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Unit 7: Agriculture // Section 8: Agriculture and Energy


Agriculture consumes significant quantities of energy, especially in industrialized countries. Farmers use energy directly to heat and cool buildings, operate equipment, pump irrigation water, and transport products to market. Agriculture also consumes large quantities of fossil fuel indirectly as inputs for fertilizer (a prime ingredient of which is natural gas) and pesticides (made from petroleum and natural gas). Food processing and long-distance shipment consume additional energy.

U.S. energy use in agriculture has declined by more than 25 percent since the oil price shocks of the 1970, thanks partly to practices such as conservation tilling that require less working of soil. Changes in pesticide practices, including rapid adoption of GE crops that are resistant to insects, have lowered total use of pesticides. In addition, pesticides themselves have become much more sophisticated, so smaller quantities are required.

Nonetheless, some agricultural practices remain extremely energy-intensive—most notably, raising livestock on grain. In traditional farming systems, animals eat local forage and crop wastes that are not usable as food for humans. However, large-scale livestock farmers typically feed animals grains and other protein byproducts because the animals grow to market weight more quickly and it requires less land area than grazing, so meat production can take place closer to population centers (Fig. 12).

Raising animals on grain consumes fossil fuel as inputs for the pesticides and fertilizers needed to grow feed crops. Fattening one steer on corn to market weight can consume the equivalent of 35 gallons of oil (footnote 13). Some critics argue that this is an inefficient way to use food resources because animals convert only a fraction of the energy in their feed grain to growth (for background, see Unit 4, "Ecosystems"). According to one estimate it requires two kilograms of grain to produce one kilogram of poultry, four to make one kilogram of pork, and seven to produce a kilogram of beef (footnote 14).

Feedlot cattle

Figure 12. Feedlot cattle
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Source: United States Environmental Protection Agency.

Other agricultural sectors could become energy resources in the coming decades. High oil and gas prices since the late 1990s have spurred worldwide interest in making liquid biofuels from plant sources such as forestry waste and fast-growing energy crops. Biofuels produce fewer atmospheric pollutants and greenhouse gases than fossil-based fuels when they are combusted, although the net effect in terms of CO2 depends on energy use during production and subsequent processing of the crop.

Biofuels are valuable substitutes for imported oil in the transport sector because they can be used in most conventional engines with minor adjustments. They include ethanol, which is grain alcohol fermented from grain crops like corn (and soon from woody plants), and biodiesel, a natural version of diesel fuel made from oil crops such as soybean, sunflower, and rapeseed. (For more details, see Unit 10, "Energy Challenges.")

Several countries have made significant investments in biofuels. Most notably, all gasoline sold in Brazil is at least 25 percent ethanol made from local sugar cane. U.S. producers currently make about 4.5 billion gallons of ethanol per year from corn, equal to 3 percent of national gasoline consumption, with production scheduled to rise to 7.5 billion gallons per year by 2012. Most ethanol plants and fuel pumps are located in Midwestern corn-growing states.

Corn ethanol is the first type of ethanol to be commercialized in the United States because corn kernels and sugar cane juice are made up of simple carbohydrates that are easy to ferment, so the production process is relatively cheap. There is growing interest in making ethanol from the cell walls of fast-growing plants such as switchgrass and willow and poplar trees, as well as corn stalks. These feedstocks are made up of complex polymers such as cellulose, hemicellulose, and lignin, which contain more energy (Fig. 13).

Simplified model of a primary cell wall

Figure 13. Simplified model of a primary cell wall
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Source: United States Department of Energy Genome Programs/ genomics.energy.gov.

Corn ethanol has benefited U.S. farmers by increasing demand and driving up corn prices, but it only delivers modest environmental benefits. According to the U.S. Department of Energy, using corn ethanol only reduces greenhouse gas emissions by about 18 to 29 percent compared to gasoline because fertilizer and other inputs required to grow corn are made from fossil fuels. However, cellulosic ethanol could reduce greenhouse gas emissions by as much as 85 to 86 percent compared to gasoline (footnote 15).

Cellulosic plant materials are difficult to break down, and no method has been developed to date for fermenting lignin, so making cellulosic ethanol is more expensive and technically challenging than producing corn ethanol. Government agencies, universities, and private investors hope to commercialize cellulosic ethanol production in the United States as soon as 2012. If it develops into a large-scale industry, cellulosic ethanol could create new markets for farmers to grow energy crops that require fewer chemical inputs than corn and can be raised on land unsuited for food crops (Fig. 14). However, extending the footprint of agriculture in this way might also reduce biodiversity by converting more land into managed ecosystems.

Geographic distribution of potential biomass energy crops

Figure 14. Geographic distribution of potential biomass energy crops
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Source: United States Department of Energy Genome Programs/ genomics.energy.gov.

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