Homospermic versus heterospermic insemination of zona‐free hamster eggs to assess fertility of fluorochrome‐labeled acrosome‐reacted bull spermatozoa

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Abstract
Fresh spermatozoa from six bulls, with fertility ranging from 64% to 78%, (based upon 59‐day nonreturn rates for 159,448 cows inseminated) were mixed with zona‐free hamster eggs in 15 heterospermic pair inseminations. Five of the bulls were used in homospermic insemination studies. Prior to incubation, spermatozoa from each bull were labeled with contrasting fluorescent stains pretested for effects on spermatozoa. Equal numbers of spermatozoa were mixed and treated with liposomes of dilauroylphosphatidylcholine to induce the acrosome reaction. Spermatozoa from split ejaculates within a male competed against each other equally in the hamster egg test, indicating that the staining procedure did not affect egg penetration rates. Bulls differed in their egg penetration rates when their sperm were inseminated either homospermically or heterospermically, but the differences in the homospermic inseminations were not significantly correlated with sire fertility. The number and percentage of sperm which penetrated eggs, and the number of eggs penetrated in the heterospermic competitive tests were highly correlated with fertility (r ≥ 0.86). Therefore, egg penetration rates from heterospermic inseminations appear to be valuable indicators of fertility and much more sensitive predictors than results from homospermic inseminations.