LH Release in Response to GnRH during the Postpartum Period of Dairy Cows

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Abstract
Postpartum cows (n=25) were assigned randomly to receive an intramuscular injection of gonadotropin releasing hormone (GnRH; 100 µg) at either 3, 10, 20, 30 or 40 days postpartum (five cows per group). Blood samples were collected via jugular catheter at 10-min intervals from −30 to +180 min and at 30-min intervals from +180 to +420 min after GnRH. Plasma levels of luteinizing hormone (LH), estrone (E1), estradiol (E2), progestin (P4) and prolactin (Prl) were estimated by radioimmunoassay. Luteinizing hormone time responses to GnRH among groups were described by fifth order regression. Response curves among groups were not parallel (P<.01) and orthogonal comparisons indicated that day 3 release of LH was less than on days 10, 20, 30 and 40 (P1, E2, P4 and Prl) showed that only pretreatment E2 was associated appreciably with peak LH levels after GnRH in the postpartum cow (P<.01). Gross and within treatment correlations of pretreatment E2 and peak LH were +.57 and +.67, respectively. Mean pretreatment levels of LH on day 3 were lower (Pvs .91 ng/ml). In summary LH release in response to GnRH treatment was not restored fully until after 10 days postpartum. Copyright © 1978. American Society of Animal Science . Copyright 1978 by American Society of Animal Science.