Cover crops in the upper midwestern United States: Simulated effect on nitrate leaching with artificial drainage

Authors: Malone, Robert - Rob; Jaynes, Dan; Kaspar, Thomas; Thorp, Kelly; KLADIVKO, EILEEN - Purdue University; Ma, Liwang; James, David; SINGER, JEREMY - Former ARS Employee; MORIN, XENIA - Rutgers University; SEARCHINGER, TIMOTHY - Princeton University

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2014
Publication Date: 7/1/2014
Citation: Malone, R.W., Jaynes, D.B., Kaspar, T.C., Thorp, K.R., Kladivko, E.J., Ma, L., James, D.E., Singer, J.W., Morin, X.K., Searchinger, T.D. 2014. Cover crops in the upper midwestern United States: Simulated effect on nitrate leaching with artificial drainage. Journal of Soil and Water Conservation. 69:292-305.

Interpretive Summary: Nitrate losses from agricultural lands in the Midwest flow into the Mississippi River Basin and contribute to hypoxia in the Gulf of Mexico. Previous work has shown that winter cover crops can reduce nitrate loadings, but the potential impact across the Midwest is currently unknown. We used the Root Zone Water Quality Model (RZWQM) to simulate the use of winter rye as a cover crop and estimated its impact on nitrate losses from drained fields at 41 sites across the Midwest. One of the simulated treatments was winter rye overseeded (aerial seeded) into a no-till corn-soybean rotation. On average, this treatment reduced simulated nitrate loss in drainage 42.5% over the sites compared to systems without winter rye. Regression analysis of the RZWQM variables from these sites showed that temperature and precipitation during winter rye growth, nitrogen-fertilizer application rates to corn, and simulated corn yield account for 99% of the simulated site-to-site variability in nitrate loss reductions in tile flow due to winter rye. For example, greater nitrate loss reductions were estimated from adding winter rye at sites with warmer temperatures and less precipitation because of more cover crop growth and more soil nitrate available for cover crop uptake. We estimate that winter rye has the potential to reduce nitrate losses from drained fields by 166 million kilograms per year or about 20% of the total nitrate load in the Mississippi River considering just the area of 5 Midwestern states that drains into the Mississippi River (MN, IA, IL, IN, OH). We estimate that the cost per kilogram of nitrate removed from drainage water by winter rye adoption in corn-soybean systems ranges from $2.08 to $4.13 (U.S. dollars), a cost quite competitive with other management practices for reducing nitrate loads in surface waters especially if the other benefits of winter rye are considered such as erosion control. This research will be helpful to policy makers and scientists interested in hypoxia in the Gulf of Mexico and nitrate loading to surface waters from agricultural fields by providing the following estimates associated with adopting cover crops to tile drained fields in corn and soybean in the U.S. Midwest: 1) the potential nitrate load reduction to the Mississippi River; 2) the cost per kilogram of nitrate removed and 3) the influence of weather and management variables on cover crop impact.

Technical Abstract: Fall-planted winter cover crops are an agricultural management practice with multiple benefits that includes reducing nitrate losses from artificially drained fields. While the practice is commonly used in the southern and eastern U.S., little is known about its efficacy in Midwestern states where artificial subsurface drainage is widely used in corn-soybean systems (Zea mays L.- Glycine max [L.] Merr.) and winters are longer and colder. We used a field-tested version of the Root Zone Water Quality Model (RZWQM) to simulate the use of cereal rye (Secale cereale L.) as a winter cover crop and estimate its impact on nitrate losses from drained fields at 41 sites across the Midwest. One of the simulated treatments was winter rye overseeded (aerial seeded) into a no-till corn-soybean rotation at simulated main crop maturity. On average, this treatment reduced simulated N loss in drainage 42.5% over the sites compared to systems without winter rye. Regression analysis of the RZWQM variables from these sites showed that temperature and precipitation during winter rye growth, N fertilizer application rates to corn, and simulated corn yield account for 99% of the simulated site-to-site variability in nitrate loss reductions in tile flow due to winter rye. For example, greater N loss reductions were estimated from adding winter rye at sites with warmer temperatures and less precipitation because of more cover crop growth and more soil N available for cover crop uptake. We used soil and land cover spatial databases to estimate that a minimum of 2.3 million ha (5.7 million ac) of drained land were used to grow corn in rotation with soybean within five Midwestern states (OH, IN, IL, IA, MN) and would be immediately suitable for winter rye because they were in no till or ridge till systems. Because fall-tillage complicates timely winter rye planting in corn-soybean systems, an additional 6.1 million ha (15.1 million ac) would be suitable for winter rye by delaying tillage until spring on these lands. If winter rye were overseeded at cash crop maturity on all 8.4 million ha (20.8 million ac) of these lands, we estimate nitrate-N losses in drainage would be reduced by approximately 195 million kg yr-1 (430 million lb yr-1) by combining the spatially variable RZWQM results with the land area estimates. Considering just the area of the five states that drains into the Mississippi River, winter rye has the potential to reduce nitrate-N losses from drained fields by 166 million kg yr-1 (366 million lb yr-1) or about 20% of the total nitrate-N load in the Mississippi River. We estimate that the cost per kg (2.2 lb) of nitrate-N removed from drainage water by winter rye adoption in corn-soybean systems ranges from $2.08 to $4.13 (U.S. dollars), a cost quite competitive with other management practices for reducing nitrate loads in surface waters especially if the other benefits of winter rye are considered.