Optimal rates of litter retention for maximizing annual aboveground net primary productivity (ANPP) were investigated in native mixed-grass rangelands in Saskatchewan, Canada. The study was conducted on 18 independent study sites on brown and dark brown Chernozem (Mollisol) soils during the 2011 and 2012 growing seasons. Each site contained 30 treatment plots with two control plots and 28 treatment plots receiving between 1 and 2 290 g/m² of added plant litter sourced from a reference site. Soil moisture values were recorded for each plot in May, June, and July, while forb, graminoid, and total ANPP were harvested at the end of the growing season. Soil moisture-litter relationships were driven by weather patterns with significant differences between years. Graminoid ANPP-litter relationships were nonlinear with peaks in productivity between 54 and 412 g/m² of litter mass in the brown soil zone and between 27 and 157 g/m² of litter mass in the dark brown soil zone. Forb ANPP was highest at the lowest litter masses and was not significantly influenced by study year or soil zone. The recommended litter retention rates in provincial range health assessment guidelines are accurate for the dark brown soil zone but are lower than the levels identified here for the more arid brown soil zone.