Duration of gonadotropin support influences follicle growth and oocyte developmental competence in prepubertal calves.

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.