On-farm lambing outcomes after transfer of vitrified and slow frozen embryos

Abstract The aim of the present study was to determine the most suitable embryonic stage and embryo freezing technique for commercial implementation of frozen embryo trading by small-scale sheep producers. There was a 2 × 2 factorial design utilized for conducting the study consisting of two embryo stages (2-8 cells or morula/blastocyst) and two cryopreservation protocols (vitrification or slow-freezing). For the in vivo produced embryos, there were treatments of crossbred donor ewes to induce superovulation. Embryos were recovered surgically on either Day 2 or 5.5 after estrous onset. The embryos were cryopreserved using either a vitrification or slow-freezing method before there was transfer to recipients. Ovarian response, embryo survival and lambing outcomes were analyzed. There were no differences in number of recovered and fertilized embryos at the two embryonic developmental stages. There were no effects of embryonic stages and cryopreservation methods on pregnancy rate, twinning rate, fetal birth weights and lamb weight at 1 month of age. When there was use of vitrified embryos for transfers, there was a greater lamb weight at 2 months of age (8.38 ± 0.20 compared with 7.78 ± 0.21 kg; P =  0.044) than when there was transfer of embryos cryopreserved using slow freezing procedures. Considering economic and practical benefits to small-scale sheep farms, morula/blastocyst stage-embryo collection and transfer into the uterus is more efficacious than transferring 2-8 cells embryos into the oviduct. Results of this study may contribute to the genetic improvement in the flocks of small-scale sheep producers.