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Study shows advantages of longer corn-soy crop rotation cycle
Iowa Correspondent

AMES, Iowa — A newly released study from three Midwest agricultural researchers reveal there are significant advantages to implementing longer crop rotations with corn and soybeans.
“Balancing productivity, profitability and environmental health is a key challenge for agricultural sustainability,” said Adam Davis, USDA Agricultural Research Service weed ecologist in Urbana, Ill.; Iowa State University agronomist Matt Liebman, who leads the research project; Jason Hill, an environmental scientist at the University of Minnesota; Craig Chase, economist and interim program leader at ISU’s Leopold Center for Sustainable Agriculture; and Ann Johanns, ISU extension economist.

The findings were published on Oct. 10 in the peer-reviewed online journal PLOS ONE. Data collected since 2003 at ISU research plots comparing two-year corn-soybean rotations with longer-term rotations reveal many advantages, including “higher yields, lower energy use and effective weed and pest management with far fewer chemicals, while providing comparable economic returns.
“Substantial improvements in the environmental sustainability of agriculture are achievable now, without sacrificing food production or farmer livelihoods,” the authors wrote.

“More diverse cropping systems can use small amounts of synthetic agrichemical inputs as powerful tools with which to tune, rather than drive, agroecosystem performance, while meeting or exceeding the performance of less diverse systems,” the authors note in the journal, calling it “synergizing effects of cropping system diversification.”

Funding for the study was provided by the USDA National Research Initiative, the Leopold Center, the Iowa Soybean Assoc. and the Organic Center in Boulder, Colo. The project consisted of 36 replicated research plots at the ISU Marsden Farm west of Ames, Iowa, that compare three crop production systems:

•Two-year conventional corn-soybean rotation

•Three-year rotation of corn, soybean and small grain/red clover (during startup, triticale was the small grain; oats have been used as the small grain since 2006)

•Four-year rotation of corn, soybean, small grain/alfalfa and a fourth year of alfalfa

According to the study, the diverse rotations received clover and alfalfa residues and composted cattle manure, with 80-86 percent less synthetic nitrogen applied than in the conventional system during 2003-11.

Yields up, weeds down

“The diverse rotations also reduced herbicide use during the corn and soybean years by application in 15-inch bands rather than broadcast spraying, and cultivation between rows,” the article states. “The two-year system required more than twice the fossil energy inputs of other rotations, primarily due to increased herbicide and fertilizer usage.

“These differences contributed to a healthy bottom line for the diverse rotations. Although there were no statistically significant differences in profitability among the different systems, the three-year rotation came out numerically on top, netting $194 an acre in returns to management, compared to $187 for the two-year system and $171 for the four-year rotation.”

In addition, the longer rotations had higher yields – an average of 4 percent greater for corn in the three- and four-year rotations than in the two-year rotation, and 9 percent greater for soybeans during the nine years of the study.

But the biggest difference over time among the three systems was in weed management.

The study said weeds were suppressed as effectively in the longer rotations as in the two-year rotation, with declining soil seed banks and negligible weed biomass. Yet, herbicide inputs in the longer rotation plots were 6-10 times lower.

Moreover, during the startup phase of the project (the first three years), there was only a twofold difference between the two-year system and the longer rotations in potential toxicity to freshwater organisms, based on analyses evaluating eight of the 10 active chemical ingredients applied. During the past six years, however – 2006-11 – potential aquatic toxicity was 200 times less in the longer rotations than in the two-year system.

The authors note although corn and soybeans are grown less frequently in longer, more diverse rotations, “this will not compromise the ability of such systems to contribute to the global food supply, given the relatively low contribution of corn and soybean production to direct human consumption and the ability of livestock to consume small grains and forages.”

Drought a lingering influence

Steven D. Johnson, ISU extension farm and agribusiness management specialist, said with the early 2012 harvest, Iowa and the Corn Belt will likely not plant as many acres of corn in 2013 as this year.

“Referring to what some call the drought hangover, drought gets in people’s minds and lingers for years,” he said. “Many farmers want to get their crop rotations back in balance after planting more corn-on-corn in recent years.”

With relatively tight U.S. marketing year ending stocks for both corn and soybeans by August 2013, Johnson said any problems in global production, such as South American weather, could push farmers to plant one crop over another by spring.

“Observers suggest many factors may have contributed to the shift to more planted corn acres in the past, including improved corn genetics, disease/pest challenges in soybeans, new improved tillage equipment and crop insurance considerations,” he said.
“Farmers need to evaluate their own individual circumstances. That includes everything from land costs, crop rotation issues and price expectations. I think the lack of soil moisture and the drought experience will weigh heavily on farmers’ minds in making 2013 planting decisions.”