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Study Results Show Significant Nitrate Reduction and Unchanged Crop Yields

November 5, 2018

The results of a five-year study conducted by the Iowa Learning Farms, Iowa State University Extension and Outreach, and Practical Farmers of Iowa reinforced that cover crops added to a corn-soybean rotation have no negative effect on yield and result in statistically significant reductions in nitrate concentration in subsurface water. The details of the study are included in an infographic available for download at

no till soybean field.This study helps to dispel the commonly held belief that cover crops reduce yield in the following cash crops. Throughout the 22 site-years of yield data, there was no significant difference in cash crop yields between control strips without cover crops and those planted with cover crops. The report notes that planter settings may impact yield if not properly managed to accommodate residue from the cover crops.

Iowa soils are highly vulnerable to nitrate losses between April and June when natural nitrate production exceeds typical crop demands. The analysis of water samples from those three months showed a statistically significant reduction in nitrate concentration in the cover crop strips. The data collected for water quality was comprised of 17 site-years. The most significant reductions were measured in the single species treatments – 61 percent reduction for rye and 23 percent for oats when compared to the no cover treatment in the same field.

“We are excited to see this significant reduction in nitrate concentration when cover crops are present, as addressing nitrate levels is a key component to reaching our Iowa Nutrient Reduction Strategy goals,” stated Liz Juchems, conservation outreach specialist for Iowa Learning Farms who oversees the data collection and plot management for the project.

The project started in 2013, with plots at six Iowa State University research farms in no-tillage corn and soybean rotation fields. The plots were prepared with three treatments: single species, mixture and no cover crop. Each treatment was replicated four times. 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.

For all project sites, spring and fall cover crop biomass and cash crop yield data were collected to evaluate the establishment of the cover crops and potential yield impacts. To evaluate impacts on water quality, suction lysimeters were installed at five sites. Lysimeter pore water samples were taken and analyzed for nitrate once every two weeks over the growing season.

“Since this project had locations located throughout the state, we were able to see how the different cover crop species performed in different soil regions and weather patterns. We observed consistent establishment and biomass production of the rye and oats at all sites and gained the largest reduction in nitrate concentration from those single species treatments. Rye and oats provided the most biomass and had the lowest cost of establishment, helping make them the top choice for cover crops in Iowa,” said Juchems.

Proper management is key when incorporating cover crops into a corn-soybean rotation and maintain cash crop yield. Effective termination of overwintering species with herbicide requires an actively growing plant and is recommended to be completed 7-10 days before planting corn and at the time of planting for soybeans.

Source: Liz Juchems, Iowa State University