A John Deere 9750 STS combine slowly made its way through an Iowa State University research field, all the while dumping a crop of corn kernels into the combine's hopper and blowing a crop of stalks, cobs and leaves into a trailing wagon.

That dual-stream, single-pass harvesting system was developed by Stuart Birrell, an Iowa State associate professor of agricultural and biosystems engineering, and graduate students Mark Dilts and Ben Schlesser. They're working to design, build and test machinery that will harvest corn stover -- the stalks, cobs and leaves -- when farmers bring in their grain. The stover could be the source of plant fiber that feeds the next generation of ethanol plants.

The researchers ran their latest version of a stover harvester through about 50 acres of corn near Ames this fall. Birrell recently showed some video of the tests on his office computer and explained how the system works.

The researchers are developing stover attachments that can be used on standard combines. The result would be an additional cost to farmers of about $10,000 to $15,000 instead of the six figures it would take for a separate combine to harvest stover. The attachments would also allow farmers to harvest grain and stover with one pass through a field.

The system the researchers have come up with includes a modified row crop header and corn reel attached to the front of the combine and a chopper and blower attached to the back.

The header and reel feed leaves and stalks into the combine so the biomass can be harvested before it touches the ground and is contaminated with soil. The chopper cuts stover into 2-inch pieces. And the blower throws the chopped stover into a wagon.

Although tests with the prototype machine have been successful, Birrell said there is more development work to do:

• Harvest capacity
  The stover harvesting equipment is capable of speeds equal to a normal grain harvest when less than 50 percent of the stover is collected. When all of a field's stover is collected, harvest speeds are about half of a normal grain harvest. Birrell said that would be unacceptable to farmers. And so he's working to get the speed to at least 80 percent of a normal grain harvest -- no matter how much stover is collected. That would allow farmers to decide how much stover they want to harvest without significantly affecting the time it takes to harvest their fields. 

• Transportation
  Birrell said researchers need to figure out how to pack the harvested stover so it can be economically transported. He said stover comes off the combine at a density of about 3 to 4 pounds per cubic foot; it needs to be about 10 to 12 pounds per cubic foot for efficient trucking.

• Storage
  Birrell said researchers need to figure out how huge quantities of biomass can be stored. He said the U.S. Department of Energy has estimated a biorefinery would need at least 2,000 tons of biomass per day. A year's supply would cover 100 acres with 25 feet of biomass.

• Fertility
  Birrell said researchers need to determine how much stover can be removed from fields while still returning sufficient organic matter to the soil and protecting the soil from winter erosion.

Birrell's stover harvesting research has been supported by a three-year, $180,000 grant from the U.S. Department of Agriculture and the U.S. Department of Energy and a two-year, $50,000 grant from Deere & Company of Moline, Ill.

Birrell said development of a stover harvesting system has been constrained by a lack of research funding.

"Significant resources have been dedicated to the process of converting cellulose into ethanol," he said. "But very little has gone into answering how do you get a supply of stover from the field to the biorefinery. This will be critical to the success of the bioeconomy."