Effect of ovarian activity and endogenous reproductive hormones on GnRH-induced ovarian cycles in postpartum dairy cows.

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 micrograms 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 greater than or equal to 1.0 ng/ml on day 9 post-treatment in contrast to only four (20%) saline-treated cows (P less than 0.05). Seventeen of 20 cows ovulating in Group II exhibited a palpable follicle greater than or equal to 10 mm in diameter prior to treatment compared to 3 of 4 cows in Group I. Prior to treatment, plasma concentrations of LH, estradiol-17 beta and progesterone were not different (P less than .05) between cows in Groups I and II. In Group II, preinjection levels of estradiol-17 beta and LH for cows ovulating in response to GnRH were higher (P less than .01) than in cows not ovulating. Postinjection concentrations of progesterone in plasma on days 7 to 9 were higher (P less than .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 less than .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-17 beta 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.