Lee McClune thinks he's found a better crop — and a more economical, on-farm process — for making ethanol.

Sweet sorghum has been raised for centuries to produce sorghum syrup for human consumption. But its drought-tolerance and super-high sugar content make it a fine candidate for ethanol production, says McClune, of Knoxville, IA.

“Sweet sorghum contains about twice the fermentable sugar in its sap that corn produces, on a per-acre basis,” he says. “Right now we can harvest between 400 and 600 gallons of what we call ‘sorganol’ per acre of sweet sorghum. But it's conceivable that figure could double once we get more experience with the process and have access to better varieties.”

Studies by Iowa State University agronomist I.C. Anderson have yielded somewhat promising results. Sweet sorghum is a relatively low-input crop, and usually has minimal insect and disease pressures. It yields best in hot, dry summers, producing stalk syrup with 15-17% sugar content, according to Anderson's studies.

McClune says sweet sorghum is similar to forage sorghum in its focus on stalk rather than seed production. To maximize sugar retention in the stalk and minimize seed development, he plants Southern varieties on his farm.

“At harvest in late September or October, the plants are almost 80% moisture and the heads still fairly immature,” he says.

Growing the crop is the easy part. Building a harvesting machine was a bit more complicated. McClune started with the header and feed rolls of an old forage chopper, then built a platform behind them with a press that crushes the stalks and extracts the sorghum juice. A tank on top of the machine collects the filtered juice.

The harvester runs pressed stalks back onto the ground, where McClune leaves them to dry for three to five days before baling them. He plans to burn the bales to generate heat for his distilling machine.

“This would allow us to produce our ethanol with near-zero fossil fuel inputs — really just the fuel used for the planting and harvesting equipment,” he says.

The pressed stalks are still rich in sugar and other nutrients, he notes, and could easily be ensiled for livestock. “But for the time being, we're just returning them to the soil to improve organic matter levels.”

After harvest, sulfuric acid is added to the juice to lower the pH and prevent bacteria from growing. Yeast is then mixed in, and the fermentation process is complete in just three to five days.

One problem is the volume of material that must be handled. The harvested stalks can weigh up to 40 tons per acre, which makes transport to a central processing plant cost-prohibitive.

“That's why the concept of an on-farm fermentation and distillation unit came to be,” recalls McClune. “Dr. Anderson thought it was feasible, and now I'm trying to make it a reality.”

A mobile distillation unit that could be moved from farm to farm is the centerpiece of McClune's on-farm sorganol production concept. But time and money limitations have kept it in the development stage, he says.

“Distillation is an old process and doesn't need any special technology. It's really just a matter of building a machine to the proper scale and making it mobile.”

He's currently seeking investors and grant money to complete his on-farm system.

Interest in sweet sorghum is picking up, he notes, and the availability of seed should improve with the recent establishment of breeding programs at the University of Kentucky and Texas A&M University.

“Varieties now in development could boost yields significantly, which could mean getting upwards of 1,000 gallons of fuel per acre,” says McClune. “That could lower the overall cost of sorganol to about 50¢ a gallon. So the economics are there.”