<I>In vitro</I> production of embryos from high performance cows and the development of frozen-thawed embryos after transfer: a field study

In this field study, embryos were derived from genetically highly valuable cows excluded from breeding due to reproductive disorders. Cows, 5 to 10 years old, of Czech Siemmental, Holstein Dairy and Beef Cattle breeds were used as oocyte donors. Oocytes were obtained either in the growth phase of the first follicular wave from cows with synchronized oestrus or in any other phases of follicular development from cows without oestrus synchronization. The embryos were prepared by a standard protocol described previously. The mean number of usable oocytes, transferable and freezable embryos per donor, and the mean percentage of usable, transferable and freezable embryos were assessed. The results were analyzed by Student’s-<I>t</I> and Chi-squared tests. The embryos were frozen according to a slow freezing protocol. After thawing, they were transferred to recipients on Day 7 after oestrus. Irrespective of the breed, the mean numbers of usable oocytes and transferable and freezable embryos collected per donor were significantly higher (<I>P</I> < 0.01) for the synchronized than for the nonsynchronized donors (20.4 vs 11.7, 4.3 vs 1.0 and 3.2 vs 0.8, respectively). Similarly, the mean percentages of usable oocytes, transferable and freezable embryos were significantly higher (<I>P</I> < 0.01) for the synchronized than for the nonsynchronized donors (28.5% vs 20.5%, 20.9% vs 9.0% and 15.8% vs 6.5%, respectively). On comparison of the synchronized and nonsynchronized donors of each breed, the difference in the mean percentage of usable oocytes was significant (<I>P</I> < 0.01) in cows of all three breeds, the difference in the mean percentage of transferable embryos was significant in Czech Siemmental and Holstein Dairy cows (<I>P</I> < 0.01) and the difference in the mean percentage of freezable embryos was significant only in Holstein Dairy cows (P < 0.01). After the transfer of 41 frozen-thawed embryos and 43 fresh embryos, 20 heifers and 24 heifers became pregnant, respectively. In conclusion: (a) higher number of oocytes from infertile, genetically valuable cows was recovered in the growth phase compared with the other phases of follicular development; (b) greater development of these oocytes resulted in more embryos for transfer and cryopreservation; (c) the transfer of frozen-thawed and fresh embryos resulted in pregnancy rates of 48.8% and 55.8% , respectively.