The basic estrous cycle of the cow averages 21 days in duration (a range of 18 to 24 days) and extends from one period of heat (estrus) to the next. There are four parts to the estrous cycle. Proestrus lasts for one to three days immediately before the cow comes into estrus or heat. Metestrus is the three to four days after estrus, and diestrus is the 12 to 15 days in the middle of the cycle.
During proestrus, the pituitary gland of the brain secretes the follicle stimulating hormone (FSH) that causes the ovary to develop follicles. Several follicles develop in what is termed follicular waves, but only one dominant follicle and egg mature for ovulation (rupture of the follicle and release of the egg).
As the follicle develops, estrogen is secreted by the ovary. The egg grows larger as more estrogen is secreted by the ovary. Estrogen causes estrus (“heat”) and contractions of the uterus to aid in transport of sperm.
The pituitary gland of the brain secretes lutenizing hormone (LH) usually in a pulsatile rhythm during the estrous cycle. High estrogen from the ovary causes an LH surge or spike that causes the ovarian follicle to rupture and release the egg. During metestrus, the site of where the follicle ruptured becomes a corpus luteum (CL) that secretes progesterone (the calming hormone) to inhibit estrus and, if the cow is pregnant, maintains pregnancy. Some cows may have a bloody vaginal discharge during metestrus indicating they were in heat a day or two earlier. Once the CL matures, the cow is in diestrus. If the uterine wall does not get a signal by days 16 to 18 of the estrous cycle indicating that a fertilized egg has been implanted in the uterine wall, the hormone prostaglandin is secreted from the uterus, and the CL regresses between days 18 and 20 to complete the reproductive cycle.