Producer Question from 2012
Q: What does a GnRH injection do for the cow at the time of CIDR insertion? (May 2, 2012)
A: GnRH will cause the release of LH (luteinizing hormone) from the anterior pituitary. Examples of GnRH that producers would use in an estrous synchronization program would be Cystroelin, Factrel, Fertagyl, and Ovacyst. GnRH causes a surge of LH within 2 to 4 hours after injection.
If the cow or heifer have large follicles (at least 10 mm in diameter) present on the ovary, GnRH will induce ovulation of these follicles about 24 to 30 hours after GnRH injection. This ovulation will occur without you seeing these females showing signs of estrus or heat because the corpus luteum is producing progesterone and/or you have a CIDR inserted keeping the female from showing the visual signs of heat.
GnRH in the synchronization protocol allows you to control what's going on with the follicles on the ovary and cause follicles to "turn over".
For more information, please see our Estrous Cycle Learning Module or the UNL Extension publication Synchronizing Estrus in Beef Cattle, EC283 (PDF 1.18MB).
Dr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska–Lincoln, Lincoln, NE