Can you harvest corn residue for biofuels without depleting soil organic matter and sacrificing yields?

New research from Illinois suggests that farmers could collect about half of their corn stover — if yields are high and corn is grown continually in a minimum tillage system, says Jeff Coulter, an Extension corn specialist at the University of Minnesota.

Besides offering another farm product to sell, residue harvest has other advantages for continuous corn, says Coulter, formerly at the University of Illinois. The soil warms up faster in the spring, disease pressure is lower and less nitrogen (N) is tied up by soil microorganisms.

Most significantly, no-till corn yields match conventional-tillage yields if some of the plant biomass is removed. “When you bale corn stalks, continuous-corn yields with no-till are just as good as those with a chisel-plow system in the central Corn Belt,” Coulter says. “That's important because farmers are reluctant to use no-till for continuous corn.”

A 200-bu./acre corn crop produces about 4.5 tons/acre of stover, Coulter says. This abundant biomass could be one of the main feedstocks for cellulosic ethanol, now in development.

However, leaving corn residue in the field has a lot of important benefits: It helps control water and wind erosion, builds soil organic matter, recycles nutrients and improves soil structure and productivity. Farmers need to know how much residue they could harvest for biofuel without giving up these benefits, Coulter says.

In a corn-soybean rotation with conservation tillage, you need to retain about 3.5 tons/acre of corn residue to maintain soil productivity, so “it becomes difficult to justify taking any residue off,” he says.

CONTINUOUS CORN generates much more residue, offering more promise for biomass harvest. But the key to sustainable removal is conservation tillage, Coulter says.

In continuous-corn studies in central and northern Illinois in 2006 and 2007, Coulter compared three different residue harvest rates with no-till and chisel-plow tillage. He also compared four N rates.

Among the key findings:

  • With no-till, surface residue coverage met the 30% conservation standard for erosion protection. “You just can't collect all the residue with a rake and baler,” Coulter says.

  • The soil warmed up faster when residue was removed. “If you take half or all of the residue off, soil temperature at 2 in. over the first four weeks after planting is 2-3° warmer under no-till, but not under tillage,” Coulter says.

  • Removing residue raised yields in no-till, but not in conventional tillage. Averaged over N rates, residue removal did not significantly increase yields in the chisel-plowed plots. In the no-tilled plots, yields rose by 7-10% when half or all the residue was harvested. Residue keeps the soil colder, slowing emergence and early vegetative growth. It also harbors inoculum for corn diseases. “As we remove more residue, yield reductions associated with no-till for continuous corn production diminish,” says Coulter.

  • Harvesting residue cut N fertilizer need. The optimum N rate dropped 13% when half or all the residue was harvested. “Corn residue has a high carbon-to-nitrogen ratio, causing soil microorganisms to tie up N in the soil,” Coulter says.

Bottom line: Growers could sustainably harvest about half of their corn residue, but only if “corn is grown continuously, yields are good and minimum tillage is used,” Coulter says.