| Abstrakt: |
Anestrous postpartum (PP) Hereford cows (n =20) were used to determine the effects of repeated injections of human chorionic gonadotropin (hCG) on the progesterone (P4) secretion and functional lifespan of gonadotropin-releasing hormone (GnRH)-induced corpora lutea (CL). Suckling was reduced to once a day from Day 21 to Day 25 PP, and all cows received injections of 200 µg GnRH at 1500 h on Day 24 PP to induce ovulation. Treated cows (HCG, n = 10) received 200 IU hCG b.i.d. from 1900 h on Day 27 PP to 1900 h on Day 33 PP; control cows (CTRL, n=10) were not injected. Blood was collected on Days 21, 23, 25, and 27 to 33, 35, 37, and 39 PP. Serum P4concentration was measured by radioimmunoassay and used to classify luteal lifespan and the associated estrous cycle as short (SHORT) or normal (NORM) in duration. Treatment with hCG resulted in more (p<0.01) cows with SHORT cycles (7 of 9 vs. 4 of 9). Serum P4concentrations were similar (p>0.20) between groups from 4 days before until 6 days after GnRH injection. Cows with NORM cycles (n = 7) had greater serum P4concentrations (p<0.05) on Days 7 to 11 after GnRH than cows with SHORT cycles (n = 11). By Day 39 PP, all cows with SHORT cycles appeared to have undergone a second ovulation. Charcoal-stripped serum pools from before (PRE) and during hCG injection (INJ) were assayed for total luteinizing hormone-like bioactivity (LH-BA) using a dispersed mouse-Leydig cell bioassay. HCG-INJ sera had approximately 10 times greater LH-BA than HCG-PRE, CTRL-PRE, and CTRL-INJ sera (p<0.001). Twice-daily injections of hCG on Days 1 to 7 after GnRH-induced ovulation in PP beef cows increased serum LH-BA but did not stimulate additional P4secretion and appeared to increase, not decrease, the incidence of SHORT cycles. Reduced serum LH-BA during the early luteal period does not appear to be the cause of SHORT cycles following GnRH-induced ovulation in PP cows. |
