OVARIAN SUPERSTIMULATORY RESPONSE RELATIVE TO FOLLICULAR WAVE EMERGENCE IN HEIFERS

Two experiments were designed to evaluate the responsiveness of beef h eifers to superstimulatory treatments administered during the first fo llicular wave. Heifers were examined daily (Experiment 1) or twice dai ly (Experiment 2) by ultrasonography to determine the status of follic ular wave development and the day of initiation of superstimulatory tr eatment. Heifers in both experiments were superstimulated with a total dose of 10 ml Folltropin (equivalent to 200 mg of NIH-FSH-P1), divide d into 10 equal intramuscular injections over 5 days. On the last day of treatment, heifers received 500 mug of cloprostenol after each inje ction of Folltropin to induce luteolysis. In the respective groups, su perstimulatory treatments were initiated on Day -1, Day 0 (day of ovul ation) or Day +1 for Experiment 1, and on Day -1, Day 0, Day +1 or Day +2 for Experiment 2. In Experiment 1, the number of ovulations in eac h ovary was assessed by ultransonography and by counting the number of corpora lutea (CL) in each ovary at slaughter. The correlation betwee n both techniques for assessing ovulatory response was high (r = 0.98; P<0.0001), and there was no significant difference in the mean number of ovulations detected by ultrasound (5.7+/-1.1) versus the mean numb er of CL counted at slaughter (6.2+/-1.2). In Experiment 1, the mean ( +/- SEM) number of CL counted at slaughter in heifers treated on Day - 1 (9.4+/-3.8) and Day 0 (7.3+/-1.6) was higher (P<0.05) than that of h eifers treated on Day +1 (0.7+/-0.3). The mean number of follicles gre ater-than-or-equal-to 7 mm in diameter on the last day of treatment wa s also higher (P<0.05) in the Day -1 group compared with the Day +1 gr oup; the Day 0 group was intermediate. In Experiment 2, the mean numbe r of ovulations was higher (P<0.05) in the Day 0 group (18.4+/-3.4) th an the Day -1 (9.5+/-2.3), Day +1 (6.7+/-.2) or Day +2 (6.5+/-2.3) gro ups. Heifers in the Day -1, and Day 0 groups had more (P<0.05) follicl es greater-than-or-equal-to 7 mm at the end of treatment compared with heifers in the Day +1 or the Day +2 group. The stated hypothesis was supported: exogenous FSH treatment initiated at the time of wave emerg ence, near the expected time Of the endogenous wave-eliciting FSH surg e, has a positive effect on the superstimulatory response. A higher su perstimulatory response was elicited when treatments were initiated on the day of, or the day before, wave emergence compared with that of l ater treatments.