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Energy output soars with switchgrass-based ethanol

In terms of its productivity as an energy crop, switchgrass may be at the top of the heap.

The results of a National Academy of Science (NAS) study indicate switchgrass, as a biomass crop, yields more than 500% the energy it consumes.

The study, the results of which were released Monday, was based on switchgrass production at 10 farm sites in Nebraska, South Dakota and North Dakota. Prior to this research, not enough switchgrass production data existed on which to base energy output numbers.

The five-year study, in which farmers were paid to grow switchgrass under typical crop management, shows a clear trend in the crop's energy consumption. Switchgrass is a perennial crop and, in this study carried out by a team led by University of Nebraska agronomist and USDA-ARS researcher Kenneth Vogel, was not harvested in the first year following planting. That means the bulk of the energy spent to bring the crop to maturity was used in the first "establishment" year of the five-year cycle. After farmers began harvesting the crop each year following its establishment, that's when energy gains were seen.

"Total agricultural inputs were less during the establishment year than in postplanting years because nitrogen fertilizer, a major agricultural energy input, was not applied per recommended management practices," according to the study results. "In the establishment year, herbicides, diesel fuel and seed were the top agricultural energy inputs. Nitrogen fertilizer, diesel fuel and herbicides accounted for the majority of agricultural energy inputs for postplanting harvest years."

As with any crop, conditions like weather and management practices create variability in yields. In this study, for example, switchgrass yields were higher the further east the farms were located because of higher rainfall amounts. Though there will always be this type of variability in results, one clear trend can be seen.

"The results of this study demonstrate that switchgrass grown and managed as a biomass energy crop produces more than 500% more renewable energy than energy consumed in its production and has significant environmental benefits," according to the NAS report. "Compared with low-input prairies, switchgrass grown and managed as a biomass energy crop can produce significantly greater biomass per hectare, which makes it a more feasible system for providing meaningful supplies of biomass to meet energy demands."

Because of this net energy gain achieved by producing ethanol with switchgrass biomass, the crop meets the primary requirements to be a staple of the nation's energy sector, according to the NAS study.

"For an alternative transportation fuel to be a substitute for conventional gasoline, the alternative fuel should have superior environmental benefits, be economically competitive, have meaningful supplies to meet energy demands and have a positive net energy value," according to the study text.

Looking into the future on the production side, the study's authors say that energy output will only increase, as switchgrass becomes a greater farm staple. In this way, it's expected to mirror the rise of corn production in the past four decades.

"It is expected that biomass conversion rates will be improved in the future because of both genetic modifications of biomass feedstocks and improvements in conversion technology, which should result in improvements in net energy for switchgrass," according to the NAS report. "Current corn production has increased 160% in the U.S. in the last 40 years because of increased grain yields and expansion of crop area. Approximately 50% of the increase in grain yield of corn during this period was attributable to improved hybrids.

"Only a fraction of the research effort that has produced these significant improvements in corn genetics and management has been available for switchgrass and other potential perennial herbaceous biomass species."

In terms of its productivity as an energy crop, switchgrass may be at the top of the heap.

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