Proteins present in the seminal plasma and sperm influence sperm function and fertilization. The present study was carried out to screen breeding bull semen samples for the presence of fertilityassociated 28-30 kDa heparin-binding protein (HPB) and its effect on in vitro sperm characters and fertility. Semen samples were collected from 22 breeding bulls and the sperm proteins were extracted by Triton X detergent extraction method. HBPs were eluted, and the molecular weight of the proteins was assessed by discontinuous sodium dodecyl sulphate polyacrylamide gel elecrophoresis. Based on the presence/absence 28 kDa HBPs, bulls were categorized into group I and group II. Frozen semen samples were evaluated for in vitro sperm characters at immediate post thaw, 60, 120 and 180 min post-thaw incubation. To assess the field fertility of the bulls, 50 frozen semen straws/bull were used for insemination. Results indicated that only 50% of the bulls screened had 28-30 kDa HBPs in their sperm. Bulls positive for fertility-associated protein had better in vitro sperm characters, better protection against oxidative stress, readily underwent capacitation induction by heparin and had 13% higher conception than the bulls lacking the protein. So, it can be concluded that the bulls positive for of 28-30 kDa HBPs in sperm had higher chance of fertility and screening for its presence can be included in the regular breeding soundness examination for selection of bulls. ARTICLE HISTORY

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