Advanced BioFuels USA

by Paige Short (Michigan Technological University)  Switchgrass is a promising biofuel alternative to corn, but farmers, environmentalists and biofuel developers, find deciding on the right time to harvest particularly thorny.

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The current viable harvest window for switchgrass extends from peak biomass, when the grass is at its biggest before it begins to degrade from weathering, to the first killing frost, when the plant shuts down completely for the winter. The longer the plant stays in the field, the more nitrogen returns to the soil—a clear environmental benefit—but this extra time reduces the amount of biomass harvested from the crop. Future biofuel manufacturers may want to harvest multiple times per season to efficiently use storage facilities, while farmers must consider weather and the harvest times of their other crops. A conundrum.

Nailing down a definitive answer to the switchgrass harvest debate is something Rebecca Ong, assistant professor of chemical engineering, is exploring in her research. Working with researchers from the University of Tennessee, the University of Wisconsin-Madison and Michigan State University, Ong monitored and sampled several stands of Wisconsin switchgrass to better understand the costs and benefits to harvesting at various points in the season. The work is part of the Great Lakes Bioenergy Research Center.

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Comparing data from samples collected every two to three weeks from the switchgrass fields against biofuel yield data from each site, Ong discovered something rather unusual: As the crop aged past senescence, the point at which plants begin to go dormant for the winter, ethanol yields from the harvested crop became more efficient rather than less so.

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Previous researchers at Purdue University have engineered yeast, the microbe involved in switchgrass fermentation, to break down xylose, but the process can be fickle and sensitive to inhibition.

“Something we’re trying to do [in our lab],” Ong says, “is break up that polymer, break all the links so that we have those individual sugars and then convince the microbes that they want to actually eat them.”

In the later samples from Ong’s study, the yeast didn’t need convincing. The xylose had become consumable.

Ong believes this change is due to plant senescing, which is when plants begin to go dormant and dismantle compounds in their stems and leaves for storage and reuse the following growing season. When the switchgrass was harvested before senescence, one or more of these compounds might have been interfering with the microbes’ ability to digest the xylose. Giving the plants more time in the field may have made the switchgrass sugar more appealing to the yeast.

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Once identified, they can deploy strategies to mitigate the effects, like engineering the biofuel-making microbes to be more tolerant, giving farmers more room to harvest switchgrass during good weather.     READ MORE