EFFECT OF HOMOLOGOUS AND HETEROLOGOUS SEMINAL PLASMA ON THE FERTILIZING ABILITY OF EJACULATED BULL SPERMATOZOA ASSESSED BY PENETRATION OF ZONA-FREE BOVINE OOCYTES

The ability of seminal plasma to influence the fertility of ejaculated bull spermatozoa was examined using a sperm penetration assay for zon a-free bovine oocytes. Washed, ejaculated spermatozoa from bulls of be low (low) or above average (high) fertility were mixed with seminal pl asma from the same bull, or with seminal plasma from a bull of contras ting fertility. Treated spermatozoa were stained with different fluoro chromes and competed to penetrate oocytes after heterospermic insemina tion in vitro. Washed spermatozoa exposed to seminal plasma from bulls of high fertility penetrated more oocytes than those spermatozoa mixe d with seminal plasma from bulls of low fertility (P < 0.01). Mixing l ow fertility spermatozoa with high fertility seminal plasma generally improved penetrating ability compared with low fertility spermatozoa m ixed with low fertility seminal plasma (P = 0.05). Washed spermatozoa from a bull of low fertility mixed with his own seminal plasma had gre ater ability to penetrate oocytes than did washed spermatozoa from a b ull of high fertility mixed with seminal plasma from a bull of low fer tility (P < 0.01). The bias associated with using low fertility semina l plasma from the bull providing the spermatozoa was removed by repeat ing this experiment using pooled seminal plasma from different subfert ile bulls. After combination with pooled seminal plasma from bulls of low fertility spermatozoa from bulls of high or low fertility penetrat ed oocytes in a similar way, but high fertility spermatozoa had a slig htly higher penetration rate (P = 0.12). In conclusion, the penetratio n of zona-free oocytes by ejaculated spermatozoa from bulls of low fer tility was marginally improved by seminal plasma from bulls of high fe rtility, but penetration by high fertility spermaotoza was decreased b y exposure to low fertility seminal plasma. Seminal plasma from bulls of low fertility similarly affected the penetrating ability of high an d low fertility spermatozoa if the seminal plasma used was foreign to the spermatozoa being tested.