Poet-DSM collects what it calls a “second-pass cob bale,” which contains about 30% cobs, 45% husks and 20% stalks. “This gives a desirable feedstock, lower in ash, and with more components of the plant from above the ear, where there are fewer nutrients,” Wirt says. Growers are asked to turn off the combine’s chopper-spreader and make a windrow. The baler gathers about 1 dry ton of residue per acre, or roughly 25% of the residue from a 180-bushel corn crop. “Our low take is sustainable, even in a corn-soybean rotation.”

For the past 4 years, grain and cattle producer Rick Elbert, Emmetsburg, has supplied about 600 tons of corn stover to Poet-DSM, some of it collected from fields in a corn-soybean rotation. Elbert farms across an 11-mile radius, which gives him flexibility to choose the flattest and most productive fields each season for stover removal. Since he’s been harvesting residue, he has intensified his nutrient management. “I really watch it carefully.”

DuPont’s target harvest rate is 2 tons per acre on fields with 180-bushel yield, or about 50% of the residue. “We recommend limiting harvest to 3 out of 4 years in continuous corn,” Penland says, “for an average removal rate over 4 years of 1.5 tons per acre — well below the 4-ton limit.” In a corn-soybean rotation, stover collection should be restricted to 2 in 5 years, Penland says, for an average annual removal rate of 0.8 tons per acre over 10 years.

When corn yields drop below 180 bushels per acre, as in 2012, “we reduce harvest rates,” Penland says. Growers report grain yields, and custom harvesters adjust collection, field-by-field. “This is still an art. There’s a lot of judgment involved.”

On Dave and Dan Struthers’ farm near Collins, Iowa, corn stover production in 2012 dropped along with grain yields. Stover removal fell from a goal of 3 square bales per acre to 1.5 – 2 square bales per acre, Dave Struthers says.

The Strutherses, who raise hogs in hoop-style barns, have baled cornstalks for years as bedding. Struthers has observed, “there is quite a difference in biomass among hybrids.” Eventually, he says, “biomass may become a factor in hybrid selection” for growers who want to sell residue.