Tillage For Corn and Soybean
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Since the beginning of settled agriculture, farmers have spent huge amounts of energy and money tilling the soil in preparation for planting crops. Over the millennia, most tillage systems were designed for extensive soil disturbance, with secondary tillage operations that resulted in loose, pulverized soil around the seed at planting. This made possible adequate seed-to-soil contact, which is critical for water uptake by the seed and subsequent germination and emergence. Residues from the previous crop were often removed at harvest or tilled into the soil, in part so they would not disrupt planting. Burial of residue also helped to reduce the spread of residue-borne plant pathogens. Weed seeds were also buried, and fewer of them emerged in the next season.
With the development of herbicides for weed control, better disease resistance in crop varieties, and new equipment over the past 50 years, it has become possible to plant crops without tillage, or at least with less tillage, and still obtain the seed-to-soil contact at planting that is so critical to uniform emergence. Predictions in the 1960s and 1970s that tillage would disappear did not, however, come to pass, and there are recent indications that tillage is "back," though it is often modified to serve the increased awareness of the soil-conserving advantages of having some crop residue on the surface of the soil. We want to look at current definitions of tillage, consider how tillage affects both the soil and crop residue, and examine the results of some recent tillage research in Illinois.
Tillage Definitions
Much of the following information (including direct quotations) comes from the Conservation Tillage Information Center, which serves as a clearinghouse for information about tillage and crop residue management. Crop Residue Management (CRM) "includes all field operations that affect residue amounts, orientation and distribution throughout the period requiring protection. Site-specific residue cover amounts needed are usually expressed in percentage but may also be in pounds." In practice, CRM is considered to involve the use of conservation tillage systems, so this term means "keeping residue on the surface" in most cases.
Conservation Tillage is "any tillage and planting system that covers 30 percent or more of the soil surface with crop residue, after planting, to reduce soil erosion by water." The following three types of tillage and planting systems are all considered conservation tillage:
- No-till/strip-till is a tillage (and planting) system in
which
"the soil is left undisturbed from harvest to planting except
for strips up to 1/3 of the row width (strips may involve only residue
disturbance or may include soil disturbance). Cultivation may be
used for emergency weed control. Other common terms used to describe
No-till include direct seeding, slot planting, zero-till, row-till,
and slot-till."
- Ridge-till is a system in which "the soil is left undisturbed
from harvest to planting except for strips up to 1/3 of the row width.
Planting is completed on the ridge and usually involves the removal
of the top of the ridge. Residue is left on the surface between ridges.
Ridges are rebuilt during row cultivation."
- Mulch-till is "full-width tillage involving one or more tillage trips which disturbs all of the soilsurface and is done prior to and/or during planting. Tillage tools such as chisels, field cultivators, disks, sweeps or blades are used." This system differs from "reduced" or "conventional" tillage only because it leaves more than 30 percent residue cover after planting.
Tillage systems that leave less surface residue than conservation tillage include:
- Reduced-till, which is "full-width tillage involving
one or more tillage trips which disturbs all of the soil surface
and is performed prior to and/or during planting. There is 15 to
30 percent residue cover after planting."
- Conventional-till or intensive-till, which is "full-width tillage which disturbs all of the soil surface and is performed prior to and/or during planting. There is less than 15 percent residue cover after planting. Generally involves plowing or intensive (numerous) tillage trips."
After increasing as conventional tillage decreased through the 1970s and 1980s, conservation tillage as a percentage of U.S. crop acres has stabilized at about 35 to 37 percent over the past decade. Of this, about half is no-till/strip-till and half is mulch-till, though in recent years, the area of no-till/strip-till has increased. Ridge-till has remained at only about 2 percent of U.S. crop acres and has shown little movement. Overall, the use of conventional tillage has not changed much since the early 1990s, and in the past two years, it appears to have increased .
Conventional tillage is the most common system used for corn, while no-till is the most common system for soybean production. In the U.S., about 36 percent of the corn crop is grown using some form of conservation tillage, while this percentage is about 55 for soybean.
While deep tillage (also called ripping or deep ripping) is not defined as a separate tillage system, the use of this technology has grown rapidly in the Midwest in the last five years or so. Deep ripping is generally defined as tillage done to a depth of more than 12 inches, usually with the goal of shattering the soil beneath the surface and so relieving soil compaction effects on crops. Its development has accompanied the use of larger tractors. Depending on the tillage standards and soil-engaging points used, deep tillage may result in little disruption of surface residue, or it may bury a substantial amount of this residue.
Recent Tillage Research in Illinois
For the past four years (1999 to 2002), we have been conducting a comparison of strip-till, mulch-till, and no-till at three locations, all on productive soils in central and northern Illinois. In addition to tillage/planting system comparisons, we also tried to find out if changing tillage might change the response to starter fertilizer, and whether or not timing and placement of N fertilizer should change when changing the tillage/planting system. Strips were made in the fall, with or without N placement at the time they were made. Mulch-tillage was also accomplished in the fall. Soybean was the previous crop in all cases.
Somewhat surprisingly, even with relatively early planting, we saw very little effect of tillage on corn yield, averaged over 10 site-years . While no-till soils were slightly cooler than tilled soils at planting, the spring weather in all three years was relatively warm; soil temperatures at planting ranged from the mid 50s to the mid 60s among all site-years. Both N placement beneath the row and the use of starter fertilizer increased early-season plant growth, but did not result in yield increases.
We are also conducting a study designed to look at effects of deep tillage in different soils in different locations in Illinois. To do this, we established trials to compare no-till, chisel plow (conventional tillage, as measured by residue after planting the next spring), and deep tillage, which generally qualified as conservation tillage, at least when it followed corn harvest. These primary tillage systems were split in the spring, with no secondary (spring preplant) tillage in one half, and the use of a disk or field cultivator in the other half of each plot.
Over a wide range of soils, from silt loams with claypan in southern Illinois to deep, dark-colored prairie soils in northern Illinois, deep tillage did not produce a significant yield response, with or without secondary tillage, in both corn and soybean The lack of response to annual deep tillage suggests that this practice, which is often done on intervals longer than one year, may provide limited benefit in some of the fields where it is being done in Illinois. That does not mean that it does not influence surface compaction and permeability, but such improvements clearly do not guarantee higher crop yields.
Finally, the Illinois Department of Agriculture has sponsored some 25 trials in farmer fields over the past three years under the SOILS (Save Our Illinois Soils) project. Treatments consist of no-till, strip-till, and spring tillage of soybean residue before planting corn. Through 2001, full-width tillage produced slightly higher yields than strip-till and no-till, but this yield increase may not have provided an economic return due to higher costs of this tillage practice.
Summary
While work to compare tillage systems will continue in Illinois, it seems rather clear that most currently used tillage systems, if carefully deployed, are probably not affecting crop production and yields very much. We think that this is largely because equipment companies and producers are very much aware of the need to place seed into favorable soil and moisture conditions, regardless of tillage system, and are working effectively to do this. We have well learned the lesson that getting a good corn stand is of paramount importance, and some of the increase in tillage for corn is likely due to problems with getting adequate stands in the colder, wetter soils that often result from reduced tillage. Soybean seed, on the other hand, is usually planted into warmer soils, and seeding rates are such that stand losses of 15 or 20 percent, which would seriously affect corn yield, usually have little effect on soybean yields.
We also need to acknowledge that the definitions of tillage systems, based as they are on percent residue cover after planting, are considerably influenced by factors such as previous crop, and not just by what equipment we use for tillage and planting. As an example, soybean and corn generally follow each other in the rotation, and the same tillage system that leaves more than 30 percent residue after soybean is planted following corn will often leave much less after corn planted following soybean. It certainly pays to look beyond such definitions when we are trying to asses how tillage might affect seed placement and soil conditions, both around the seed at planting and also in the rooting zone of the crop.