Efficiency of piglet production after transfer of the vitrified embryos at the blastocyst and the expanded blastocyst stages collected individually from breeding sows

The introduction of breeding pigs from a breeding farm to a commercial farm is generally carried out by the transportation of live animals. The objective of this study was to construct a system using the technique of embryo transfer (ET) for the introduction of breeding pigs with minimal risk of disease transmission and transportation cost. Blastocysts and expanded blastocysts from Duroc sows were collected at the National Livestock Breeding Center located in Fukushima Prefecture and were virified using the micro volume air cooling (MVAC) method without direct contact with liquid nitrogen (LN). After cryopreservation, the embryos were put in the LN container and transported to Saga Prefectural Livestock Experiment Station. As the basic solution for cryopreservation (vitrification solution con taining 6.0M ethylene glycol, 0.6 M trehalose and 2% (w/v) polyethylene glycol and warming and dilition solution containing 1.8 M ethylene glycol and 0.3 M trehalose), 20 mM Hepes buffered porcine zygote medium (PZM)-5 or porcine blastocyst medium (PBM) was used. Warming vitrified embryos was transferred surgically and pregnancy results after the transfer was investigated. When PBM was used as a basic solution for vitrification of expanded blastocysts, the litter size (5.3+-0.9) and the efficiency of piglet production (35.6% ; percentage of number of piglets born based on the number of embryos transferred) of requesting it as ratio of to number of transplant embryos were higher (P<0.05) than those using PZM-5 (2.7+-0.3 and 14.8%). The results demonstrated that PBM was superior to PZM-5 as a basic solution of vitrification of porcine expanded blastocysts by the MVAC method. Even in the use of PBM, the efficiency of piglet production after transfer of vitrified blastocysts (14.3%) was lower (P<0.05) than that of the expanded blastocysts. However, the zona pellucida of blastocysts is thicker than that of expanded blastocysts and may be prevented from being damaged as the processes of vitrification and warming. Thus, considering the reduction of the risk of disease transmission, the use of blastocysts on embryo transfer may be effective for the introduction and production of breeding pigs.