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Effect of Ovarian Activity and Endogenous Reproductive Hormones on GnRH-Induced Ovarian Cycles in Postpartum Dairy Cows^1,2,

University of Missouri, Columbia, 65211⁴

Abstract

The objective was to determine the relationship of prior follicular development to GnRH-induced cyclic ovarian activity in dairy cows postpartum. Sixty lactating Holstein and Guernsey cows from the University of Missouri dairy herds were assigned at random to one of two groups. Group I consisted of 20 cows (control) given a single intramuscular (IM) injection of saline. Group II consisted of 40 cows given a single injection of 100 /µg gonadotropin releasing hormone (GnRH) IM. Treatments were administered 12 to 14 days postpartum. All cows were palpated per rectum to monitor ovarian activity prior to treatment (time 0) and 2 to 3 and 7 to 9 days post-treatment. Blood was collected via tail vessel puncture at time 0 and 7 to 9 days post-treatment. The percentage of cows exhibiting a follicle 10 mm or greater in diameter prior to treatment was not different between Group I (50%) and Group II (43%). However, twenty (50%) of the GnRH-treated cows initiated cyclic ovarian activity following treatment as evidenced by palpable CL and plasma progesterone 1.0 ng/ml on day 9 post-treatment in contrast to only four (20%) saline-treated cows (P<.05). Seventeen of 20 cows ovulating in Group II exhibited a palpable follicle 10 mm in diameter prior to treatment compared to 3 of 4 cows in Group I. Prior to treatment, plasma concentrations of LH, estradiol-17ß and progesterone were not different (P<.05) between cows in Groups I and II. In Group II, preinjection levels of estradiol-17ß and LH for cows ovulating in response to GnRH were higher (P<.01) than in cows not ovulating. Postinjection concentrations of progesterone in plasma on days 7 to 9 were higher (P<.01) in cows ovulating than in cows not ovulating. Six (30%) cows in Group I developed ovarian cysts prior to conception as compared to 12.5% (5) of the cows in Group II (P<.05). The present study suggests: (1) GnRH treatment as early as 12 to 14 days postpartum may initiate cyclic ovarian activity in normal dairy cows; (2) elevated preinjection concentrations of estradiol-17ß and follicular growth are important for GnRH induced ovulations; and (3) GnRH treatment 12 to 14 days postpartum may be useful in reducing abnormal ovarian activity.

¹ Journal Paper No. 8279. Missouri Agr. Exp. Sta., Univ. of Missouri-Columbia. Approved by the Director. The authors express appreciation to Janet Liebert, for assistance with hormone assays; to Carta Perkins, for secretarial assistance in typing of the manuscript; and to Ken Bolte, Lewis Croy and Mark Neinhueser, for their assistance with the data collection.

² This research was supported in part by Abbott Lab., North Chicago, IL.

³ Present address: Al-Fateh University, Faculty of Agr., Dept. of Anim. Sci.

⁴ Dept. of Dairy Husbandry and Vet. Med. and Surgery.

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