Acute progesterone administration regresses persistent dominant follicles and improves fertility of cattle in which estrus was synchronized with melengestrol acetate.

Experiments were conducted to determine whether acute progesterone administration would regress persistent follicles and improve fertility in heifers and postpartum cows fed melengestrol acetate (MGA). In Exp. 1, heifers (n = 13) were fed MGA for 11 d (1st d of MGA = d 1). Prostaglandin F2 alpha (PGF2 alpha) was administered to heifers on d 2 to regress the corpus luteum (CL). On d 9, heifers were randomly assigned to receive an injection of either 200 mg of progesterone (PROG) or no hormone (vehicle; VEH). Neither growth of the persistent follicle nor plasma estradiol concentrations were altered by administration of VEH, and the persistent follicle ovulated after cessation of MGA feeding. Administration of PROG regressed the persistent follicle, reduced (P < .05) systemic estradiol concentrations, and resulted in ovulation of a newly recruited follicle. In Exp. 2, heifers were fed MGA for 14 d and were administered either PROG (n = 30) or VEH (n = 31) on d 12. When the CL was absent on d 12, synchronization conception rate (SCR) and pregnancy rate (PR) were greater (P < .05) for heifers administered PROG than for those administered VEH. Neither SCR nor PR were different among treatments when the CL was present on d 12. In Exp. 3, cows (n = 49) were fed MGA for 14 d and were administered either PROG or VEH on d 12. In cows lacking a CL (n = 32), administration of PROG increased (P < .05) SCR, but not PR. We conclude that acute PROG administration induces turnover of persistent follicles and may increase fertility when estrus is synchronized with MGA.