The effect of duration of FSH support on follicle growth and oocyte developmental competence in prepubertal calves was investigated. Experiment 1 tested the effects of duration of FSH support (4 vs 7 days) and method of oocyte maturation (in vitro vs. in vivo) and provided a comparison with sexually mature heifers. Despite the compromising effect of unexpected ovulations in some prepubertal calves, embryo development from oocytes collected following 4 days of FSH support and in vitro maturation was similar to that from sexually mature heifers after 7 days of FSH support and in vivo maturation. In Experiment 2, prepubertal calves were given 4, 6 or 7 days of FSH support and oocytes were matured in vitro. At the time of oocyte collection, the number of follicles ≥ 6 mm was greater (P = 0.03) in calves given 7 days of FSH treatment than 4 days (37.3 ± 5.5 vs. 14.7 ± 2.5), but the number of cumulus-oocyte complexes collected did not differ among groups (P = 0.1). Oocytes from prepubertal calves given 6 days of FSH treatment had higher rates of cleavage and blastocyst formation than 4 or 7 days of treatment (cleavage = 73.2 vs. 51.3 vs. 47.2 % respectively, P = 0.01
blastocyst = 40.9 vs. 20.5 vs. 20.2 % respectively, P = 0.02). In conclusion, six days of FSH support under controlled endogenous LH release by exogenous progesterone in 5-month-old prepubertal calves was associated with greater developmental competence of oocytes than 4 or 7 days.