EFFECT OF ESTRADIOL VALERATE ON OVARIAN FOLLICLES, EMERGENCE OF FOLLICULAR WAVES AND CIRCULATING GONADOTROPINS IN HEIFERS

An experiment was designed to examine the effect of estradiol valerate (EV) on the growth and regression of follicles of a wave and on the e mergence of the next follicular wave. Twenty-six beef heifers were exa mined daily by ultrasonography and randomly allocated to 1 of 4 treatm ent groups at the time of ovulation (Day 0): untreated control heifers and those that received 5 mg EV intramuscularly on Day 1, Day 3 or Da y 6. Maximum diameter of the dominant follicle was greater (P < 0.05) in control heifers than in heifers treated on Day 1 or Day 3. Mean day of onset of regression of the dominant follicle was later (P < 0.05) in control heifers than in heifers treated on Day 1 but was not differ ent from heifers treated on Day 3. In heifers treated on Day 6, cessat ion of growth, maximum diameter and onset of regression were not diffe rent from that of control heifers. The emergence of the next follicula r wave was earlier (P < 0.05) in heifers treated on Day 1 than in cont rol heifers, whereas wave emergence was delayed (P < 0.05) in heifers treated on Day 3 or Day 6. The mean day of maximum concentration of FS H prior to the emergence of the next wave was earlier in heifers treat ed with EV on Day 1 and later in heifers treated on Day 3 or Day 6 com pared with that of the controls (P < 0.05). Treatment on Day 1 or Day 3 resulted in a significant LH surge in 8/13 heifers, whereas no LH su rges were detected in control heifers or in heifers treated on Day 6. The hypothesis that EV suppresses the growth of the dominant follicle, was supported. Estradiol valerate treatment resulted in early emergen ce of the next follicular wave in heifers treated on Day 1, but treatm ent on Day 3 or Day 6 resulted in delayed emergence of the next follic ular wave.