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Be careful with cornstalk harvest

October 31, 2008
Wayne Schoper Brown/Nicollet Extension Educator

After the corn harvest complete, the next step is to do something with the corn stalk residue is. In most fields, corn stalk residue remaining after grain harvest is incorporated into the soil with tillage or is left on the soil surface. Some corn producers have been selling their corn stalks by baling them and selling them to livestock producers for use as bedding. Another future possibility is the production of ethanol from corn stalk residue. However, we do know that soil productivity (synonymous with soil carbon) will be reduced if all corn residue in a field is harvested regularly and there is not another source of carbon being returned to the soil to replace the carbon removed with the crop residue. Good sources of carbon include: a) manure; b) bi-products from industrial processes such as ash; and c) winter cover crops. Increased fertilization in fields where residue is harvested will help replace some of the nutrients removed with the residue, but it will not compensate for the lost carbon. In addition, nitrogen fertilizer rates in continuous corn should actually be reduced following corn residue harvest.

So, why is soil carbon so important? Carbon is important because it is the backbone of soil organic matter. 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. Since plants are at the top of the food chain, they are the initial source of all soil organic matter. For producers, soil organic matter is important because: a) it is a bank of nutrients that will be slowly released over time; b) it improves the water-holding capacity of the soil; and c) it promotes the aggregation of soil particles. Aggregation is important because it promotes water infiltration, increases the rooting ability of plants, and allows the soil to be tilled with less horse power. As a result, soil organic matter (or soil carbon) is synonymous with soil productivity. The light-colored forest-derived soils in the eastern Corn Belt contain about half the organic matter that our dark prairie-derived soils contain in Minnesota. As a result, crop water stress is often common in these soils, even though rainfall in the eastern Corn Belt is generally greater than that in Minnesota.

So, should we be harvesting corn stalk residue? The answer, of course, is that it all depends. The amount of crop residue that can be sustainably harvested in the absence of supplemental carbon in the form of manure or cover crops depends on the crop rotation and tillage system. Research shows that a 200 bushel corn crop produces 4.22 tons of dry matter per acre. In a corn-soybean rotation where corn residue is moldboard plowed, the amount for corn residue that needs to be retained is greater than the amount produced with a 200 bushel corn crop. Thus, it is not sustainable to harvest corn residue in this system, and this system is actually reducing soil productivity over time. On the other hand, if continuous corn is grown with moldboard tillage, the amount of corn residue that needs to be retained leaves enough carbon behind to allow light harvesting ( .84 tons per acre) of corn stalk residue.

So here are the main points to take home. Corn residue harvest is best suited to continuous corn systems. If you are in a corn-soybean rotation, be sure to replace harvested crop residue with organic matter, preferably, manure. Also, do not remove the entire crop residue; try to leave half of the material in the field. And a final point, if you are harvesting crop residue, rotate residue harvest among all of your fields and avoid harvesting from the same fields year after year.



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