Most farm fields in the U.S. Midwest lay bare over the winter. From the time of corn and soybean harvests—sometime between September and December—until the planting season in spring, valuable topsoil is often unprotected. Wind, snowmelt, and rain can remove topsoil and nutrients from fields and carry them into the Mississippi River, where, instead of providing a foundation for crop production, they contribute to water pollution and flow into the Gulf of Mexico.
Cover crops are increasingly planted by farmers between growing seasons to reduce soil erosion and fertilizer runoff. Planting cover crops has been found to protect soil, improve soil carbon sequestration, reduce nitrogen and phosphorous runoff, and reduce the need for chemical weed control. But the practice has hindered yields of primary cash crops, a recent study has found. The study, led by NASA Harvest researchers, analyzed the productivity of farm fields across the U.S. Midwest and found that cover crop adoption was associated with an average yield hit of 5.5 percent for corn fields and 3.5 percent for soybean fields.
The map shows the yield impacts for cover crop adoption for corn (maize) fields in 2019 and 2020 in six states: Iowa, Indiana, Missouri, Ohio, Illinois, and Michigan. Red and orange areas on the map show fields with yield reductions in response to cover crop adoption. Yields were reduced the most in dark red areas.
The study identified farm fields which used cover crops for three or more years by using a recently developed dataset of cover crop presence between 2000 to 2021. The dataset fused vegetation greenness data from Landsat and NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) to identify fields using cover crops. Cover crop adoption in the six states has increased fourfold during the past decade, the analysis revealed: from 1.8 percent of total crop acreage in 2011 to 7.2 percent in 2021.
The researchers then calculated farm-level yields on fields using cover crops, and used a machine learning method to compare those fields to similar fields that went without cover crops. They found that almost all farmers using cover crops for three or more years saw reduced yields compared to those who let the soil lay bare. Yields were especially hindered on corn fields and on drier fields; this may be because cover crops compete with primary crops for water and nutrients.
“I’ve observed similar sentiments among farmers—many things need to go just right to avoid yield impacts, and that’s difficult to do,” said Jillian Deines, lead author of the study and now an earth scientist at Pacific Northwest National Laboratory. “This study is a rare, quantitative look at how agricultural conservation practices perform in the real-world, versus in tightly controlled experimental plots,” Deines added. Remote sensing data allowed Deines and colleagues to look at the field-level impact that management practices had on crop yields, instead of relying on county-level statistics.
In the United States, federal and state policies encourage cover crop planting by defraying the cost of adoption. Cover crops can provide environmental benefits including reduced soil erosion and reduced runoff of nitrogen and phosphorous into waterways, which contribute to algal blooms and the dead zone in the Gulf of Mexico. But cover crops can be labor- and cost-intensive to implement, and as this research shows, they can affect yields.
“Agriculture is a very tricky business to get right, and things typically don’t work out as planned,” said David Lobell, who is a co-author of the study and leads crop yield studies for NASA Harvest. Lobell added: “The combination of satellite data and powerful machine learning methods can help us make better informed decisions about conservation practices and any policies needed to support their adoption.”
NASA Earth Observatory images by Joshua Stevens, using data courtesy of Deines, J. M., et al. (2022) and Zhou, Q., et al. (2022). Story by Emily Cassidy.