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Liquid Transportation Fuels from Coal and Biomass: Technological Status, Costs, and Environmental Impacts F Estimating the Amount of Corn Stover That Can Be Harvested in a Sustainable Manner The use of national average corn grain yields to estimate available amounts of corn stover for producing biofuels provides a general guideline for decision making by both industry and landowners, but it is sometimes inappropriate because of the site-specific nature of agricultural production. For example, the amount of stover needed to minimize erosion and maintain soil organic matter for a specific land area depends on several factors, including the predominant landscape (for example, rolling or flat), soil type or series, climate, tillage and crop-management practices, and yield. As a result, regardless of the type of tillage being used, some locations in a given field cannot spare any crop residue without risking degradation of soil resources. In some other locations, crops grown in rotation with corn will actually benefit from partial removal of stover. National Agricultural Statistics Service data (USDA-NASS, 2008) on five important corn-producing states—Illinois, Iowa, Indiana, Nebraska, and Minnesota—were used to illustrate the complex relationships among seasonal weather patterns (climate), crop yields, and the multiple uses for corn stover, including mitigation of soil erosion, sustaining of soil organic matter, and biofuel feedstock. Those five states were chosen because in 2007 they accounted for 50 of the 86.5 million acres of harvested corn. In 2003–2007, average corn yields were 143–180 bushels/acre in Illinois, 157–181 bushels/acre in Iowa, 146–168 bushels/acre in Indiana, 146–166 bushels/acre in Nebraska, and 146–174 bushels/acre in Minnesota. Variations in yield were attributed to seasonal differences in weather. Two scenarios were constructed to help to determine a baseline amount of stover that could be harvested in a sustainable manner. First, the 5-year average yield in the five states (161 bushels/acre) was used to project 55.9 million tons
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Liquid Transportation Fuels from Coal and Biomass: Technological Status, Costs, and Environmental Impacts of harvestable stover. Second, the average of the highest grain yields achieved in each state from 2003 to 2007, 173.8 bushels/acre, was computed for the projection. That approach increased the estimated harvestable yield to 71.2 million tons, or 94 percent of the national projection based on the 2007 average grain yield. Recognizing that the high yields occurred in different states during a given year because of weather differences, the panel discussed reducing the estimate to 70 million tons. Ultimately, the consensus was to use 76 million tons on the basis of the national corn grain yield. The panel used the high harvestable value because it took a conservative approach to estimating the amount of stover that has to be left in the field to maintain soil. The panel also assumed that crop yields increase as a result of genetic improvement to enhance a crop’s stress-tolerance. Some may consider the panel’s baseline too low because many producers in Iowa and Illinois are already achieving corn yields of 208 bushels/acre, which is 20 percent higher than the 2007 average in those two states. Furthermore, if 70 percent of the corn growers in Iowa, Illinois, Indiana, Nebraska, and Minnesota grow corn continuously at that yield level, the projected amount of feedstock that could be harvested in a sustainable manner in just those states would increase to 112 million tons. That scenario justified the use of 112 million tons as the panel’s 2020 estimate. Similarly, though fewer, some producers in the five states are already achieving average yields of 230 bushels/acre by using good management practices (Elmore and Abendroth, 2008). According to the same procedure as before, that level of production could provide 135 million tons of stover per year as a biofuel feedstock. It also demonstrates the genetic potential of corn hybrids that are already commercially available. On the basis of the research and extension service reports published up to 2008, the panel chose to use 135 million tons as its projection for 2035 because it can be achieved by simply maintaining the 30-year trend of an average increase of 1.964 bushels/acre per year in the five leading corn-producing states. Achieving that nationally would increase the amount of stover that could be harvested in a sustainable manner to 232 million tons/year. Finally, the panel computed the corn grain yield that would be needed to produce enough harvestable stover to meet the Energy Independence and Security Act of 2007 goal of at least 16 billion gallons of cellulosic biofuel. Given the conservative estimate of 50 million acres and the 70–30 distribution between continuous and rotated corn, respectively, the average grain yield would have to increase to 293 bushels/acre to meet that goal. That is not beyond some projections, but it most likely will not be required, because as feedstock demand increases, more
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Liquid Transportation Fuels from Coal and Biomass: Technological Status, Costs, and Environmental Impacts landowners will want to participate, tillage intensity will probably be decreased, and the area that can be harvested in a sustainable manner will probably increase. In fact, achieving 228 bushels/acre on 86 million acres would also meet that goal. REFERENCES Elmore, R., and L. Abendroth. 2008. Are we capable of producing 300 bu/acre corn yields? Available at http://www.agronext.iastate.edu/corn/production/management/harvest/producing.html. Accessed January 14, 2009. USDA-NASS (U.S. Department of Agriculture, National Agricultural Statistics). 2008. National Agricultural Statistics. Available at www.nass.usda.gov. Accessed June 18, 2008.
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