As more farmers consider planting a field in corn for multiple years, they need to consider the yield loss associated with that decision.

Economic factors may favor growing corn over soybeans based on federal farm payments and actual pricing, but a University of Kentucky grains specialist advises farmers to remember the nearly 10 percent yield loss a second consecutive year of corn in the same field can cause. This needs to be factored in as they consider the economic advantage of corn versus soybeans.

“It seems that more of our producers are getting away from rotating crops and are favoring, or the economics to them seem to favor, growing more corn meaning two or three years of corn rather than rotating corn and soybeans or corn, wheat and soybeans,” said James Herbek, UK grains specialist. “Most of our farmers were doing some kind of rotation but now are growing more second and third year corn. There are a lot of questions being asked about what it is doing to corn yields or what can be done to negate some of that loss.”

It is well established that rotating crops provides a yield increase, he said, while continuous corn results in yield decreases. This is probably due to less disease, insect and weed pressure in rotated crops because the cycle is being broken and they aren’t allowed to build up.

There also is a rotation effect that increases yields whether or not there are pest pressures, Herbek said. Some of the theories surrounding the phenomenon include: chemical compounds in the stalk residue or roots that inhibit growth of the same crop; chemical compounds in a different crop that actually stimulate the other crop; and unfavorable microbes that increase over favorable ones in soils where a continuous crop is grown.

There are studies looking at these theories and it may be a combination of all three or other factors that is resulting in the yield reductions, Herbek said.

“Generally, we say you are going to have a 10 to 15 percent reduction in your yields if you grow the same crop two years or more in succession,” he said. “Some years you may only have a 5 to 10 percent reduction, some years you may have as high as a 20 percent reduction. This is due to differences in the environment and/or the growing season.”

A study at the UK Research and Education Center in Princeton some 15 years ago on crop rotation found there is about a 12 bushel per acre yield advantage in corn by rotating and a five-bushel increase in soybeans. Tests at other universities are consistent with this study.

The biggest drop in yield comes in the second year and then levels off but does not bounce back. A study is under way at the UKREC to look at some agronomic management practices that may combat this reduction.

The study is looking at soil insecticides, shredding stalks, a wheat cover crop, a Bt hybrid, some minor tillage, and extra nitrogen. These treatments are being studied separately and one plot contains all the treatments. In 2002, the first year of the study, none of the treatments increased the yield on second year corn above the check treatment, which was a non-Bt form of the hybrid and standard practices. First year corn of the same hybrid planted following soybeans resulted in a 15 percent yield increase.

“It doesn’t look too hopeful but we are going to continue to this study for several more years,” Herbek said.