Pituitary response to exogenous GnRH on day 7 postpartum in high-producing dairy cows

To examine the pituitary function in the release of luteinizing hormone (LH) in response to gonadotropin-releasing hormone (GnRH) in postpartum dairy cows, 22 Holstein-Friesian cows were intramuscularly given 50 mu g of fertirelin acetate, GnRH analog, on day 7 postpartum. Blood samples were taken 60 min and immediately before, and 60, 120 and 300 min after the GnRH injection. Parity, induced parturition, dystocia, retained placenta, milk yield, energy balance and body-condition score (BCS) were also studied for their possible influences on LH release after GnRH administration. Of 22 cows, 12 (55%) responded well with a five-fold increase in LH after the GnRH injection. The other 10 (45%) showed a poor LH response (less than four times the basal level in seven cows), and there was no response in three cows. The change in body-condition score (BCS), which occurred between days 7-10 antepartum and day 7 postpartum, and milk yield are involved in the regulation of the pituitary function on day 7 postpartum. Thus, about half the cows at 7 days postpartum had an insufficient resumption of the pituitary function, and it was thought that some nutritional factors may have affected the pituitary function in early postpartum dairy cows.