PERSISTENCE OF THE DOMINANT FOLLICLE DURING MELENGESTROL ACETATE ADMINISTRATION AND ITS REGRESSION BY EXOGENOUS ESTROGEN-TREATMENT IN BEEF-CATTLE

Experiments were conducted to test the hypothesis that estrogen treatm ent will regress a persistent dominant follicle developed during melen gestrol acetate (MGA) treatment in the absence of a functional corpus luteum (CL) with normal fertility following development and ovulation of a newly recruited follicle. In Exp. 1, nonlactating beef cows (n = 31) were administered .5 mg . cow(-1). d(-1) of MGA (d 0) for 14 d wit h 25 mg of prostaglandin F-2 alpha (PGF(2 alpha)) administered on d 6 and 8 to regress the CL. On d 11 of treatment, approximately half the MGA-treated cows received 5 mg of estradiol valerate (EV) i.m. (MGAEV, n = 14) and the remainder were maintained on MGA (n = 17). Ovaries we re checked with ultrasound on d 8, 10, 12, and 14 of MGA treatment and every day until ovulation. A persistent dominant follicle developed i n 90% of the MGA-treated cows by d 10 of treatment. Most of the MGA-tr eated cows ovulated the persistent dominant follicle (n = 13/17), wher eas EV treatment regressed the persistent dominant follicle (n = 10/14 ) with the recruitment of a new follicle that ovulated (n = 8/10). Dia meter of the ovulatory follicle was larger (P < .05) for the MGA (19.8 +/- .6 mm) than for the control (15.1 +/- .8 mm) and MGAEV (14.8 +/- .7 mm) cows. In Exp. 2, nonlactating, multiparous beef cows (n = 97) a nd yearling heifers (n = 38) were equally allotted to either a control , MGA alone, or MGA + estradiol-17 beta (MGAE) group with the same dos e of MGA as administered in Exp. 1. The Ist d of MGA feeding was the I st d of treatment. On d 10 of treatment half the MGA-treated animals w ere injected i.m. with 5 mg of estradiol-17 beta. In controls, behavio ral estrus was detected and animals were artificially inseminated (AI) for 5 d (d 10 to 14 of experiment). All controls not exhibiting estru s by d 15 of experiment were injected with 25 mg of PGF(2 alpha). The remaining controls and all MGA cows were observed for behavioral estru s and AI commenced for 7 d following withdrawal of MGA (d 15 to 21 of experiment). More (P < .05) controls (90.3%) than MGA (84.8%) or MGAE (63.6%) cows showed estrus within 7 d after MGA withdrawal. The percen tage of animals conceiving to the synchronized estrus did not differ ( P > .05) among treatments. The data support our hypothesis that a pers istent dominant follicle developed and can be regressed with exogenous estrogen treatment followed by the recruitment and ovulation of a new follicle after MGA withdrawal and fertility of that estrus does not s eem to be significantly compromised.