The aim of this study was to examine the effect of a single administration of human chorionic gonadotrophin (hCG) during the establishment of the corpus luteum (CL) on progesterone (P4) concentration and pregnancy per artificial insemination (P/AI) in lactating dairy cows. Postpartum spring-calving lactating dairy cows (n = 800; mean ± SD days in milk and parity were 78.5 ± 16.7 and 2.3 ± 0.8, respectively) on 3 farms were enrolled on the study. All cows underwent the same fixed-time AI (FTAI) protocol involving a 7-d progesterone-releasing intravaginal device with gonadotrophin-releasing hormone (GnRH) administration at device insertion, prostaglandin at device removal followed by GnRH 56 h later, and AI 16 h after the second GnRH injection. Cows were blocked on days postpartum, body condition score, and parity and randomly assigned to receive either 3,000 IU of hCG 2 d after FTAI or no further treatment (control). Blood samples were collected on d 7 and 14 postestrus by coccygeal venipuncture on a subset of 204 cows to measure serum P4 concentration, and pregnancy was diagnosed by ultrasonography approximately 30 and 70 d after FTAI. Administration of hCG caused an increase in circulating P4 concentrations compared with the control treatment on d 7 (+22.2%) and d 14 (+25.7%). The P/AI at 30 d after FTAI was affected by treatment, farm, body condition score, and calving to service interval. Overall, administration of hCG decreased P/AI (46.3% vs. 55.1% for the control). Among cows that did not become pregnant following AI, a greater proportion of control cows exhibited a short repeat interval (≤17 d) compared with cows treated with hCG (8.6% vs. 2.8%, respectively). In addition, the percentages of cows pregnant at d 21 (59.6% vs. 52.0%) and d 42 (78.3% vs. 71.9%) were greater in control than in hCG-treated cows. The overall incidence of embryo loss was 10.7% and was not affected by treatment. There was a tendency for an interaction between treatment and CL status at synchronization protocol initiation for both P4 concentration and P/AI. In conclusion, administration of hCG 2 d after FTAI increased circulating P4 concentrations. Unexpectedly, cows treated with hCG had lower fertility; however, this negative effect on fertility was manifested primarily in cows lacking a CL at the onset of the synchronization protocol.
Food Quality & Sensory Science