Published online 1 January 2007
Published in Agron J 99:1-11 (2007)
DOI: 10.2134/agronj2005.0222
© 2007 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Biofuels
Current and Potential U.S. Corn Stover Supplies
R. L. Grahama,*,
R. Nelsonb,
J. Sheehanc,
R. D. Perlacka and
L. L. Wrighta
a Environ. Sci. Div., Oak Ridge National Lab., P.O. Box 2008, Oak Ridge, TN 37831-6407
b Engineering Extension Programs, Kansas State Univ., 1048 Rathbone Hall, Manhattan, KS 66506
c National Renewable Energy Lab., 1617 Cole Blvd., Golden, CO 80401
* Corresponding author (grahamrl{at}ornl.gov)
Received for publication July 28, 2005.
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ABSTRACT
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Agricultural residues such as corn (Zea mays L.) stover are a potential feedstock for bioenergy and bio-based products that could reduce U.S. dependence on foreign oil. Collection of such residues must take into account concerns that residue removal could increase erosion, reduce crop productivity, and deplete soil carbon and nutrients. This article estimates where and how much corn stover can be collected sustainably in the USA using existing commercial equipment and estimates costs of that collection. Erosion constraints to collection were considered explicitly, and crop productivity and soil nutrient constraints were considered implicitly, by recognizing the value of residues for maintaining soil moisture and including the cost of fertilizer to replace nutrients removed. Possible soil carbon loss was not considered in the analysis. With an annual production of 196 million Mg of corn grain (
9.2 billion bushels), the USA produces 196 million Mg of stover. Under current rotation and tillage practices, 30% of this stover could be collected for less than $33 Mg–1, taking into consideration erosion and soil moisture concerns and nutrient replacement costs. Wind erosion is a major constraint to stover collection. Analysis suggests three regions of the country (central Illinois, northern Iowa/southern Minnesota, and along the Platte River in Nebraska) produce sufficient stover to support large biorefineries with one million Mg per year feedstock demands and that if farmers converted to universal no-till production of corn, then over 100 million Mg of stover could be collected annually without causing erosion to exceed the tolerable soil loss.
Abbreviations: HI, harvest index • hp, horsepower • NASS, National Agricultural Statistics Survey • NRCS, National Resource Conservation Survey • T, tolerable soil loss (Mg ha–1 yr–1)
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INTRODUCTION
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BIOENERGY AND BIOBASED PRODUCTS are essential elements of the U.S. national policy to increase energy supplies and reduce dependence on foreign oil (USDOE-USDA, 2002a, 2002b). The United States Department of Energy is actively supporting research to lead to the development of future biorefineries, which convert ligno-cellulosic biomass feedstocks to ethanol, power, and biobased chemicals (USDOE, 2003). The Biomass Research and Development Act of 2000 (Title III of the Agricultural Risk Protection Act) has fostered USDA and U.S. Department of Energy bioenergy research solicitations in recent years.
Existing agricultural residues, such as stover from corn grain production, are an obvious source of biomass especially for the near term. The collection and removal of these residues that would otherwise be left in the field must be done in a sustainable fashion and not impair the productivity of the land, diminish water quality, or result in unwanted carbon emissions. Two recent articles examine the issues of sustainable collection of corn stover (Wilhelm et al., 2004; USDA-NRCS, 2003). Both studies suggest that some collection of corn stover is sustainable but do not indicate how much or where in the USA collection might be sustainable.
The objective of this analysis was to estimate the location and quantity of corn stover that might be sustainably collected in the United States and the cost of collecting that stover. With regard to sustainability, the analysis focused on limiting water and wind erosion. Potential effects on crop productivity were partially addressed by considering preservation of soil moisture and the replacement cost of nutrients lost with stover removal. The need to maintain soil carbon was not considered. Although soil carbon and crop productivity are important considerations, the knowledge base for quantitatively assessing the amount of stover that needs to remain on the field to maintain soil carbon or crop productivity is limited, although it is growing (see Wilhelm et al., 2004; USDA-NRCS, 2003; Linden et al., 2000). The model Century (Sheehan et al., 2004) has been applied to address soil carbon aspects of stover removal but not under all the corn production situations that occur within the USA. The USDA Soil Conditioning Index (USDA-NRCS, 2006) can be used to assess the effect of stover removal rates on soil carbon. However, in its current form with manual input, the Soil Conditioning Index is not practical to run for the thousands of corn production situations that occur in the USA.
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METHODS
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Using readily available data from the USDA, we sought to capture the complexities of corn production practices in the USA and their effect on corn stover supplies. Crop yields, crop rotations, soils, and tillage practices were considered at the finest spatial resolution available, generally the county. The supply of sustainably collectable stover in the USA was estimated by (i) calculating the stover produced per hectare of corn production in a county, (ii) calculating the amounts of stover required to stay in the field (Mg ha–1 yr–1) to meet sustainability and operational constraints for the suite of typical corn rotation and tillage practices in the county, (iii) calculating the collectable quantities of stover (Mg ha–1 yr–1) under each of those same practices, (iv) calculating the cost of collecting stover ($ Mg–1) under those same practices, and (v) estimating quantities (and their respective collection cost) of collectable stover in a county given the corn production practices of that county. These five steps are described in the following text.
Stover Production
The amount of stover produced per hectare of corn grain production was estimated using corn grain yield values and a stover mass to grain mass ratio of 1:1 (i.e., a dry weight harvest index [HI] of 0.5) as reported by Gupta et al. (1979). Because corn grain production is reported by the USDA in units of bushels and acres, for conversion to units of mass it was assumed that a bushel of corn had dry grain mass of 21.5 kg (56 lb at 15.5% moisture) (Wilcke and Wyatt, 2002). Stover production was calculated with the following equation:
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The amount of residue produced by other crops grown in rotation with corn was also calculated using crop-specific residue to grain ratios and bushel to dry mass conversion factors (Table 1). Estimates of other crop residues were used in the wind and water erosion constraint calculations for different rotations. County grain production and area harvested for 1995 through 2000 were obtained from the USDA National Agricultural Statistics Survey (USDA-NASS, 2003) and used to calculate an average grain yield by dividing the sum of the grain production over those years by the sum of the harvested acres.
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Table 1. Residue to grain ratios, associated residue harvest index, and factors used to convert USDA values of grain production from bushels to dry mass.
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Constraints to Stover Collection
To determine the amount of stover that could be collected (collectable stover), three constraints were considered.
Equipment Constraints
Collection operations leave some stover in the field. The amount left in the field is a function of the equipment used to collect the stover and the condition of the stover. For this analysis, it was assumed that at least 25% of the stover would be left in the field because of current equipment collection limitations; thus, no more than 75% of the stover could be collected under any condition (Montross et al., 2002; Schechinger and Hettenhaus, 2004). Working in experimental Kentucky fields, Montross et al. (2002) reported round bale collection efficiencies of 38%, 55%, and 64% using three strategies, respectively: bale only; rake and bale; and mow, rake, and bale. Schechinger and Hettenhaus (2004) reported collection efficiencies of 40 to 50% without raking and 70% with raking in large-scale stover collection operations in Nebraska and Wisconsin.
Soil Moisture Constraints
In some regions of the country, under rainfed agriculture all stover must be left on the field to maintain soil moisture for the next crop. Under the assumption that these regions coincide with where local wind erosion climatic factor is greater than 50 in April (R. Follette, personal communication, 2005), Allmaras' (1983) map of climatic factors was used to locate counties with a soil moisture constraint that precluded any stover collection (Fig. 1
). This constraint was not applied to stover collection from irrigated corn production. Data from the USDA National Agricultural Statistical Survey (USDA-NASS, 2003) were used to determine area and yield of irrigated corn production in the counties where stover from rainfed corn production could not be collected (Fig. 2
).
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Fig. 1. U.S. counties where the need to leave stover to conserve soil moisture constrains all collection of stover except under irrigated corn production (based on Allmaras, 1983).
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Water and Wind Erosion Constraints
The stover needed to remain in the field (Mg ha–1) to assure that erosion did not exceed the tolerable soil loss value (T) was estimated using the approach of Nelson (2002), Nelson et al. (2004), and Sheehan et al. (2004). This approach considers cropping rotation, tillage, local climate conditions, and the suite of agricultural soil types found in a county. Only corn rotations that accounted for more than 15% of the hectares in corn production in the state were evaluated (Padgitt et al., 2000). Corn–corn and corn–soybean [Glycine max (L.) Merr.] rotations were evaluated for all states. Corn–small grain rotations (wheat [Triticum aestivum L.], barley [Hordeum vulgare L.], and oat [Avena sativa L.]) were also evaluated for North and South Dakota. Three tillage practices were considered for each rotation: conventional tillage (moldboard plow followed by disking and cultivation), mulch till (chisel disking followed by cultivation), and no-till production systems. For each combination of rotation and tillage practice, soil type-specific erosion constraint values were calculated and converted to a single county-level value by weighting all the soil type-specific values by their land area relative to the county's total cropland soil base. Water erosion constraints to collection were considered in all states east of the Rocky Mountains, and wind erosion constraints were considered only in the western states shown in Fig. 3
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Collectable Stover
For each rotation and tillage practice combination, the collectable stover (dry Mg ha–1) was estimated by subtracting the maximum collection constraint from the amount of stover produced per hectare in the county.
 | [2] |
where CQ = collectable stover per ha in county c under rotation r and tillage t (Mg ha–1 yr–1), c = county, r = rotation, t = tillage practice, Stover = the amount of stover produced per hectare in county c (Mg ha–1 yr–1), and Max constraint = the amount of stover left in the field that meets all erosion, moisture, and equipment constraints in county c under rotation r and tillage t (Mg ha–1 yr–1).
With the exception of counties with soil moisture constraints, stover production was calculated on the basis of all corn production in the county. In counties where soil moisture constrained collection of stover from rainfed corn production but corn was produced with irrigation, stover production was calculated on the basis of stover produced under irrigated corn production, and the soil moisture constraint and the water erosion constraint were not included in determining the maximum constraint value. Data from the National Agricultural Statistical Survey (USDA-NASS, 2003) were used to determine area and yield of irrigated corn production in the counties where stover from rainfed corn production could not be collected (Fig. 2).
Cost of Collecting Stover
The farmgate cost of collecting stover was calculated using an engineering approach (American Association of Agricultural Economics, 2002) and was based on the work of Perlack and Turhollow (2003), Sokhansanj and Turhollow (2002), and Sokhansanj et al. (2002). The costs are in 2002 dollars and reflect replacement of nutrients removed with the stover estimated at $7.17 Mg–1 ($6.50 ton–1) (Gallagher et al., 2003) and all resources associated with collecting stover and delivering it to the side of the field in the form of large round bales, wrapped in mesh. Depending on the amount of stover to be collected, one of three collection scenarios was assumed (Table 2). For each collection scenario, collection costs were estimated for a range of stover collection quantities and a regression equation was developed that related collection costs with amount of stover collected (Fig. 4
). These equations were used to associate a collection cost for each unique value of CQc,r,t.
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Table 2. Collection scenarios as function of amount collected. Under all situations, bales are moved to the field edge for storage using a 89 kW (120 hp) tractor and bale wagon (Inland) with a capacity of 17 bales.
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Fig. 4. Curves used to estimate stover collection cost as a function of stover collected in the field. Curves include $7.17 Mg–1 nutrient replacement cost ($6.50 ton–1) (Gallagher et al., 2003). "Bale windrow" refers to the collection method assumed to be used when collecting less than 2.69 Mg ha–1. "Rake/windrow & bale" refers to the method assumed used when collecting between 2.69 and 3.36 Mg ha–1. "Two operations–Shred/rake and bale" refers to the method assumed used when collecting greater than 3.36 Mg ha–1. The three methods are described in Table 2.
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Estimating County Stover Supply
Each county's supply (Mg yr–1 at a given cost) of collectable stover was calculated for the following four scenarios: (i) current tillage practices and constraining erosion to less than T, (ii) universal no-till and constraining erosion to less than T, (iii) universal mulch till and constraining erosion to less than T, and (iv) current tillage practices and constraining water erosion to less than 1/2 T (only calculated for states without wind erosion).
Current Supply and Erosion less than T
The following equation was used to estimate collectable stover for the first scenario:
 | [3] |
where S = collectable stover supply in county c under rotation r and tillage t (Mg yr–1), CQ = collectable quantity of stover under rotation r and tillage t in county c (Mg ha–1 yr–1), Land = hectares of harvested corn in the county c (ha yr–1), Tillage = % of corn under tillage scenario t in county c, and rotation = % of corn in rotation scenario r in state s.
For this calculation, the value of Landc was based on harvested corn hectares in a county between 1995 and 2000 as reported by USDA (USDA-NASS, 2003). In counties with soil moisture constraints, only irrigated corn hectares were considered. The value of Tillaget,c was based on county-level data from the National Crop Residue Management Survey (CTIC, 2004) on corn tillage practices between 1995 and 2000. The Survey covers five tillage categories: conventional till, reduced till, ridge till, mulch till, and no-till. Conventional and reduced till hectares were grouped together to define corn produced under conventional tillage. Likewise, ridge and mulch till hectares were grouped to define corn produced under mulch tillage.
Collection costs associated with each CQc,r,t were calculated using the regression equation. These costs were paired with their appropriate supplies (Ss,c,r,t), and the quantities of collectable corn stover available at farmgate costs less than $27.56, $33.07, $38.58, and $44.09 per Mg ($25, $30, $35, and $40 per ton) were calculated.
Potential Stover Supply under Universal No-Till or Mulch Till Practices
The impact of tillage practices was evaluated by estimating the supply of stover that could be collected if all current corn production was in no-till or mulch till. No change in yield was assumed. Equation [3] was used, but only no-till (or mulch till) values of CQ were considered, and Tillage was set to 100%.
Supply of Collectable Stover if Water Erosion Is Constrained to less than 0.5 T
To evaluate the impact of more stringent sustainability requirements, the amount of stover that must be left in the field under current tillage practices to assure that water erosion did not exceed 1/2 T was calculated. This was done for all rotations and tillage practices. CQ was recalculated taking into account the stover needed in the field under the more stringent constraint, and then S was calculated. This calculation was done only for states where wind erosion was not evaluated.
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RESULTS AND DISCUSSION
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The national supply of collectable stover is directly related to the quantity of grain produced. Figure 5
shows the amount of stover (196 million Mg) annually produced from corn grain production assuming 196 million Mg of corn grain production (the average annual U.S. corn production between 1995 and 2000) and a stover HI of 0.5. To put this number in context, U.S. corn production from 1993 to 2005 has ranged from 136 to 254 million Mg yr–1 (6.3–11.8 billion bushels yr–1) with an upward trend but significant year to year variation. Corn production exceeded 215 million Mg (10 billion bushels) in 1994, 2003, 2004, and 2005 but hovered between 194 and 213 million Mg between 1996 and 2002 and dropped to 159 million Mg in 1995 (USDA-NASS, 2006).
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Fig. 5. Annual production of corn stover in the United States. Values were derived as described in text using 1995–2000 corn production statistics from USDA.
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The amount of stover produced is influenced not only by corn grain production but also by the stover HI. Although the stover HI is commonly assumed to be 0.5 (i.e., 50% of the dry mass of the corn plant is grain and 50% is stover, the value used in this analysis), various reports applied the 0.5 HI but then used the weight of a bushel of #2 corn at 15.5% moisture rather than the dry weight to calculate the mass of stover per bushel of harvested corn grain. Failure to adjust for the water in the standard weight of #2 corn grain when computing stover production from grain production and stover HI results in inflated estimates of stover quantities. The use of stover HI to compute stover mass is problematic in that stover HI is variable. Linden et al. (2000) reported an average corn grain HI (grain dry weight to total plant dry weight) of 0.561 (SD = 0.079) or a stover HI of 0.439 for corn production over 14 yr in east central Minnesota. Montross et al. (2002) found a stover HI of 0.47 to 0.52 on a farm in Kentucky. Both studies reported that as grain yield went up, stover HI went down. Patterson et al. (1995) also noted variability in residue HI for wheat. This issue is important because it significantly affects the amount of stover that is estimated to be collectable regardless of what constraints to collection are assumed for meeting sustainability needs. It is also the source of some of the variability in estimates of collectible stover.
Stover Supply under Current Corn Production Practices
Given the constraints discussed previously, the total annual collectable stover in the USA was estimated at 58.3 million Mg (64.2 million dry tons). This value was nearly 30% of all stover produced. Nearly all the collectible stover (93%) came from land where at least 2 Mg of stover could be collected per hectare and collection costs were less than $33.07 Mg–1 ($30 ton–1). More than half the supply of harvestable stover was from fields where 4 Mg ha–1 or more could be collected at a cost of less than $27.56 Mg–1 ($25 ton–1) (Table 3). Most of the supply (62%) came from three major corn-producing states: Iowa, Minnesota, and Illinois. The supply from Nebraska, another major corn-producing state, was comparatively low because of the impact of wind erosion on collection, a constraint not considered in the other three states but likely to be critical in some areas of these states. Wind erosion constrained collection significantly. Wherever wind erosion was calculated, stover could be collected only from lands managed using mulch or no-till practices.
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Table 3. Hectares of harvested corn, quantities of collectable stover in Mg (000) by collection cost, and quantity of stover produced. The quantity of stover available at a cost >$44.09 Mg–1 is the same as the total amount of collectable stover. Stover values are based on current corn grain production and management practices.
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Figure 6
shows at a county level where 54 million Mg of stover could be collected for less than $33.07 Mg–1 ($30 ton–1). Three regions stood out as suitable for stover collection in large quantities: central Illinois/Indiana, northern Iowa/southern Minnesota, and along the Platte River in Nebraska (Table 4). Table 3 characterizes these three regions. They have high corn yields, are flat, and, in the case of Nebraska, irrigated. Collection constraints require that at least a third of the stover be left on the field. Conservation tillage (mulch or no-till) is used on 22% of the central Illinois/Indiana land, 29% of the Iowa/Minnesota land, and 61% of the Nebraska land. These regions are capable of supporting biorefineries demanding large quantities of stover annually within a short hauling distance (USDOE-USDA 2002a).
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Fig. 6. Sustainably collectable corn stover for less than $33.07 Mg–1 ($30 ton–1) under current (1995–2000) tillage practices.
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Table 4. Characteristics of the three major corn stover supply regions in the United States assuming current (1995–2000) corn management practices.
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The estimate of current collectable stover supplies is significantly less than previous estimates: 89.8 million Mg (98.9 million dry tons) (Gallagher et al., 2003) and 108.9 million Mg (120 million dry tons) (Walsh et al., 2000). The Gallagher et al. (2003) estimate was based on (i) a higher HI, (ii) all corn produced using mulch till practices, (iii) no equipment constraints to collection, and (iv) a minimum of 1.6 Mg ha–1 (0.72 tons acre–1) of stover left on the field. Gallagher et al. (2003) estimated delivered bale costs generally less than $16.53 Mg–1. Their lower cost is attributable to lower assumed labor costs, the assumption that harvest costs were fixed per hectare, and the assumption that stover could be collected efficiently without raking. The Walsh et al. (2000) estimate assumed 25.4 kg of stover were produced per bushel (rather than 21.5 kg as in this study) and no equipment constraints to collection.
Effect of Increased Adoption of Conservation Tillage
Changing tillage management significantly affected the estimate of collectible stover, especially in western states where wind erosion is problematic (Fig. 7
). Collectable stover in Nebraska more than doubled under the assumption of universal no-till corn production. If all U.S. corn production were under mulch till and if the assumed annual corn production area of 28.5 million hectares did not change, the total collectable stover supply (at any price) would rise to 68.9 million Mg (76 million tons yr–1). Similarly, if no-till practices were universally adopted, the total collectable stover supply would increase to 101.2 million Mg (111 million tons). Even with these tillage practices, nearly 50% of stover would remain uncollectible. The location of sustainably collectable stover at less than $33 Mg–1 (Fig. 8
) showed almost no overlap with the 59 million hectares (146 million acres) of land classed as highly erodible cropland in 1997 by NRCS (Heimlich, 2003). This indicates that the highly erodible land, defined as land where the erosion potential is at least eight times its T value, would not be a source of sustainable stover biomass, even with no-till management. The additional stover under reduced tillage scenarios came from the highest corn yield production areas (compare Fig. 5 and 8) and moderately expanded the three regions previously identified as being suitable for large biorefineries (Fig. 6). Under no-till scenarios, the inability of equipment to collect more than 75% of the stover became the maximum constraint to collection in many areas.
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Fig. 7. Top eight states for producing collectable corn stover. Three tillage scenarios were considered: current tillage practices, universal mulch till, and universal no-till. Collection was constrained by soil moisture and equipment consideration and limiting erosion to less than tolerable soil loss.
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Fig. 8. Sustainably collectable corn stover for less than $33.07 Mg–1 ($30 ton–1) assuming universal no-till corn production.
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Effect of More Restrictive Constraints on Water Erosion
If more stringent water erosion constraints were assumed (i.e., that stover could be collected only when removal would not increase erosion to more than 0.5 T), the collectable supply decreased significantly regardless of tillage assumptions (Fig. 9
). Under this scenario (which addressed only states assumed not to have wind erosion constraints), only 22 of the 145 million Mg of stover produced in these states was available for collection. However, if universal no-till were adopted, the available stover would more than double to 57.5 million Mg per year. Although T is commonly assumed to be an acceptable erosion rate, it may not be sustainable (Mann et al., 2002). Thus, there is merit to exploring the impact of more stringent erosion constraints on stover collection. Such constraints would also contribute to sustaining soil carbon.
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Fig. 9. Collectable stover from states without wind erosion considerations (see Fig. 3). Collectable stover was calculated as a function of differing combinations of water erosion constraints (limiting erosion to < tolerable soil loss [T] or less than 0.5 T) and tillage assumptions (current practices, universal mulch till, or universal no-till). Curves are cumulative over the cost range.
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SUMMARY
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Given current corn production practices, less than 28% of the stover produced in the United States could be sustainably collected at a farmgate cost less than $33.07 Mg–1 ($30 dry ton–1). More stringent soil loss constraints would lower this value considerably. However, if farmers chose to universally convert to no-till corn management and total stover production did not change, the sustainable supply would almost double. Given the current (1997–2000 average) U.S. corn production, 91.8 to 106.1 million Mg (105–117 million dry tons) of stover would be potentially collectable if farmers wished to manage their corn lands to produce harvestable grain and stover. These values have considerable uncertainty because they do not account for variation in stover HI and do not factor in the need to maintain or enhance soil organic matter and tilth. The latter consideration may require more stover left on site than was estimated in this analysis, which considered only soils erosion and soil moisture constraints. The estimate may also be high because current equipment may not be able to collect stover as efficiently as was assumed in this analysis. Nonetheless, they suggest that sufficient corn stover could be sustainably collected to support the development of corn-stover–based biorefineries in the Midwest.
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ACKNOWLEDGMENTS
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We appreciate the helpful comments of anonymous reviewers and the editors at the Agronomy Journal. We also wish to acknowledge the assistance of Ron Follett, USDA, in defining regions with soil moisture constraints. This research was funded by the U.S. Department of Energy's Office of Biomass with the Office of Energy Efficiency and Renewable Energy. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the US Department of Energy under contract DEAC05-00OR22725.
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ABSTRACT
INTRODUCTION
