Summary. Progestagen-impregnated vaginal sponges + PMSG were used to synchronize oestrus in crossbred adult ewes which were inseminated 56 h after sponge removal with 0·5 ml diluted semen containing 400, 200, 100, 50 or 25 × 106 spermatozoa per insemination. The diluent was skim milk-citrate or pooled seminal plasma.
There was no difference in reproductive performance due to the insemination medium. Fertility (no. of ewes lambing) after insemination of 400 or 200 × 106 spermatozoa was 68% and was similar to that observed after natural service at progestagen-induced oestrus. When ≤ 100 × 106 spermatozoa were inseminated, fertility fell markedly and the number of lambs per ewe inseminated decreased. A decrease in litter size also occurred. The data indicate that insemination of 200 × 106 spermatozoa, i.e. less than 10% of the number in a single ram ejaculate, allows normal conception rates in progestagen-treated ewes.