Short-term relationships between plasma LH, FSH and progesterone concentrations in post-partum dairy cows and the effect of Gn-RH injection

Abstract

Jugular venous blood samples were obtained from 7 dairy cows every 10 min for 10-19 h during the early- or mid-luteal phase of the estrous cycle, and each cow was given 1 or 2 i.v. injections of 100 .mu.g synthetic Gn-RH [gonadotropin-releasing hormone, gonadoliberin]. Of these cows, 4 were also sampled in a different cycle with no treatment being administered. Peaks of plasma LH [luteinizing hormone lutropin], FSH [follicle stimulating hormone, follitropin] and progesterone were detected in each animal in the absence of treatment; those of LH and progesterone often occurred in parallel. Injection of Gn-RH was always followed by a significant increase in plasma LH and progesterone concentrations and in most cases by a significant FSH increase. There was a significant temporal relationship between the peaks of all 3 hormones. A further 8 cows were sampled during the first 10 days postpartum when the mean plasma progesterone concentration was low. An i.v. injection of 200 .mu.g synthetic Gn-RH was given to each animal causing a significant increase in plasma LH and FSH concentrations, but in only 1 cow was the Gn-RH injection followed by a significant increase in plasma progesterone concentration. The LH response to Gn-RH injection was significantly less in cows injected on or before day 5 postpartum than in cows injected on days 7-10 postpartum.