Grain and Dual Purpose Production: System Efficiencies, Limitations, and Potential.

From a farmer perspective, annual grain systems have numerous desirable attributes for biomass production when compared to second generation, perennial crops such as switchgrass and Miscanthus. Equipment and management are familiar, revenues accrue in the first year of production and the system maximizes both potential market diversity (grain, silage, stover, biomass…) and year-to-year flexibility in land use. Likewise, from an industrial perspective, annual sources will be important to synchronize feedstock availability with capacity at conversion facilities. Yet, both the species and managements in these systems have been designed and optimized for production of food and feed. Switching to a fuel-production focus requires reassessment of optimization and efficiency metrics. With energy as a predominant input in intensive management, net energy balance must be positive but such an accounting entirely overlooks major resources that currently have no energy equivalency value. The purpose of this presentation is to review resource accounting in annual grain production systems from the bioenergy perspective. Certainly, known biophysical limits to nutrient (especially N), water and radiation use efficiency have and will continue to constrain primary productivity. Over the past 70+ years, yields of widely grown commodities such as maize have increased dramatically due to a combination of genetic improvement and management intensification realized against increasing atmospheric CO₂ concentrations but the extent to which these yield increases are the product of increased system efficiencies is the subject of ongoing debate. Regardless, a bioenergy context for annual crop performance requires a higher standard for all efficiencies and an unprecedented exactness in resource accounting. Concomitantly, novel valuation systems with equivalencies for multiple ecosystem services must be developed (e.g. air versus water quality) and used to guide decision makers. This presentation will focus on efficiency interactions and tensions in past/current annual cropping systems and discuss potential and key limitations for efficiency improvements via management and/or crop improvement.