Use of computer-assisted sperm analyses (CASA) and flow cytometry to explain Angus bull field fertility differences

The objective was to evaluate whether computer-assisted sperm analysis (CASA) and flow cytometry (FC) could explain differences in field fertility of five Angus bulls. The hypothesis was that high fertility bulls would exhibit the highest values for total motility (TM), progressive motility (PM), and intact plasma membranes, intact acrosomes, and normal calcium influx (VANCa), and lowest values for DNA fragmentation index (DFI). Two 0.5-mL straws (10 to 40 million sperm/straw) from each bull and same collection date were thawed simultaneously, pooled, and assayed in duplicate (n=92). Samples for CASA were stained with Hoechst 33342 and evaluated for TM and PM. In FC, the percentage of cells with VANCa were identified using a multiparametric staining procedure (Hoechst 33342, propidium-iodide, fluorescein isothiocyanate-conjugated peanut agglutinin-647 and fluo-3-acetomethoxy ester) and reported as a single population. DNA stability was assayed with acridine orange and reported as the percentage of single-stranded DNA (DFI). A generalized linear mixed model was used to analyze bull, duplicate, and their interaction as fixed effects, while collection date was considered a random effect and a beta distribution was assumed (SAS 9.4). There was no effect of duplicate or interaction (P>0.1). CASA and FC characteristics were different between bulls (P<0.05; Table 1). Bulls with the highest fertility (A and B) did not display the highest values of TM, PM and VANCa, nor the lowest value of DFI. Bull D, however, showed the highest values in TM, PM and VANCa, and lowest in DFI. Bull C, which had the lowest field fertility, did not present the lowest values in sperm analyses, whereas bull E showed the poorest in vitro values with an intermediate field fertility. In conclusion, CASA and FC were not able to completely explain the difference in field fertility between bulls.