EFFECT OF MELENGESTROL ACETATE (MGA) OR PROGESTERONE-RELEASING INTRAVAGINAL DEVICE (PRID) ON FOLLICULAR DEVELOPMENT, CONCENTRATIONS OF ESTRADIOL-17-BETA 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 (E(2)) and progesterone (P-4), 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-releasing 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 Pq concentrations declined 3 d earlier in Control-2 and MGA-2 cows than in Control-3, MGA-3 or PRID-3 cows. Throughout the treatment period (d 18 to 24) serum E(2) concentrations were higher (P < .05) in MGA-2 cows than in MGA-3 or PRID-3 cows. Mean and baseline LH concentration and amplitude of LH pulses did not differ (P > .10) among treatments on d 17, 20, and 23. The LH pulse frequency did not differ (P > .10) among treatments on d 17 or 23; however, LH pulse frequency was greater (P < .03) on d 20 in MGA-2 cows than in MGA-3 or PRID-3 cows (4.3 +/- .6 vs 2.6 +/- .3 and 3.2 +/- .4, respectively). In conclusion, treatment with MGA extended the growth phase of ovulatory follicles, which resulted in a premature increase in both serum E(2) concentration and LH pulse frequency, whereas the follicle that was dominant at the beginning of PRID treatment underwent atresia and a preovulatory follicle emerged after the initiation of PRID treatment.