The researchers studied how planting oat and hairy vetch cover crops in winter affects maize growth, soil erosion, water run-off, and nitrate leaching in Japan. Maize grown after hairy vetch had similar yield to the control with less fertilizer, while maize after oat had lower yield. Using cover crops reduced runoff and erosion significantly. Nitrate leaching was highest in the control, followed by hairy vetch-maize, hairy vetch + oat-maize, and oat-maize. Hairy vetch-maize had better nitrogen supply timing for maize growth compared to oat-maize or conventional cultivation.