In vivo oocyte recovery as a means of improving bovine embryo and calf production
The rate of genetic change in cattle can be increased using multiple ovulation and embryo transfer techniques (MOET). In the SAC beef MOET scheme 15 embryos must be produced in 12 weeks assuming that a pregnancy/live birth rate of at least 50% is achieved. This study compared 1) the effects of several protocols for oocyte donors used for transvaginal ultrasound guided follicular aspiration, in vivo oocyte recovery and subsequent in vitro embryo production, 2) yield of embryos produced in 12 weeks from oocyte donors by in vitro techniques v conventional in vivo embryo recovery, and 3) pregnancy rates and calving outcome achieved using in vivo (frozen/thawed) and in vitro (fresh and frozen/thawed) embryos. Pretreatment of Simmental heifers with exogenous steroids prior to follicular aspiration had no effect on oocyte recovery or embryo production. FSH pretreatment or increasing the frequency of aspiration from once to twice per week enabled almost twice as many transferable embryos to be produced. The total number of transferable embryos produced over a 12 week period was increased approximately 3 fold by in vitro techniques or a combination of in vivo and in vitro techniques compared in vivo embryo recovery alone. There was no significant difference between pregnancy rates achieved by transferring in vivo frozen/thawed, in vitro fresh or in vitro frozen/thawed embryos but the overall pregnancy rate was lower than normally observed at this centre. There were differences between in vivo frozen/thawed and in vitro produced embryos in terms of gestation length, calving difficulty and the sex ratio of calves produced. It was concluded that in vivo oocyte recovery and in vitro embryo production allows more embryos to be produced and more pregnancies achieved in a fixed period of time than conventional superovulation and embryo recovery. However, transfer of in vitro produced embryos may increase the risk of problems at calving compared to transfer of in vivo produced embryos.