Impact of Manure on Costs, Returns, and Energy Use on Corn Fields in Iowa
Michael Duffy Most manure studies compare manure to commercial fertilizer using standard, replicated methodologies. These studies have been conducted on farms, at research farms, and with varying rates of manure and so forth, but for the most part they have presented a controlled comparison of manure and commercial fertilizer. This paper reports on the impacts of manure use shown in a cross-sectional survey. The purpose of this study was to determine whether or not manure, regardless of the source or rate, had any impacts on corn crops. The impacts examined are costs, returns, and energy use. Data for the study comes from a 1996 random sample of 990 cornfields across Iowa. The original survey was part of the USDA Cost and Return Survey. The Leopold Center for Sustainable Agriculture at Iowa State University provided funds to increase the sample size and allow more detailed analysis. The survey, conducted by the USDA Agricultural Statistics Service during the fall of 1996, used personal enumeration. The chosen farmers were asked questions about the cropping practices followed on a selected field that they farmed. The practices included machinery, fertilizer, pesticides, seeding, and other activities on the field. The cornfields selected were first divided according to the previous crop grown. Of the 990 fields, 222 or 22 percent had corn following corn. There were 687 fields, 69 percent, where corn followed soybeans. The remaining 81 fields were corn following some other crop or following fallow. To minimize the effects of rotation, only the corn following corn and corn following soybean fields results will be presented and discussed. The corn following cornfields were almost equally divided between those that did and did not receive manure. Of the 222 fields, 54 percent did not receive any manure while 46 percent did. Manure use for corn following soybeans showed greater variation. Manure was applied to 22 percent of the corn following soybean fields. Previous studies in Iowa have shown that only about 50 percent of the farmers report adjusting commercial fertilizer on fields where they applied manure. Table 1 shows the comparison of fertilizer use in 1996 based on the previous crop and whether or not manure was applied. Manured corn after corn fields showed an average of 14 pounds less nitrogen use, 15 pounds less phosphorus, and 19 pounds less potassium. For corn following soybeans, the manured fields received 10 pounds less nitrogen, 12 pounds less phosphorus, and 17 pounds less potassium. In a simple t-test, all of the differences were significant at the 99 percent level. Table 1 shows that, on the average, Iowa farmers are adjusting commercial fertilizer usage. When manure was applied, the farmers reported using a significantly lower amount of all three primary crop nutrients. Table 1: Fertilizer Use Comparison on Corn with and without Manure, 1996
The difference in total costs reported in Table 2 is significant. However, the only individual cost category that was significant was the expense for fertilizer. Table 3 shows the average yield and returns for 1996 based on whether or not manure was applied. For corn after corn, the yield difference was 10.6 bushels per acre and this difference was significant at the 99 percent level. The yield difference for corn after soybeans was not significant. Table 3 uses an assumed corn price of $2.60 per bushel. The return to land, labor, and management reported simply subtracts the total costs reported in Table 2 from the gross revenue. The return to management subtracts the per acre labor charge reported in FM1712 and assumes a land charge equal to $.89 per bushel of reported yield. The returns reported in Table 3 are significantly different at the 99 percent level. This is true for both corn after corn and corn after soybeans. The fertilizer cost differences were enough to cause significant variations in returns for corn after soybeans even though the yields were not significantly different. Table 2: Corn Cost Comparisons with and without Manure
For pesticide energy, a simple conversion factor of one gallon of diesel fuel equivalent per one pound of active ingredient is assumed. The actual energy use will vary by formulation but such refinements are not pursued here. Fertilizer energy use is assumed to be 5.01 pounds per gallon for nitrogen, 22.42 pounds per gallon for P2O5, and 26.53 pounds per gallon for K2O. Energy use in fertilizer production is the subject of much debate and there have been several alternative estimates proposed. These estimates are the most recent ones available from a U.S. Department of Energy study (Energy in Synthetic Fertilizers and Pesticides: Revisited by M.G. Bhat, et al., Oak Ridge National Laboratory, ORNL/Sub/90-99732/2, January, 1994). Fertilizer is the single biggest energy use category in corn production. Nitrogen is the single biggest source of energy used. When manure is substituted for commercial nitrogen, the energy balance significantly improves. Although Iowa corn production is still consuming more fossil fuel energy than is produced in the form of animal feed, the use of manure significantly improves the overall energy balance. Table 4: Energy Summary Comparison for Corn with and without Manure
Iowa farmers do appear to be adjusting commercial fertilizer use based on whether or not they apply manure. For corn after corn, Iowa farmers used 12 percent less nitrogen and for corn after soybeans, they used 8 percent less. The change in fertilizer use produced a level of yield close to equivalent in 1996. The result is that costs, returns, and energy use are significantly improved with manure use. This was not a standard research comparison of manure versus commercial fertilizer use. However, this study does show that, in practice, when using manure fertilizer use and costs can be reduced without sacrificing yields or returns.
|
