The current technique for artificial insemination (AI) of ewes during the breeding season necessitates the synchronisation of oestrus with progestagen sponges and PMSG, and involves depositing semen into the posterior cervix at a fixed time after sponge removal. Extensive field trials over a number of years in the UK have indicated that a single insemination using fresh diluted semen 56 h after sponge removal generally results in a conception rate of 70%, while conception rates following a single insemination of frozen-thawed semen 57 h after sponge removal have ranged from 19% to 34% (mean 28%). Giving two inseminations of frozen-thawed semen at 50 h and 60 h after sponge removal increased the overall mean conception rate, but only up to 48%. This inability to achieve acceptable conception rates with frozen-thawed semen nullifies many of the potential benefits of AI in sheep flocks.

It is well established that the cervix presents a major barrier to sperm transport in the ewe, particularly when oestrus has been synchronised with progestagens and PMSG.