Pre-freeze Bull Sperm Head Morphometry Related to Post-thaw Fertility

Artificial insemination using cryopreserved semen is a common management tool of the contemporary livestock producer. The ability to determine post-thaw fertility from pre-freeze sperm function parameters would be of major benefit to producers. In this study computer-aided sperm head morphometry was used to determine whether there is any association between pre-freeze bull sperm head measurements and post-thaw non-return to oestrus rates. Sixteen commercial artificial insemination bulls were used for this study. Of the 16 bulls, eight had > or =69% non-return to oestrus rates while the remaining eight bulls had <69% non-return to oestrus rates, based on artificial insemination of cryopreserved semen from a minimum of 748 inseminations per bull. Microscope slides of extended or cryopreserved and thawed semen were prepared and stained by the MGZIN procedure. The morphometric dimensions for length, width, width/length, area and perimeter for a minimum of 200 sperm heads were analyzed from each slide by ASMA and the mean measurements and intra-analysis coefficients of variation (CV) recorded. The post-thaw non-return to oestrus rates for all bulls were correlated with pre-freeze measurements of width (r=0.53, P<0.05), and the change in width/length after cryopreservation (r=0.61, P<0.05). There were no significant differences in the mean pre-freeze or post-thaw sperm head measurements between the two groups. However, the mean post-thaw change in width/length was significantly (P=0.003) different between the groups. The mean post-thaw intra-analysis variability for width was significantly lower (P=0.03) in bulls with non-return to oestrus rates (NRR)> or =69%. Overall, the data suggests that pre-freeze and post-thaw bull sperm head morphometry measurements of individual bulls may have minimal predictive value of post-thaw fertility as determined by non-return to oestrus rates between groups of bulls with limited variance in non-return to oestrus rates. The clinical relationship of sperm head morphometry and fertility in bulls with a greater, clinically significant range of non-return to oestrus, remains to be studied.