A gronomy J our n al • Volume 10 9, I ssue 2 • 2 017 P ost-harvest seeding of cover crops reduces the risk of wind erosion and nutrient loss through leaching and runoff during the non-growing season, while increasing the biodiversity and resilience of prairie cropping systems (Martens et al., 2015). Yet, few studies have directly investigated late-summer to-fall seeded cover crop management practices in semiarid regions (e.g., Moyer and Blackshaw, 2009;Blackshaw et al., 2010;Liebig et al., 2015; Th omas et al., 2016a), which constrains their adoption by farmers (Liebig et al., 2015). A better understanding of how cover crop management practices impact subsequent annual crop performance could provide important knowledge to advance the use of cover crops in semiarid regions of North America.Th orup-Kristensen (1993) coined the term "pre-emptive competition" to describe the eff ect whereby cover crops assimilate N, but then decompose and mineralize too slowly to supply the retained N to the succeeding crop. Th us, the N supply to the subsequent crop depends on complex interactions among the cover crop characteristics, soil and climatic properties, and the succeeding crop itself (Th orup- Kristensen et al., 2003). For instance, oilseed radish decomposed more quickly than barley (Hordeum vulgare L.) in southern Manitoba (Halde and Entz, 2016), while glyphosate [N-(phosphonomethyl)glycine]-killed fall rye consistently reduced unfertilized spring wheat yield relative to no cover crop in southern Alberta (Moyer and Blackshaw, 2009). Whether oilseed radish may scavenge N as effi ciently as fall rye and subsequently decompose and mineralize to supply N to the succeeding crop has not been determined in southern Alberta.Fall rye and oilseed radish are two contrasting cover crops. Fall rye is a perennial monocotyledon with an extensive fi brous root system, while oilseed radish is an annual dicotyledon with a large taproot. Fall rye must be killed prior to planting the succeeding cash crop, chemical burndown (herbicide treatment) in spring being a common method, whereas oilseed A quadratic function explained 93% of the variability between pre-plant soil NH 4 -N plus NO 3 -N (0-7.5-cm depth) and spring wheat grain yield in 2014, indicating that the N supply limited spring wheat grain yield. We conclude that fall rye scavenged residual NO 3 -N better than oilseed radish during the non-growing season, particularly during the spring period when this perennial species assimilates N, but under semiarid conditions it may decompose and mineralize too slowly to supply N at the right time for the subsequent crop, while oilseed radish tended to boost spring wheat grain yield.