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590,000 bales of corn stover

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Stacked on top of each other, the bales would be about 300 miles high – high enough to reach the International Space Station in orbit.

In the short-term, cellulosic ethanol is the future of biofuel. The biomass of choice for conversion is currently corn stover; it’s commonplace, plentiful, and less controversial than using the grain itself. But the amount needed seems somewhat high; to make 30 million gallons of ethanol per year, the DuPont Nevada Site Cellulosic Ethanol Facility alone needs 590,000 bales of stover. Stacked on top of each other, the bales would be about 300 miles high – high enough to reach the International Space Station in orbit.

In South Dakota, where crop production is roughly comparable to Iowa’s one generation ago, USDA Agricultural Research Service scientists have discovered that even judicious stover removal isn’t always for the best in terms of soil health.

In semi-arid, lower-production locations such as South Dakota, the researchers found that removing stover can have a pronounced negative effect on soil aggregation.

In one of the longest running studies on the subject, scientists with USDA Agricultural Research Service in Brookings, South Dakota, operating as part of the Renewable Energy Assessment Project or REAP, have examined the effects of stover removal since 2000. In 2005, they added a cover crop to their corn-soybean rotation, but the results of removing stover from the field since then are telling, as ARS Scientist Shannon Osborne explains.

In semi-arid, lower-production locations such as South Dakota, the researchers found that removing stover can have a pronounced negative effect on soil aggregation. However DuPont Pioneer Agronomy Research Manager Andy Heggenstaller says Iowan soil doesn’t fall into those categories.

Heggenstaller points out that in higher-production areas, such as Iowa, too much stover can become a residue management issue. It can block out sunlight, causing soil to stay cooler longer, and can prevent seed from getting where it needs to in the soil during planting. Some farmers are tempted to intensify tillage to deal with stover, but Heggenstaller cautions against that as well; he says that stover removal is a better solution for a wider range of soil types.

…a yield of 175 bushels per acre is the baseline at which enough carbon is headed back into the ground to make stover harvest a viable option…

ARS Researcher Doug Karlen at the National Laboratory for Agriculture and the Environment in Ames says there’s a limit, however, to how much stover can be taken from a field at one time.

Karlen cautions against generalizing when considering stover removal, but for perspective he explains that in central Iowa, including the area surrounding the DuPont plant in Nevada, a yield of 175 bushels per acre is the baseline at which enough carbon is headed back into the ground to make stover harvest a viable option without damaging soil aggregation or microorganisms. At an average yield per acre over 200 bushels, Karlen says taking stover off the field is best for residue management, but warns against aggressive tillage practices that he says is detrimental to soil health.

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