168FACTORS AFFECTING ON EMBRYO TRANSFER PREGNANCY RATES OF IN VITRO-PRODUCED BOVINE EMBRYOS

Abstract

The objective of this study was to analyze factors affecting the pregnancy rates after transfer of IVF-derived Japanese Black embryos. Holstein cows and heifers (n=7250) were selected as recipients, and embryo transfers were performed for 3yr (between 1998 and 2000). The IVM-IVF procedure was performed according to a method previously described (Hamano S and Kuwayama M 1993 Theriogenology 39, 703–712). IVF-derived embryos that developed into expanded blastocysts (grade 1, manual of IETS) after 7 to 8 days (insemination=Day 0) were used for this study. Some of these embryos were frozen in TCM-199 supplemented with 1.4M glycerol, 20% calf serum, and 0.25M sucrose. The embryos were seeded at −6°C, held at −6°C for 10min, and then cooled to −25°C at a rate of 0.33°Cmin−1. Frozen embryos were thawed in a 30 to 35°C water bath after 10s of air thawing. Fresh (n=3952) or frozen-thawed (n=3298) embryos were nonsurgically transferred to recipients on Days 6 to 9 of the estrous cycle. Data collected at the time of embryo transfer included recipient parity (cow or heifer), whether recipient estrus was natural or synchronized with PGF2α, cloprostenol or CIDR, methods of estrous confirmation (showing standing heat, rectal palpation of ovary without standing heat, or showing only mucous vulvular discharge), number of examinations of the CL by palpation per rectum (twice on the day before embryo transfer and the day of embryo transfer, or once on the day of embryo transfer), type of embryos (fresh or frozen), and day of the estrous cycle at the time of embryo transfer. CATMOD procedures of SAS were used to determine the factors affecting the pregnancy rate. Overall pregnancy rates were 37.3% (n=2704). Whether recipient estrus was natural or synchronized and the type of embryos did not influence the pregnancy rates. Heifers had significantly higher pregnancy rates than cows (44.0% v. 33.0%, respectively, P<0.05). Pregnancy rates among the subset of heifers and cows showing standing heat were significantly higher than those showing only mucous vulvular discharge (39.5% v. 33.5%, respectively, P<0.05). Examining the CL twive had a significantly higher pregnancy rate than did a single examination of the CL (41.1% v. 35.6%, respectively, P<0.05). Pregnancy rate on Day 8 (38.4%, 1358/3533) of the estrous cycle at the time of embryo transfer was significantly higher than on Days 6 (27.7%, 23/83) and 7 (36.2%, 1235/3408) (P<0.05), and the pregnancy rate on Day 6 of the estrous cycle at the time of embryo transfer tended to be lower than on Day 9 (38.9%, 88/226) (P<0.08). These results demonstrate that confirming standing heat, performing CL examination twice before embryo transfer, freezing high quality embryos, and performing embryo transfers on Day 8 resulted in an improved pregnancy rate for the transfer of IVF-derived embryos.