Follicular oocytes were aspirated from bovine ovaries collected at a local abattoir. The cumulus-intact oocytes were matured, fertilised and subsequently cultured in vitro. Of 2297 oocytes exposed to in vitro procedures during a 30-day experimental period, 92 per cent matured, 83 per cent were fertilised, 73 per cent cleaved, 48 per cent developed to the morulae and 14 per cent developed to the expanded blastocyst stage. During this experimental period, 300 similar embryos fertilised in vitro were frozen at different post in vitro block developmental stages. After approximately one year of storage in liquid nitrogen, 98 of these embryos were thawed and cultured either for up to four hours or for two days in tissue culture medium-199. Culturing the embryos for up to four hours was not as successful for in vitro development as culturing for two days. Of the 40 embryos cultured for two days, 67 per cent of early blastocyst stage embryos developed to expanded blastocysts in vitro and 46 per cent of morula stage embryos developed to expanded blastocysts, whereas only 8 per cent of 16-cell stage embryos developed to expanded blastocysts. A 50 per cent pregnancy rate resulted when frozen-thawed embryos were co-cultured for two days before transfer compared with 20 per cent for frozen-thawed embryos cultured for up to four hours before transfer. Five calves were born after a normal gestation period with birthweights ranging from 37.3 to 54.5 kg.
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