Longer crop rotations could provide the key to successfully fighting Sudden Death Syndrome (SDS), the soybean disease that decimated yields in many Iowa soybean fields in 2010.

The exceptionally wet weather in many areas of Iowa during the growing season contributed to the losses from SDS. But there was one noticeable exception. Soybeans grown in three-year and four-year rotations with corn, oats and alfalfa were healthier than beans grow in a two-year rotation with corn.

Iowa State University agronomist Matt Liebman has been studying various aspects of extending the conventional corn-soybean rotation with small grains and forages for the past eight years. He has documented many benefits of the longer rotations, including the need for fewer purchased inputs made from fossil fuels while maintaining high levels of crop production.

Use longer crop rotation to control SDS in soybeans

Soybeans in three-year rotations with corn, oat and red clover, and in four-year rotations with corn, oat and alfalfa, seemed to escape the worst effects of SDS in 2010. This disease, caused by a soil fungus that infects soybean roots soon after planting, can lead to significant drops in yield.

"We have had SDS in these plots, but it was very dramatic this year," Liebman notes. "It's definitely a rotation effect because everything else is the same, but we do not know why it's happening."

In Liebman's test plots at the ISU Marsden Farm west of Ames, 90% of the soybean plants in a two-year rotation were severely affected by SDS, while less than 10% were affected in the three- and four-year rotation plots. A defoliation rating of the plots showed similar effects: on average, the plots in a two-year rotation had 62.5% defoliation, compared to 5% and 4.4% for the three- and four-year rotations, respectively.

Beans in 3-year and 4-year crop rotations yielded more

The plots were harvested October 5, and yields determined from an area of 6 rows by 275 feet in the four replicates of each treatment. Soybean yields did not differ between the three-year and four-year rotations and were not affected by variety (both Roundup Ready and non-Roundup Ready soybean varieties were planted). Average yield of the two varieties was 54.5 bushels per acre.

In contrast, soybean yields in the two-year rotation were significantly lower than in the longer rotations and were significantly affected by variety. In the two-year rotation, Kruger K-287RR/SCN produced 42.1 bushels per acre, whereas Kruger K-2918/SCN produced 21.6 bushels per acre. (Yield data was reported at a seed moisture level content of 13%.)

All soybeans in the experiment follow corn. All were planted under similar conditions, on similar days, and grew with similar rainfall. The longer rotations have received substantially fewer external inputs, relying on red clover, alfalfa and cattle manure for weed control and nutrients.

What is causing these differences in SDS severity?

ISU plant pathologist Leonor Leandro visited the plots in early September and has looked at what might be causing the differences. "We don't know the exact mechanism," she says, "but the differences in SDS severity are impressive."

ISU researchers have shown that SDS fungus can survive in corn kernels and roots, but they have not looked at oats, wheat or alfalfa. Other microbes or bacteria in the soil could be suppressing the pathogen, or allowing roots to grow with fewer problems.

Liebman has been comparing the different rotations the past eight years, thanks to an initial grant and continued support from the Leopold Center and USDA. "We know there are many benefits of using longer rotations," says Jeri Neal, who works with the Leopold Center's Ecology Initiative. "Farmers who are willing to explore different rotations may significantly reduce their risks and costs associated with SDS, and save money on other energy and input costs."