Effect of a Radish Cover Crop On Nitrogen Availability to Corn Following Small Grains in Minnesota.

Nitrogen losses from crop fields reduce efficiency of use of this critical nutrient and can contribute to greenhouse gas emissions and reductions in water quality. One proposed solution is the use of cover crops as catch crops to take up excess plant-available nitrogen after a main crop finishes growing and release it to the next main crop. To evaluate the effectiveness of radish (Raphanus sativus L. cultivar "Groundhog") as a catch crop, studies were initiated in fall 2010 and 2011 at two sites in southern Minnesota. In August, radish cover crops were planted into oat stubble. Before radish planting, urea was applied to ensure available nitrogen levels of at least 67 kg ha^-1, after which the whole field was disked, field cultivated, and harrowed or cultivated with a Roterra soil stirrer. The radish cover winterkilled in November to December. The following spring, the plots were subdivided and nitrogen was applied at 0, 45, 90, 135, and 179 kg ha^-1 in the form of urea. Corn (Zea mays L.) was planted as a test crop. Soil and biomass samples were collected at key points during the cover crop and corn growing seasons. Although the radish cover crop reduced late fall soil nitrate significantly relative to the control in two out of four site-years, it did not affect V8 soil nitrate level in the zero-nitrogen treatment significantly in any site-year. Results from the first year of the study also show no effect of the cover crop on corn nitrogen uptake at V8 in the zero-nitrogen treatment or on corn grain yield. These results suggest that in a small grain-corn rotation in Minnesota, a radish cover crop may not function effectively as a catch crop even with moderate to high levels of available soil nitrogen in the fall.