Buserelin overcomes the effect of the corpus luteum on ovarian follicular development in postpartum cycling cows

Follicular development after treatment with a gonadotropin-releasing hormone 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 μg of buserelin. Both ovaries were collected on day 3 or day 6 (n = 4 per group per day) and follicles over 1.57 mm in diameter were observed histologically. Compared with day 3 in the saline group, there was a greater decrease in the percentage of Class I total (1.57–3.67 mm; P < 0.08) and Class 1 atretic follicles (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 buserelin 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, follicular responses in CLO and NCLO were compared by daily ultrasonography 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 indicate that treatment with buserelin overcame most of the local effects of 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.