Soil tilth lab studies corn stover harvesting

By Ryan C. Christiansen | November 03, 2008
Web exclusive posted Nov. 5, 2008 at 9:49 a.m. CST

The top portion of the corn plant, including the cob but minus the grain, is the best portion of the corn stover to use in the production of cellulosic ethanol, according to new findings from the USDA Agricultural Research Service's National Soil Tilth Laboratory in Ames, Iowa.

As much as 100 to 150 million tons of corn stover are left in U.S. fields annually, the USDA said. Because of this, corn stover is seen as a potential go-to feedstock for producing cellulosic ethanol. Since 2005, the USDA soil lab has been harvesting corn stover at four different heights to measure the amount and quality of stover that might be harvested using different removal strategies.

The scientists found the lowest 30 inches of cornstalk are 64 percent water, which makes that portion of the stover more expensive to harvest, store, and transport - and also less efficiently converted to ethanol due to more soil contamination and higher concentrations of silica and lignin, according to Douglas Karlen, a research scientist in the USDA-ARS Soil and Water Quality Research Unit.

However, the corn plant material above the lowest 30 inches of cornstalk, the portion which includes the cobs and husks, produces significantly more ethanol and similar ethanol yields are possible using "normal-cut" stover (16 inches above the ground), which suggests that using a standard harvester is the fastest, most convenient, and least expensive method for harvesting stover.

No matter how much corn stover is removed from the field, the removal of stover would result in having to apply more fertilizer to the soil for future crops, the USDA said. Depending on whether you remove the whole plant or just the top half of the plant, removing corn stover from the field results in a per-acre loss of up to 45 pounds of nitrogen, 2 to 4 pounds of phosphorus, and 23 to 38 pounds of potassium, which translates into $25 to $30 per acre in additional fertilizer costs. Other soil nutrients including calcium, magnesium, iron, zinc, copper, and manganese are also lost and in some soil types, the loss of potassium might result in long-term potassium deficiencies that would reduce crop productivity.

In 2009 researchers will begin testing the impact of removing only the grain and the cobs from the field, Karlen said. The cobs make up between 15 and 23 percent of the above-ground, non-grain biomass of the corn plant, which means that if only the grain and the cobs are taken during harvest, between 77 and 85 percent of the corn plant can be returned to the soil, he added. Preliminary tests indicate there is a relatively small amount of nitrogen in a corn cob, which means fertilizer replacement costs would be minimal if only grain and cobs are taken. Karlen's team will be examining the impact of a grain-and-cob-only harvest on plots that will be harvested Nov. 6 during the Poet LLC Project Liberty Field Day in Emmetsburg, Iowa. Poet's 50 MMgy grain-to-ethanol plant in Emmetsburg is being converted to an integrated corn-to-ethanol and cellulose-to-ethanol biorefinery, which will produce 125 MMgy of ethanol, 25 MMgy from corn cobs.