivity (i.e. estrus or heat) to the next. In cows, the estrous cycle is typically 21 days but can as short as 17 and as long as 24 day. The estrous cycle begins with the expression of estrus by the bovine female. This period of sexual receptivity typically lasts 6-24 hours. Estrus behavior in cows is stimulated by the production of estrogen secreted from a large follicle. Follicles are the fluid-filled, balloon-like structures on the ovaries that contain the oocyte (egg). The large amount of estrogen produced by the follicle also stimulates a massive release of a hormone called luteinizing hormone (LH). This LH surge generally occurs at the same time as the onset of standing estrus, and stimulates the rupture of the large follicle (termed ovulation) and release of the oocyte (egg). Ovulation of the large follicle occurs approximately 24-30 hours after the onset of estrus. During ovulation, the oocyte is released, the follicle wall collapses, and a corpus luteum (CL or yellow body) is formed. After ovulation, the CL increases in size and increases its production of the hormone progesterone. Progesterone inhibits final maturation of ovarian follicles and estrus behavior. Approximately 16-17 days after estrus, the uterus releases a hormone called prostaglandin that stimulates regression of the CL and a reduction in progesterone production. Declining progesterone production allows final maturation of a large follicle that stimulates the subsequent estrus. Like the CL, follicles also grow and regress throughout the estrous cycle. Follicle growth in cattle occurs in a wave-like pattern and females generally have either two or three waves of follicular development during an estrous cycle. Emergence, growth and regression of a follicular wave generally take 8-10 days.