Cervical artificial insemination with frozen-thawed semen in sheep: the secret is in the cervix of Norwegian ewe breeds

In sheep, cervical artificial insemination (AI) of frozen-thawed semen is not practiced worldwide due to the unacceptably low pregnancy rates achieved (typically <30%). The exception to this is in Norway where farmers perform shot-in-the-dark vaginal AI with frozen-thawed semen and achieve pregnancy rates over 60%. Research in Ireland has shown that this is due to the breed of the ewe used in Norway and specifically the ability of sperm to traverse the cervix in some ewe breeds (such as those in Norway) but not others. This review summarizes the published studies performed over the last two decades, which have focused on characterizing the biological differences in the cervix of ewe breeds of divergent fertility. It not only discusses how this unique ewe breed effect can be used to develop our understanding of how frozen-thawed semen can cross the ovine cervix but also provides new in vivo opportunities through the addition of specific biomolecules that will protect sperm, thus improving the success of cervical AI using frozen-thawed semen in sheep.