INFLUENCE CORPUS-LUTEUM AND INDUCED OVULATION ON OVARIAN FOLLICULAR DYNAMICS IN POSTPARTUM CYCLIC COWS TREATED WITH BUSERELIN AND CLOPROSTENOL

The influence of the corpus luteum (CL) at the time of buserelin treat ment and of buserelin-induced ovulation on the dynamics of ovarian fol licular development was determined in 18 postpartum cyclic beef cows i njected i.m. with 8 mu g of buserelin on d 0 (d of treatment) and with 500 mu g of cloprostenol (PGF) 6 d later. From d 0 to onset of estrus , ovaries were examined ultrasonographically, and blood samples were c ollected daily. Number of medium (5 to 10 mm) follicles and diameters of the two largest (F1 and F2) follicles in each cow were recorded. Co ws were grouped according to the presence (CL+) or absence (CL-) of an active CL before buserelin injection and the presence (OV+) or absenc e (OV-) of a buserelin-induced ovulation (OV). Three groups were forme d: CL-OV+ (n = 8), CL+OV+ (n = 6), and CL+OV- (n = 4). Buserelin induc ed an ovulation in all CG cows (progesterone [P-4] < 3 ng/mL) and in C L+ cows that had P-4 < 4 ng/mL but did not in CL+ cows that had P-4 > 8 ng/mL. Within CL+ cows, buserelin-induced ovulation (OV+) was associ ated with a smaller (P < .0001) increase in F1 and F1 - F2 diameters a nd a transient increase in the number of medium follicles that was gre ater (P < .03) and of a longer duration (P < .01) than in OV- cows. Af ter PGF, the increase in F1 and F1 - F2 diameters was still greater in OV+ than in OV- cows (day x OV; P < .05). In all cows, the selection of the preovulatory follicle occurred before PGF injection. Results in dicate that a buserelin-induced ovulation was dependent on P-4 concent rations at the time of treatment. Subsequently, ovarian follicular dyn amics were altered by a GnRH-induced ovulation, but emergence and sele ction of a large growing follicle occurred in all cows within 6 d of t reatment. This follicle became the preovulatory follicle following PGF -induced luteolysis.