Biofumigation with Brassicaceae Cover Crops: Evaluation of Performance in Vegetable Cropping Systems

Brassicaceae cover crops can perform unique ecosystem functions in cropping systems due to the ability to accumulate large amounts of glucosinolates in their tissue.  In addition to other cover crop benefits, Brassicaceae species serve as biofumigants for soil disinfection when glucosinolates are hydrolyzed to isothiocyanates and other compounds after incorporation into the soil.  However, current literature suggests performance of Brassicaceae cover crops has remained unpredictable.  Understanding conditions that enhance the biofumigation effect of these cover crops would help enhance their ecosystem functions.  Therefore, multiple independent field studies were conducted in Michigan between 2003 and 2009 to evaluate performance of Brassicaceae cover crops in various vegetable cropping systems.  Cover crops included brown mustard (Brassica juncea ‘Common brown’), oilseed radish (Raphanus sativus ‘Defender’), yellow mustard (Sinapis alba ‘Tilney’), and Oriental mustard (Brassica juncea ‘Forge’). Vegetable crops tested included celery and onion (on muck soil) and cucumber, summer squash, muskmelon, and eggplant (on mineral soil).  Using weed suppression, crop growth and yields as examples of traits of interest, we observed large variations in cover crop performance among cropping systems.  Weed suppression in the range of 10 to 40% (mineral soil) and 30 to 90% (muck soil) was generally achieved especially during the first month following cover crop incorporation.  However, supplemental weed control was always needed for acceptable weed management.  Crop establishment seemed to be a major problem especially with direct seeded species.  In 2007, for example, we observed almost 100% loss in muskmelon stand when the crop was seeded within the first two weeks following biofumigation.  It appears that under Michigan growing conditions, allowing sufficient time between biofumigation (cover crop residue incorporation) and crop planting is critical.  The greatest increases in crop yield (5 to 20%) were observed for celery and onion when biofumigation was conducted in the fall and the cash crop planted the following spring/summer.  To optimize beneficial ecological functions of Brassicaceae cover crops, it is critical to take into account the agroecosystem under consideration, the cash crop and planting material (seed vs. transplant), soil type, soil biological properties, timing of biofumigation relative to cash crop planting, cover crop species, and cover crop growth stage.  Finally the practice of biofumigation with Brassicaceae cover crops should follow all basic rotational restrictions.