LH RELEASE IN RESPONSE TO GnRH D U R I N G THE POSTPARTUM PERIOD OF D A I R Y COWS 1

Postpartum cows (n=25) were assigned randomly to receive an intramuscular injection of gonadotropin releasing hormone (GnRH; 100 /~g) at either 3, 10, 20, 30 or 40 days postpartum (five cows per group). Blood samples were collected via jugular catheter at lO-min intervals from 3 0 to +180 rain and at 30-min intervals from +180 to +420 min after GnRH. Plasma levels of luteinizing hormone (LH), estrone (El) , 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~.O1) and orthogonal comparisons indicated that day 3 release of LH was less than on days 10, 20, 30 and 40 (P<.01). Furthermore, release of LH on day 10 was less than on days 20, 30 and 40 (P<.05). Stepwise regression analyses of pretreatment hormone levels (LH, El , 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 E 2 and peak LH were +.57 and +.67, respectively. Mean pretreatment levels of LH on day 3 were lower (P<.01) than levels on 10, 20, and 40 days postpartum (.32 vs .91 ng/ml). In summary LH release in response to GnRH treatment was not restored fully until after 10 days postpartum. (