Rotating Mid-South corn and soybeans

The impendent importance of ethanol as a fuel alternative in the United States has pushed corn to the forefront as a money-making crop for Mid-South farmers. The question is: does the Mid-South have the background to immediately grow corn to its fullest yield and/or profit potential?

The U.S. Corn Belt has long used a corn/soybean rotation to enhance production of the two crops. However, there is no general agreement on the specific reasons for this enhancement.

Nitrogen availability is usually identified as responsible for the greatest proportion of increased corn yields following soybeans, but other factors such as decreased weed, disease, and insect pressures may also be important for both crops in this rotation.

Several summaries of results from numerous long-term studies conducted in the Midwest conclusively prove that yield of corn grown in a 1:1 rotation with soybeans produces greater yields than corn grown following corn.

A review published on a Purdue University Web site ( presents yield results from 16 studies, each conducted for six years or more throughout the Corn Belt. The average increase in corn yield following soybeans in these studies was 8.5 percent (154 versus 142 bushels per acre).

A summary of 20 years of rotation research conducted in northeast Iowa ( showed the same trend. From 1979 to 1998, yields of corn following soybeans exceeded yields of corn following corn by 33 percent (141 versus 106 bushels per acre) when 80 pounds of nitrogen fertilizer per acre were used for corn.

Yields of corn following soybeans exceeded corn following corn yields by 15.5 percent (148 versus 128 bushels per acre) when 160 pounds of nitrogen fertilizer per acre were used for corn, and by 11.8 percent (151 versus 135 bushels per acre) when 240 pounds of nitrogen per acre were applied to the corn. These results indicate that corn following soybeans requires less fertilizer nitrogen than corn following corn.

A report in Agronomy Journal (Varvel and Wilhelm, Vol. 95, p. 1220-1225, 2003) utilized yield response data from two long-term studies (20 years dryland and 10 years irrigated) conducted in Nebraska. The dryland results showed that corn following soybeans yielded 17 percent more than corn following corn (137 versus 117 bushels per acre) when 80 pounds per acre of nitrogen fertilizer were applied to the corn, and 4.5 percent more (135 versus 129 bushels per acre) when 160 pounds of nitrogen per acre were applied to the corn.

With irrigation, corn following soybeans yielded 9.5 percent more (184 versus 168 bushels per acre) and 4.6 percent more (183 versus 175 bushels per acre) when 90 and 135 pounds per acre of nitrogen were applied to the corn, respectively.

The Nebraska scientists went an important step further in their work. They determined that the amount of nitrogen supplied by soybeans to the following corn crop in both the dryland and irrigated studies was approximately 60 pounds per acre per year. They stated that this additional nitrogen does not become available until late in the growing season, thus making it difficult to detect with late-fall or early-spring soil testing.

They concluded that this amount of additional nitrogen available to corn following soybeans must be considered when fertilizer recommendations are made. Failure to do this results in excessive nitrogen application to corn, thus increasing expense and potential nitrogen loss.

It is unreasonable to assume that the Midwestern results will directly transfer to the Mid-South.

First, Mid-South soil properties present a much different environment for off-season maintenance of soil nitrogen levels (higher soil temperatures, frequent long-term soil saturation resulting in anaerobic soil conditions, etc.) that can result in greater losses of soil nitrogen during the winter months.

Second, higher temperatures in the Mid-South during the winter months will result in greater decomposition of crop residues between harvest and next season’s planting. Both of these factors will affect residual soil nitrogen levels.

Third, lower dryland crop yields in the Mid-South will presumably result in different nitrogen use patterns by corn and less crop residues. The magnitude of these differences is not known. (Growing dryland corn in the lower Mid-South is not recommended.)

There is anecdotal evidence that corn yields will be greater following soybeans in the Mid-South, and this naturally leads to the assumption that rotation of the two crops will change the dynamics of their production. However, there is a lack of long-term research that documents just how a corn/soybean rotation will perform outside the Midwest.

The Delta’s corn and soybean researchers need to make a concerted and cooperative effort to initiate and conduct research that will document the perceived potential benefits of a corn/soybean rotation system of production.

Two things are certain regarding efforts to ascertain the potential benefits of rotating the Mid-South’s grain crops.

First, the recent surge in commodity prices and costs for fuel, fertilizer, genetically modified seed, and other inputs make it imperative that agronomic findings be supplemented by economic analyses to determine the monetary ramifications of rotations.

Second, previous research in this area is probably obsolete because of the recent changes in commodity prices, input costs, and technology.

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