GNRH INDUCES OVULATION OF A DOMINANT FOLLICLE IN PRIMIPAROUS DAIRY-COWS UNDERGOING ANOVULATORY FOLLICLE TURNOVER

This trial examined the effect of an injection of gonadotropin-releasi ng hormone (GnRH) administered approximately 3 weeks postpartum on the induction of a luteinising hormone (LH) surge and ovulation of large (over 10 mm) follicles in the ovaries of lactating primiparous dairy c ows likely to have an extended postpartum period of anovulation. At 2 weeks postpartum daily transrectal ultrasound of ovarian follicles com menced in 20 primiparous dairy cows which were randomly assigned to be injected i.m. with 250 mu g GnRH or saline. Treatment was given the d ay after the largest growing follicle was over 10 mm in diameter, Bloo d samples were drawn hourly following treatment to measure plasma conc entrations of LH. Milk samples were taken three times per week from 2 weeks postpartum to 3-4 weeks after treatment for analysis of progeste rone concentration. Every primiparous cow treated with GnRH had an LH surge (over 10 ng ml(-1) maximum concentration) which was maximal 2 h after treatment. Nine of the ten primiparous cows treated with GnRH an d one saline-treated cow ovulated within 4 days of treatment, However, only three of the primiparous cows treated with GnRH continued to ovu late following the first, short (less than 10 day) luteal phase. These data indicate that sufficient releasable pituitary stores of LH were present at the time of treatment with GnRH and that large follicles ar e able to be ovulated, Hypothalamic release of GnRH, rather than pitui tary or ovarian insufficiency, appears to be the factor limiting resum ption of cyclic activity in these primiparous cows. Even though ovulat ion can be induced, it may not lead to the resumption of normal cyclic ity.