Agricultural biofuels require the use of scarce land, and this land has opportunity cost. We explore the objective function of a social planner who includes a land constraint in the optimization decision to minimize environmental cost. The results show that emissions should be measured on a per acre basis. Conventional agricultural life cycle assessments for biofuels report carbon emissions on a per gallon basis, thereby ignoring the implications of land scarcity and implicitly assuming an infinite supply of the inputs needed for production. Switchgrass and corn are then modeled as competing alternatives to show how the inclusion of a land constraint can influence life cycle rankings and alter policy conclusions.