Gonadotropin-releasing hormone at estrus: luteinizing hormone, estradiol, and progesterone during the periestrual and postinsemination periods in dairy cattle. Biol Reprod

Administering gonadotropin-releasing hormone (GnRH) improved conception rates in our previous studies. Our objective was to determine if the effect of GnRH was mediated through serum luteinizing hormone (LH) and/or by altered secretion of serum progesterone (P) and estradiol-1 7f3 (E) during the periestrual and post-insemination periods. Cattle were given either GnRH (n=54) or saline (n_-r55) at 72 b and inseminated artificially (Al) 80 b after the second of two injections of either prostaglandin F2a or its analog, cloprostenol. Progesterone and E were measured in blood serum collected during 3 wk after A! (estrus) from 60 females. Blood was collected for LH determinations via indwelling jugular cannulae from 14 cows and 11 beifers. Collections were taken every 4 b from 32 to 108 b after the second PGF injection (PGF-2) (periestrual period) and at more frequent intervals during 240 mm after administration of GnRH (n=18) or saline (n=7). Ten females bad a spontaneous preovulatory LH surge before GnRH treatment (GnRH-spontaneous), whereas GnRH induced the preovulatory LH surge in six females. A spontaneous LH surge appeared to be initiated in two beifers at or near the time of GnRH treatment (spontaneous and/or induced). The remaining seven cows bad spontaneous LH surges with no subsequent change in LH after saline treatment. Serum P during the 21 days after estrus was lower (p<O.05) in both pregnant and nonpregnant (open) cattle treated previously with GnRH compared with saline. Serum