Estimation of Minimum Retainable Residue Levels

for Continuous-Crop Rotation

In general, crop residue removal was affected by wind erosion more than rainfall erosion in the western two-thirds of Kansas, Nebraska, and South Dakota. Rainfall erosion was the dominant erosive force in the east­ern one-third of these three states, as well as all the other seven states considered. Equation 8 presents WEQ:

E = f (WI, WK, WC, WL, WV) (8)

in which E is the average annual soil loss (Mg/[ha-yr]); WI is the wind erod — ibility index (a measure of soil susceptibility to detach and be transported by wind) and varies by individual soil type; WK is the soil ridge-roughness factor and describes the condition of the field surface at a particular time;

WC is the climate factor and represents the amount of erosive wind energy present at a particular (county-level) location; W L is a function of wind direction, field length, and width and is the unsheltered median travel dis­tance of wind across a field; and WV is the vegetative factor. The relationship between E and the other variables is highly nonlinear.

The amount of residue potentially available for removal with respect to applying WEQ was determined by analyzing total soil loss attributable to wind forces in each field-management period (time between each field operation) for all individual soil types and then summing across all field — management periods including crop growth. These values were then com­pared to erosion values obtained in the rainfall erosion analysis, and the greater required minimum residue level at harvest was chosen. A detailed discussion of the application of WEQ to agricultural crop residue removal is provided in an article by Nelson (9).

Results

Tables 7 and 8 present data concerning maximum quantities of corn stover and wheat residue, respectively, that could potentially be removed from agricultural cropland in each of the 10 largest corn-producing states in the United States subject to the constraints of the tillage scenarios, pro­duction yields, soil types, and field topologies considered in this analysis. The removable residue quantities presented in this article reflect remov­able residue with respect to only soil erosion and no accounting/method — ology was performed with respect to the impact removing residue would have on, e. g., soil tilth and nutrients. These are amounts that could be removed if all agricultural cropland (not just those in corn, wheat, and soybeans) in each state were planted to a particular rotation and subject to conventional, reduced/mulch, and no-till field-management practices (till­age scenarios), and the counties achieved crop yields equivalent to the 1997-2001 5-yr average. The quantities are the lesser of the two quantities that can be removed under the rain and wind erosion analyses.

For example, if all agricultural cropland in Iowa were managed in a continuous-corn, no-till rotation, 84.4 million dry Mg of corn stover could be harvested annually. If all the agricultural cropland were managed in a corn-winter wheat rotation using mulch till practices, 30.2 million dry Mg of corn stover and 13.4 million dry Mg of wheat residue could be harvested annually. (Note, that if a county did not produce a specific crop during 1997-2001, then it is assumed that crop is not produced in that county and thus any rotation with that crop is also not produced in that county; the same constraint is not applied regarding to tillage practices.) An assump­tion was made that all tillage practices are possible in all counties.

In addition, it should be noted that not all estimated residue quantities will actually be removed owing to the potential of some farmers being unwilling to remove residues from their fields, as well as weather condi­tions that may prohibit collection. Other factors may come into play as well.

Maximum Removable Corn Stover Quantities (Million Metric Tons at Harvest) by Rotation
and Tillage Practice When All Cropland Acres in Each State Are Planted to the Rotation"

Maximum Removable Wheat Straw Quantities (Million Metric Tons at Harvest) by Rotation
and Tillage Practice When All Cropland Acres in Each State Are Planted to the Rotation"

Quantities can be adjusted for these factors by assuming that only a certain percentage of the estimated quantities is actually removed.

The estimated quantities of removable corn stover and wheat straw presented in Tables 7 and 8 conform to intuitive expectations in that as tillage operations become less intensive (i. e., go from conventional to no-till), the amounts of removable residue increase across all rotations in all states. Differences in estimated removable quantities among states is a function of several factors including production location (whether the majority of production occurs in areas that have highly erodible soils and field topology nonconducive to removal), climatic/erosive conditions at the locations of production, and actual yields at these specific locations among others. These factors must be considered before residues can be removed at any specific location.

Conclusion

A methodology was developed to assess the amount of agricultural crop residue that can be removed without exceeding the tolerable soil-loss limit in both single and multicrop (2-yr) rotations. Application of this methodology to select corn — and wheat-based cropping rotations on land capability class I-VIII soils subject to conventional, reduced/mulch, and no-till field-management practices in Iowa, Illinois, Nebraska, Minnesota, Indiana, Ohio, Kansas, South Dakota, Missouri, and Wisconsin indicates that significant removable quantities of corn stover and wheat straw exist, but there is considerable variation in the amounts of removable residue with respect to each tillage scenario across all states analyzed. These amounts only consider the need to keep erosion to a tolerable level and do not encompass soil carbon considerations.

References

1. Energy Information Administration. (2003), in Annual Energy Outlook 2003 with Pro­jections to 2025, US Department of Energy, Washington, DC, p. 18.

2. Energy Information Administration. (2003), Annual Energy Outlook 2003 with Projec­tions to 2025, US Department of Energy, Washington, DC.

3. Energy Information Administration. (2003), in Annual Energy Outlook 2003 with Pro­jections to 2025, U S Department of Energy, Washington, DC, p. 4.

4. Energy Information Administration. (2003), in Annual Energy Outlook 2003 with Pro­jections to 2025, US Department of Energy, Washington, DC, p. 3.

5. Nelson, R. G., Enersol Resources. (2001), Resource Assessment, Removal Analysis, Edge — of-Field Cost Analysis, and Supply Curves for Corn Stover and Wheat Straw in the Eastern and Midwestern United States, National Renewable Energy Laboratory, Golden, CO.

6. Larson et al. (1979), in Journal of Soil and Water Conservation, Special Publication No. 25, Soil Conservation Society of America, Ankeny, IA.

7. US Department of Agriculture. (1997), Predicting Soil Erosion by Water: A Guide to Conservation Planning with the Revised Universal Soil Loss Equation (RUSLE), Agricul­tural Handbook Number 703, US Department of Agriculture, Agricultural Research Service.

8. Skidmore, E. L. (1988), in Soil Erosion Research Methods, Soil and Water Conservation Society of America, Ankeny, IA.

9. Nelson, R. G. (2002), Biomass Bioenergy 22, 349-363.

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