GnRH induced LH release in suckled beef cows. 1. The effects of days postpartum and estradiol-17β concentrations on the release of LH following administration of GnRH

Abstract

Twenty suckled Charloix × Hereford beef cows (5 cows/group) were assigned at random to receive 100 μg GnRH (IM) at either 2 to 3, 7 to 8, 15 to 16, or 31 to 32 days postpartum, Groups 1 through 4, respectively. Blood samples for hormone determinations were collected at time 0 (pre-GnRH), every half hr for 3 hr, and at 4.0 hr and 6.0 hr post-GnRH. Mean plasma LH, estradiol-17β, or progesterone concentrations were not different among groups prior to GnRH. Plasma LH increased (P < .05) following GnRH in Groups 2, 3 and 4, but not in Group 1. Peak GnRH induced LH release was greater (P < .05) in Groups 3 and 4 than in Groups 1 or 2. Correlation coefficients between days postpartum and peak LH release (r = .72), and estradiol-17β concentrations and time of LH peak (r = −.42) were significant (P < .05). These data indicate that LH release in response to GnRH, in suckled beef cows is not fully restored until 15 to 16 days postpartum.