211 A COMPARISON OF PREGNANCY MAINTENANCE AFTER EMBRYO TRANSFER OF DAY 7 OR 13 BOVINE EMBRYOS AND THE RELATIONSHIP BETWEEN INTERFERON-TAU PRODUCTION OF DAY 13 BOVINE EMBRYOS AND PREGNANCY SUCCESS

Cattle embryo transfer (ET) involves the transfer of Day 7 embryos. Recent advances in molecular techniques mean that these techniques are employed in combination with ET. One example is the use of PCR to determine the sex of embryos, which requires the biopsying of blastomeres. This process, however, can compromise embryo development. The identification of embryonic markers to predict whether a pregnancy will survive to term also remains elusive. In cattle, interferon (IFN)-tau is well established as the principle factor for the maintenance of pregnancy. The transfer of Day 7 embryos a time at which minimal IFN-tau production is occurring means that using IFN-tau as a predictor of pregnancy success is limited. Compared to Day 7 embryos, Day 13 embryos are larger, with an increased number of cells that would reduce the risks of biopsy and the measurement of IFN-tau production. Although these characters are beneficial to ET, there are few reports of ET using embryos at this stage. Hence, the aims of our study were to compare the success of pregnancy maintenance after ET of Days 7 and 13 bovine embryos and determine whether IFN-tau production by Day 13 embryos prior to ET could be used as a marker to predict survival to term. Japanese Black cows (n = 10) were superovulated and inseminated (Day 0). On Days 7 (n = 5) and 13 (n = 5), embryos were nonsurgically recovered. Good quality Day 7 embryos were selected and frozen by the conventional slow freezing method until transfer. Day 13 embryos were collected, and their dimensions were recorded; they were subsequently cultured individually for 1 h in TCM-199 with 10% FBS (100 �L). The amount of IFN-tau in the medium was determined by antiviral activity assay using the NIH international standard. Frozen–thawed Day 7 and fresh Day 13 embryos were transferred into synchronized recipient cows (D7ET: n = 39, D13ET: n = 37). Following ET, estrus was monitored twice daily. Ultrasound diagnosis was performed on Days 30, 40, 50, 60, 70, and 80 post-estrus. All data were analyzed by Student's t-test. In total, 20 cows in the D7ET group and 18 cows in the D13ET group went to term. In the animals that did not maintain pregnancy, the percentage of cows that returned to estrus before Day 25 was significantly lower in the D13ET group than in the D7ET group (16.2% vs. 38.5%; P < 0.05). However, the number of cows that returned to estrus after Day 30 post-ET was significantly higher in the D13ET than the D7ET group (27.0% vs. 2.6%; P < 0.05). In the D13ET group, of the 10 cows that did not return to estrus by Day 50, half had no fetus present but fetal membranes were evident. No significant difference in IFN-tau production or the length of Day 13 embryos was determined between those that went to term and those that did not survive (432 ± 154 vs. 1448 ± 1045 pg/embryo, P = 0.36; 1.65 ± 0.34 vs. 1.40 ± 0.17 mm, P = 0.55, respectively). In conclusion, the use of Day 13 vs. Day 7 embryos in ET did not improve the pregnancy rate to term although it helped avoid early embryo death, and there was no relationship between IFN-tau production of Day 13 embryos and their pregnancy success.