Longer rotations could provide key to fighting SDS in soybean

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October 13, 2010

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

Another benefit of longer rotations has surfaced, puzzling him and other researchers at Iowa State. Soybean 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 Sudden Death Syndrome (SDS). 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 said. “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 percent of the soybean plants in a two-year rotation were severely affected by SDS (soybeans to the right of the flag in image), while less than 10 percent were affected in the three- and four-year rotation plots (soybeans to the left of the flag). A defoliation rating of the plots showed similar effects: on average, the plots in a two-year rotation had 62.5 percent defoliation, compared to 5 and 4.4 percent for the three- and four-year rotations, respectively.

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™ 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 percent.)

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.

ISU plant pathologist Leonor Leandro visited the plots in early September and was eager to look at what might be causing the differences. “We don’t know the exact mechanism,” she said, “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 competitive grant from the Leopold Center and continued with support from the Leopold Center and the U.S. Department of Agriculture.

“We know there are many benefits of using longer rotations,” said 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.” 

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The Leopold Center was established by the 1987 Iowa Groundwater Protection Act as a research and education center at Iowa State University to develop sustainable agricultural practices that are both profitable and conserve natural resources. Iowa State does not discriminate on the basis of race, color, age, religion, national origin, sexual orientation, sex, marital status, disability, or status as a U.S. Vietnam Era Veteran. Any persons having inquiries concerning this may contact the Director of Affirmative Action, 3210 Beardshear Hall, (515) 294-7612.

This page was found at http://www.leopold.iastate.edu/news/10-13-2010/longer-rotations-could-provide-key-fighting-sds-soybean on 01/21/2013