FOLLICULAR DEVELOPMENT AND REPRODUCTIVE ENDOCRINOLOGY DURING AND AFTER SUPEROVULATION IN HEIFERS AND MATURE COWS DISPLAYING CONTRASTING SUPEROVULATORY RESPONSES

To understand the causes for poor response to superovulation in mature cows of high genetic potential, endocrine and follicular events durin g and after superovulation were compared in heifers (<2 yr old) yieldi ng large numbers of embryos and cows (9 to 13 yr old) known to be poor embryo donors. Follicular development was monitored by daily ultrason ography. Blood samples were taken 2 to 3 times a day for the measureme nts of P4, E2, FSH and LH by RIA. Intensive blood collections at 15-mi n intervals for 6 h were also performed during preovulatory and luteal phases. The number of embryos produced in the heifers (15.2 +/- 2; me an +/- SEM) and the cows (0.6 +/- 0.4), was similar to the number of o vulatory follicles derived from ultrasonographic observations in the h eifers (16.2 +/- 3.7), but not in the cows (7.8 +/- 2.8). Contrary to that observations in heifers, there was no increase in the number of 4 - to 5-mm follicles in cows during superovulation. The number of large r follicles (>5 mm) increased during superovulation in both cattle gro ups, but it was significantly lower in cows than in heifers. During su perovulation, the maximal E2 concentration was greater (P<0.0001) in h eifers than in cows. One cow showed delayed luteolysis during superovu lation, while another had abnormally high FSH (>10 ng/ml) and LH (>3 n g/ml) concentrations following superovulation. All the cows had a post ovulatory FSH rise which was not detected in the heifers. The results showed that attempts to improve superovulatory response in mature gene tically valuable cows are hampered by a number of reproductive disorde rs that are not predictable from the study of the unstimulated cycle.