The effect of the number of transferred embryos, the interval between nuclear transfer and embryo transfer, and the transfer pattern on pig cloning efficiency

To improve the efficiency of producing cloned pigs, we investigated the influence of the number of transferred embryos, the culturing interval between nuclear transfer (NT) and embryo transfer, and the transfer pattern (single oviduct or double oviduct) on cloning efficiency. The results demonstrated that transfer of either 150–200 or more than 200 NT embryos compared to transfer of 100–150 embryos resulted in a significantly higher pregnancy rate (48 ± 16, 50 ± 16 vs. 29 ± 5%, p < 0.05) and average litter size (4.1 ± 2.3, 7 ± 3.6 vs. 2.5 ± 0.5). In vitro culture of reconstructed embryos for a longer time (40 h vs. 20 h) resulted in higher (p < 0.05) pregnancy rate (44 ± 9 vs. 31 ± 3%) and delivery rate (44 ± 9 vs. 25 ± 9%). Furthermore, double oviductal transfer dramatically increased pregnancy rate (83 ± 6 vs. 27 + 8%, p < 0.05), delivery rate (75 ± 2 vs. 27 + 8%, p < 0.05) and average litter size (6.5 ± 2.8 vs. 2.6 ± 1.2) compared to single oviductal transfer. Our study demonstrated that an improvement in pig cloning efficiency is achieved by adjusting the number and in vitro culture time of reconstructed embryos as well as the embryo transfer pattern.