Abstract 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 m2) and estimated inputs. We managed switchgrass as a biomass energy crop in field trials of 3–9 ha (1 ha = 10,000 m2) 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 Mg·ha−1 with a resulting average estimated net energy yield (NEY) of 60 GJ·ha−1·y−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 requirements [petroleum energy ratio (PER)] to measure the sustainability of a biofuel. In initial analyses, switchgrass was estimated to have a net energy balance of 343% when used to produce biomass ethanol (7). More recent energy model analyses that used simulated biomass yields and estimated agricultural inputs indicate that switchgrass could produce >700% more output than input energy (8–10), whereas GHG have been assumed to be near zero (1) or estimated to be slightly positive (8) for ethanol derived from switchgrass.

Lignocellulosic feedstocks such as switchgrass, woody plants, and mixtures of prairie grasses and forbs have been proposed to offer energy and environmental and economic advantages over current biofuel sources, because these feedstocks from perennial plants require fewer agricultural inputs than annual crops and can be grown on agriculturally marginal lands (11). An estimated 3.1 × 106 to 21.3 × 106 ha (1 ha = 10,000 m2) of existing agricultural land in the U.S. is projected to be converted to perennial grasses for bioenergy based on theoretical market prices (1). The majority of land for perennial grass production is projected to come from the reallocation of existing cropland, with land currently enrolled in the Conservation Reserve Program (CRP) and pastures being second and third, respectively. The CRP was authorized by the Food Security Act of 1985 and had a goal of removing highly erodible marginal cropland from crop production by paying farmers and land owners to revegetate the land with perennial grasses and trees. The cropland base predicted to be converted to perennial grass biomass systems will be similar to existing CRP land (12).

Unlike corn, for which long-term data on grain yield and agricultural inputs in the U.S. are available, data for switchgrass and other perennial herbaceous plants grown and managed as bioenergy crops are limited and are based largely on small-plot research, in which plots are typically <5 m2. To obtain relevant field-scale information for switchgrass managed as a biomass energy crop, we conducted trials using fields on 10 farms in the midcontinental U.S. (Fig. 1) for 5 yr to obtain production information for use in net energy and economics analysis. Adapted switchgrass cultivars were grown and managed as a biomass energy crop in fields on four farms each in Nebraska and South Dakota and two farms in North Dakota using management practices developed in previous small plot research.

Fig. 1. Switchgrass field locations managed for bioenergy (filled circle) and human-made prairie plots (+) with average annual precipitation zones for 2000–2005 (13).

Cooperating farmers, who were paid for their work and land use, documented all production operations and field biomass yields. This study provided 5 yr of production and management information from each farm, which we used to estimate net energy, petroleum inputs to ethanol outputs, and GHG emissions.

Discussion In this study, we used actual farm information to determine energy inputs. The lower energy inputs for biomass we are reporting in comparison to the estimates reported previously clearly highlight discrepancies that can occur when analyses are based on small-scale research plots and misassumptions. In the prairies of the U.S., precipitation and species richness follow an east–west gradient, with highest levels of precipitation (Fig. 1) and species richness (29) occurring in the east. Mean above-ground net primary production of grassland systems and mean annual precipitation have a positive correlation (r = 0.90) for the Great Plains (30). In this study, farms in the east produced greater switchgrass biomass yields than farms in the western part of the study region. Based on precipitation, the low-input prairie in Minnesota was in a higher biomass production zone than the fields in this trial. In addition to having low net-energy yields, the Minnesota prairie plots (19) represent an artificial system, because they were hand-seeded, hand-weeded, and irrigated during establishment; only 10-cm-wide strips within a plot were hand-harvested to determine biomass yields; and the same strips were never reharvested. Low-input subsistence agriculture has low outputs, because essential factors needed to optimize capture of solar energy are lacking. The addition of nitrogen to undisturbed and restored high-diversity prairies has been shown to increase above-ground biomass production (31, 32). These results demonstrate a similar situation likely exists for perennial biomass energy crops. Switchgrass managed as a biomass energy crop with moderate inputs including N fertilizer can be as net energy efficient as low-input systems but can produce significantly greater quantities of energy per unit of land. For an alternative transportation fuel to be a substitute for conventional gasoline, the alternative fuel should (i) have superior environmental benefits, (ii) be economically competitive, (iii) have meaningful supplies to meet energy demands, and (iv) have a positive NEV (11). The results of this study demonstrate that switchgrass grown and managed as a biomass energy crop produces >500% more renewable energy than energy consumed in its production and has significant environmental benefits, as estimated by net GHG emissions as well as soil conservation benefits (1). In this study, we used a constant previously published conversion rate. 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 improvement in net energy for switchgrass. 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; it also has fully equivalent NEV. Current corn production has increased 160% in the U.S. in the last 40 yr because of increased grain yields and expansion of crop area (2). In Iowa, corn grain yields increased >80 kg·ha−1 per year during the period from 1930 to 1994 (33). Approximately 50% of the increase in grain yield of corn during this period was attributable to improved hybrids, whereas the remaining improvement was due to improved management practices and inputs. 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. This is a baseline study that represents the technology available for switchgrass in 2000 and 2001, when the fields were planted. It clearly demonstrates that managed switchgrass production systems have the potential to produce significantly more energy than is used in production and conversion. Traditional breeding techniques have increased yield performance of switchgrass by 20–30% from existing parent types (34). It is expected that further improvements in both genetics (hybrid cultivars, molecular markers) and agronomics (production system management practices and inputs) will be achieved for dedicated energy crops such as switchgrass, which will further improve biomass yields, conversion efficiency, and NEV (35). As an indicator of the improvement potential, switchgrass biomass yields in recent yield trials in Nebraska, South Dakota, and North Dakota (36–38) were 50% greater than achieved in this study. The Green Revolution greatly enhanced the capacity of agriculture to increase food supplies throughout the world by the use of improved genetics and management inputs (39). Green energy goals of nations likewise can be met in part through improved genetics and agronomics. The environmental and ecological effects of the conversion of cropland to CRP were largely positive. It is expected that results will be similar for conversion of land to perennial grasses such as switchgrass for bioenergy. However, environmental and ecological assessments should continue to be made at both the micro and macro scales.

Methods Locations. We conducted trials on 10 farms in the northern Great Plains for 5 yr to obtain field-scale production information for use in net energy and economic analysis. The 10 farms were located in areas where previous economic model analyses indicated switchgrass grown as a biomass energy crop would be economically feasible (40). The cooperating farmers and farms and fields used in this study were selected based on recommendations of U.S. Department of Agriculture Natural Resource Conservation Service (USDA-NRCS) staff for the three states and site visits by K.P.V. The USDA-NRCS provides technical land eligibility determinations, conservation planning, and practice implementation for the CRP. Rainfed fields represent a range of biomass production environments that occur in this geographical region and have marginal cropland characteristics that could have qualified them for enrollment in the CRP. Adapted switchgrass cultivars were grown and managed as a biomass energy crop in fields on four farms each in Nebraska and South Dakota and two farms in North Dakota using management practices developed in previous research (41). Farms are identified by the name of the nearest town (Fig. 1). The selected switchgrass cultivars were developed primarily for use in pastures. Seeding rates were based on pure live seed (PLS) per unit area (30 PLS m2), which was ≈10 kg·ha−1. The Nebraska fields were established in 2000, except for the Atkinson field, which was reestablished in 2001 because of drought conditions in 2000. The South and North Dakota fields were established in 2001. Total area planted to switchgrass was 67 ha. Fields used in this study were existing cropland being used for grain or oilseed production. Soil samples were taken on each field before switchgrass establishment to assess initial soil fertility and quality. Field sizes, soil characteristics, and previous cropping history are described (42). Field size ranged from 3 to 9.5 ha and averaged 6.7 ha. Cooperating farmers, who were paid for their work and land use, documented all production operations and machine-harvested field biomass yields. A U.S. Department of Agriculture agronomist visited each field at least twice during each growing season to monitor switchgrass management, stands, and biomass yields. In midsummer, before harvest, 1.1-m2 quadrants were clipped at 16 locations within each field, and the harvested samples were dried and weighed to verify machine-harvested yields. In our analysis, fields not harvested in the establishment year had their previous agricultural energy inputs added to the first harvested year. After a killing frost, fields with yields >1.1 Mg·ha−1 and with minimal weed populations were harvested in the establishment year. Harvesting costs would exceed biomass value for yields below this threshold value. After the establishment year, cooperators had the option to harvest at emerged inflorescence to postanthesis stage of development or after a killing frost. Most cooperators chose to harvest at emerged inflorescence to postanthesis (early to mid-August) in postestablishment years, except for the Bristol, SD, and Munich, ND, farmers, who harvested after a killing frost. Harvests were done with conventional hay equipment. Modern balers are engineered to deliver very uniform bales, so cooperators weighed a subset of bales for yield determinations and sampled the bales with a provided bale-coring probe to obtain bale samples for determining baled biomass dry matter concentration. All yields were adjusted to a dry-weight basis. Life Cycle Bioenergy Analysis. Energy and Resources Group Biofuel Analysis Meta-Model (EBAMM) calculates cellulosic (switchgrass) agricultural inputs and yields based on previous switchgrass small-plot research, modeled transportation costs, embodied energy of ethanol plant materials, and current agricultural inputs for corn (9, 10, 14, 34, 43). We were able to update EBAMM in this study by: (i) basing agricultural diesel consumption on actual field operations, (ii) eliminating agricultural electricity use based on known inputs, (iii) basing embodied energy of farm machinery on field operations, (iv) basing packaging energy on the material that was used, (v) incorporating switchgrass seed energy costs, and (vi) crediting carbon sequestered by switchgrass to GHG emissions based on field-scale yields (see SI Table 2). A hydrolysis/fermentation biorefinery was the model cellulosic ethanol plant for EBAMM, with cogeneration power/export being the average of a steam Rankine cycle power system and a gas turbine combined cycle system (9, 44). Energy output was based on the ethanol energy value of 21.2 MJ·liter−1 (low heating value) and an electricity export of 4.79 MJ·liter−1. In this analysis, biorefinery energy, ethanol conversion yield, and byproduct energy were kept constant, whereas agricultural inputs and crop yield varied by field and harvest year. Seed energy values were based on agriculture inputs from U.S. Department of Agriculture–Agriculture Research Service (USDA-ARS) (Lincoln, NE) switchgrass seed fields (see SI Table 6). Agricultural inputs from any nonharvest year were added to the first harvestable year to determine NEV, NEY, PER, and GHG displacement. Farmers at individual locations did not report diesel consumption but reported all field operations. Farm machinery was considered the same across all locations to make comparisons among locations (see SI Tables 2 and 7). Any tillage inputs in the establishment year were added to the embodied energy and diesel use requirements for each location. Biomass production system diesel use was estimated based on the number and type of field operations at each location in a given year (see SI Table 7). Nitrogen fertilizer rates recommended to farmers in this study were 10 kg of N per Mg·ha−1 of expected yield (45) with a recommended maximum of 112 kg·ha−1·y−1. Nitrogen fertilizer application varied by postestablishment harvest years and locations because of farmer management decisions based on soil moisture conditions. Applied N ranged from 0 kg·ha−1 to 212 kg·ha−1 with a mean application rate of 74 kg·ha−1·yr−1 across all farms (harvest years 2–5). Farm labor energy was included in farm machinery costs and was not separated into an individual agricultural input. Agricultural inputs used for the human-made prairie study were based on reported values from a previous study (19). Agricultural inputs and yields from the human-made prairie study were inserted into EBAMM to make accurate comparisons among studies. A default energy requirement in EBAMM for bale transportation to a cellulosic biorefinery was removed from the human-made prairie study to eliminate duplication. Corn grain yields for Nebraska are from 2000 to 2004, whereas South and North Dakota corn grain yields are from 2001 to 2005 (20).

Acknowledgments This research was funded and conducted under U.S. Department of Agriculture–Agricultural Research Service and University of Nebraska–Lincoln Specific Cooperative Agreement no. 58-5440-0-305. Funds to initiate the research were from the U.S. Department of Energy Biomass Feedstock Development Program, Oak Ridge National Laboratory, contract no. DE-A105-900R2194.