Recent advances in artificial insemination (AI) and semen preparation have led to an increase in the availability of equine genetics globally. Generally, less than half the stallion population produce semen of suitable robustness to maintain acceptable fertility post-freezing, and research is needed to improve not only the numbers of stallions whose semen can be successfully frozen, but also to increase the number of collections that meet fertility standards. The number of progressively motile sperm present during ovulation determines successful conception in horses. AI practitioners recommend that 500 million progressively motile sperm should be introduced into the mare’s uterus to ensure best chance of pregnancy (Matson and Pycock, 2006), and progressive motility is currently accepted as suitable for determining fertility in stallions. Established laboratory quality assessments are used to identify stallions with suitably robust semen for freezing and worldwide distribution, however fertility traits can be influenced by many factors, e.g. age, level of competitive performance or workload, nutrition, disease and stress (Tucker and Morris, 2006). In order to investigate the impact of such factors on semen quality, it is first important to determine the level of variation in semen quality between stallions that are currently used for frozen semen collections, which formed the main objective of the following study. This baseline can then be used to compare improvements in semen quality from various nutritional and management interventions, which form the basis of our ongoing research program.