Previous work has show poor relationships between the maintenance of sperm motility and fertility after AI of ram semen. New laboratory tests of sperm competence have been developed but their relationship to fertility needs to be established. Two large scale trials involving the laboratory evaluation of straws of frozen semen from different rams and the field AI of ewes with the same batches of semen were conducted. In Trial A, 1671 ewes were AI`d with 52 batches of semen from 26 rams on 6 farms. In Trial B 6872 ewes were AI`d with 137 batches of semen from 34 rams on 25 farms. In Trial A, there was no linkage between farms while in Trial B the same batch was used on more than one farm and most farms had more than 1 batch from more than 1 ram. Laboratory evaluation involved: visual motility assessment post thaw and time taken to drop to 5% motility when incubated at 38°C, 8 parameters of velocity and direction of motility using CASA; % live and % acrosome intact and sperm concentration. These parameters were measured immediately post-thaw and in Trial B a repeat measurement was made after 6h at 38°C. In addition, in Trial A the relative IVF value was determined. In Trial A there were significant (P<0.05) between farm and ram within farm effects on fertility (% pregnant) but no ejaculate within ram effect. Significant (P<0.001) differences were seen in all semen parameters except initial motility, time to 5% and concentration. Parameters correlated with fertility were IVF and % live and together these accounted for 44 % of the variation. In Trial B, there were effects of farm on fertility (P<0.001) and breed of ram (P<0.05), residual ram variation was non-zero but not significant. There were significant differences between rams and between ejaculates within rams in concentration, visual motility, % live, velocity and directional parameters. The number of live and motile sperm inseminated was correlated (P<0.001) with fertility. Significant (P<0.05) correlations were also obtained between two CASA parameters; the % live and % visual motility, after 6h of incubation. Multiple regression equations were derived that accounted for between 62 and 75 % of the variation between rams.
Proceedings of the New Zealand Society of Animal Production, Volume 58, , 181-185, 1998
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