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USDA Corn Ethanol Energy Balance

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    www.usda.gov/oce/reports/index.htm. Contact: Brenda Chapin 202-720-5447 bchapin@oce.usda.gov 2008 Energy Balance for the Corn-Ethanol Industry The Agricultural
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      www.usda.gov/oce/reports/index.htm.

      Contact:
      Brenda Chapin
      202-720-5447
      bchapin@...

      2008 Energy Balance for the Corn-Ethanol Industry

      The Agricultural Resource Management Survey of corn growers for the year 2005 and
      the 2008 survey of dry mill ethanol plants are used to estimate the net energy balance of
      corn ethanol. This report measures all conventional fossil fuel energy used in the
      production of 1 gallon of corn ethanol. The ratio is about 2.3 BTU of ethanol for 1 BTU
      of energy inputs, when a portion of total energy input is allocated to byproduct and fossil
      fuel is used for processing energy. The ratio is somewhat higher for some firms that are
      partially substituting biomass energy in processing energy.
      Authors
      H. Shapouri, Agricultural Economist, Office of Energy Policy and New Uses, Office of
      the Chief Economist, USDA
      Paul W. Gallagher, Associate Professor, Economics Department, Iowa State University
      Ward Nefstead, Associate Professor, Applied Economics Department, University of
      Minnesota
      Rosalie Schwartz, Program and Recruitment Director, Agricultural Economics
      Department, University of Nebraska (Lincoln)
      Stacey Noe, Program Coordinator, Agricultural Entrepreneurship Initiative, Iowa State
      University
      Roger Conway, Former Director, Office of Energy Policy and New Uses, Office of the
      Chief Economist, USDA
      Contents
      Energy Consumption by Corn Producers ......................................................................................3
      2008 Ethanol Producer Survey ......................................................................................................4
      Conclusion .....................................................................................................................................6
      References ......................................................................................................................................7
      Tables ............................................................................................................................................ 8
      Appendix ......................................................................................................................................10
      2
      2008 Energy Balance for the Corn-Ethanol Industry
      The ratio of energy in a gallon of ethanol relative to the external fossil energy required to produce
      the corn and process and ship the ethanol is an important measure of sustainability of the corn-
      ethanol industry (Pimentel). Recent updates of energy balance calculations have verified
      enhanced industry performance and identified methods that could yield further improvement
      (Shapouri, et al; Gallagher and Shapouri). The 2008 updates presented in this report record the
      effects of current practices used by corn producers and ethanol processors. Current fertilizer
      practices are taken into account using recent data collected by the USDA. The current standards
      of electrical and processing energy use in the corn-ethanol industry are taken from a recent survey
      of ethanol producers.
      Energy Consumption by Corn Producers
      Corn producers use most energy products (gasoline, diesel, natural gas, liquid petroleum gas, and
      electricity) directly in planting, harvesting, and drying their crop. There is also considerable
      energy embodied in the commercial fertilizers applied to enhance plant growth.
      Table 1 and Table 2 provide a summary of new USDA data on energy components and totals.
      The trends for components and total energy are summarized with data at 5-year intervals over the
      last 25 years. Agricultural Resource Management Survey (ARMS) is the source of data used to
      estimate total direct and indirect energy inputs used in production of corn. Energy inputs used in
      production of corn are derived from the responses of 1,814 corn farmers in 19 States to a survey
      on corn production practices and costs as part of the 2005 ARMS. The target population for the
      corn survey was farmers who planted corn with the intention of harvesting corn for grain. The
      USDA National Agricultural Statistics Service (NASS) and the Economic Research Service
      collect production and cost data once every 5-8 years for each commodity on a rotating basis in
      the ARMS. The State data from the survey are also weighted to represent all U.S. corn acreage
      (see Appendix Tables A1 and A2).
      Importantly, the largest energy components for corn production are nitrogen and direct energy for
      fuel and electricity. Nitrogen use measured on a per bushel basis has declined by about 20
      percent since the mid-90s. Similarly, all direct energy components have declined by about 50
      percent since the mid-90s. Together, the nitrogen and direct energy reductions result in a 30
      percent decline in the energy required to produce a bushel of corn. Overall, 65,285 BTU/bu
      (British thermal unit per bushel) were required for corn production in 1996, whereas 41,029
      BTU/bu were required in 2005.
      Lastly, it is the energy in corn that is actually used for ethanol production, expressed per gallon of
      ethanol, which is important for an evaluation of ethanol production. Ethanol yields have
      increased by about 10 percent in the last 20 years, so proportionately less corn is required
      14,866 BTU/gal in 2005 (see Table 2). Further, only the starch fraction of the corn plant (66
      percent) is used for ethanol production.1 So the net corn energy used for ethanol production is
      9,811 BTU/gal in Table 2. The corn energy required for ethanol production is shown in the
      bottom two rows of Table 2. The corn energy input to ethanol production declined to 9,811
      BTU/gal from 16,346 over the most recent 10-year period.
      1
      To see this, notice that a bushel of corn weighs 56 lbs, and yields 17.5 lbs of distilled grains (the protein,
      fiber and oil components of the corn plant). The starch component is 38.5 lbs = 56-17.5, so the starch
      fraction of the corn plant is 38.5/56 = .688, that is, the starch (ethanol-making) component is about 2/3 of
      the corn. In many cases the 2/3 allocation rule is very conservative – in some plants, the corn oil is
      removed from the distilled grain and used for biodiesel processing (a contribution to energy output).
      3
      2008 Ethanol Producer Survey
      A short survey of ethanol producers in Iowa, Minnesota, Nebraska, and eastern South Dakota was
      conducted in the fall of 2008 and the winter of 2009. This survey was conducted by the students
      from the National Agricultural Marketing Association Chapters of Iowa, Minnesota, and
      Nebraska. The survey was designed by staff at USDA and Iowa State University. The survey
      included questions about the extent of thermal and electric energy use, the type of energy used,
      the type of ethanol production process used, and processing yield (the Survey Questionnaire is
      given in the Appendix). Sixteen plants responded to the survey, which is marginally enough to
      discuss the results.2
      We report only survey summary measures, to accommodate the confidentiality of individual
      firms. Some of the data are summarized with means, group means, and standard deviations.
      Other results are more conveniently expressed using regressions.
      First, the ethanol processing yield of survey respondents has a mean of 2.76 gal/bu of corn.
      Further, the standard deviation is 0.07 gal/bu. Hence, processing yields continue a slow but
      steady improvement—the 2009 average yield is 3.7 percent higher than the average for the 2002
      USDA survey, and 4.7 percent higher than the 1998 survey.
      Second, the survey responses on external thermal energy showed that several firms were using
      between 10 percent and 50 percent biomass power. Hence, we summarize the thermal energy
      results using the following regression:
      btui = 22953 ddmi + 29421 dddi + 28313 dwi - 26920 fbi
      (23.8) (29.4) (28.3) (6.1)
      Adj. R2 = .98 Dep. Var. Mean = 24840 rmse = 2973 CV = 12.0%
      Numbers in parentheses are t-values
      Where:
      btui = thermal energy used, in BTU/gal
      ddmi = 1 for dry mills selling modified distiller's grains (dg), and 0 otherwise
      dddi = 1 for dry mills selling dry dg, and 0 otherwise
      dwi = 1 for wet mills, and 0 otherwise
      fbi = the fraction of thermal energy from biomass, 0/1
      i indicates data for firm i
      Twenty-two observations were used for the process energy regression, because some firms that
      produce dry and modified dg reported energy use for both configurations.
      The regression coefficient for each dummy variable indicates the average energy use of a specific
      processing configuration when the biomass fraction is zero. Also, a 50-percent biomass fraction
      would reduce the energy use of a dry mill with dry instead of wet dg by about half—a very
      sensible result.
      The standard deviation (SD) of the sample mean (  x ) is related to the population S.D. (  ) and sample
      2
      x  n . With the conventional sample, n=30,  x  0.18 . A sample with n =
      size (n) as follows:
      16 is almost as accurate:  x  0.25 . For the farm sample, n = 1,814, sampling error of the mean is
      virtually eliminated, i.e.,  x  0.023 .
      4
      The thermal energy regression estimate also can be used to infer the heat increment associated
      with producing dry dg versus wet dg, even though results are only estimated for modified and dry
      dg. First, the additional thermal energy required to process dry dg instead of modified dg is the
      difference between the coefficients for ddd and ddm: 6,468 BTU/gal. Second, modified dg
      consists of a 50:50 mix of wet and dry dg. So the heat required to dry dg is 6,485/0.5 = 12,936
      BTU/gal, which amounts to 44 percent of the energy required for ethanol production with dry dg
      with typical moisture standards.
      For comparison, the USDA process model of a dry mill, the USDA-Agricultural Research
      Service Eastern Regional Research Center "corn dry mill process and cost model" (Kwiatkowski,
      et al.), suggests a similar but slightly higher estimate. They calculate that 51 percent of the
      thermal energy used in an ethanol plant is attributable to producing dry dg with 10 percent
      moisture content. In subsequent calculations of the byproduct credit in corn-energy balance, we
      examine the survey estimate and the process model estimate.
      Third, the survey responses also suggest that electricity use is related to the fraction of dg that is
      dried:
      eleci = 0.60214 + 0.15476 fdi
      (9.1) (1.5)
      Adj. R2 = .095 Dep. Var. Mean = 0.6858 rmse = 0.1311 CV = 19.2%
      Where
      eleci = electricity use, in kWhr/gal
      fdi = fraction of dg that is dried, 0/1
      Energy Balance Estimate
      Table 3 contains an update for the energy balance results given in Gallagher and Shapouri.3 The
      revision includes latest data for corn energy use from the USDA survey, for ethanol conversion
      from the National Agribusiness Marketing Association (NAMA) survey, and transportation from
      the GREET model (version 1.8). A dry mill with dry distiller's grains is the reference case of
      Table 3. The columns report energy use with conventional fossil fuel power (Columns 1 and 2)
      and with biomass power (Columns 3 and 4). Finally, the byproduct credit is the heat used to
      prepare dry dg—we compare the regression estimate using the NAMA data (Column 2) and the
      engineering model estimate (Column 1).
      For the conventionally powered dry mill, shown in Columns 1 and 2, the ethanol conversion
      estimate of heat content, 40,019 BTU/gal, is the sum of electricity and thermal energy from the
      NAMA regression estimates, appropriately configured. Additionally, survey reported numbers
      are all adjusted to an energy input basis. The corn production estimate is also the same, at 9,811
      BTU/gal from the 2005 USDA data given in Table 2. The byproduct credit in Column 1 of
      20,409 BTU/gal is based on the ASPEN model. But the Column 2 estimate of byproduct credit is
      inferred from the regression estimate. The various components of energy use are compared to the
      heat content of ethanol (76,300 BTU/gal). Together, the recent energy use estimates show that
      the ratio of energy in ethanol to the external energy used to produce ethanol is about 1.4, even
      without allowing for the processing component of the byproduct credit. After fully allowing for
      heat used to produce byproducts, the energy ratio is between 1.9 and 2.3.
      3
      Our analysis updates the energy balance picture for a representative firm, using methods developed
      previously. At the market level, we assume that the representative firm is not part of an industry expansion
      that is large enough to displace land from previous uses.
      5
      Biomass power reduces the external fossil energy needed to produce ethanol. In the case of corn
      stover, some of the fossil energy used to produce corn-biomass is recovered. Energy required for
      stover harvest and fertilizer replacement is taken into account in Column 3. In a typical dry mill,
      biomass power would replace natural gas, but market purchases of electricity would likely
      continue. At the upper range of survey responses shown in Column 3, external thermal energy
      reduces by about one-half, to 15,961 (BTU = 29421-26920 * 0.5) BTU/gal on an output basis,
      and external energy for electricity would remain at 8,720 BTU/gal. Under these circumstances,
      the energy balance ratio increases to 2.8, even using the lower byproduct credit from the
      regression results. Similar calculations that used a short rotation woody crop (willow) instead of
      stover yielded similar energy balance estimates.
      In Column 4, complete replacement of external processing energy for thermal energy and
      electricity extends beyond the range of survey responses. But the possibilities are interesting.
      Corn residues, which contain about the same energy (BTUs) as the corn, are presently discarded.
      But residues represent enough energy to replace all of the process heat and electricity needed for
      ethanol, and combined heat and power plants are capable of producing the required process heat
      and electricity. Hence, the energy balance could increase to about 25.7 when half of the
      renewable energy produced in corn production (the residue) is no longer discarded.
      Conclusion
      A dry grind ethanol plant that produces and sells dry distiller's grains and uses conventional fossil
      fuel power for thermal energy and electricity produces nearly two times more energy in the form
      of ethanol delivered to customers than it uses for corn, processing, and transportation. The ratio
      is about 2.3 BTU of ethanol for 1 BTU of energy in inputs, when a more generous means of
      removing byproduct energy is employed.
      Some dry mills are already using up to 50 percent biomass power. The energy output for these
      plants is near 2.8 times energy inputs, even using the conservative byproduct allowance. As
      processors master the logistics of handling bulky biomass, the energy balance ratio could reach
      26 BTUs of ethanol per BTU of inputs used.
      Overall then, ethanol has made the transition from an energy sink, to a moderate net energy gain
      in the 1990s, to a substantial net energy gain in the present. And there are still prospects for
      improvement.
      6
      References
      Economic Research Service Staff, "Briefing Room: Agricultural Resource Management Survey
      (ARMS)," http//www.ers.usda.gov/Briefing/ARMS/, U.S. Department of Agriculture, accessed
      10/7/2009.
      Gallagher, P., M. Dikeman, J. Fritz, E. Wailes, W. Gauthier, and H. Shapouri, Biomass from
      Crop Residues: Some Cost and Supply Estimates, Agricultural Economic Report Number 819,
      U.S. Dept of Agriculture, January 2003.
      Gallagher, P. and H. Shapouri, "Improving Sustainability of the Corn-Ethanol Industry," in
      Biofuels, W. Soetaert and E. Vandamme, eds, John Wiley, West Sussex (UK), December 2008.
      Kwiatkowski, Jason, McAloon, Andrew, Taylor, Frank, Johnston, David; "Modeling the Process
      and Costs of Fuel Ethanol by the Dry-Grind Process, Industrial Crops and Products 23(2006),"
      pp. 288-296.
      Pimentel, David, "Ethanol Fuels: Energy Security, Economics, and the Environment," Journal of
      Agricultural and Environmental Ethics 4(1991):1-13
      Shapouri, Hosein, James A. Duffield, and Michael Wang, "The Energy Balance of Corn Ethanol:
      An Update," U.S. Department of Agriculture, Office of the Chief Economist, Office of Energy
      Policy and New Uses, Report No. 813(July 2002).
      Shapouri, H., and P. Gallagher, USDA's 2002 Ethanol Cost-of-Production Survey, U.S.
      Department of Agriculture, Office of Energy Policy and New Uses, Agricultural Economic
      Report No. 841(July 2005).
      7
      Tables
      Table 2. Total energy requirements of farm inputs for nine-State
      Table 1. Energy-related inputs used to grow corn, nine-State weighted average average
      Energy Used
      a
      Conversion factors:
      Energy inputs to BTU in BTU/bu
      1991 1996 2001 2005 corn: 1991 1996 2001 2005
      c
      Seed 784 860 663 394
      Pounds/acre 19.62 19.61 22.11 18.29 394.26 BTU/pound
      Fertilizer:
      Nitrogen Pounds/acre 124.5 129.38 133.52 133.39 24,500 BTU/pound 25,023 25,358 23,477 20,464
      Potash Pounds/acre 52.77 59.25 88.52 61.26 3,000 BTU/pound 1,299 1,422 1,899 1,151
      Phosphate Pounds/acre 58.17 48.16 56.81 54.36 4,000 BTU/pound 1,909 1,541 1,631 1,362
      b
      Lime 1,109 1,706 1,402 1,937
      Pounds/acre 242.18 382.18 350 554.36 558 BTU/pound
      Energy Input:
      Diesel Gallons/acre 6.85 8.6 6.85 5.81 152,372 BTU/gallon 8,562 10,483 7,491 5,539
      Gasoline Gallons/acre 3.4 3.09 1.7 1.92 144,211 BTU/gallon 4,022 3,565 1,759 1,735
      LPG Gallons/acre 3.42 6.36 3.42 3.2 85,895 BTU/gallon 2,410 4,370 2,108 1,722
      BTU/ft3
      Natural gas Cubic ft/acre 246 200 245.97 208.9 1,046 2,111 1,674 1,846 1,368
      Electricity kWh/acre 33.59 77.13 33.59 20.41 9,365 BTU/kWh 2,581 5,779 2,258 1,197
      Custom work Dollars/acre 6.68 15.07 10.12 8.45 1,590 3,340 1,581 648
      Chemicals Pounds/acre 3.99 3.49 2.66 2 154,150 BTU/pound 5,049 4,304 2,941 1,928
      Custom drying Dollars/acre 1.79 0 2.09 1,030 0 0 642
      Purchased water Dollars/acre 0.18 0.08 136 75
      Total: 58,095 65,285 49,819 41,029
      Yield, 3-year average bu/acre 121.9 125 139.34 159.7 conversion to BTU/gallon ethanol:
      BTU/bu 58,095 65,285 49,819 41,029
      a
      Includes energy loss and transmission
      gal/bu 2.50 2.64 2.66 2.76
      loss (LHV)
      b
      Lime use in 1996 is an average of 1991, 2001, and
      BTU/gal 23,238 24,767 18,715 14,866
      2005
      c
      Seed conversions calculation shown
      starch fraction 0.66 0.66 0.66 0.66
      below:
      seeds per acre 25,501 25,495 28,739 23,771 ethanol's share 15,337 16,346 12,352 9,811
      (BTU/gal)
      pounds of seed/acre 19.62 19.61 22.11 18.29
      bu seed/bu corn 0.0029 0.0028 0.0028 0.002
      BTU/bu corn seed 166.94 182.91 141.14 83.89
      magnification factor 4.7 4.7 4.7 4.7
      BTU/bu corn, adjusted 784 859 663 394
      Abbreviations: DDG: dry distillers grains; NG: natural gas; GREET: Greenhouse Gasses, Regulated Emissions, and Energy Use is Transportation Model;
      SRWC: short rotation woody crops, and LPG: liquefied petroleum gas.
      8
      Corn Ethanol Dry Mill producing dry distillers grains (ddgs): Energy use and net energy
      Table 3 value
      Processing Power Configuration Natural Gas & Purchased electricity Biomass Power, Replace 50% Biomass power, Replace 100%
      w/Corn
      ASPEN DDG credit Survey DDG credit of Natural Gas (NG) Stover of NG & elec w/ Corn Stover
      in BTU / gallon
      Corn Production 9,811 9,811 9,811 9,811
      Corn Transport 1,430 1,430 1,430 1,430
      1
      26,767 4, 2 2
      40,019 2,135
      Ethanol Conversion 40,019
      Ethanol Distribution 1,470 1,470 1,470 1,470
      5
      1,055 1,055 1,055
      Farm Machinery 1,055
      Total Energy Used 53,785 53,785 40,533 15,901
      3
      Byproduct Credit 20,409 12,936 12,936 12,936
      Energy Used, Net of Byproduct Credit 33,375 4,0849 27,597 2,965
      Ethanol Energy Produced 76,300 7,6300 76,300 76,300
      Energy Ratio, w/o Byproduct Credit 1.42 1.42
      Energy Ratio, w/ Byproduct Credit 2.29 1.87 2.76 25.73
      Footnotes: (sources and calculation details)
      1 kwhr elec /gal eth BTU elec /kwhr elec BTU out/gal BTU elec/BTU input BTU out/ BTU in BTU in /gal
      Electricity: 0.757 3,413 0.30 8,720
      Power: 29,421 0.94 31,299
      Total: 40,019
      Source: Gallagher and Shapouri (2006)
      2 corn-stover harvest energy, BTU/ gal (direct+energy embodied in machinery) from GREET model,Wang 912
      Corn-stover fertilizer replacement requirement BTU/ gal from Gallagher, Dikemen, and Shapouri (2003) 1,223
      Total: 2,135
      3 51.0 percent of total energy used for ddg preparation. See McAloon, et al., ASPEN model.
      4 kwhr elec /gal eth BTU elec /kwhr elec BTU out/gal BTU elec/BTU input BTU out/ BTU in BTU in /gal
      Electricity: 0.757 3,413 0.30 8,720
      Power: 15,961 0.94 16,980
      Corn-stover harvest: 456
      Corn-stover fertilizer replacement: 611
      Total: 26,767
      5 energy in farm machinery (steel and tire production, assembly, repair parts): 2,913 BTU/ bu from GREET model, Wang
      9
      Table A1. Energy-related inputs used to grow corn in nine States and nine-State weighted average, 2005
      Nine-State
      Weighted
      IL IN IA MI MN NE OH SD WI Avg.
      Yield (2004-06) 161.8 159.61 173.34 141.4 164.61 157.34 153.09 116.77 142.55 159.70
      Bushels/acre
      0
      Seed 24,180 23,660 24,700 23,010 24,700 21,710 24,050 20,540 24,960 23,771
      Kernels/acre
      Fertilizer: Pounds/acre
      Nitrogen 147.51 146 128.87 119.43 129.18 130.42 161.37 107.5 101.7 133.39
      Pounds/acre
      Potash 94.19 104.91 60.64 72.55 54.85 6.59 79.05 10.73 51.1 61.26
      Pounds/acre
      Phosphate 64.68 67.84 46.87 40.19 52.66 60.06 59.31 36.48 34.8 54.36
      Pounds/acre
      Lime 836 858 714 644 112 188 722 2 676 554.36
      Pounds/acre
      Energy Inputs:
      Diesel 4.50 4.20 3.00 4.70 4.10 17.00 3.70 2.60 6.60 5.81
      Gallons/acre
      Gasoline 1.80 2.50 1.70 2.70 2.10 1.70 2.20 1.60 2.00 1.92
      Gallons/acre
      LPG 2.00 4.10 2.60 1.60 8.10 1.70 3.70 0.40 3.60 3.20
      Gallons/acre
      Natural gas 90.00 40.00 0.00 80.00 30.00 1,200.00 100.00 10.00 70.00 208.90
      Cubic ft/acre
      Electricity 25.70 14.40 6.80 10.90 16.70 50.60 7.00 11.40 32.00 20.41
      kWh/acre
      Custom work 7.09 7.05 6.81 8.69 8.99 11.87 8.05 8.78 13.38 8.45
      Dollars/acre
      Custom drying 2.18 0.20 3.34 2.60 2.98 0.25 2.10 0.24 3.90 2.09
      Dollars/acre
      Purchased water 0.00 0.00 0.00 0.00 0.00 0.57 0.00 0.00 0.00 0.08
      Dollars/acre
      Chemicals 2.47 1.71 1.75 2.22 1.71 2.27 2.85 1.34 1.60 2.00
      Pounds/acre
      2005 production 1,708,850 88,8580 2,162,500 288,860 1,191,900 1,270,500 464,750 470,050 429,200 8,875,190
      1,000 bushels
      10
      Table A2. Total energy requirements of farm inputs for nine States and nine-State weighted average, 2005
      Nine-State
      Weighted
      IL IN IA MI MN NE OH SD WI Avg.
      BTU to produce 1 bushel of corn
      Seed 412 408 300 437 355 506 469 386 474 394
      Fertilizer:
      Nitrogen 22,336 22,411 18,215 20,693 19,227 20,308 25,825 22,555 17,479 20,464
      Potash 1,746 1,972 1,049 1,539 1,000 126 1,549 276 1,075 1,151
      Phosphate 1,599 1,700 1,082 1,137 1,280 1,527 1,550 1,250 976 1,362
      Lime 2,883 3,000 2,298 2,541 380 667 2,632 10 2646 1,937
      Energy Inputs:
      Diesel 4,238 4,010 2,637 5,065 3,795 16,463 3,683 3,393 7,055 5,539
      Gasoline 1,604 2,259 1,414 2,754 1,840 1558 2,072 1,976 2,023 1,735
      Liquefied
      Petroleum Gas 1,062 2,206 1,288 972 4,227 928 2,076 294 2,169 1,722
      Natural gas 582 262 0 592 191 7,978 683 90 514 1,368
      Electricity 1,488 845 367 722 950 3,012 428 914 2,102 1,197
      Custom work 537 541 481 753 669 924 644 921 1,150 648
      Custom drying 661 62 946 903 889 78 673 101 1,343 641
      Purchased water 0 0 0 0 0 533 0 0 0 75
      Chemicals 2,353 1,652 1,556 2,420 1,601 2,224 2,870 1,769 1,730 1,928
      Input hauling 1,213 1,279 966 1,099 277 479 1,129 70 1,172 868
      Total energy 42,714 42,606 32,601 41,627 36,679 57,310 46,284 34,003 41,909 41,029
      Energy in custom work accounts for 16%, diesel fuel is used as source of energy
      Energy in custom drying account for 80%, propane is used as source of energy
      Energy in purchased water account for 90%, electricity is source of energy
      Energy used for seed is 4.7 times higher than energy used for corn
      Weighted average of insecticides and herbicides are used to estimate energy in pesticides
      Energy used to haul input to farm is equal to weight of lime, diesel fuel, gasoline, LPG and pesticides multiplied by 218.5 BTU per pound
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