Insemination time and dilution rate of cooled and chilled ram semen affects fertility

Abstract

Adult Merino ewes ( n=448) were apportioned into two groups and inseminated with: extended at 30 °C with skim milk and stored for 6 h at 15 °C (cooled semen) or extended with skim milk-citrate trisodium with egg yolk and stored for 24 h at 5 °C (chilled semen). Each group was further subdivided according to the time of cervical insemination at 42, 46 and 50 h after pessary (MAP-60 mg) removal and according to the dilution of the semen (120×10 6 spermatozoa in 0.05, 0.1 and 0.2 ml). The pregnancy rate after insemination with cooled semen was 50% better than that after chilled semen (56.7 vs. 37.5%; P<0.001). Pregnancy rate was not affected by the volume of insemination; however, there was a tendency of increased lambing rate with an insemination dose of 0.1 cc (1:2, dilution), especially when the ewes were inseminated with cooled semen. The effect of time on insemination was significant only in ewes inseminated with chilled semen at 5 °C ( P<0.01). Insemination carried out 46 h after pessary removal resulted in higher pregnancy and lambing rate (36.5, 31.1; 52.0, 45.3; and 24.0, 20.0 at 42, 46 and 50 h, respectively). Pregnancy of ewes inseminated with chilled semen at 46 h after pessary removal was similar to that obtained using cooled semen (52.0 vs. 56.7%). From this study, it is concluded that advancing the time of insemination with chilled semen at 5 °C improves pregnancy and that the lambing obtained under these conditions is similar to the one obtained with cooled semen.