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Updates on the Benefits of Crop Rotation

Crop rotation is a term used to describe the pattern of growing two or more crop species in a given field in some consecutive order. Soybean is commonly rotated with corn, wheat, rice, or grain sorghum in the U.S. Reasons given for growing soybean in rotation rather than continuously are: 1) higher yields of one or both crops; 2) a decreased need for N fertilizer on the grain crop following soybean; 3) increased residue cover; 4) mitigation of pest and weed cycles; and 5) improved economic potential.

The perception is that rotation of soybean with a grain crop provides positive agronomic, environmental, and economic benefits. This is based on long-term soybean:corn rotation research that has been conducted in the Midwest. Research is now being conducted in the Midsouth to verify and/or confirm those results at Midsouth locations.

One such study was recently conducted at the Lon Mann Cotton Research Station in Marianna, Arkansas [lat. 34°46' (roughly equivalent to extreme North Miss.)] under the direction of Dr. Jason Kelley, Professor and Extension Agronomist with the Univ. of Arkansas Cooperative Extension Service. Highlights of that research are summarized in a video and in a Delta Farm Press article by Alaina Dismukes. Click here for the objective results that appear in the Arkansas Soybean Research Series publication. Below are details about the conduct of and results from the corn/soybean portion of theresearch.

•   The soil at the site was a Memphis silt loam.

•   Plots were conventionally tilled before planting corn and soybean.

•   Soybean variety each year was a MG 4.6-4.9 that was planted in April.

•   Corn hybrids were 112-117 days maturity.

•   Both crops were irrigated each year.

•   Soybean yields following corn vs. following soybean were increased in 4 of the 6 years of the study by an average of 10.8 bu/acre.

•   Corn yields following soybean vs. following corn were increased in 5 of the 6 years of the study by an average of 9.6 bu/acre.

•   These results indicate that both corn and soybean yields are better when grown in rotation with each other than when grown as monocrops.

•   Although data are not shown, the authors allude to finding a lower number of cysts of soybean cyst nematode following corn than following soybean. Their yearly sampling did not find any root knot nematodes in soil at the study site.

The University of Arkansas presented virtual/online Soybean and Corn Field Days in Sept. 2020. Check out the presenters and their presentations at Univ. of Ark. 2020 Virtual Soybean Field Day and Univ. of Ark. 2020 Virtual Corn Field Day. This is an excellent opportunity to view pertinent research being conducted on these two major crops in the Midsouth.

A study conducted in Illinois addressed a little-researched facet of the corn-soybean rotation. Results from that study are reported in an article titled “Long-term crop rotation and tillage effects on soil greenhouse gas emissions and crop production in Illinois, USA” that is authored by Behnke et al. It is published in Agriculture, Ecosystems and Environment 261 (2018) 62-70 (https://doi.org/10.1016/j.agee.2018.03.007). Pertinent points about the rotation part of the research and its results follow.

•   Experimental plots were established in 1996. Measurements of greenhouse gas emissions (GHG) from soil were taken weekly during the 2012-2015 growing seasons, or 17 through 21 years after the plots were established.

•   Soils at the site were primarily silty clay loam and silt loam.

•   Crop rotations were continuous corn (CCC), corn-soybean (CS), corn-soybean-wheat (CSW), soybean-corn (SC), continuous soybean (SSS), and wheat-corn-soybean (WCS). Only results from the CCC, CS, SSS, and SC rotations are presented here.

•   Corn yields from the CS rotation were about 46 bu/acre greater than those from the CCC rotation.

•   Soybean yields from the SC rotation were about 4.5 bu/acre greater than those from the SSS rotation.

•   Daily nitrous oxide (N2O) emissions were higher from all rotations with corn compared to rotations with soybean. This was likely due to N fertilizer applications to corn. The conclusion is that decreasing the number of corn years in a rotation will lower N2O emissions.

•   CCC had greater daily CO2 emissions compared to SSS. This was attributed to greater residue amounts in the CCC rotation.

•   The results from this study confirm that a CS or SC rotation will result in increased yields of both crops compared to each grown as a monocrop.

•   The results from this study also indicate that using a crop rotation with less corn can be an effective strategy for reducing GHG emissions, especially N2O.

•   These results add a valuable contribution to the positive effects of long-term crop rotations that involve corn–i.e., not only will yields of both corn and soybean be increased, but having less corn in rotations will help in mitigating GHG emissions.

For insight into the significance of N2O as a greenhouse gas, and contributors to its emission and the importance of its mitigation, read the article titled “Measuring Greenhouse Gases Starts in Soil“ from North Carolina State University’s Dept. of Crop and Soil Sciences.

Annual statistics from the USDA-NASS confirm the decades-long dominance of corn and soybean as the major row crops in the U.S.–i.e., by far, they occupy the most acreage in U.S. farming systems. The above results further confirm that production potential of both crops is enhanced by their rotation with each other. In addition, rotating these two crop with each other apparently will mitigate GHG emissions from the sites on which they are grown.

Composed by Larry G. Heatherly, Oct. 2020, larryheatherly@bellsouth.net