During harvest, many are concerned with the high amount of corn residue appearing within fields. One of the main reasons that we are seeing a higher amount of residue is because of the higher yields. A 200 bu/a corn crop can leave around 5 tons or 12,000 – 16,000 lb/a of residue on the soil surface. Corn produces over twice the amount of residue in comparison to other crops and more than twice the residue necessary to provide 100% soil cover. There are other factors that have occurred over the past several years that have caused higher levels of corn residue such as higher plant populations, foliar fungicides, Bt traited products with higher stalk quality, reduced tillage, high fertility or productive soils, corn-on-corn rotation, as well as favorable environmental conditions that affect the rate of microbe decomposition.
If residue is allowed to build on the soil surface, many problems can occur. A high percent of residue on the soil surface can cause soil temperatures to remain cooler in the spring, which can result in emergence issues. Poor stands due to delayed germination can also result due to residue in the seed furrow, especially in corn on corn rotations, because of poor soil to seed contact. Many diseases are harbored within residue and in some cases, alleopathy or chemicals leaching out from corn trash might delay early crop growth. Lastly, nutrients, such as nitrogen can be tied-up within residue.
Residue can be a very important source of nitrogen, however, if a high amount of corn residue is present, immobilization will take place, which means that plant usable forms of nitrogen are tied up within the soil. Various soil microbes, depending on temperature, available oxygen, and moisture will decompose this residue into humus, and then organic matter. Once residue is decayed, mineralization can take place, and plant available sources of nitrogen, such as ammonium (NH4), become available to plants. Corn stalks consist of a carbon to nitrogen (C:N) ratio of 40 – 80 and in order to be broken down to humus by microbes, the C:N ration must be brought down to 10.
The first point of trash escape into the field is via the combine at harvest. Stalks, cobs, and chaff should be distributed evenly across the field as much as possible. A dense mat of trash on the soil surface in the spring can delay soil moisture loss and soil warming. The goal is to avoid thick residue in areas and bare soil showing in other areas of the field. Choppers, batt, and chaff spreaders can help to spread and evenly distribute trash, which in turn, can increase the decomposition rate. However, a chopper head can cost more, weigh more, and possibly over-process residue, which can lead to erosion issues. Crushing knife rolls or tapered snapping rolls on the combine head can aid in processing residue by crimping, chopping, or crushing of stalks, which allows many points for decomposers to enter the stalk. Trash reduction kits added to a combine can help to reduce power usage and possibly help to increase speed at harvest. Set the corn head higher, so that taller stalks remain standing to increase air movement down rows to encourage decomposition, but in a no-till operation, standing stalks can cause planting issues. This also allows the stalk to remain intact and anchored into the soil, which helps to eliminate erosion.
Some might choose to chop or mow down stalks, but one of the biggest problems is that trash can form a flat mat on the soil surface. If trash is matted between rows, it is more likely to plug tillage and planting equipment later. Be sure to choose a mower that will ensure an even residue distribution such as a flail mower and not a rotary mower, which can cause increase windrowing of residue that can hinder residue distribution. Chopped residue can be more easily washed away; therefore, erosion can ensue. Some recommend that if you want to chop residue, just let the combine do this job.
Fall grazing or bailing corn residue are options for growers with livestock. Along the same lines and another “hot topic”, is harvesting fodder for biofuel. This can provide some additional income and can solve some planting equipment challenges. The two issues that come along with these options are erosion as well as nutrient and carbon removal. Corn stover can consist of around 17lbs of nitrogen (N), 4lbs of phosphorus (P), and 20lbs of Potassium (K) per ton and if removed, these nutrients will need to be replaced. Carbon is critical when it comes to maintaining soil quality and productivity. If you are a livestock grower, the removal of carbon and nutrients from the soil can replaced by adding manure. However, those with just cash crops will need to replace nutrients by purchasing costly commercial fertilizer. However, carbon is not as easily replaced in the absence of manure, especially in a corn-on-corn operation that is intensively managed. Other problems that can result from fall grazing can be disease, compaction, reduced soil temperature in the spring, as well as germination problems.
There are some who believe that liquid nitrogen, such as urea or ammonium sulfate broadcasted over the top of corn stalks or residue in the fall or with UAN in the spring, will increase their decompositions rate. This theory is based on the process of nitrogen immobilization. Corn stalks consist of a higher amount of carbon over nitrogen. In order for microbes to build their populations and decompose the corn stalks, they are in need of nitrogen to build their cells. Research has shown that the addition of nitrogen to residue in the fall does not help to increase microbial populations, because temperatures are too cold for microbial residue breakdown to occur. Therefore, if adding nitrogen to corn stalks in the fall, the limiting factor is not nitrogen, but low temperatures. When liquid nitrogen is placed over the top of high residue in the spring, it can leach quickly and if temperatures are warm, up to 20% nitrogen loss can occur due to volatilization. Soil microbes need to get their nitrogen from the soil; therefore it is recommended that sources of nitrogen be incorporated or injected to encourage decomposition.
There has also been some talk of placing sugars or numerous other products, sometimes with liquid nitrogen, over the top of corn residue. The idea is that the sugars provide a food source for microbes to feed on, while decomposing corn residue. These sugars or other products are applied during the spring or fall in hopes that trash will breakdown before the next planting season.
The application of sugars often occurs south of I-70. Some claim they work better in the south because there is a longer duration of warm temperatures for decomposition by microbes to take place. There has not been much research or evaluation done of sugars or other products. Some say certain products work better with incorporation, but this could be because tillage alone encourages the breakdown of residue.
When managing residue, cultivation is most important because the incorporation of tillage within a farm operation will significantly increase decomposition rates by soil microbes. Fall tillage decomposition rates are around 5 – 10% higher than spring tillage. Additional tillage is often needed after the second year of corn to incorporate residue and speed up decomposition. With cultivation, benefits can be warmer soil temperatures in the spring and less interference of residue during planting operations. But, there also can be drawbacks when it comes to cultivation. Some tillage equipment might not be equipped to handle high residue levels and cultivators equipped for high residue levels can cost several more dollars per acre if compared to conventional models. In general, with tillage, there comes increased production costs and more soil erosion can occur with the decrease of residue. Continuous no-till operations can also improve bio and microbial activity in fields, breakdown crop residue, and release nutrients back into the soil. Of course, a no-till operation is much easier within a corn and soybean rotation. The drawbacks of no-till are poor emergence during cold or wet springs, lower spring soil temperatures, and uneven crop debris across the soil surface.
Ultimately, we should strive for “healthy soil” and one sign of a well maintained soil is earthworms. Some ways that we can obtain a good healthy soil is a continuous no-till operation with a crop rotation, continue to return organic residues such as manure or compost to soil, and the use of cover crops when fields are fallow. Cover crops can create a “micro climate” under its canopy that creates a moist environment, full of potential decomposers of crop residue. Not only do cover crops help to speed up the decomposition process, they also add biodiversity to corn residue, so they can feed microbes that, in turn, will feed on corn residue. However, one must ensure that cover crops do not dry out soil or create additional, unwanted residue.
If residue is not managed, we must find ways to deal with increased trash on the soil surface at planting, so that it does not hinder crop emergence. Some ways to deal with extra residue at planting is to mount tillage tools such as cleaning disks, sweeps, brushes, rolling finger, or plows. The basic goal is to make sure that at least 1/3 of the row area consists of less than 10% residue. In a continuous corn operation, row cleaners are needed to avoid seed contact with trash. With strip-till, the use of residue cleaners or managers are used at planting in order to obtain an 8 inch zone that is free of residue. If anhydrous is involved with a strip-till operation, there should be a 6 - 8 inch tilled zone to prevent nitrogen loss or other problems with seed germination. In no-till, there can be issues with standing stalks when planting, so coulters, openers, gauge wheels, and press wheels might be needed.
Some claim that those with no-till operations should be weary of coulters, which cut residue, loosen soil, and reduce wear on seed furrow opener in abrasive soil. If coulters are set too deeply, they can cause air pockets to occur in the seed zone. If residue levels are too high in a no-till situation, some might run a residue mover up front and then follow that by a spoked residue mover. Be sure that residue movers are not doing more harm than good, as some can move residue over the row which may hinder crop emergence. There is no single management technique that will make this breakdown of residue occur, instead an integrated approach throughout the growing season during planting, nutrient application, and harvesting will be needed to be incorporated into a farming operation.