Agricultural production consumes large amounts of energy, either directly through combustion of fossil fuels, or indirectly through use of energyintensive inputs, especially fertilizer. Over 2005-08, expenses from direct energy use averaged about 6.7 percent of total production expenses in the U.S. farm sector, while fertilizer expenses represented another 6.6 percent. However, these averages mask much greater energy intensities for major field crops. Several factors can influence energy prices faced by U.S. agriculture: availability of natural gas, world oil prices, energy taxes, or a greenhouse gas policy designed to reduce carbon dioxide emissions.