There is increased interest in composting as a means of handling the large amounts of manure generated by southern Alberta feedlots. However few studies have been conducted on the changes in physical properties of manure as it becomes compost and the impact of these changes on haulage requirements. Additionally, there is a perceived constraint to overwinter composting in southern Alberta, due to extremely low air temperatures. This study examined active (mechanically turned) and passive (passive aeration system) windrow composting during winter and summer. Dry matter mass reductions were in the range of 21-30%. Bulk density increased 3-4 fold with both types of composting. Volume loss during the thermophilic phase was of the order: summer-active (72%) > summer-passive (55%) > winter-active (51%) > winter-passive (34%) with further smaller losses during the curing phase. Water mass loss was as high as 83% for active composting during summer. Active composting generally led to larger changes in physical properties of manure than passive composting. Winter composting was feasible despite ambient air temperatures < -30 degrees C. The results demonstrate the ability of composting to substantially reduce the mass, volume and water content of manure that needs to be transported for land application.