Perennial herbaceous plants such as switchgrass (Panicum virgatum L.) are being evaluated as cellulosic bioenergy crops. Two major concerns have been the net energy efficiency and economic feasibility of switchgrass and similar crops. All previous energy analyses have been based on data from research plots (<5 m 2 ) and estimated inputs. We managed switchgrass as a biomass energy crop in field trials of 3-9 ha (1 ha ‫؍‬ 10,000 m 2 ) on marginal cropland on 10 farms across a wide precipitation and temperature gradient in the midcontinental U.S. to determine net energy and economic costs based on known farm inputs and harvested yields. In this report, we summarize the agricultural energy input costs, biomass yield, estimated ethanol output, greenhouse gas emissions, and net energy results. Annual biomass yields of established fields averaged 5.2 -11.1 Mg⅐ha ؊1 with a resulting average estimated net energy yield (NEY) of 60 GJ⅐ha ؊1 ⅐y ؊1 . Switchgrass produced 540% more renewable than nonrenewable energy consumed. Switchgrass monocultures managed for high yield produced 93% more biomass yield and an equivalent estimated NEY than previous estimates from human-made prairies that received low agricultural inputs. Estimated average greenhouse gas (GHG) emissions from cellulosic ethanol derived from switchgrass were 94% lower than estimated GHG from gasoline. This is a baseline study that represents the genetic material and agronomic technology available for switchgrass production in 2000 and 2001, when the fields were planted. Improved genetics and agronomics may further enhance energy sustainability and biofuel yield of switchgrass.agriculture ͉ bioenergy ͉ biomass ͉ biomass energy ͉ greenhouse gas A renewable biofuel economy is projected as a pathway to reduce reliance on fossil fuels, reduce greenhouse gas (GHG) emissions, and enhance rural economies (1). Ethanol is the most common biofuel in the U.S. and is projected to increase in the short term because of the voluntary elimination of methyl tertiary butyl ether in conventional gasoline and in the long term because of U.S. government mandates (2, 3). Maize or corn (Zea mays) grain and other cereals such as sorghum (Sorghum bicolor) are the primary feedstock for U.S. ethanol production, but competing feed and food demands on grain supplies and prices will eventually limit expansion of grain-ethanol capacity. An additional feedstock source for producing ethanol is the lignocellulosic components of plant biomass, from which ethanol can be produced via saccrification and fermentation (4). Dedicated perennial energy crops such as switchgrass, crop residues, and forestry biomass are major cellulosic ethanol sources that could potentially displace 30% of our current petroleum consumption (5).Net energy production has been used to evaluate the energy efficiency of ethanol derived from both grain and cellulosic biomass (6). Typically, studies have used net energy values (NEV), net energy ratios, and net energy yield (NEY) and have compared biofuel output to petroleum requ...

The economic potential of cellulosic biomass from switchgrass has heretofore been evaluated using estimates of farm costs based on extrapolation from experimental data and budget estimates. The objective of the project reported here was to estimate the cost of production that would be experienced by farmers on commercial production situations. Switchgrass was produced as a biomass crop on commercial-scale fields by ten contracting farmers located from northern North Dakota to southern Nebraska. Results showed a wide range of yields and costs across the five production years and ten sites, with an overall average cost of $65.86 Mg −1 of biomass dry matter, and annualized yield of 5.0 Mg ha −1 . The lowcost half of the producers were able to produce at an average cost of $51.95 Mg −1 over the 5-year period. When projected to a full 10-year rotation, their cost fell further to $46.26 Mg −1 . We conclude that substantial quantities of biomass feedstock could have been produced in this region at a cost of about $50 Mg −1 at the farm gate, which translates to about $0.13/l of ethanol. These results provide a more reliable benchmark for current commercial production costs as compared to other estimates, which range from $25 to $100 Mg −1 .

Life-cycle assessments (LCAs) of switchgrass (Panicum virgatum L.) grown for bioenergy production require data on soil organic carbon (SOC) change and harvested C yields to accurately estimate net greenhouse gas (GHG) emissions. To date, nearly all information on SOC change under switchgrass has been based on modeled assumptions or small plot research, both of which do not take into account spatial variability within or across sites for an agro-ecoregion. To address this need, we measured change in SOC and harvested C yield for switchgrass fields on ten farms in the central and northern Great Plains, USA (930 km latitudinal range). Change in SOC was determined by collecting multiple soil samples in transects across the fields prior to planting switchgrass and again 5 years later after switchgrass had been grown and managed as a bioenergy crop. Harvested aboveground C averaged 2.5± 0.7 Mg C ha −1 over the 5 year study. Across sites, SOC increased significantly at 0-30 cm (P=0.03) and 0-120 cm (P=0.07), with accrual rates of 1.1 and 2.9 Mg C ha −1 year −1 (4.0 and 10.6 Mg CO 2 ha −1 year −1 ), respectively. Change in SOC across sites varied considerably, however, ranging from −0.6 to 4.3 Mg C ha −1 year −1 for the 0-30 cm depth. Such variation in SOC change must be taken into consideration in LCAs. Net GHG emissions from bioenergy crops vary in space and time. Such variation, coupled with an increased reliance on agriculture for energy production, underscores the need for long-term environmental monitoring sites in major agro-ecoregions.

Highlights d 347 site-years of yield data from 11 experiments show benefits of diversification d Rotation diversification increased maize yields under putative droughts d More diverse rotations also showed yield benefits across all growing conditions d Diverse rotations accelerated maize yield gains over time