In boars, heat stress is recognised to reduce fertility, libido and sperm parameters such as morphology, motility, viability and DNA integrity. Evidence of seasonal sub-fertility is largely anecdotal, although the impact of high environmental temperatures, particularly in tropical climates, on sperm motility is acknowledged. In this study, ejaculates from stud boars (n=9) from an AI centre in south-east Queensland, Australia, were assessed weekly from November (late spring) to May (late autumn). The objective was to determine whether a decline in sperm function could be associated with seasonal change. Routine procedures were followed for semen dilution in BTS extender, transport and storage, at ambient temperature. Samples were washed over a discontinuous percoll gradient (35% and 70%) and diluted in modified Tyrodes-based medium acclimated to 37˚C. CASA sperm motility assessment was used to evaluate motility parameters on day of collection (Day 0) and post-storage (Day 7) for before and after motility activation using 5 mM bicarbonate. Individual sperm trajectories (n=245,457) from CASA analysis was then statistically clustered into four sub-populations according to velocity and linearity of their trajectories. The sub-population with non-linear-low velocity, cluster mean 40 µm/s, trajectories showed a consistent proportion of approximately 45% and 50% of the motile population, on Day 0 and 7, respectively, throughout the study period. The sub-population with linear-high velocity, cluster mean 80 µm/s, trajectories showed a sharp decline from November, 20% and 10%, to mid-January, 4% and 2%, for the Day 0 and Day 7 sample respectively. This sub-population then remained below 2-4% until the study was concluded. Ejaculates appeared to be less sensitive to motility activation by bicarbonate from mid-January. These initial results suggest that mechanisms that control sperm motility are affected by seasonal heat stress and fertilising potential of spermatozoa cannot be restored rapidly after the seasonal heat stress subsides.