Effect of days postpartum and endogenous reproductive hormones on GNRH-induced LH release in dairy cows.

Forty-eight Holstein and Guernsey cows were assigned according to age and breed to one of seven groups based on days following parturition. Cows in Groups II to VII received an intramuscular injection of 100 ~tg gonadotropin releasing hormone (GnRH) on either 1 or 2, 3 or 4, 5 or 6, 7 or 8, 12 or 13, or 18 or 19 days postpartum, respectively (six cows per group). An injection of the carrier vehicle for GnRH was given to cows in Group I (controls), which included two cows per each postpar tum treatment period (12 cows). Prior to treatment, plasma LH increased (P<.05) from a mean concentration of 1.1 + .1 ng/ml at 1 or 2 days postpar tum to 3.5 -+ .6 ng/ml at 18 or 19 days postpartum. Preinjection estradi01-17/3 concentrations tended to increase with days postpar tum (r = .31; P<.07), however, plasma progesterone did not change during the postpar tum interval studied. Following treatment, plasma LH did not increase (P>.05) for cows in Groups I, II, III and IV but was significantly increased in cows of Groups V, VI and VII (P<.05). Peak LH concentrations following GnRH were higher (P<.05) for cows in Groups V to VII (14.1 + 2.7, 11.2 + 2.1 and 13.6 + 2.3 ng/ml, respectively) than those in Groups II to IV (3.1 +1.4, 2.8 + .8 and 4.2 + .6 ng/ml, respectively).