Effect of melengestrol acetate (MGA) or progesterone-releasing intravaginal device (PRID) on follicular development, concentrations of estradiol-17β and progesterone, and luteinizing hormone release during an artificially lengthened bovine estrous cycle

Two trials were conducted to determine whether 7-d progestogen treatment beginning on d 17 of the estrous cycle altered 1) ovarian follicular development, 2) serum concentration of estradiol-17 beta (E2) and progesterone (P4), and 3) patterns of release of luteinizing hormone (LH). In both trials, Angus, Angus x Holstein, or Holstein cows 2 to 6 yr of age were randomly assigned to receive either melengestrol acetate (MGA, .5 mg.animal-1.d-1; n = 23), a progesterone-releaseing intravaginal device (PRID, n = 26) or to serve as untreated Controls (n = 14). Ultrasonography and blood sampling were performed daily throughout the experiment beginning on d 3 (Trial 1) or d 9 (Trial 2) of the estrous cycle. In Trial 2, blood samples were collected every 15-min for 6 h on d 17 (all cows) and d 20 and 23 (MGA and PRID cows) for determination of LH. Estrous cycle length was 3 to 5 d greater (P < .05) for MGA- and PRID-treated cows characterized by two (MGA-2 and PRID-2) or three (MGA-3 and PRID-3) dominant follicles than for control cows exhibiting two (Control-2) or three (Control-3) dominant follicles. A greater proportion (P < .05) of MGA- than of PRID-treated cows ovulated the follicle that was dominant at the beginning of treatment on d 17 (10 of 23 vs 1 of 26). Serum P4 concentrations declined 3 d earlier in Control-2 and MGA-2 cows than in Control-3, MGA-3 or PRID-3 cows.(ABSTRACT TRUNCATED AT 250 WORDS)