The aim of the present study was to investigate whether dietary supplementation with antioxidants affects semen fertility parameters in New Zealand Standardbred stallions. Fourteen Standardbred stallions of varying fertility, from 3 studs located throughout New Zealand, were allocated to one of 3 treatments: minerals (Se, Cu, Zn, Mn) and vitamin E supplement, oil (canola) supplement and control (no supplement). The studs were provided with the supplements as aliquots to be added to each feed once per day. Stallions from one stud were fed a different basal diet from the other two studs. At least 3 semen samples were collected from each stallion and sent to Equibreed NZ Ltd, before, and after feeding the supplements for around 60 days. Spermatozoa fertility parameters evaluated included total motility, progressive motility, total progressive motility, morphology (normal, loose heads, head defects, mid-piece defects, tail defects), acrosome status (FITC-PNA), membrane integrity (hypo-osmotic swelling test), and concentration. These parameters were assessed at various times including 6-8h and 24h after collection of semen, and immediately and 30min after thawing, frozen semen. Blood levels of Se, Cu and Zn were measured before and after supplementation. Per cycle pregnancy data was also obtained from questionnaire responses from studs A and B at the end of the trial. This study demonstrates that there was a statistically significant effect of feeding oil as a dietary supplement on sperm motility at 24h after collection (longevity) and also on the per cycle pregnancy rates when treatment groups were combined. Consequently, we were able to elucidate a difference in the actions of mineral and vitamin E supplementation when compared with oil supplementation on fertility parameters measured in this trial. Other beneficial effects of antioxidant supplementation on sperm parameters were suggested from the results of the fourteen stallions on two separate diets, but were not found to be statistically significant.