The CL secretes progesterone for about two weeks, whether or not mating has occurred and whether or not pregnancy has occurred. The declining progesterone allows another follicle to develop, another estrous cycle to occur, and, ultimately, another opportunity to establish pregnancy. When PGF production by the uterus is suppressed, then the CL can continue to secrete progesterone to support the growth and development of the conceptus. If PGF is produced inappropriately during pregnancy, or if the conceptus is not able to suppress it sufficiently, then the pregnancy will be lost. Conversely, if PGF is not produced in the right amounts and the right time at the end of the estrous cycle, then the CL is maintained inappropriately and there is a delay to the next ovulation.
Time is lost before the mare can be bred again.
In sheep and cattle, at the end of the estrous cycle, the CL produces the hormone oxytocin. Uterine PGF, in turn, stimulates the CL to produce more oxytocin. Studies in sheep have shown that if oxytocin production from the CL is blocked, then luteal regression is delayed and there is a subsequent delay in the amount of time it takes for the ewes to return to estrus. In addition, it is believed that the sheep or cow conceptus is able to uncouple the positive feedback loop between luteal oxytocin and uterine PGF in order for maternal recognition of pregnancy to occur. The CL of the mare produces little, if any, oxytocin.