Abstract
To investigate the influence of post-thawed embryonic developmental stage and post -thawed morphological quality on pregnancy rate, frozen embryos, which had been collected from Japanese Black beef cattle and qualified as morphologically excellent before freezing, were thawed by the stepwise method and transferred non-surgically to Holstein heifers on day 7 (day 0=onset of estrus). The transferred embryos ranged in development from compacted morula to blastocyst stage and were evaluated as either morphologically excellent or good quality just after thawing. Pregnancy rates of 62.0% (49/79), 66.7% (26/39), and 81.0% (34/42) were achieved in the transfers of the compacted morula, early blastocyst, and blastocyst stages, respectively, for the transfer of the excellent quality embryos. The pregnancy rate obtained with blastocyst stage embryos was significantly (P<0.05) higher than that obtained with compacted morula embryos. The pregnancy rate of morphologically excellent quality embryos of the compact morula stage was 62.0% (49/79), and that of the good quality embryos of the compacted morula stage was 58.3% (7/12) revealing no significant difference between the two morphological qualities. These results indicate that the pregnancy rate increases as the embryonic developmental stage advances from compacted morula to early blastocyst, and that post-thawed morphological qualities, excellent and good quality, have no influence on pregnancy rates for frozen-thawed embryos that are transferred to bovine recipients on day 7 of estrus.
