Decomposition of cover crop mulches has received little attention in the scientific literature, particularly in the context of the organic rotational no-till systems adapted for the climatic conditions of the northern Great Plains of Canada. The objective of the study was to determine the effect of plant species and mulch application rate on cover crop decomposition and mulch quality parameters over time. Using the litter bag technique, six plant species and two mulch application rates were tested twice in field studies at Carman, Manitoba, Canada. Among the plant species tested, decomposition was the fastest with oilseed radish (Raphanus sativus L.) and the slowest with barley (Hordeum vulgare L.). The effect of application rate on mulch biomass was not consistent between experiments. Mulches released a large amount (46.4%) of their initial N content after only 30 days of field placement, for all levels of plant species and mulch application rates combined. Forty-four percent of initial N content still remained in the mulches by early May (Day 250), and may be available for the subsequent crops seeded in the spring or later in the crop rotation. This research provides us with valuable information on nutrient release, soil cover, and potential weed control from mulches.