The usefulness of combining traditional sperm assessments with in vitro heterospermic insemination to identify bulls of low fertility as estimated in vivo.

To date, no single in vitro assessment can estimate bull fertility. This research was aimed at evaluating the ability of a series of laboratory assessments to assign 50 Holstein Friesian bulls grouped as low (ER-NRR<-1.5), medium (-0.5<ER-NRR<+0.5) and high (ER-NRR≥+1.5) fertility based on estimated relative non-return rates (ER-NRR), to the two categories of low and medium-high fertility. Heterospermic insemination with a Piedmontese reference bull was employed to define an index of competitive binding ability (CBI) to the zona pellucida using fluorochrome-labeled sperm, and a competitive fertility index (CFI) using embryo paternal assignment by single nucleotide polymorphism (SNP). Furthermore, kinetic parameters, membrane integrity and sperm DNA/chromatin integrity (% DFI) were assayed. Low fertility bulls had lesser (P<0.05) values for total motility and membrane integrity, and a greater value of % DFI as compared to medium and high fertility groups. A modest (P<0.001) correlation was reported among ER-NRR and total motility (r=0.30), progressive motility (r=0.26), membrane integrity (r=0.43) and % DFI (r=-0.26). While % DFI alone allowed the identification of 70% lowly fertile bulls, combining membrane integrity, average path velocity and CBI allowed for identification of 78% of the lowly fertile sires. Paternal assignment by SNPs resulted in 96% of successful assignments and could provide an alternative support to microsatellites for in vivo studies based on heterospermic fertilization for estimating fertility.