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Influence of the Ovary on Estradiol-Induced Luteinizing Hormone Release in Postpartum Milked and Suckled Holstein Cows^1,2,

Kansas State University, Manhattan 66506 and and North Carolina State University, Raleigh 27650

Abstract

Holstein cows were assigned at parturition to be milked (n = 10) or suckled by four calves each (n = 10). One-half of each group was ovariectomized bilaterally (ovex) at the end of wk 1 and the remaining one-half was ovariectomized unilaterally (intact) at the end of wk 2. Cows were challenged with 400 /µg estradiol-17β (im) at the end of wk 1, 2, 3 and 4 (24 h after surgery) and blood was collected at 0, 12, 14, 16, 18, 20, 22, 24 and 26 h after estradiol to determine luteinizing hormone (LH) concentration. Blood also was collected every 3 d until 6 wk to estimate ovulation in intact cows by monitoring serum progesterone concentration. Calves were weaned from suckled cows after the third blood collection period (wk 3). Total ovarian follicles (16.5 ± 4 vs 34.2 ± 4) and follicular fluid weight (.7 ± .2 vs 1.4 ± .2 g) were less (P<.05) in suckled than in milked cows during wk 1, respectively. But the reverse trend appeared to occur during wk 2, although total ovarian follicles (26.4 ± 6 vs 16.4 ± 6) and follicular fluid weight (.9 ± .3 vs .8 ± .3 g) were similar in suckled and milked cows, respectively. Fewer suckled than milked cows responded to estradiol (LH concentration 1 SD above the average pre-estradiol LH concentration) at the end of wk 1 and 2 (P<.05). The first indication of a normal luteal phase (progesterone concentration >1 ng/ml for 12 d) occurred 1 wk earlier (P<.05) in milked than in suckled cows. Ovariectomy did not influence the proportion of cows responding to estradiol until wk 4, when more (P<.05) ovex than intact cows had an estradiol-induced LH response. None of the intact cows (suckled or milked) showed an estradiol-induced LH release when serum progesterone exceeded 1 ng/ml. Interval to peak LH declined (P<.05) and magnitude of peak LH increased (P<.05) from wk 1 to 4 postpartum in cows that responded to estradiol. We concluded that suckling compared with milking reduces the proportion of cows with an estradiol-induced LH release until after 2 wk postpartum. Although the ovary did not prevent an estradiol-induced LH release until serum progesterone exceeded 1 ng/ml, these data reveal an association between time postpartum when estradiol caused an LH peak and when the first sustained luteal function was initiated.

¹ Contribution No. 82-522-J, Kansas Agr. Exp. Sta., Manhattan 66506. The authors express appreciation to M. K. Schmidt for assistance in performing this study. We wish to thank Dr. G. D. Niswender for providing the bovine LH antiserum; Dr. L. E. Reichert, Jr., for providing purified bovine LH (LER 1716-2) for iodination, and the Hormone Distribution Office, NIAMDD, Bethesda, MD, for donating the LH (NIH-LH-B10) for standards. This research was conducted as a contribution to regional research project NC-113, "Methods for Improvement of Fertility in Cows Postpartum."

² Dept. of Anim. Sci. and Ind., Kansas State Univ.

³ Dept. of Surg, and Med., Kansas State Univ.

⁴ Dept. of Anim. Sci., North Carolina State Univ.