Switchgrass produces almost twice as much renewable energy than previously thought and is comparable to corn in terms of energy yields, according to the first large-scale study at field production level. (PNAS 2007, 105, 464-9)
‘All previous estimates (on switchgrass) have been completed using data from small research plots and estimated inputs,’ according to lead researcher Ken Vogel at the University of Nebraska. ‘We used real world farms and equipment for a five-year period,’ he says.
The current standard is corn-based ethanol. But corn must be grown from seed every year, requires fertilisers, pesticides/herbicides and machine combine harvesting. In contrast, switchgrass is a perennial that can be grown on marginal soils with little use of fertilisers, insecticides or irrigation.
Previous analyses suggested that switchgrass has a net energy balance of 343%, but this study gives a figure of 540% and the authors suggest that this could be doubled with improved cultivars, varieties, or hybrids with improved biomass yields and improved conversion properties. ‘But this will take time and resources,’ says Vogel, ‘It takes longer to breed perennial than annual crops.’
Vogel’s group collected data from 10 farm sites in Nebraska and North and South Dakota. Farmers grew switchgrass in fields ranging from 7 to 23 acres over a five-year period and kept track of the diesel fuel, seed, and fertiliser used, as well as the dry weight of switchgrass harvested each year. The results depended on the region and on rainfall, but on average, the fields produced biomass equivalent to 320 gallons of ethanol/acre [60 gigajoules/hectare].
Vogel says: ‘The main point is that 5.4 times more bioenergy is produced as ethanol than non-renewable energy to produce it; managed energy crops with inputs including fertiliser produce double the energy yield of low input systems and have equivalent net energy values; and substantial improvements can be made with improved agronomics and improved cultivars or varieties.
Since switchgrass absorbs carbon dioxide from the atmosphere as it grows, ethanol from this feedstock would produce almost zero greenhouse gases.