What does a GnRH shot do for the cow at the time of CIDR insertion?

Gonadotropin-releasing hormone (GnRH) will cause the release of LH (luteinizing hormone) from the anterior pituitary within 2 to 4 hours of treatment. Luteinizing hormone stimulates ovulation of a large follicle (at least 10 mm in diameter) if present on the ovary. Treatment with GnRH will induce ovulation of these follicles about 24 to 30 hours after GnRH injection. This ovulation will typically occur without signs of estrus or heat because the corpus luteum is producing progesterone. Use of GnRH in a synchronization protocol is designed to program follicle growth so that a newly recruited dominant follicle is present on the ovary at the conclusion of the synchronization protocol which improves the synchrony of estrus and conception rate.

Examples of GnRH that producers would use in an estrus synchronization program would be Cystroelin, Factrel, Fertagyl, and Ovacyst.