Effect of GnRH Injection at - 72 h in MGA- PGEstrus Synchronization Protocol

Yearling beef heifers from 2 locations were synchronized with melengestrol acetate (MGA)- prostaglandin F2α (PGF) fi xed time AI (TAI) protocol. At PGF administration 72 h before AI, heifers were randomly assigned to receive either 0 or 5 μg gonadotropinreleasing hormone (GnRH). Th e administration of 5 μg GnRH at PGF did not increase estrus activity or improve TAI pregnancy rates at either location (Location 1, 56% (GnRH) vs. 57%; Location 2, 59% (GnRH) vs. 53%). Administering GnRH at PGF increased (74% vs. 63%) pregnancy rates for heifers inseminated during a follow- up heat detection period at one location. A low dose of GnRH administered 72 h prior to AI in a 14 d MGA- PGF synchronization protocol does not increase pregnancy rates or estrus expression in yearling, beef females bred with TAI when compared to the normal MGAPGF synchronization protocol