This study was carried out to determine suitable selection factors for recipients and embryos which could improve pregnancy rates following bovine embryo transfer. The experiment included 52 surgical transfers from February, 1985 through June, 1986 performed on Kyongbuk Breeding Center in southern Korea. The pregnancy rate was highest when recipients were in estrus within 6 hours before the donor to 12 hours after the donor (78.3 % versus 50 % for recipients in estrus earlier or later). Pregnancy rates were acceptable following culture under field conditions for up to 17 hours. More recipients over 15 months of age (76.1 %) remained pregnant than those under 15 months (66.7 %). Embryos transferred during the months from February to July resulted in higher pregnancy rates than those transferred during the remaining 6 months (77.3 % versus 57.1 %). Transferrable embryos were classified A (best) to C (worst); those graded A or B resulted in significantly higher pregnancy rates than those graded C (81.8 % and 73.3 % versus 25.0 %, p0.05). Pregnancy rates among recipients of the Korean native breed tended to be higher than among Holstein recipients (100 % versus 71.1 %). Similarly, when the embryo was transferred to the right uterine horn, pregnancy rates tended to be higher than when it was transferred to the left (81.3 % versus 65 %). Pregnancy rates did not differ according to the stage of development of the embryo; they were for morulae, tight morulae, blastocysts, and advanced blastocysts, respectively : 75.0 %, 66.7 %, 75.0 %, and 77.4 %

This study was performed to investigate the effects of stage and quality of embryo, synchrony between donor and recipient and difficulty of transfer on pregnancy rate following non-surgical transfer of frozen-thawed bovine embryos. The overall pregnancy rate of this experiment was 63.4 % and that of heifers (73.1 %) was higher than that of cows (46.7 %). The pregnancy rates of recipients transferred with morulae, early blastocysts and blastocysts were 50.0 %, 64.7 % and 71.4 %, respectively. The pregnancy rate of recipients transferred with good embryos (67.9 %) was higher than that of recipients transferred with fair embryos (53.8 %). The pregnancy rates of embros transferred to left and right uterine horn were 63.2 % and 63.6 %, respectively. The pregnancy rate of recipients in estrous synchrony 0 (76.2 %) was higher than those of recipients in synchrony -1 (55.6 %) and +1 (44.4 %). The pregnancy rate of recipients transferred with 2 embryos (71.4 %) was higher than that of recipients transferred with 1 embryo (61.8 %). The pregnancy rate of embryos transferred to uterine tip (72.0 %) was higher than that of embryos transferred to uterine base (50.0 %). Ease of transfer was ranked to a scale of one to three on the basis of increasing difficulty. Transfers ranked as ease score 1 accounted for 77.8 % of pregnancies and had higher pregnancy rate than ease score 2 (66.7 %) or 3 (45.5 %). The pregnancy rate of recipients with excellent corpus luteum (CL) (70.0 %) was higher than those of recipients with good CL (61.1 %) or fair CL (61.5%). In reviewing above results, it was considered that the factors such as embryo stage, embryo quality, estrous synchrony, corpus luteum quality, tranfer site within uterus, recipient's parity and ease score affected the pregnancy rate after non-surgical transfer of frozen-thawed bovine embryos