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Using crop residue for renewable fuels

Using crop residue for renewable fuels

Because the effect of residue removal is not apparent in the short-term, multiple variables can impact results. Tillage, crop rotation and yield level are factors that will dictate how much crop residue can be harvested and still ensure sustainability of the system.

The buzz about crop residue is increasing after the U.S. Department of Energy's Office of Biomass Program’s announcement to increase the use of crop residue as a source of biomass for renewable fuel production.

“Crop residue can be fermented, burned, charred, or gasified to produce energy,” said Fabián Fernández, University of Illinois Extension specialist in soil fertility and plant nutrition. “However the residue is used, one thing is certain — it needs to be taken out of the field.”

Fernández said it’s difficult to determine the amount of corn stover (all above-ground corn plant material except grain) that can be removed without adverse consequences to the soil's level of organic matter, or physical and chemical properties, and to successive crop yields. Because the effect of residue removal is not apparent in the short-term, multiple variables can impact results. Tillage, crop rotation and yield level are factors that will dictate how much crop residue can be harvested and still ensure sustainability of the system.

“Removing grain means removing nutrients from the soil,” Fernández said. “The export of plant nutrients from a field when crop residue is being removed is also an important point to consider.”


Increase feedstock production

As popularity increases in using corn cobs as a bioenergy feedstock, Fernández said removing only corn cobs may be a way to increase bioenergy feedstock production while minimizing the long-term effect of residue removal on soil productivity.

Corn cobs represent only 20 percent of the total residue produced in a corn field, he said. Corn cobs have the advantage of being more consistent than other parts of the crop residue in terms of density and moisture and can be collected in one operation during grain harvest. It’s also easier to store corn cobs because they are less susceptible to decomposition.

Phosphorus and potassium

“Ultimately, the amount of phosphorus and potassium present in corn cobs is far less than that in stalks and leaves, so removal of corn cobs represents less removal of nutrients from the field,” Fernández said.
However, removal of corn cobs alone is not practical for all farmers and many are interested in removing as much residue as is sustainably possible, Fernández said.

While there are many factors that can influence the amount of nutrient removal when crop residue is baled, the way to calculate actual removal is straightforward. Following are a few simple steps that can be used to calculate nutrient removal and value when stover is taken out of the field.

Calculate the amount of stover produced

The first step in determining total nutrient removal in stover is to calculate how much stover is produced. This is typically estimated from a harvest index (also known as residue-to-corn grain ratio). The most widely used dry weight ratio is 1:1 residue:grain. Using this 1:1 ratio to calculate the pounds of dry residue produced, the grain yield (in bushels per acre) is multiplied by 47.3. The value can then be divided by 2,000 to obtain the number of dry tons produced.

Just an estimate

“Of course, this is just an estimate,” he said. “This calculation will tend to over-estimate stover quantity in high-yield fields (more than 180 bushels per acre) and tend to underestimate stover quantity in low-yield fields (less than 100 bushels per acre).

Determine amount of stover being removed

The most reliable method of determining the total amount of stover is to directly measure the weight and water content of the residue being removed. Doing this would eliminate the first step and give the most accurate information. Since this approach is not always feasible, approximate removal amounts need to be determined in relation to the harvest method.

Data from Iowa State University showed that with shredding and raking stover, 80 percent of the total will be removed; raking alone will remove 65 percent, and collecting stover from the combine windrow will remove 50 percent. To estimate the total amount of stover removed, multiply the estimated total stover produced (step 1) by the percent removed by the method of harvest used.
Calculate stover’s nutrient content

The third step in calculating how much nutrient is removed in stover is to determine the stover's nutrient content. The best approach is to analyze the nutrient content of a sample from the bale.

If that’s not possible, use ballpark values (7 pounds of phosphate and 30 pounds of potash per ton). Keep in mind that the actual amount of nutrients present in the stover can vary significantly depending on growing season conditions, hybrid, and general fertility of the soil. In addition, nutrients are also affected by the time elapsed and the amount and frequency of precipitation since the crop reached maturity, and the time the stover was removed from the field.

Estimate the value of stover

Finally, to calculate the estimated stover value, multiply the amount of nutrients removed in stover (step 3) by the current price of the corresponding nutrient. The impact of increased removal of these nutrients and organic carbon through removing stover is not as obvious in the short-term as for phosphorus and potassium, but it will definitely carry consequences. While calcium, magnesium, sulfur, and micronutrients are not typically provided through fertilization in Illinois, greater stover removal can accelerate depletion of these nutrients in the soil.

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