The comparative efficiency of two semen extenders, eggyolk-Illini Variable Temperature extender (E) and reconstituted milk (M), in the maintenance of fertilizing ability was tested by heterospermic insemination using semen from Black (B) and White (W) males whose offspring were distinguishable. Semen from the different males was combined just before insemination to give a mixture of equal numbers of spermatozoa from each. Each of the male/semen combinations BE+WE, BM+WM, BE+WM and BM+WE was inseminated into one of four groups of females.
There were no significant differences between groups in the proportion of does kindling or in litter size. When the BE+WE combination was used, 69% of progeny were W and when the BM+WM combination was used, 86% were W. When the BM+WE combination was inseminated, 96% of progeny were W. However, when BE+WM was used, only 32% of progeny were W. Spermatozoa stored in E were associated with a significantly greater proportion of offspring than spermatozoa from males stored in M when BE was compared with WM (P <0·001) and when WE was compared with WM (P<0·01). It is concluded that the competitive advantage enjoyed by spermatozoa stored in E is due to the superiority of the E extender in maintaining fertilizing ability. This finding suggests that appropriate heterospermic inseminations could be used to distinguish between semen handling techniques and extenders.
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