ESTRUS SYNCHRONIZATION WITH AN ORAL PROGESTOGEN PRIOR TO SUPEROVULATION OF POSTPARTUM BEEF-COWS

Ovarian follicular dynamics and steroid secretion patterns were monito red in postpartum beef cows that were synchronized for estrus with mel engestrol acetate (MGA) or prostaglandin F-2 alpha (PGF) prior to supe rovulation. Twenty-four multiparous Angus cows were stratified by numb er of days postpartum to an MGA or PGF treatment prior to superovulati on. Cows in the MGA group were fed 0.5 mg MGA/d for 14 d in a grain ca rrier. Superstimulatory treatments began 14 d after withdrawal of MGA from feed or 11 d after administering a single injection of 500 mu g c loprostenol (PGF). Supersthnulatory treatments (FSH) were administered twice daily in decreasing doses (7.5, 5, 5, 2.5 mg) over 4 d. Sixty a nd 72 h after initiating the superstimulatory treatments, all cows wer e treated with 750 mu g and 500 mu g PGF, respectively. Cows were inse minated at 0, 12, and 24 h from the onset of standing estrus with seme n from 2 proven sires. Cows within treatment were inseminated with 1, 2 and 1 (single) or 2, 4 and 2 units (double) of semen at the designat ed insemination times. Blood sampling and transrectal ultrasonography of ovaries were performed daily beginning 2 d prior to the initiation of FSH treatment and were continued through embryo recovery. Ovaries w ere examined daily to determine the number and size of follicles. Plas ma samples were analyzed for progesterone and estradiol. Follicles wer e counted and categorized based on a 5 to 9 mm range or greater than o r equal to 10 mm. At the end of superovulatory treatment there were mo re (P less than or equal to 0.01) follicles greater than or equal to 1 0 mm among cows that were estrus synchronized with MGA (7.5 +/- 1.2) t han with PGF (3.9 +/- 1.2). These differences were reflected in higher (P less than or equal to 0.05) subsequent concentrations of plasma pr ogesterone, suggestive of differences in ovulation rate between treatm ents (MGA = 8.5 ng/ml; PGF = 5.6 ng/ml). There were no significant dif ferences between treatments in concentrations of estradiol, total numb er of ova (MGA=11; PGF=9), fertilized embryos (MGA=9; PGF=7), or trans ferable embryos recovered (MGA=8; PGF=6). Furthermore, double insemina tion offered no significant improvement in the number of recovered fer tilized ova. These data identify differences in follicular and endocri ne response when cows were estrus synchronized with MGA versus PGF pri or to superovulation. (C) 1997 by Elsevier Science Inc.