Effect of narrow sperm head shape on fertility in cattle.

Seven experiments were done in feedlot heifers to determine the importance of various degrees of narrowness of the sperm head on fertility in feedlot heifers. Frozen semen used in these experiments was selected to be normal in all respects except for very high numbers of a single specific type of sperm head aberration. Semen with the sperm aberration in question and control semen were selected to be as similar as possible in dose and postthaw viability so that differences in fertility would be attributable to the morphological variant under study. Fertilization rates were determined by collecting embryos from the reproductive tracts of superovulated heifers which had been slaughtered seven days after insemination. Pregnancy rates and rates of embryonic loss were studied in estrus-synchronized heifers by repeated transrectal ultrasound examinations from day 22 to day 55 after insemination. Reproductive tracts were collected and examined after slaughter at 60 days postinsemination.The combined results of these experiments show that a moderate degree of sperm head narrowness, in the absence of other seminal signs of a disturbance of spermatogenesis, is not detrimental to fertility. However, extreme narrowness of the postacrosomal region of the sperm head of most spermatozoa, as was found in two bulls without other seminal signs of a disturbance of spermatogenesis, resulted in significantly reduced fertility. The data suggest that, although a decision between normal and abnormal sperm morphology may contain a degree of subjectivity, of the defects studied only sperm with extreme narrowness of the post-acrosomal region are likely to reduce fertility.