In vivo fertility of rams in relation to sperm-zona pellucida binding and sperm-zona pellucida penetration of ovine oocytes

Sperm-zona pellucida (zona) binding and sperm-zona penetration have been suggested for use as in vitro bioassays of fertility since both are essential steps in the fertilization process. The correlations of sperm-zona binding and sperm-zona penetration with the in vivo fertility of sheep were investigated in this study. In vivo fertility was estimated from a heterospermic insemination trial using cryopreserved ram semen. Neither zona binding, zona penetration nor the ability to undergo an acrosome reaction was significantly correlated with the in vivo fertility of the rams (P = 0.78, P = 0.66, and P = 0.85, respectively). These results suggest that the zona binding and zona penetration bioassays may not be useful estimators for assessing cryopreserved ram sperm fertility.