1ST POSTPARTUM LUTEAL FUNCTION IN DAIRY-COWS AFTER OVULATION INDUCED BY PROGESTOGEN AND GONADOTROPIN-RELEASING-HORMONE

The objective was to determine the effects of progestogen treatment on the lifespan of the first corpus luteum induced by GnRH in periparturient-milked cows. Dairy cows (n = 55) were assigned randomly following normal parturitions to receive either a progestogen implant (6 mg of norgestomet) or a blank implant (control) for 6 d beginning 2 to 5 d after calving. Fifty micrograms of GnRH were administered i.m. 72 h after implant removal to induce ovulation. Concentrations of LH and FSH in serum from 24 to 30 h and from 66 to 72 h after implant removal were similar among treatments. The magnitude of LH released after GnRH injection was higher in progestogen-treated cows (7.6 +/- .9 ng/ml) than in controls (5.3 +/- .9 ng/ml). Concentrations of estradiol in serum from the beginning of the implant period until 3 d after GnRH injection were higher in cows receiving progestogen (9.1 +/- 1.7 pg/ml) than in controls (5.6 +/- 1.7 pg/ml). The proportion of cows that responded to GnRH (elevated concentrations of progesterone in serum greater than .5 ng/ml within 3 to 5 d after injection) tended to be higher in cows receiving progestogen (24 of 28) than in control cows (19 of 27). By definition, interval to first ovulation after GnRH injection was shorter in cows responding to GnRH (3.8 +/- .5 d) than in those failing to respond (20.2 +/- .9 d). Fewer cows had short (3 to 10 d) luteal phases when treated with progestogen implants (3 of 22) when compared with those of controls (7 of 19). Concentrations of 13,14-dihydro-15-keto-prostaglandin F2-alpha in serum were lower in progestogen-treated cows than in controls for 12 d after GnRH injection. We conclude that progestogen treatment had a priming effect on hypothalamic-pituitary function resulting in increased LH release after GnRH injection. These progestogen-mediated changes in LH secretion followed increased secretion of estradiol by preovulatory follicles and resulted in decreased concentrations of prostaglandin F2-alpha metabolite in periparturient milked cows that was associated with prolonged postovulatory luteal function.