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