Effect of variable suckling intensity and estrogen administration upon serum luteinizing hormone in Brahman cows

Ten mature Brahman cows were randomly allotted within calving intervals to either a suckled (S) or nonsuckled (NS) treatment group. All cows received a 20 mg intramuscular injection of estradiol-17beta (E2), suspended in 2 ml of corn oil, to determine the effect of suckling on the estrogen induced LH surge. Starting on day 21 postpartum the S cows were suckled at six hour intervals for 24 hours, at which time they were challenged with a 20 mg E2 injection. The suckling regimen was continued for 48 hours postinjection. The NS cows were separated from their calves on day 21 postpartum and received no suckling stimulus for 72 hours. At 24 hours after calf separation, the NS cows were challenged with a 20 mg E2 injection. Blood samples were removed at two hour intervals beginning 10 hours post E2 injection until 36 hours postinjection, at which time blood samples were removed at four hour intervals until 48 hours postinjection. Blood samples were processed to yield serum and assayed for luteinizing hormone (LH) via radioimmunoassay. The injection of a 20 mg dose of E2 induced an LH surge in all cows. The NS cows were found to exhibit a longer (P<.05) duration of the estrogen induced LH surge than the S cows, 15.6 +/- .98 and 12.4 +/- .75 hours, respectively. The timing parameters (time to start of LH surge, time to peak LH value and time to end of surge) and LH concentration parameters (LH concentration at start of LH surge, peak value of LH surge and LH concentration at end of LH surge) were not different between suckling regimens. No blockage of the LH response to estrogen challenge was found on day 22 postpartum. Suckling did depress the duration of the LH surge indicating some blockage due to suckling stimuli.