Integrating Brassica Cover Crops Into Onion Cropping Systems: Implications for Plant Population, Stand Establishment, and Yield

Use of cover crops in vegetable production has become an indispensable tool to increase soil organic matter, reduce soil erosion, suppress weeds, and improve soil biological activity. Selection of cover crops largely depends upon intended agro-ecological service. Brassica cover crops produce substantial amount of above- and below-ground biomass and have the potential to suppress plant parasitic nematodes and soil borne diseases. Due to these positive attributes brassica cover crops are widely used and may help improve onion cropping systems. This research investigated the impact of oilseed radish (Raphanus sativus ‘Defender’) and yellow mustard (Sinapis alba ‘Tilney’) on onion stand establishment, growth, yield, and grade distribution on a muck soil at the Muck Soils Research Farm, Michigan State University. A split plot experimental design was set up with cover crop treatment (oilseed radish, yellow mustard, or bare soil) as the main plot and five onion planting densities (647500, 562500, 512500, 430000, and 350000 plants/ha) as sub plots. Cover crops were seeded in late summer before planting of onion crop the following year in 2007, 2008, and 2009. Onion stand establishment and yield responded positively to brassica cover crops. In 2008, overall onion stand was about 44% greater in the cover crop plots compared with the control without cover crop.  The impact of the cover crops on marketable yield varied with onion planting density.  Although cover crops increased total yields, at highest onion plant density (647,500 plants/ha), good stand establishment in cover crop plots, significantly increased the quantity of small bulbs, which were non-marketable. As onion plant density decreased, the benefit of using the cover crops became more evident.  At an onion plant density of 430,000 plants/ha which is 75% of normal planting density, cover crops increased marketable yield by 25% as compared to the control plot. This could be attributed to excellent onion stand count in cover crop plots as compared to the control plot. Onion stand establishment or marketable yield was similar for both cover crops (oilseed radish or yellow mustard). Positive impact of cover crop on onion stand establishment has direct implication on onion seeding rate. This work suggests that onion growers using brassica cover crops on muck soils should lower onion seeding rates. Often use of brassica cover crops are cost-prohibitive; however, savings in onion seed cost could help offset some of the cost associated with the use of those cover crops.