Resynchronized Ovulation in Lactating Dairy Cattle of Unknown Pregnancy: Occurrence and Timing of Gonadotropin-Releasing Hormone 1

The objectives were to determine if the first gonadotropin-releasing hormone (GnRH) injection of a resynchronization protocol is essential to achieve acceptable fertility and if the timing of the standard second GnRH injection would improve pregnancy rates per AI (P/AI) when administered at 56 versus 72 h after prostaglandin F 2α (PGF 2α ). Lactating dairy cows of unknown pregnancy status (n = 763) were treated with either GnRH or saline to initiate ovulation resynchronization, which was completed 7 d later when cows were diagnosed as nonpregnant. Within treatments, nonpregnant cows were administered PGF 2α and then were either injected with GnRH 56 h later and inseminated 16 h later or injected and inseminated concurrently at 72 h. Injection of GnRH at 56 h produced more (P < 0.001) P/AI than injection at 72 h when AI was administered at 72 h (29.9 vs. 14%, respectively). Further, beginning the resynchronization with GnRH improved the resulting P/AI but was time dependent. When the nonpregnant status was determined between d 29 and 35 after previous AI, the GnRH injection (7 d before a nonpregnant diagnosis) did not increase the resulting P/AI compared with saline (27.4 vs. 24.8%, respectively). In contrast, when a nonpregnant diagnosis occurred after d 35 and GnRH was given 7 d earlier, P/AI were greater (P = 0.036) with GnRH than with saline (32.4 vs. 15.6%, respectively). Injection of GnRH 7 d before a pregnancy diagnosis in pregnant cows had no effect on subsequent pregnancy loss. Administering the second GnRH injection at 56 h rather than 72 h after PGF 2α, with AI at 72 h, improved the pregnancy outcome. The first GnRH injection of the ovulation resynchronization program did not improve the pregnancy outcome when the nonpregnant diagnosis was determined before d 35, but after d 35, the first GnRH injection improved the resulting fixed-timed P/AI compared with saline.