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Diverse Rotations Slash Erosion

Small grains and legumes add a longer season for living roots.

The results of Iowa State University’s (ISU) long-term Marsden Farm experiment showed the soil-saving benefits of adding forage legumes and small grains to a corn-soybean rotation, even when tillage was part of the production system.

Add no-till to the equation, and the results are magnified. That’s the finding of the Union of Concerned Scientists (UCS) in an analysis of the Iowa study.

The UCS, a nonprofit science advocacy organization, superimposed no-till production practices on the Marsden Farm study’s results and found as much as a 91% decrease in soil erosion if extended crop rotations including small grains and legumes were practiced “in the 25 Iowa counties with the most erodible soils . . .” says a UCS press release.

With or without tillage, the inclusion of small grains and legumes in extended rotations holds the key to decreased soil erosion.

“In longer rotations, living roots hold the soil down for longer periods of the growing season,” says ISU agronomist Matt Liebman. “Here in Iowa, small grains can be planted early – in March and April. Of course, red clover and alfalfa are perennials. Having living cover on the [surface] and living roots in the soil reduces the erosion. That’s true for any tillage system.”

The Marsden Farm experiment, which began in 2001 and continues today, compares three systems, all incorporating some tillage. The study includes a two-year corn/soybean rotation; a three-year corn/soybean/oat-and-red clover rotation; and a four-year corn/soybean/oat-and-alfalfa, and alfalfa rotation. The three- and four-year systems periodically receive cattle manure.

“We found that diversification of conventional corn-soybean systems with small grains and forage legumes, coupled with integration of those systems with livestock, can allow for large reductions in the use of mineral fertilizers and herbicides and lead to less environmental damage, equivalent profitability, improved soil quality, and higher crop productivity,” says Liebman.

Following is a listing of key findings from the Marsden Farm study.

Mineral nitrogen fertilizer use was 86% and 91% lower, and herbicide use was 96% and 87% lower in the three- and four-year rotations, respectively, than in the two-year system.

Corn yield averaged 4% higher and soybean yield averaged 16% higher in the more diverse systems compared with the two-year system.

Weed management was not affected by rotation.

Incidence and severity of sudden death syndrome, a disease affecting soybeans, was significantly lower in the longer rotations than in the two-year rotation.

Three indicators of soil quality – particulate organic matter carbon, microbial biomass carbon, and potentially mineralizable nitrogen – were 22% to 51% higher in the three- and four-year rotations than in the two-year rotation.

March through May concentrations of nitrate in drainage water collected from corn in the more diverse systems were 57% lower than from corn in the two-year system.

Soil erosion was 50% lower, fossil energy use was 59% lower, and freshwater toxicity associated with herbicide use was 93% lower in the more diverse systems than in the conventional cropping system.

Increases in rotation length led to greater labor requirements and decreased gross revenue. 

“However, production costs also dropped substantially as cropping system diversity increased,” says Liebman. “Consequently, net returns to land and management did not differ statistically among systems, though profitability tended to rise as rotation length increased.”

Nearly $200 Million in Savings 

n a broader stage, says the UCS, the soil-conserving benefits of more diverse rotations managed under no-till and their decreased use of fertilizers and herbicides could potentially have the effect of saving nearly $200 million annually in water-pollution cleanup costs in the 25 Iowa counties with the most erodible soils.

The catalyst driving farmers’ adoption of more diverse rotations is the marketplace. “We need better marketing opportunities for small grains and for meat and milk produced with environmentally friendly practices,” says Liebman. “We are starting to see some companies interested in doing that.

“With our research,” he continues, “we’re trying to give such companies the confidence that farmers who use more diverse rotations are indeed having a beneficial effect on the environment.”

In the meantime, as these new markets begin to take root, the existing marketplace can potentially absorb some increases in production of small grains, for instance, without price downturns. According to the UCS analysis, diverse crop rotations could be adopted over time on 20% to 40% of Iowa’s farmland without changes in crop prices driving farmers back to predominantly corn-soybean production.

Diversify Rotations with Alfalfa

Looking for a way to reduce pest pressure and boost soil health by diversifying crop rotations? “Forages like alfalfa are a great crop for soil health,” says Marisol Berti, a North Dakota State University (NDSU) forage and biomass specialist. “Alfalfa is a great nitrogen fixer and can reduce soil compaction and increase infiltration.”

One way to get a jump on including alfalfa in a rotation is by interseeding it into corn. NDSU trials show interseeded alfalfa can yield 2½ tons per acre more than if seeded in the spring. The following year, alfalfa hay yields have doubled to 5 tons per acre in NDSU trials. 

Interseeded alfalfa can clip corn yields that first year, particularly in dry years, says Berti. If corn yields are reduced by 30 bushels per acre at $3 per bushel, that’s a $90-per-acre reduction. 

However, doubling next year’s alfalfa yield from 2½ to 5 tons per acre could ease those concerns. 

Alfalfa also needs to be fertilized with phosphorus (P) and potassium (K). Establishing interseeded alfalfa also requires farmers to forgo established preemergence corn herbicides and, instead, rely on postemergence ones, such as glyphosate used in Roundup Ready corn and alfalfa systems. Farmers also need to plant leafhopper-resistant alfalfa varieties.  

“Without them, leafhoppers will weaken alfalfa to the point to where it won’t survive under the corn canopy,” she says. Spraying is sometimes still required for leafhoppers, even with a leafhopper-resistant variety.

Learn More 

Matt Liebman

515/294-7486

mliebman@iastate.edu

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