title: Persistence of the dominant follicle during melengestrol acetate administration and its regression by exogenous estrogen treatment in beef cattle.
abstract: Experiments were conducted to test the hypothesis that estrogen treatment will regress a persistent dominant follicle developed during melengestrol 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 with 25 mg of prostaglandin F2 alpha (PGF2 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 were checked with ultrasound on d 8, 10, 12, and 14 of MGA treatment and every day until ovulation. A persistent dominant follicle developed in 90% of the MGA-treated cows by d 10 of treatment. Most of the MGA-treated cows ovulated the persistent dominant follicle (n = 13/17), whereas EV treatment regressed the persistent dominant follicle (n = 10/14) with the recruitment of a new follicle that ovulated (n = 8/10). Diameter 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) and yearling heifers (n = 38) were equally allotted to either a control, MGA alone, or MGA + estradiol-17 beta (MGAE) group with the same dose of MGA as administered in Exp. 1. The 1st d of MGA feeding was the 1st d of treatment. On d 10 of treatment half the MGA-treated animals were injected i.m. with 5 mg of estradiol-17 beta. In controls, behavioral estrus was detected and animals were artificially inseminated (AI) for 5 d (d 10 to 14 of experiment). All controls not exhibiting estrus by d 15 of experiment were injected with 25 mg of PGF2 alpha. The remaining controls and all MGA cows were observed for behavioral estrus 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 percentage of animals conceiving to the synchronized estrus did not differ (P > .05) among treatments. The data support our hypothesis that a persistent 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 seem to be significantly compromised.
authors.affiliation: Department of Animal Science, Oklahoma State University, Stillwater 74078-0425, USA.
authors.forename: J V
authors.initials: JV
authors.lastname: Yelich
authors.forename: R D
authors.initials: RD
authors.lastname: Geisert
authors.forename: R A
authors.initials: RA
authors.lastname: Schmitt
authors.forename: G L
authors.initials: GL
authors.lastname: Morgan
authors.forename: J P
authors.initials: JP
authors.lastname: McCann
mesh_terms: D000818:Animals; D002417:Cattle / Q000097:blood / Q000502:physiology*; D015237:Dinoprost / Q000494:pharmacology; D004357:Drug Synergism; D004958:Estradiol / Q000494:pharmacology; D004967:Estrogens / Q000494:pharmacology*; D004971:Estrus / Q000187:drug effects / Q000502:physiology; D004973:Estrus Synchronization / Q000187:drug effects / Q000502:physiology; D005260:Female; D003341:Luteolysis / Q000187:drug effects* / Q000502:physiology; D008552:Melengestrol Acetate / Q000494:pharmacology*; D006080:Ovarian Follicle / Q000502:physiology*; D010053:Ovary / Q000000981:diagnostic imaging / Q000502:physiology; D010060:Ovulation / Q000187:drug effects / Q000502:physiology; D011247:Pregnancy; D018873:Pregnancy Rate; D011374:Progesterone / Q000097:blood; D045167:Progesterone Congeners / Q000494:pharmacology*; D011897:Random Allocation; D013997:Time Factors; D014463:Ultrasonography
chemical_list: D004967:Estrogens; D045167:Progesterone Congeners; D011374:Progesterone; D004958:Estradiol; D008552:Melengestrol Acetate; D015237:Dinoprost