Effect of Days Postpartum and Endogenous Reproductive Hormones on GNRH-Induced LH Release in Dairy Cows

Abstract

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 /μg 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 postpartum 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 postpartum to 3.5 ± .6 ng/ml at 18 or 19 days postpartum. Preinjection estradiol-17ß concentrations tended to increase with days postpartum (r = .31; P<.07), however, plasma progesterone did not change during the postpartum 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). Combining all treatment groups, peak LH concentrations increased with days postpartum (r = .61; P<.01), increasing preinjection estradiol-17ß concentrations (r = .64; P<.01) and increasing preinjection LH concentrations (r = .50; P<.01). Preinjection plasma progesterone concentrations were not related to peak LH levels (r = .09; P>.10). The inclusion of both days postpartum and preinjection estradiol-17ß in step-up multiple regression significantly improved the coefficient of determination (R2 = .59) in comparison to either variable alone. Preinjection LH and progesterone, however, did not significantly improve the model.