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Ovulation, Estrus and Endocrine Response after GnRH in Early Postpartum Cows¹

Michigan State University², East Lansing 48824

Abstract

Twenty lactating Holstein cows each received as a subcutaneous-implant in an ear, a No. 5 gelatin capsule containing gonadotropin releasing hormone (100 µg) or saline on day 14 postpartum. Serum luteinizing hormone (LH) increased from 1.9 ± 0.4 ng/ml at GnRH administration to 5.7 ± 1.3, 13.8 ± 2.2 and 15.0 ± 1.9 ng/ml at 1, 2 and 4hr. and declined to 4.7 ± 0.6 ng/ml at 6 hours. Serum LH was unchanged during 6 hr. after treatment of 10 cows with saline. One cow which was to receive GnRH had ovulated on day 9 postpartum; all others receiving GnRH (9) ovulated on day 15, 1 day after treatment. Interval to first ovulation postpartum for GnRH-treated cows was 14.4 ± 0.6 days compared to 23.6 ± 2.6 days for eight of 10 saline-treated animals. Two saline-treated animals developed ovarian follicular cysts and did not ovulate prior to day 65. Serum progesterone in GnRH-treated cows averaged 0.3 ± 0.1 ng/ml on day 14 postpartum, increased to 1.3 ± 0.2, 2.2 ± 0.3 and 2.7 ± 0.6 on days 18, 21 and 25 and decreased to 2.0 ± 0.7, 1.5 ± 0.6 and 1.2 ± 0.4 ng/ml on days 29, 32 and 35 postpartum, respectively; a response which mimicked progesterone changes during the normal estrous cycle. Serum progesterone in saline-treated cows increased linearly from 0.4 ± 0.1 ng/ml on day 14 postpartum to 2.9 ± 0.9 ng/ml on day 35 postpartum. Serum estradiol averaged 5.0 ± 0.5 and 3.8 ± 0.5 pg/ml prior to treatment on day 14 for cows treated with saline and GnRH, respectively, and these concentrations were maintained for 4 hours. In cows treated with saline and GnRH serum estrone increased from 11.0 ± 0.8 and 11.3 ± 1.0 pg/ml, respectively, to 19.9 ± 8.3 and 16.5 ± 8.5 pg/ml at 4 hours. GnRH administered 2 weeks postpartum induced LH release and ovulation and these cows continued to have normal estrous cycles until day 65 postpartum, when the study was terminated.

¹ Published with the approval of the director of Michigan Agriculture Experiment Station as Journal Article No. 6740.

² Animal Reproduction Laboratory, Department of Dairy Science.

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