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bioweb.sungrant.org » Technical » Biomass Resources » Agricultural Resources » Crop Residues » Corn Stover
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Between 2000 and 2005, an average of 71 million acres of corn were harvested each year in the U.S. (range between 68.8 and 75.1 million acres), with national average yields ranging between 129.3 bu/ac and 148 bu/ac. Production is concentrated in the Midwest (about 80%), but occurs in nearly every state except New England (USDA-NASS, 2007). Corn stover is the above ground, non-grain portion of the corn plant (stalks, leaves, corn husks, and cobs). While corn grain is already used to produce ethanol and organic chemicals, the corn stover can also be collected and used for bioenergy and bioproducts.
The quantities of corn stover available depend on the quantities produced, minus the quantities that must remain on the field. Crop residues play a vital role in maintaining soil characteristics (e.g., soil organic matter and soil moisture), controlling erosion and chemical runoff, and ensuring the long-term productivity of the soil. Sufficient residue quantities must be left to maintain these functions.
In the absence of data for crop residue yields, the quantities of corn stover produced per acre are estimated by multiplying the corn grain yield by a residue-to-grain ratio (i.e., a harvest index). Most studies assume a residue-to-corn grain ratio of 1:1 (Brown, 2003; Heid, 1984; Larson, 1997a, 1997b). Additionally most studies, but not all, assume a corn grain weight of 56 lbs/bu, which is the industry standard weight at 15.5% moisture.
The quantities of stover that must remain on the field to maintain soil characteristics depend on a number of factors including whether corn is produced in a continuous cropping system or in rotation with other crops, the timing and type of management practices used (particularly tillage operations), the physical characteristics of the soil (soil type and erodibility), field characteristics (slope), and climate.
Corn typically produces higher quantities of residue relative to other crops that it is produced in rotation with, resulting in more corn stover being available under continuous corn production than in systems using crop rotations, but continuous systems often have greater pest and disease vulnerabilities. Currently, about 80% of corn is produced in rotation with another crop, typically soybeans (75%), although other crops are also used (e.g., wheat, rye, barley, and oats) (Kim and Quimby, 2003; USDA-ERS, 2006).
Available corn stover quantities are greater with the use of less intensive tillage practices, such a no-till. Corn tillage practices vary substantially by region; at a national level, 62% of corn acres used conventional till operations, 18% used reduced till operations, and 20% employed no-till methods in 2004 (Conservation Technology Information Center, 2004).
Residue cover plays a key role in limiting water and wind soil erosion. Nelson (2002, 2003, 2004), using the Revised Universal Soil Loss Equation (RUSLE) for rain erosion (Renard, 1996) and the Wind Equation (WEQ) for wind erosion (Skidmore, 1970, 1979, 1980), developed a methodology to estimate stover quantities needed to keep water and wind erosion below the tolerable soil loss level (T), which is the maximum rate of soil erosion that will not lead to prolonged soil deterioration and/or a loss of productivity. Quantities that must remain were estimated for each crop rotation and tillage practice combination for every soil type in all counties where corn is grown.
Crop residue decay returns carbon (organic matter) to the soil. Soil organic matter is crucial to the long term productivity of soil, as it affects soil processes such as cation exchange, aggregate stability, water holding capacity, and soil microbial activity. A national assessment of the amounts of stover needed to maintain soil organic matter is not available. Sheehan (2002, 2004) used the Nelson erosion data and the CENTURY model (a soil nutrient cycling model; CSU, 2001) to evaluate soil impacts resulting from the use of corn stover to produce ethanol in Iowa. Under most scenarios examined, the quantities of residues sufficient to maintain erosion at or below T were also sufficient to maintain soil organic matter levels, but exceptions occurred. This indicates that under some conditions, soil organic matter (rather than erosion) will be the binding constraint with respect to corn stover quantities that can be collected.
Limited data is available regarding the impacts of corn stover removal on long-term soil productivity. A recent review (Wilhelm, 2004) noted that some studies found significantly reduced yields with residue removal, while others found no impact. This indicates that initial conditions and regional variations will be important considerations. Other factors (e.g., soil moisture, microbial activity, equipment constraints) have not been systematically evaluated, but could also affect the quantities of removable residues.
The estimated quantities of corn stover available for bioenergy and bioproducts depends critically on the assumed corn acres, corn grain yields, crop rotation, tillage practice, and quantities that must remain on the soil. Based on average harvested acres and yields from 2000-2004, 280.8 million dry tons of corn stover were produced (Walsh, 2006) (figure 1).
Nelson (2004), using average acres and yields from 1997 to 2001, estimated 65 to 597 million dry tons of corn stover could be available in the Midwest if every cropland acre (not just corn) was in the specified rotation (continuous corn, corn-soybean, corn-spring wheat, corn-winter wheat) and tillage combination (no-till, conventional, reduced till), and sufficient stover quantities were left to control for wind and water erosion at the tolerable soil loss level. Both Perlack et al. (2005) and Walsh (2006) use Nelson’s erosion analysis to estimate the quantities of corn stover that could be available. Perlack et al. estimated that 74.8 million dry tons of corn stover are available currently, based on 68.8 million harvested acres and grain yields of 138.2 bu/ac, a continuous corn rotation, current mix of tillage practices, and controlling for soil erosion. Walsh estimates that 207.1 million dry tons of corn stover could be available (assuming average acres and yields for 2000-2004) if all acres are in a continuous corn rotation and use no-till operations, and erosion constraints only are accounted for, but that only 57.6 million dry tons of corn stover are available if all corn acres are in a corn-soybean rotation, use the current mixture of tillage practices, and both erosion and soil organic matter constraints are accounted for (the existing situation) (figure 2). Soil organic matter constraints are based roughly on the Residue Equivalent Value quantities (the quantities of residue that must be returned to soil annually to maintain a constant level of organic matter), as estimated by the Soil Conditioning Index model developed by USDA (Lightle, 1997, 1999).
Few studies estimating national corn stover quantities include an economic cost component. Those that do, generally assume that the stover is baled, as this method has been demonstrated at a commercial scale (Schechinger, 2004; Glassner, 1998), involves off-the-shelf technology, and uses widely available equipment. Baling readily fits into the existing infrastructure and can be used now without major modifications. However, baling involves multiple passes over the field which occur following grain harvest. This can be problematic in areas with short collection periods (due to the onset of winter, which limits field access) and may increase the amount dirt contained in the stover. New approaches that simultaneously harvest and collect the grain and stover are being explored, but not currently available (Atchison, 2004; ISU, 2006). These approaches are expected to reduce collection costs.
Gallagher (2003a, 2003b) (using 1997 corn acres, corn yields, and input costs) estimated that 102.4 million tons of corn stover could be available for bioenergy uses at a price of less than $42/dt, with 90 million dry tons available at prices of less than $14.50/dt. The analysis assumed that sufficient corn stover quantities were left to provide 30% coverage of the field surface (1,430 lbs/ac). Gallagher’s projections included acres for which the erosion level was below the tolerance level when 1,430 lbs/ac of residues were left. The analysis was based on representative soil types by major crop production region. Prices included the cost of chopping and baling the stover, on-farm hauling of bales, and a fertilizer replacement cost of $6.47/dt of corn stover removed, and accounted for the use of corn stover as livestock forage.
Rooney (1998) estimated that 233.6 million dry tons of corn stover could be available based on 1996 acres and yields. The analysis assumed 2 dry tons of stover per acre needed to remain on the field, and eliminated quantities for areas where, after leaving 2 dt/ac, the quantities available for collection were less than 1 dt/ac. He estimated average corn stover costs (including fertilizer replacement cost of $5.33/dt) as $25.26/dt (range of $21.42 to $31.63/dt, depending on location).
Projected quantities of corn stover that could be available in the future are based on multiplying projected corn grain yields by a residue-to-grain ratio. Most analyses assume the current residue-to-grain ratio remains unchanged as corn grain yields increase in the future. In the absence of data, the assumption is reasonable, but it should be noted that some breeding approaches might alter the ratio. Future grain yield projections typically rely on a combination of factors, including genetic improvements, more efficient use of crop inputs (e.g. fertilizers and herbicides), and improvements in the machinery used to produce and harvest the grain (FAPRI, 2007; USDA, 2007; RCA III Symposium, 1997). The removal of corn stover alters the assumed management practices and increases the uncertainty of applying future grain yield projections to estimate future stover quantities.
A workshop of crop and livestock experts (RCA III Symposium, 1997) projected that under the mostly likely scenario, average national corn grain yields of 215 bu/ac and 260 bu/ac could be achieved by 2030 and 2050, respectively (compared to the 1990-1992 yields of 120 bushels/ac). This projected yield increase is attributed to a combination of genetic, management, and equipment improvements. The USDA projects average national yields of 170.2 bushels/ac and 82.8 million harvested acres by 2016 (USDA-OCE, 2007). The Food and Agricultural Policy Institute projects average national corn grain yields of 172.6 bu/ac and 82.4 million harvested acres by 2016 (FAPRI, 2007). Industry sources indicate the potential of substantially higher grain yields. Corn grain yields have increased an average 1.8 bu/ac/yr between 1965 and 2005 (CAST, 2006).
Few studies estimate future quantities of corn stover. Perlack et al. (2005) estimated that by mid-century, 169.7 million dry tons of corn stover could be available under moderate corn grain yield increases (172.8 bu/ac), and 256.1 million dry tons under high corn grain yield increases (207.3 bu/ac). Both estimates assumed 76.6 million harvested acres of corn, all acres in a continuous corn rotation, all acres using no-till practices, and that quantities of residues needed to maintain erosion at the tolerable soil loss level are left on the field. No economic analysis was conducted.
Walsh (2006) estimated county corn stover supply curves (i.e., quantities of stover available as a function of price) for the years 2005, 2010, 2015, 2020, and 2025. The analysis used a dynamic model of the U.S. agricultural sector (POLYSYS; De La Torre Ugarte, 2000) to estimate corn acres over time. The analysis assumes a corn-soybean rotation, and that (over the time period evaluated) corn grain yields increased from 147.4 to 183.3 bushel/ac, acres using reduced and no-till practices increased from 40 to 75%, and collection costs decreased to 75% of current round baling costs. The analysis controlled for water and wind soil erosion at the tolerable soil loss level (using Nelson’s work) and included a rough estimate of the quantities needed to maintain soil organic matter at existing levels, based on the Residue Equivalent Values in the Soil Conditioning Index (Lightle, 1997, 1999). Costs include expenses for fertilizer replacement, stover collection, and movement of the bales to the edge of the field (but not transportation costs to a user facility), and differ regionally as a result of regional differences in input costs. County supply curves were aggregated to obtain estimated national corn stover supplies. National supply curves for select prices are shown in table 1.
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