The effect of GnRH at time of insemination on initiation of LH pulses and subsequent progesterone1

Research has indicated that luteinizing hormone (LH) pulses play a vital role in corpus luteum (CL) formation and subsequent progesterone concentrations. Therefore, our objectives were to determine: 1) when LH pulses begin following onset of estrus, 2) the effect an injection of gonadotropin releasing hormone (GnRH) would have on initiation of LH pulses, and 3) the effect LH pulse initiation had on subsequent plasma progesterone concentrations. Cows were synchronized with the Select Synch + Controlled Internal Drug Releasing device (CIDR) protocol (d -7 100 μg GnRH and CIDR; d 0 25 mg prostaglandin (PG) and removal of CIDR; estrus detected with HeatWatch). Following detection in estrus, a jugular catheter was inserted in each cow (n = 10). Based on initiation of estrus, cows were allotted into two treatments: 1) GnRH given 12 h (12.5 ± 1.2 h) after the initiation of estrus (n = 5; 100 μg) and 2) Control (n = 5). Blood samples were collected at 15-min intervals for 6 h at 12 h (bleed 1), 26 h (bleed 2), 40 h (bleed 3), 54 h (bleed 4), and 68 h (bleed 5) after the onset of estrus. The interval from onset of estrus to bleed 1 and ovulation was similar between treatments. The GnRH cows tended to have a greater area under the LH curve for bleed 1 compared to control cows. No differences were detected in bleeds 2, 3, 4, or 5. Average concentration of LH for GnRH cows in bleed 1 tended to be greater than control. No differences were detected in bleeds 2, 3, 4, or 5. No differences were detected in pulse frequency between treatments in bleeds 1, 3, 4, or 5, but in bleed 2, control tended to have more pulses than GnRH (2.5 ± 0.5 vs 1.4 ± 0.4). The GnRH-treated cows tended to have greater subsequent progesterone concentrations; however, GnRH-treated cows that had no LH pulses during bleed 2 had lower progesterone concentrations than cows with pulses (control or GnRH). In summary, injecting cows with GnRH approximately 12 h after the onset of estrus tended to reduce LH pulses 26-32 h following initiation of estrus, and elimination of LH pulses between 26-32 h resulted in decreased concentrations of progesterone during the subsequent cycle.