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Farmers in Nebraska and the Dakotas brought the U.S. closer to becoming a biofuel economy, planting huge tracts of land for the first time with switchgrass—a native North American perennial grass (Panicum virgatum) that often grows on the borders of cropland naturally—and proving that it can deliver more than five times more energy than it takes to grow it.
Working with the U.S. Department of Agriculture (USDA), the farmers tracked the seed used to establish the plant, fertilizer used to boost its growth, fuel used to farm it, overall rainfall and the amount of grass ultimately harvested for five years on fields ranging from seven to 23 acres in size (three to nine hectares).
Once established, the fields yielded from 5.2 to 11.1 metric tons of grass bales per hectare, depending on rainfall, says USDA plant scientist Ken Vogel. "It fluctuates with the timing of the precipitation,'' he says. "Switchgrass needs most of its moisture in spring and midsummer. If you get fall rains, it's not going to do that year's crops much good."
But yields from a grass that only needs to be planted once would deliver an average of 13.1 megajoules of energy as ethanol for every megajoule of petroleum consumed—in the form of nitrogen fertilizers or diesel for tractors—growing them. "It's a prediction because right now there are no biorefineries built that handle cellulosic material" like that which switchgrass provides, Vogel notes. "We're pretty confident the ethanol yield is pretty close." This means that switchgrass ethanol delivers 540 percent of the energy used to produce it, compared with just roughly 25 percent more energy returned by corn-based ethanol according to the most optimistic studies.
