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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.
Britt et al. (Fri,) studied this question.