FOLLICULAR DEVELOPMENT AND REPRODUCTIVE ENDOCRINOLOGY DURING A SYNCHRONIZED ESTROUS-CYCLE IN HEIFERS AND MATURE COWS DISPLAYING CONTRASTINGSUPEROVULATORY RESPONSES

Ovarian follicular development and plasma concentrations of progestero ne (P4), estradiol-17 beta (E2), luteinizing hormone (LH), and follicl e-stimulating hormone (FSH) were compared during a synchronized estrou s cycle between heifers and mature cows displaying contrasting superov ulatory responses. Six heifers <2 years old with a history of good res ponses to superovulatory (SOV) treatment and six cows 9 to 13 years ol d with poor responses to SOV treatments were used. Follicular developm ent was monitored by daily ultrasonography. Blood samples were collect ed two to three times daily for P4 and E2 and thrice daily for LH and FSH analysis. Intensive sampling (samples every 15 min for 6 hr) was p erformed at critical periods of foilicular development to analyze the pulsatile secretion of gonadotropins. In both cattle groups, a transie nt increase (P = 0.0001) in E2 occurred 4 to 5.7 d after the preovulat ory LH surge or 2.3 d before the dominant follicle reached its maximum size. FSH concentrations increased (P = 0.006) before the emergence o f the second cohort of follicles and then decreased despite no change in the concentration of E2. Contrary to our expectation and despite di fferences between groups in terms of age, number of previous SOV treat ments, and divergent responses to superovulation, follicular developme nt was similar in both groups. However, during the luteal phase, conce ntrations of E2 and FSH and LH pulse amplitudes were less (P less than or equal to 0.05) in cows than in heifers. Therefore, follicular deve lopment monitored by ultrasonography and endocrine profiles during a s ynchronized estrous cycle are of limited value to predict quality of e mbryo donors.