A five-year study conducted by the Iowa Learning Farms, Iowa State University Extension and Outreach, and Practical Farmers of Iowa showed that cover crops had no negative effect on cash-crop yields but did reduce concentrations of nitrate in subsurface water significantly. The study tested both single-species cover crops and mixed cover crops prior to soybeans and corn. Before corn, the single species was oats and the mixture contained oats, hairy vetch, and radish. Before soybean, the single species was rye and the mixture contained rye, rapeseed, and radish. The largest reduction in nitrate concentration was delivered by the single-species treatments, which also had the lowest cost of establishment. The researchers noted that proper management and correct planter settings were keys to success in maintaining yields.