Composting is being increasingly adopted by the beef feedlot industry in southern Alberta. However, little is known about the characteristics of runoff (e.g. nitrogen, phosphorus concentrations) emanating from compost windrows subjected to heavy rainfall. A rainfall simulator was used to generate rainfall (111 mm hr^-1) and subsequent runoff from beef feedlot manure compost windrows. Two windrows were set up on a roofed concrete pad on August 13, 2009 (Day 0), turned on Days 13, 28, 42, 55, and 83, and rolled into two curing stockpiles on Day 109. Rainfall was generated at 2 locations per windrow on Days 18, 27, 40, 54, 81, 109 and 224 (after curing). The time to generate runoff decreased as the compost aged. The fresher material soaked-up more incoming rainfall before runoff commenced. The aged compost allowed faster infiltration of incoming water and quicker runoff: e.g. 16.9 min to generate the first 5 L of runoff on Day 18, compared to 9 min on Day 81.Nitrate-N in the runoff was initially high (35 mg L^-1, Day 18), dropping to 8-9 mg L^-1 (Days 27, 40), before increasing to 26 mg L^-1 (Day 54), 58 mg L^-1 (Days 81-109) and attaining its highest value in mature compost at (100 mg L^-1,Day 224). This trend follows nitrate-N production in the windrows as levels increased with time on conversion from ammonium-N. Water-extractable P values were highest in fresher material, (49-55 mg L^-1, Days 18-27), decreasing to 32-33 mg L^-1 (Days 40-52), and 24-25 mg L^-1 (Days 81-109) before finishing at 31 mg L^-1 in mature compost (Day 224). The study showed that runoff quantity and quality are influenced by the age of compost which has implications for the timing of rainfall events and potential risk to water quality if runoff is not contained.