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The effects of cover crops on nutrient cycling, weed suppression, and onion (Allium cepa) yield were evaluated under a muck soil with high organic matter in Michigan. Four brassica cover crops, including brown mustard (Brassica juncea ‘Common brown’), oilseed radish (Raphanus sativus ‘Daikon’), oriental mustard (B. juncea ‘Forge’), and yellow mustard (Sinapis alba ‘Tilney’), as well as sorghum sudangrass (Sorghum bicolor × S. sudanense ‘Honey Sweet’) produced similar amount of biomass and recycled similar amounts of nitrogen, phosphorus, and potassium. The brassica cover crop biomass contained more calcium, sulfur, and boron, but less magnesium, iron, manganese, copper, and zinc than sorghum sudangrass. However, soil fertility was generally similar regardless of whether a cover crop was used. This was mainly because the soil was sampled when most of the cover crop residue was not yet decomposed. Weed density during onion growth was reduced by all cover crops compared with the control with no cover crop, with yellow mustard treatment having the lowest weed density among the cover crops. Weed species composition was also significantly affected by the cover crops. Yellow mustard treatment had the lowest density of common purslane (Portulaca oleracea) and redroot pigweed (Amaranthus retroflexus), whereas sorghum sudangrass had the highest yellow nutsedge (Cyperus esculentus) density among all the treatments. However, weed suppression was not enough to eliminate normal control strategies. The brassica cover crops, especially oilseed radish and yellow mustard, increased onion stand count and marketable yield. These results suggest that brassica and sorghum sudangrass cover crops could provide multiple benefits if incorporated into short-term onion rotations under Michigan growing conditions.

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