Point II. Concerns over resource degradation and environmental damage from agriculture require a shift from crop and economic yields to sustainable yields.

Crop yield as the measure of soil value is inappropriate because it ''simplifies" the soil by treating it as a closed system in which an array of managed inputs produces a single output, crop yield (Figure 8-1a). This closed system view of soils was exemplified by the "maximum yield" (MY) concept of the 1960s in which large quantities of inputs were used to achieve increases in crop yield, often realizing only incremental increases at high levels of input. Agricultural production systems were characterized by high productivity with huge energy and material subsidies of labor, fossil fuels, pesticides, fertilizers, and irrigation. The tendency was to simplify the agricultural ecosystem by creating monocultures with minimal diversity beyond what was needed to produce maximum yields—a very short-term perspective. Continuous monocultures, particularly corn, tend to require larger subsidies of materials and energy. Such simple ecosystems tend to be ecologically unstable in their characteristic patterns of energy flow, nutrient cycling, and structural change (in terms of species composition, biomass, and spatial organization) (Cox and Atkins, 1979). During the period of MY, soil erosion was a major national problem. The resulting nonpoint source pollution and soil productivity decline associated with soil erosion stimulated the enabling legislation for the protection of soil and water resources, which began with the Water Pollution Control Act of 1965.

It was the energy crisis of the 1970s that moved agriculture to the "maximum economic yield" (MEY) concept in which inputs are limited to those that produce an economic crop yield response. Given typical input yield response curves, MEY would be less than MY. Higher production costs associated with increased energy costs and inflated land values resulted in increased interest in and use of conservation tillage, efficient nutrient management, and Integrated Pest Management (IPM). The shift from MY to MEY was relatively easy for farmers because it was value based at the farm level. The farmer needed to improve his profitability in the face of increased production costs and lower prices for his crops. Therefore, maximizing economic return was essential for those who would successfully compete.

It was clearly demonstrated both by research and on-farm demonstration that decreased inputs could improve farm profitability. It was also clear, however, that agriculture did not buy into MEY completely since farmers continued to apply more inputs than necessary, in part, perhaps, due to their perception of risks involved. For example, yield goal is the key parameter in determining the optimum rate of fertilizer addition. However, farmers apply fertilizers even when soil tests indicate more than adequate levels of these nutrients are available in soil, a sort of insurance application. Some, in fact, ignore soil testing completely. The same is true of nitrogen (N) applications. Farmers will often overestimate their yield goal, apply more than the optimum rate, and/or inadequately account for N supplied from the soil or previous crop in their determination of N fertilizer needs. Schepers et al. (1986) reported that farmers in Hall County, Nebraska, overestimated their yield goal by 40 bushels per acre, resulting in an over application of 40 pounds of N per acre. The extent to which farmers adhered to the MEY concept probably relates to their perception of risks associated with the MEY management practices.

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