Factors influencing the “sortability” of stallion spermatozoa into X- and Y-chromosome bearing populations

Intrinsic differences between stallions exist for semen traits such as motility, morphology fertility and the ability of spermatozoa to survive cryopreservation processes. Ejaculates from 11 stallions were used to test the differences between stallions when selecting X- and Y-chromosome bearing spermatozoa using a modified flow cytometer. Data on orientation and viability of spermatozoa were collected during sex-sorting, and motility characteristics of sex-sorted and non-sorted (control) spermatozoa were assessed before and after cryopreservation. An index was created to rank each stallion in order of their suitability for sex-sorting using the data generated by the flow cytometry software. Motility of spermatozoa was higher after sorting and cooling than in the fresh ejaculates, but was significantly lower after thawing in comparison to fresh semen for both sex-sorted and non-sorted spermatozoa. Semen samples with a high percentage of food dye positive, defined as dead, spermatozoa had a low sortability index and ranking. Thus, percentage of dead spermatozoa in the semen sample was identified as the most important factor determining sortability. We conclude that variation between stallions exists for the sortability of their spermatozoa and that the sortability index is a useful tool for the selection of suitable stallions for a sex-sorting program.