Competition between split and nonmanipulated embryos in the production of identical piglets.

The effect of nonmanipulated embryos on in utero survival of split porcine embryos was examined in this study. Previously, only limited success in the production of identical twin piglets has been reported. Embryos were collected from slaughtered donors (4 to 7 d post estrus) and were either split with the aid of a micromanipulator or left as whole embryos. Monozygotic pairs of split embryos were then surgically transferred to recipients with a complement of either split or nonmanipulated embryos. A total of 217 split embryos and 60 nonmanipulated embryos were transferred to 19 recipients. Nine of these recipients farrowed. In the two recipients that received only split embryos and farrowed, 31% of the split embryos survived to term, including two sets of monozygotic twins. In the remaining seven recipients, only 10% of the split embryos that were transferred along with nonmanipulated embryos survived to farrowing. This difference in split embryo survival (31 vs 10%) was significantly different (P<0.005). Sixty-nine percent of the nonmanipulated embryos survived to term in recipients that maintained pregnancy. Data presented in this study suggest that competition occurs between split and nonmanipulated embryos transferred to the same uterine environment.