GnRH causes the pituitary gland to secrete both LH and FSH. These two naturally occurring hormones, in concert with other changing levels of hormones, cause the ovulation of the dominate follicle present on the ovary. GnRH can be used to treat cows with cystic ovarian disease or cystic ovaries and also is used in some estrous synchronization programs. Injections of GnRH help to synchronize the follicular waves that result in better control over the timing of ovulation. PFG is secreted by the uterus and regulates the lifespan of the corpus luteum (CL). It moves from the uterus directly to the ovaries without circulating in the venous blood through the heart and back out to the ovaries through the arterial blood. As PFG causes the regression of the CL, it also causes a decrease in the progesterone levels as progesterone is secreted by the active CL. PFG is used in the treatment of uterine infections, to cause abortions up to 100 days after breeding, expulsion of mummified fetus and for timing of breeding. Keep in mind that there must be a CL present on one of the ovaries in order for PFG to work. For instance, when a CL is present at the time PFG is given, the cow will usually come into heat within 2-5 days. If the PFG is given early in the estrous cycle before a CL is formed and functional, no induced heat will occur.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 0 Day of Cycle GnRH day 1 Luteinization and resets follicular waves Ovulation Or causes ovulation of larger follicles
PGF day 7 Progesterone Follicular wave 0 1 2 3 4 5 6 7 8 9 10 Day of Synchronization
0 1 2 3 4 5 6 7 8 9 10 Day of Synchronization PGF day 7 GnRH Day 9 Breed 16 Hours after GnRH GnRH day 1