Effect of LH or GnRH on the dominant follicle of the first follicular wavein beef heifers

A study was designed to characterise ovarian follicular dynamics in heifers treated with porcine luteinizing hormone (pLH) or gonadotropin releasing h ormone (GnRH) on days 3, 6 or 9 (ovulation = day 0), corresponding to the g rowing, early-static, and late-static phases of the first follicular wave. Following ovulation, 65 beef heifers were assigned, by replicate, to the fo llowing seven treatment groups: 25 mg im of pLH on days 3, 6 or 9 (n = 9 pe r group); 100 mu g im of GnRH on days 3, 6 or 9 (n = 9 per group); or contr ols (no treatment; n = 11). Ovulation occurred within 36 h in 67%, 100% and 67% of heifers treated with pLH and in 89%, 56% and 22% of heifers treated with GnRH on days 3, 6 or 9, respectively (treatment-by-day interaction, P < 0.09). Combined for all treatment days, ovulation rates were 78% and 56% in pLH and GnRH-treated groups, respectively (P < 0.09). Overall, mean day (+/- SD) of emergence of the second follicular wave in heifers that ovulat ed was different from that in controls or in heifers that did not ovulate ( P < 0.05). Mean (+/- SD) day of emergence of the second wave occurred earli er (day 5.6 +/- 1.2; P < 0.05) in heifers that ovulated after treatment on day 3 (n = 14) than in controls (day 8.7 +/- 1.6; n = 11); however, wave em ergence in all heifers treated on day 6 (day 8.1 +/- 0.5; n = 18) did not d iffer from controls, regardless of whether or not ovulation occurred. In th e heifers that ovulated in response to treatment on day 9 (n = 8), the emer gence of the second follicular wave was delayed (day 10.9 +/- 0.4; P < 0.05 ). The day of emergence of the second wave in the 14 treated heifers that f ailed to ovulate, irrespective of the day of treatment (day 8.9 +/- 1.4) di d not differ from control heifers. The emergence of the second wave was mor e synchronous in day 6 heifers (regardless of whether they ovulated) and in day 9 heifers that ovulated compared to control heifers (P < 0.05). Result s did not support the hypothesis that the administration of pLH or GnRH at: known stages of the follicular wave in cycling heifers would consistently induce ovulation or atresia and, thereby, induce emergence of a new follicu lar wave at a predictable interval. New wave emergence was induced consiste ntly (1.3 days post-treatment) only in those animals that ovulated in respo nse to treatment. However, 22% of LH-treated heifers and 44% of GnRH-treate d heifers failed to ovulate. Treatments did not induce atresia of the domin ant follicle or alter the interval to new wave emergence in animals that di d not ovulate in response to treatment. (C) 1999 Elsevier Science B.V. All rights reserved.