Artificial insemination is prohibited for the breeding of Thoroughbred horses, and as such there is no way to assess the quality of a Thoroughbred stallion ejaculate prior to breeding. However, residual ‘dismount’ semen samples are routinely collected from the urethra immediately after coitus to confirm ejaculation and assess basic sperm parameters to determine the need to re-breed (‘cross-cover’) the mare prior to ovulation. The resazurin reduction assay (RRA) has previously been shown to provide a basic assessment of fertility potential in both rams and bulls, and the aim of this study was to investigate whether a modified RRA protocol would be effective in identifying sub-fertile ejaculates from stallions. Initially, the relationship between the RRA signal and sperm assessments including motility (CASA), antioxidant capacity (aldehyde dehydrogenase) and reactive oxygen species production (MitoSox Red) was established, after which a field trial was conducted in which the RRA, motility and morphology assessments were run on commercial Thoroughbred dismount samples (N=315) to compare their ability to discriminate between ejaculates which did, or did not result in pregnancies. RRA signal was influenced by sperm concentration, various CASA motility parameters and antioxidant capacity (R2 = 0.83), and the field trial revealed that breedings which resulted in pregnancies had significantly higher RRA signals than those that did not result in pregnancies (383.34±29.32 vs. 292.74±23.41 AFU; P=0.02). Interestingly, total motility (54.02±1.36 vs. 51.01±1.72; P=0.17), progressive motility (40.62±1.33 vs. 37.81±1.76%; P=0.19), and normal morphology (70.45±0.78 vs. 69.10±0.90%; P=0.37) were not significantly different between these two populations. This study indicates that integration of the RRA into dismount sample analysis would aid in improving management outcomes and welfare of both stallions and mares by minimising unnecessary re-breedings, with the potential for automation of the RRA for an on-farm device to further improve the efficiency of the Thoroughbred breeding industry.