Developmental competence of juvenile calf oocytes in vitro and in vivo: Influence of donor animal variation and repeated gonadotropin stimulation

Juvenile calf oocytes represent an untapped source of germ plasm for reprod uction. Reports on the developmental competence of calf oocytes have been c ontroversial. In this research, oocytes were recovered after gonadotropin s timulation from Holstein carves (N = 10) at 2-3 mo of age (2-mo cycle) and again at 4-5 mo of age (4-mo cycle). The in vitro developmental competence was measured, and prestimulation follicle numbers (for 2-mo cycle) and post stimulation follicle numbers (both cycles) were obtained. The number of ant ral follicles doubled after stimulation (23.4 +/- 6.1 vs, 55.1 +/- 16.1) fo r the 2-mo cycle and for the 4-mo cycle (47.4 +/- 12.4). The number of foll icles observed prior to stimulation in the 2-mo cycle was found to be highl y correlated with the poststimulation oocyte recovery for both collection c ycles (r = 0.95, 2-mo cycle; r = 0.81, 4-mo cycle). The majority (90-96%) o f recovered oocytes were found to be usable for in vitro maturation and fer tilization; of these, 41-42% cleaved and 10-11% developed to morulae or bla stocysts. Eighty-four in vitro-produced embryos were transferred to synchro nized recipients and resulted in 11 pregnancies, leading to 7 live (4 males , 3 females) and 2 dead tone male, one female) calves at full term. No sign ificant differences were observed between the 2-mo and 4-mo collection cycl es; however, 73% of the total pregnancies resulted from the 2-mo cycle. All pregnancies resulted from embryos of high-responding donors. The high corr elation between the number of follicles prior to stimulation and the postst imulation response suggests the possibility of screening calves prior to st imulation for routine embryo production.