Transitory Changes of Hormones in Plasma of Postpartum Dairy Cows

Changes in concentrations of luteinizing hormone, estradiol-17fl, and progesterone in blood plasma of 10 dairy cows were measured hourly for 10 h by radioimmunoassay on days 2 or 3, 7 or 8, and 12 or 13 postpartum.Comparisons were between mean hormone concentrations and hormone activity on each collection day postpartum.Hormone activity was expressed both as the maximum change in hormone concentrations during the collection period and as an activity index which included the magnitude, frequency, and duration of changes during each daily collection.Mean concentrations of luteinizing hormone in plasma increased from post-partumdays2or 3 to 7 or 8 to12 or13 as did the daily maximum change and activity index.Higher activity indexes for luteinizing hormone on days 7 or 8 and 12 or 13 as compared with days 2 or 3 postpartum were due primarily to increases in the magnitude and frequency of changes among individual cows whereas the increase in activity on days 12 or 13 as compared with days 7 or 8 was due to increased frequency.The magnitude of fluctuations of luteinizing hormone was variable, and changes normally persisted for only one hourly collection.Concentrations in plasma and activity of estradiol-17/3 for individual cows dur-