J. Anim Sci.
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J. Anim Sci. 1978. 46:443-448.
© 1978 American Society of Animal Science
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LH Release in Response to GnRH during the Postpartum Period of Dairy Cows1

L. C. Fernandes2, W. W. Thatcher2, C. J. Wilcox2 and E. P. Call3

University of Florida,2, Gainesville 32611 and Kansas State University,3, Manhattan 66502

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 (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, E1, 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 (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.

Footnotes

1 Florida Agricultural Experiment Station Journal Series No. 478.

2 Dairy Science Department.

3 Animal Science Department.






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