Progesterone is secreted by the CL and prevents estrus (sexual receptivity). When an exogenous progestin is administered for a period longer than the luteal phase of the estrous cycle (cow: 14 15 days), the CL regresses spontaneously and estrus is prevented by the exogenous progestin until treatment ends. In females whose CL regresses before progestin withdrawal, ovarian follicles continue to grow. One follicle often reaches greater than normal diameters (known as persistent follicle)[1] because the dose of progestin is inadequately low in many of the market-available progestins. Upon progestin withdrawal, oocytes shed from a persistent follicle usually are fertilized, but most embryos fail to develop, resulting in compromised fertility.

If the progestin treatment is shorter in duration, some females may have a functional CL upon progestin withdrawal and estrus will be delayed until after luteolysis occurs. Although a shorter duration of treatment is less effective in synchronizing estrus, fertility is less compro mised because persistent follicles are not formed during progestin treatment.

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