An understanding of changing hormonal patterns and the normal growth and regression of ovarian structures is necessary to understand how to control onset of estrus. Estrus behavior in cows is stimulated by the production of estrogen in 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 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 (a process called ovulation) and release of the egg.
Ovulation of the large follicle occurs approximately 24 to 30 hours after the onset of standing estrus (Figure 1). During ovulation, the egg is released; the follicle wall collapses and begins to form a corpus luteum (CL). After ovulation, the CL increases in size and increases its production of progesterone (Figure 1).
Progesterone inhibits final maturation of ovarian follicles and estrus behavior. Approximately 16 to 17 days after estrus, the uterus releases prostaglandin F2á (PG, which stimulates regression of the CL and a reduction in progesterone production. Declining progesteroneproduction allows final maturation of a large follicle,which stimulates the subsequent estrus.
Like the CL, follicles also grow and regress throughout the estrous cycle. Follicle growth in cattle occurs in a wavelike pattern, and females generally have either two or three waves of follicular development during an estrous cycle (Figures 2 and 3). Emergence, growth, and regression of a follicular wave generally take eight to 10 days.
Source: Kentucky Ag Extension