Crop residue management

Soil organic matter is composed of about 50 percent carbon. As a result, the terms soil organic carbon and soil organic matter are often used interchangeably. However, scientists prefer soil organic carbon because it can be measured with more accuracy.

Soil organic matter represents decaying plant and animal residues, microscopic soil organisms that decompose plant and animal residues and substances released by these organisms into the soil.

For producers, soil organic matter is synonymous with soil productivity because:

• It’s a source of nutrients that will slowly release over time.

• It promotes the aggregation of soil particles, which:

  • Improves the soil’s water-holding capacity.

  • Improves the rate of water infiltration.

  • Decreases the potential for erosion.

  • Increases plants’ rooting ability.

  • Lowers the soil’s bulk density.

  • Allows the soil to be tilled with less horsepower.

The light-colored, forest-derived soils in the eastern Corn Belt contain about half the organic matter as Minnesota’s dark, prairie-derived soils. As a result, crop water stress is often common on those soils, even though the eastern Corn Belt generally has greater rainfall than Minnesota.

More aggressive tillage results in greater residue incorporation and increased aeration, which helps soil microorganisms decompose crop residues and soil organic matter. Decomposition release the carbon in crop residues and soil organic matter into the atmosphere as carbon dioxide.

Conservation vs. moldboard plow

The potential to sustainably harvest crop residues increases when using a conservation tillage system. For example, with conservation tillage in continuous corn, up to 44 percent of the corn residue could be annually harvested if grain yields are consistently 200 bushels per acre (Table 1). In comparison, only 19 percent of the corn residue could be sustainably harvested with a moldboard plow tillage system in continuous corn.

In continuous corn with moldboard plowing, no residue harvest is recommended when grain yield is 150 bushels per acre or less, but you could harvest three large round bales per acre when yields reach 250 bushels per acre. This is because higher grain yields produce more residue.

Because yield levels can fluctuate greatly from one year to the next, producers should take this into account and possibly adjust the quantity of residue harvested from year to year.

Continuous corn vs. corn-soybean rotation

Figure 1 shows that a 200 bushel per acre corn crop produces 4.22 tons of dry matter per acre as corn residue (assuming a harvest index of 0.53).

When corn residue is moldboard plowed in a corn-soybean rotation, the amount of corn residue that needs to be retained is greater than the amount produced with a 200 bushel corn crop. This means it’s not sustainable to harvest corn residue in this system. The system actually reduces soil productivity over time.

On the other hand, if you grow continuous corn with moldboard plow tillage, the amount of corn residue that needs to be retained is about 0.84 tons per acre less than that produced with a 200 bushel corn crop. This leaves 0.84 tons of corn residue per acre (20 percent of the total residue production) that could be annually harvested, but this would require a 200 bushel yield level every year.

The potential for residue harvest is much greater when using a conservation tillage system such as no-till, strip-till or chisel plow tillage. With conservation tillage in continuous corn, you could annually harvest up to 45 percent of the corn residue if grain yields are consistently 200 bushels per acre.

Crop rotation also influences how much corn residue can be harvested. This is because continuous corn produces a greater quantity of root and shoot residue than with the corn-soybean rotation.

In addition, corn residue has a high carbon-nitrogen ratio compared to soybean residue, making it more resistant to decomposition.

In a corn-soybean rotation, you can only harvest 16 percent of corn residue with 200 bushels per acre yields and conservation tillage. Because of the small amount of residue, it would be hard to uniformly remove it. The same is true with continuous corn grown under moldboard plow tillage.

Residue removal is best suited for continuous corn grown with conservation tillage.

In continuous corn systems with conservation tillage, you could annually harvest 35 to 44 percent of the residue if grain yields are 175 to 200 bushels per acre. However, harvesting only 35 to 44 percent of the corn residue can be difficult.

Table 2 illustrates the amount of residue removed with different baling methods. If you use a rake to create windrows prior to baling, set the rake as high as possible to avoid collecting too much residue. Rakes set at normal operating height remove approximately 65 percent of the residue, which would collect too much residue.

In table 2, note that for residue removal by baling windrow, the combine’s spreader was turned off. For residue removal by raking and baling, rake set at normal operating height.

Derived from Vagts.
*Grain yield reported at 15.5 percent moisture.
**Residue harvest (bales per acre) across all methods assumes dry residue and 1,200-pound round bales.

For reference, Figures 2 and 3 show surface residue coverage with removal of none and approximately all of the corn residue in a chisel plow tillage system.

When calculating the cost of removing residue, consider the fertilizer costs for replacing the nutrients removed with the residue.

Table 3 shows the amounts of phosphorus and potassium in baled residue. The cost of nutrient replacement depend on your local fertilizer prices.

If soil test levels indicate a need for phosphorus, target manure applications rather than fertilizer for these fields. For continuous corn, reduce nitrogen fertilizer rates following corn residue harvest.

While increasing fertilization in fields that harvest residue will help replace some of the lost nutrients, it will not compensate for the lost carbon.

In agricultural fields, growers can maintain soil carbon levels by returning residue to the soil, rotating crops with pasture or perennials or adding organic residues such as animal manure, green manure or sewage sludge.

Good sources of carbon include manure, by-products from industrial processes such as ash and winter cover crops.

Derived from an article in Better Crops.

*Assuming dry residue and 1,200-pound, round bales.

A large, round bale of corn residue contains approximately 11 pounds of nitrogen, according to a Better Crops article, but this nitrogen would not be readily available to the subsequent crop had residue been returned to the soil. Instead, this nitrogen would slowly become available over time as the residue decomposes.

Another consideration is that when removing residue in continuous corn, you can reduce the nitrogen fertilizer rate for the subsequent corn crop. That’s because corn residue helps soil microorganisms tie-up (i.e., immobilize) nitrogen.

Research at three locations in northern and central Illinois on dark, prairie-derived soils showed that harvesting half or all of the corn residue in continuous corn reduced the economically optimum nitrogen fertilizer rate by 13 percent. This was consistent for both chisel plow and no-tillage systems.

A major cost of removing corn residue is harvesting and handling. Table 4 lists these costs, which were estimated by Iowa State University.

When calculating the total cost of removing residue, keep all of the expenses in mind. Consider the nutrients removed in the residue, in addition to the harvesting costs.

Don't forget about less-definable, long-term costs, such as how carbon removal impacts soil. Soil carbon plays a vital role in maintaining soil productivity.

Source: Alejandro Plastina.

Corn cobs consistently have more density and moisture than corn residue, and collecting cobs allows growers to return the remaining residue to the soil.

In addition, corn cobs are easier to handle with a one-pass grain plus cob harvest. One-pass cob collection requires less equipment, labor and trips over the field compared to baling residue. This reduction in field traffic reduces soil compaction. In addition, there’s minimal spoilage when storing corn cobs outdoors.

A typical field has approximately 1,500 pounds of dry corn cobs per acre, representing about 20 percent of all corn residue. Compared to harvesting all of the residue, removing a smaller quantity of material from the field only when the cobs are harvested reduces the impact on long-term soil productivity.

This makes cob harvest a sustainable practice in more cropping systems than residue harvest.

Table 6 lists the amount of phosphorus and potassium removed in corn cobs.

Derived from Sawyer and Mallarino.