Harvesting residues from corn and wheat will undoubtedly provide the most plentiful agri­cultural source of cellulosic biomass for the foreseeable future because of the extensive area upon which these crops are grown in the U. S.A. However, to achieve a sustainable harvest strategy only a portion of the total residue produced can be harvested and a suf­ficient amount must be left behind to meet all other critical ecosystem services and soil protection requirements. The ultimate challenge of balancing economic drivers favoring increased harvest to meet conversion demand with minimal transportation cost against the ecologically limiting factors (Figure 8.8) was well illustrated by Wilhelm et al. [20]. In fields where excess residue interferes with subsequent planting, stand establishment,

Economics

and nitrogen immobilization, partial residue harvest will likely increase subsequent yields. However, in more rolling and erosive landscapes most of the residue produced will likely be needed for soil protection. So, how can producers know whether or not they should consider harvesting their residues?

One strategy being developed with much of the REAP and RP data described above is the Residue Management Tool. This tool uses various databases and input information such as (1) the location and spatial extent of the potential harvest area, (2) crop rotations, (3) tillage management, (4) residue harvest methods, and (5) other land management practices to establish the potential for a safe and sustainable harvest. Every scenario involving these factors can be examined with the tool using an integrated systems model for which the input information can be defined. Using the location and spatial extent (which can be obtained directly from a combine using output files from the yield monitor), the site-specific crop yields, soils data, and climate data are assembled from the coupled databases. As the integrated residue removal tool executes its set of scenario runs, the data management modules are dynamically accessed to acquire and format the data needed for each of the models being coupled together. The integrated residue removal tool loops across the complete set of scenarios pushing each model output to the results database. The tool then aggregates the results calculated for each of the scenario runs.

Currently, the tool uses models such as RUSLE2 and WEPS to determine the amount of residue needed to mitigate water and wind erosion, and CQESTR or DAYCENT to monitor changes in the soil organic matter pool. Nutrient balance models (e. g. IFARM) and soil-test information help ensure those needs are being met and work is ongoing to
develop least-limiting water relationships between soil aeration, compaction, and plant response. By connecting all of these models and supporting input information, various soil and crop management scenarios can be created and used to develop and guide sustainable crop residue harvest programs.

The initial version of the Residue Management Tool has been developed and is currently being evaluated for use with corn stover feedstock systems. However, since the tool is simply a computer framework that connects user supplied information about the location and spatial extent to be investigated, crop rotations, tillage management practices, residue removal methods, and land management practices, it can be easily adapted for other cellulosic energy crops by changing or adding additional simulation models to those it currently connects. Also, by expanding the spatial scale, the tool could be used to design landscape management scenarios [21] that could utilize multiple cellulosic energy crops to achieve economically viable feedstock production goals while simultaneously providing other ecosystem services, such as erosion control, nutrient cycling, buffering and filtering, wildlife habitat, carbon sequestration, and opportunities for rural development. The need for such an integrated framework was recently recognized by the Chicago Council on Global Affairs in a report that examined not only agronomic crops but also various waste streams as potential cellulosic feedstock for sustainable bioenergy production.

We conclude that although crop residues may often be excluded from cellulosic energy crop discussions, they will undoubtedly be part of cellulosic bioenergy systems for many years. The best option from our perspective is to integrate them into an overall feedstock production and delivery system that will be economically, environmentally, and socially acceptable for many years to come.