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

Adult Merino ewes (n = 448) were apportioned into two groups and inseminated with: extended at 30 degreesC with skim milk and stored for 6 h at 15 T (cooled semen) or extended with skim milk-citrate trisodium with egg yolk and stored for 24 h at 5 T (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 x 106 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 degreesC (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 It 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 degreesC improves pregnancy and that, the lambing obtained under these conditions is similar to the one obtained with cooled semen. (C) 2002 Elsevier Science Inc. All rights reserved.,