Sourcing Stover Sustainably

Although corn stover offers a second valuable crop from the same acres, there’s a whole lot more to the equation than simply removing it from the soil.
By Katie Fletcher | September 20, 2014

Corn stover and other biomass crop residues are frequently referred to as “trash” or agricultural waste, suggesting it has little value. These terms are deceiving, now more than ever, with this “trash” being collected for valuable uses both on and off the farm. This is perhaps most evident at three commercial-scale cellulosic ethanol facilities that have been built in the Corn Belt, where the bulk of corn stover resides. Capitalizing on this location, within the next year, the facilities expect to convert the surrounding stover into biofuel. 

Over 100 million tons of corn stover is available annually, making it the largest quantity of biomass residue in the U.S. The state with the most biomass is Iowa, where DuPont and Poet-DSM have constructed cellulosic ethanol facilities. It is estimated that 75 million dry tons of stover can be sustainably harvested each year. DuPont’s facility in Nevada is expected to consume 375,000 tons of corn stover from approximately 190,000 acres each year, resulting in 30 MMgy of biofuel. Poet-DSM’s Project Liberty site in Emmetsburg will source roughly 285,000 bone-dry tons (BDT) of corn residue biomass annually, producing 20 to 25 MMgy.

Collecting the massive amounts of stover required to feed these facilities each year is no small task. How to approach this supply chain in a sustainable manner is perhaps even more daunting and, to further complicate the matter, mixed views surround this fairly new concept. Some corn growers view it as an opportunity to get more value from their agricultural land; others are slower to trial, and adopt with concerns about the impacts its removal could have on the condition of their soil. Some landlords and environmentalists share in the uncertainty. Stover has been harvested for purposes like animal feed and bedding for years, however, biofuel production at this scale is a fairly recent development. “Partial stover harvest is a new and unique tool for managing corn residues,” says John Pieper, stover feedstock workstream lead with DuPont.

This new tool has yet to be understood by many, and seems to linger as the main cause of uncertainty. Both Poet-DSM and DuPont have been working with researchers at Iowa State University, the USDA, the National Soil Conservation Service, growers, equipment manufacturers and other stakeholders to create sustainable stover removal plans. These programs are focused on partial stover removal, to not only leave behind a sufficient amount to meet the critical needs of the soil, but also remove enough to purpose it for biofuel production.

Food, Feed and Now Fuel

Partnering with farmers has been paramount to the ethanol producers getting cellulosic plants on line. Stover must be harvested from a wide number of fields to feed the plant; in fact, 400 or more farmers are currently supplying stover to each facility. “The growers themselves need to be comfortable, not only if it is something that makes sense to them economically, but also sustainably for their field and their crops,” says Steve Hartig, general manager of licensing with Poet-DSM Advanced Biofuels.

Poet-DSM has two types of biomass contracts for growers to market their biomass. The Grower Model says the grower is responsible for aligning the baling, staging, storage and transportation of biomass from the field to the biorefinery. They can choose to perform these tasks themselves, custom hire the job, or a mix of both. According to the Custom Model Contract, the grower harvests the grain forming the windrow, then informs the Poet-DSM biomass team, who take over from there. “The key is to show them this is part of having a sustainable and profitable business,” Hartig says. 

Corn farmer Mike Greenfield, is participating in DuPont’s corn stover collection program. “Really, for us, they have become a very integral part of how we manage our operation, it is a true partnership, and we’ve signed a longer-term contract with them for most of our acres,” Greenfield says.

The reason for the commitment is that, for Greenfield’s farming operation, it is a perfect match. “We’re corn-on-corn, and have been for several years,” he says. “Almost all of our ground has hog manure on it, so that makes our operation a natural fit.”

Like Greenfield, farmers with manure resources can add it as a fertilizer to their cornfields to improve organic matter levels after stover removal. Cover crops also help maintain organic matter levels in the soil.

When the Time is Right

Manure and cover crop use are elements that should be considered when deciding how much stover to remove sustainably. Other factors include slope, tillage, rotation and crop yield. “Growers need to take a look at it on a field-by-field basis, and get the help they need to work through that information,” says Marty Adkins, assistant state conservationist for special projects with the NRCS.

Studies conducted at Iowa State University say level fields are best for harvesting stover, recommending not to harvest stover in field areas where slope exceeds 2 to 3 percent. The studies also show that a corn or soybean rotation contributes less soil organic matter than continuous corn. Some believe a crop rotation is needed to maximize corn yields, Pieper says, and in effect, this way of thinking results in a slow movement to adopt a new tool like partial stover harvest. Yield level is also important because high-yielding fields consequently yield the most biomass. 

As grain yields increase, residues become difficult to manage into a successful subsequent corn crop, according to Pieper. High-yielding fields are a good candidate for stover removal. “A focus on leaving a sustainable, general 2.2 tons per acre, for conventional tillage volume, allows for an economical harvest rate—2 tons—on highly productive fields yielding greater than 180 bushels per acre of corn on average,” Pieper says.


Baling the Biomass

Everything from the way each field is assessed to how much stover is collected is essential to the sustainability of the field’s soil health. Baling practices must also be attuned to soil health. After the windrow is created, most biomass should be baled 24 to 48 hours after harvesting if weather permits.
DuPont’s biomass team works with farmers to bale only large square bales (3 foot x 4 foot x 8 foot) of corn stover at this time. Pieper explains that productive fields with a low slope producing a little over 4 tons of stover per acre can support partial stover harvest three out of four years on continuous corn fields, and two out of five corn years on corn-soybean-rotated fields, targeting two tons per acre.  

Poet-DSM is removing about one BDT per acre, which translates to about 20 percent of available biomass. The primary feedstock to Project Liberty is created from the stover into a specific bale line called the EZ Bale, which can be either round or square. “What we look at with the EZ Bale is the combination, so it has some of the cob and some of the stalk and a lot of the leaves,” Hartig said. “The composition is the ideal fuel we want to run, and the way it is gathered you tend to get a lot less rocks and dirt and organics.”

Innovation in feedstock composition is accompanied by equipment improvements and adaptations for efficient and safe stover harvests. “It’s forefront on everything we would develop, it has to be good for the farmer and the soil and be sustainable or it won’t work,” says Glenn Farris, marketing manager for biomass with Agco Corporation.

Agco and other equipment manufacturers in the space are focused on limiting the amount of passes through the field. Agco supplies some tractors and a portion of the balers for all three cellulosic projects. Currently it has a single-pass system and is developing a high-take corn header for release in 2016. One piece of equipment isn’t going to be the answer for all stover harvesting needs, Farris says. “There is never going to be one single solution, depending on conditions, land type, weather and everything else,” he adds. “There needs to be a full array of solutions to pick and choose from.”

Agco has made improvements as a result of the call to remove stover for biorefineries. The large square baler, Massey Ferguson or Challenger 2270XD, has received a 90 percent heavier flywheel and shear bolts and larger hydraulic tension cylinders and a new Opti-Form extended bale chamber for denser, square-shouldered bales among other improvements according to Ken Wagenbach, business development and equipment specialist for biomass with Agco.

Sustaining Relationships and Harvests

Once baled, the stover is usually stored in stacks on the edge of the grower’s field until transported to the biorefinery. Some stover is also stored at stackyards on site. Round bales, according to Poet-DSM, typically store better than square bales that are not tarped. Single row and pyramid are stacking options for round bales, with single rows holding the advantage when it comes to dry matter loss and maintaining the bale’s shape. It is recommended that square bales are tarped if stored long term over the summer months to avoid excessive weathering. Deliveries of biomass are based on the contract period. “What we are trying to do is make it as easy as possible for the growers to bring biomass to us,” Hartig says.

As easy as possible is the goal for now, while feedstock supply chains continue to be optimized, but a great deal of work lies ahead. “Challenges include managing soil inclusion, moisture and remaining cover while optimizing cost, optimally capitalizing on a new supply chain,” Pieper says.

This new supply chain must create a year’s supply of biomass in an approximately 70-day window, he continues, during which there are 28 days or more (97 percent of years) when harvest is possible. “All this must be done without hindering our cooperating farmers in the successful management of other fall field work that must be undertaken,” Pieper says. 

Ethanol producers recognize the importance of working with various stakeholders to sustainably acquire feedstock for their facilities. Population projections support the assertion that in the next 50 years, humans must produce as much food as has been produced throughout history. Additional food production means more of the “trash” left in the fields will need to be Managed. Maintaining soil is imperative in the continuation and boosting of mass crop production. Adkins agrees, “Working with the farmers to maintain the soil health has never been more important.”

Author: Katie Fletcher
Staff Writer, Biomass Magazine