BUSERELIN OVERCOMES THE EFFECT OF THE CORPUS-LUTEUM ON OVARIAN FOLLICULAR DEVELOPMENT IN POSTPARTUM CYCLING COWS

Follicular development after treatment with a gonadotropin-releasing h ormone agonist (buserelin) was compared in ovaries of postpartum cows bearing (CLO) or not bearing (NCLO) a corpus luteum (CL). In the first experiment, 16 cows on day 7 of the estrous cycle (day 0 of treatment ) were treated either with saline or 8 mu g of buserelin. Both ovaries were collected on day 3 or day 6 (n = 4 per group per day) and follic les over 1.57 mm in diameter were observed histologically. Compared wi th day 3 in the saline group, there was a greater decrease in the perc entage of Class 1 total (1.57-3.67 mm; P < 0.08) and Class 1 atretic f ollicles (P < 0.04) but a greater increase in that of Class 2 total (3 .68-8.57 mm; P < 0.06), Class 2 atretic (P < 0.04) and early atretic ( P < 0.05) follicles on day 6 in the CLO than in the NCLO. In the buser elin group however, all follicular responses (except for Classes 1 and 2 nonatretic follicles, P < 0.08) were similar (P > 0.1) between CLO and NCLO within 3 days after treatment, In the second experiment, foll icular responses in CLO and NCLO were compared by daily ultrasonograph y in cows that had (n = 6) or did not have (n = 4) a buserelin-induced ovulation. After buserelin treatment, the numbers of medium (5-10 mm) and large (over 10 mm) follicles were not different (P > 0,1) between the CLO and the NCLO whether ovulation occurred or not. Results indic ate that treatment with buserelin overcame most of the local effects o f the CL on the growth and atresia of ovarian follicles in postpartum cycling cows within a 6 day period and this occurred whether ovulation was induced or not.