Agricultural systems can be described in many ways. Over the years, researchers and farmers alike have used a variety of terms, such as farming system, cropping system, organic, ecological, to identify agricultural systems based on particular characteristics or definitions. Many of these common terms are outlined in Box 1.1 (p. 12). In addition to these terms, which focus on unique sets of practices, management techniques, and sometimes philosophies, other definitions (e.g., a corn–soybean system, a vegetable or hog production system) focus on the commodity being produced. For the purposes of this handbook, we will use the term agricultural system to refer broadly to any system that produces livestock and crops (food, feed, fiber and/or energy), including the social, political and economic components of that system.

When appropriate, case studies or examples that fit into the categories in Box 1.1 will be identified, but agricultural systems researchers should not feel bound by these definitions when conceptualizing their own systems for research and educational purposes. Rather, they should pick and choose from each to best develop or delineate their own systems.

Box 1.1: Nomenclature for Agricultural Systems

Over time and throughout the literature, agricultural systems have generally been defined by philosophy or management practices. For example, a farming system is defined as “the manner in which a particular set of farm resources is assembled within its environment…for the production of primary agricultural products…a unique and reasonably stable arrangement of farming enterprises that a household manages according to well-defined practices in response to the physical, biological, and socioeconomic environment and resources.” (IRRI, 2012). Definitions of some major types of farming systems in common use by agricultural researchers, policymakers and farmers follow. Conservation agriculture systems employ resource-conserving methods but are also considered high-output agricultural systems. Conservation farming typically involves the integrated use of minimal tillage, cover crops and crop rotations. Reduced- or low-input farming systems minimize the use of off-farm resources such as commercially purchased chemicals and fuels. These systems also tighten nutrient and energy cycles and use internal resources such as biological pest controls, solar or wind energy, biologically fixed nitrogen, and other nutrients from green manures, organic matter or soil reserves. Many reduced- or low-input farming systems are examples of integrated farming systems. Integrated farming systems combine methods of conventional and organic production systems in an attempt to balance environmental quality and economic profit. For example, integrated farmers build their soils with composts and green manure crops but also use some synthetic fertilizers in addition to biological, cultural and mechanical pest control practices. Alternative livestock production systems use lower-confinement housing and rely more on pastures than do conventional and industrial livestock farms. A common example in dairy farming is the use of intensive rotational grazing practices in which short-duration, intensive grazing episodes are followed by long rest periods that allow pastures or fields to recover. Integrated crop–livestock farming systems generate a significant fraction of animal feed on cropland and pastures owned or managed by the livestock farmer. These systems use the crop and livestock enterprises to efficiently recycle nutrients, promote crop rotations and insulate livestock farmers from price fluctuations in feed and input markets. Organic agriculture is both an ecological production management system and a labeling term that indicates that the food or other agricultural product has been produced using approved methods that integrate cultural, biological and mechanical practices that foster cycling of resources, promote ecological balance and conserve biodiversity. Synthetic fertilizers, sewage sludge, irradiation and genetic engineering may not be used. Ecologically based farming systems emphasize the use of ecological pest management, nutrient cycling, and natural and renewable resources to enhance soil health and protect water quality. Organic and other “natural” farming systems fall under this category, relying on many common practices such as crop rotations, biological pest control, manures and avoiding all or most synthetic fertilizers and pest controls. Food systems refer to a complex set of activities and institutions that link food production to food consumption. Food system studies often use a “commodity chain” approach to analyze production, processing, selling and consumption.

(Adapted from National Research Council, 2010)

Key Concepts of Agricultural Systems

Drawing from general systems theory and ecosystem ecology, the following concepts provide the foundation for agroecology and are essential for conceptualizing and understanding agricultural systems for research purposes (Drinkwater, 2009). Taken together, they also provide a framework for facilitating interdisciplinary research.

1. Agricultural Systems are Defined by Unique Spatial and Temporal Boundaries

Agricultural system boundaries can be fixed, as is the case with a farm, for example, but systems can also be defined using subjective boundaries. In agricultural systems research, spatial and temporal boundaries are determined by research goals, the structure of the underlying environment, socioeconomic and political structures and by land-use decisions made by farmers and farm communities. For this reason, after the research question(s) or hypothesis is developed, the first step in delineating the system under study is to identify the physical and temporal boundaries that align with the problem being addressed. In a watershed study, for example, an experiment designed to measure the impact of spring tillage and planting on water quality could have a short-term focus. If the emphasis were to quantify nutrient loads leaving the watershed, then a multiyear study would be needed to reflect seasonal and year-to-year variability. Physical boundaries for a system vary widely, from a field or management unit or the property line for a specific farm (Shreck et al., 2006), to a collection of farms, an entire watershed (Strock et al., 2005) or a county.

2. Agricultural Systems are Composed of Interacting Subsystems

All systems are composed of many smaller, interacting subsystems that interact in either a hierarchical or nonhierarchical manner. The predominance of nested hierarchies of subsystems within agricultural and ecological systems is a striking feature. Watersheds are a prime example of a system composed of a nested hierarchy of subsystems. A large river basin, for instance, includes many smaller tributaries draining smaller watersheds, each of which has its own smaller system of tributaries and watersheds. Nested hierarchies are not always smaller versions of a larger system, as is the case with watersheds. More commonly, agricultural systems are composed of subsystems that have their own unique properties. For instance, fields may be aggregated into farms, and farms into watersheds, agricultural regions or counties (Strock et al., 2005; Gentry et al., 2009). In other words, each level in the nested hierarchy is composed of smaller systems that are distinctly different from the larger system. Alternatively, subsystems can also exhibit non-hierarchical relationships as seen in an integrated farm that produces grains and animals. In this case, the two enterprises are simply interacting subsystems within the larger farming system and are connected by exchanges of crop outputs (grain and forages for animal feed) and nutrients (manure applied to fields).

3. System Processes Occur at Different Scales and Rates

Just as system boundaries have unique physical and temporal boundaries, system processes also vary in space and time. For example, processes such as nutrient cycling occur at scales from a few microns to a whole plant, and from a single field to a farming community. Similarly, time-based processes can range from minutes to centuries; decomposition of labile organic matter, or population changes in pests due to predator–prey interactions, can occur within a single growing season (Letourneau, 1997; Puget et al., 2000). In contrast, detectable changes in stabilized soil organic matter or the emergence of weed resistance to herbicides can take years or decades to manifest (Aref and Wander, 1997; Vidal et al., 2007). During major shifts in management regimes, such as the transition from conventional to organic management or from conventional tillage to no-tillage, the rate of change for certain processes can be rapid, while other processes are not detectable for years or decades. For example, replacing fallow with cover crops can affect soil decomposers long before changes in total soil organic carbon can be detected. Because different processes will not reach dynamic steady-state conditions at the same time, the time frame of these various processes needs to be considered when planning research, particularly when focusing on the transition from one management system to another, because legacy effects from the previous system can interact with newly imposed practices.

4. System Structure Determines Function

In agroecosystems, structural properties (e.g., soil type, climate, biodiversity) drive functions such as plant productivity, nitrogen retention or greenhouse gas emissions, as well as emergent properties such as stability and resilience. This relationship between structure and function provides a useful framework for designing agricultural systems to optimize particular functions or for understanding the basis for differences across agroecosystems. For example, greater biodiversity in natural ecosystems often corresponds with greater productivity and enhanced resilience of the system. Thus, intentional management of species diversity can be a key strategy for achieving sustainable farming systems (Jackson et al., 2007).

5. Agricultural Systems are Open Systems

Agricultural systems are open, meaning that energy, nutrients, organisms and information constantly cross system boundaries. Quantification of net flows among system components and into and out of systems, such as nutrient and energy budgets, mass balance calculations and life-cycle analysis, is important for understanding the movements and effects of these processes and properties. For example, quantification of nutrient flow across a predefined system boundary such as a field or watershed is essential to understanding the impact of farm management on long-term soil fertility and on the surrounding landscape.

6. Agricultural Systems Have Emergent Properties

All systems have emergent properties, or characteristics and behaviors that are only apparent at higher levels of system complexity (von Bertalanffy, 1968). In other words, these properties only emerge when the system is operating as a complex of subsystems; emergent properties do not exist when the subsystems or components are observed in isolation. For example, animal organ systems, such as the digestive, reproductive and cardiovascular systems, exist as such but are not viable in isolation; however, when combined in an animal structure, the emergent property of life becomes apparent. In agricultural systems, soil quality can be considered an emergent property because it exists only as a function of the interactions among soil biological, physical and chemical processes (Carter et al., 2004). Sustainability is also considered an emergent property, because it emerges from the multiple social and physical interactions within the system (Chase and Grubinger, 2014; Lengnick, 2015).