Presynchronization with Gonadotropin-Releasing Hormone Does Not Improve Fertility in Holstein Heifers

Holstein dairy heifers were randomly assigned to 1 of 2 treatments to determine whether a presynchronization strategy using GnRH would improve reproductive performance after synchronization of ovulation and timed artificial insemination (TAI). Heifers (n = 82) in the first treatment (GPG) received a hormonal protocol for synchronization of ovulation and TAI (100 μg of GnRH, d 0; 25 mg of PGF2α, d 6; 100 μg of GnRH + TAI, d 8); whereas heifers (n = 84) in the second treatment (GGPG) received GPG, but with the addition of a GnRH injection (100 μg) 7 d before initiation of the GPG protocol. The proportion of heifers receiving AI before d 8 was similar for GPG (4.9%) and GGPG (9.5%), and the proportion of heifers diagnosed pregnant 30 d after AI did not differ between treatments (45 vs. 51%, respectively). Treatment did not affect the proportion of heifers ovulating after the second GnRH injection or the proportion in which a corpus luteum regressed after treatment with PGF2α. In conclusion, presynchronization with GnRH 7 d before initiation of synchronization of ovulation using GnRH and PGF2α did not affect the proportion of heifers expressing estrus before TAI or improve synchronization response or fertility to the synchronization protocol.