The reproductive fitness of a male animal is not only measured by the ability of his sperm to fertilise the egg, but also the quality of his sperm's DNA, which inevitably contributes half the genetic code to his offspring. There are many factors that may influence the quality of sperm DNA, but the greatest threat comes from oxidative stress; an imbalance in the cell's biochemistry which may be counteracted via dietary supplementation with selected antioxidants. There is a large body of knowledge surrounding factors leading to oxidative stress in other species, particularly the human, and while there are very few studies directed toward the horse, we do know that the two major factors driving oxidative stress in this species are age and environmental heat. The horse is one of the very few livestock species in which the males continue to be commercially and genetically valuable as breeding animals until old age. While we understand that age-related changes are coupled with declining hormone levels, we also know that this hormonal decline can be driven by oxidative stress. Similarly, oxidative stress and subsequent sperm DNA damage are caused by heat stress, a phenomenon which was revealed by this research team under a previous AgriFutures Project. Following this research, we now aim to identify the best laboratory test to diagnose oxidative stress in aged and heat stress-susceptible stallions to allow timely interventions such as dietary antioxidant supplementation or climate control. Furthermore, we will develop a horse-specific heat index, which will allow stud farm personnel to manage stallion housing in such a way as to minimise heat stress and demonstrate improved sperm quality. The outcomes of this project will be improved breeding efficiency, foal health and stallion welfare, with resulting efficiencies set to underpin overarching economic benefits for the industry at large.