A 200-bushel per acre corn crop produces about 4.5 tons per acre of dry residue. As farmers know, this residue protects soil from water and wind erosion, supplies nutrients, and builds soil organic matter, which is essential for high productivity.

To maintain soil quality in a continuous corn system under conservation tillage, you need to leave about 2.3 tons per acre of residue on the field, says Douglas Karlen, research leader at the USDA National Laboratory for Agriculture and the Environment (NLAE) in Ames, Iowa. In a corn-soybean rotation, it’s 3.5 tons peracre.

Keep in mind that those are averages, says Jane Johnson, research agronomist at the USDA North Central Soil Conservation Lab in Morris, Minn. Minimum biomass needs vary significantly by soil characteristics and management. “It’s really site dependent.”

The Greenfields, for example, farm “flat and black” ground. About two-thirds of their continuous cornfields receive hog manure, which builds soil carbon. And yields are pretty consistently around 190 bushels per acre. “That’s a big part of why corn stover harvesting works so well for us,” Greenfield says. “Our fertility and soil organic matter are good.”

Biomass removal is not recommended in low-yield environments, or on fields that slope more than 4%, because of erosion risk, says Adam Wirt, Poet-DSM Advanced Biofuels logistics director. “We sit down with growers and look at their fields and maps and offer advice on field selection.” On flat, high-yielding ground in northwest Iowa, he says, “we have ample residue.”