One half of the forty-eight New Zealand Romney ewes to be mated with either a Romney or Southdown ram were immunologically sensitized to the ram 2 weeks before tupping. After 12 to 13 weeks of pregnancy, the ewes were slaughtered and each reproductive tract and conceptus examined. Sensitization to the sire had no effect on fecundity, fertility, placental weight or foetal size.
However, the number of placentomes per foetus tended to be larger in sensitized than in control ewes and was significantly greater in inter-breed than in intra-breed matings. It is suggested that the increased number of placentomes is due to heightened sensitivity of the maternal and foetal tissues at implantation, resulting from their antigenic differences.
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