research and policy evolution that are designed to reduce tradeoffs and enhance synergies between the four goals and to manage risks and uncertainties associated with their pursuit.

Measuring Progress Toward Sustainability

Sustainability is best evaluated not as a particular end state, but rather as a process that moves farming systems along a trajectory toward greater sustainability on each of the four goals. For this report, the committee’s definition of sustainable agriculture does not make a sharp dichotomy between conventional and sustainable farming systems, not only because farming enterprises reflect many combinations of farming practices, organization forms, and management strategies, but also because most types of farming systems can potentially contribute to achieving various sustainability goals and objectives. Pursuit of sustainability is not a matter of defining sustainable or unsustainable agriculture, but rather of assessing whether choices of farming practices and farming systems would lead to a more or less sustainable system as measured by the four goals.

Finding ways to measure progress along a sustainability trajectory is an important part of the experimentation and adaptive management process. Environmental, economic, and social indicators can be used to describe the performance of agriculture and to provide information on whether a farm, a farming system type, or agriculture at any scale is on a trajectory toward improved sustainability. Many indicators are means-based and others are outcome-based; both types have limitations and strengths. Efforts to develop indicators to assess social dimensions of agricultural sustainability are sparse. Some of the indicators being used, such as production energy costs and levels of implementation of best management practices, are useful at many levels of aggregation from farm-level assessments to regional and national accounting. Yet, there are no agreed-upon standards regarding which indicators to use under different conditions. Few indicators have been validated by scientists, farmers, and the public. Developing consistent and effective indicators would facilitate assessment of the sustainability of farming practices or systems. Understanding the relationships between sustainability indicators and the outcomes they are meant to represent is a priority for future research.

Farming systems that move toward greater sustainability on most, if not all, of the four goals generally strive to work with ecological and biogeochemical processes and cycles to maximize synergistic interactions and the beneficial use of internal resources, minimize dependence on external inputs, and use added inputs efficiently. Through those efforts, they potentially reduce discharges to the environment and additional waste disposal activities, provide economic resilience, and enhance social well-being. As exemplified in the case studies, many farmers who work toward improved agricultural sustainability manage their operations to encourage social and economic synergistic relationships on-farm and throughout the food chain. The overall sustainability or robustness of a farming system—the ability to adapt to stresses, pressures, and changes in circumstances over time—is a result of some mixture of resistance, resilience, and adaptability of the coupled biophysical and socioeconomic system.

TOWARD AGRICULTURAL SUSTAINABILITY IN THE 21ST CENTURY

Although all farms have the potential (and responsibility) to contribute to different aspects of sustainability, the scale, organization, enterprise diversity, and forms of market integration associated with different individual farms provide unique opportunities or bar-



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