Tillage and Residue Cover

Chapter 3 — Residue Management, No. 10, Fall 1988

Don Wysocki

Maintaining a residue cover to protect the soil is the primary objective of a conservation tillage system. Many growers are considering or developing conservation tillage systems in their conservation plans. Residue covers of 30 or 50 percent are the target levels for many conservation tillage systems. As growers and conservation planners consider conservation tillage practices, it is useii.d to have a reference regarding the residue cover that can be established through various tillage operations.

As part of the annual field day in June 1988, several tillage sequences were performed on a set of plots at the Columbia Basin Agricultural Research Center, Pendleton, OR. These plots were established to demonstrate different levels of residue cover and provide awareness of the residue levels that common tillage operations typically achieve.

Tillage Sequences and Equipment

Tillage operations were performed on a series of 20 by 100 foot plots, with tillage oriented to the long dimension. The initial condition was 7,500 to 8,500 pounds/acre uniform standing stubble of Stephens wheat on a Walla Walla silt loam, O to 2 percent slopes. Soil water content was less than half of field capacity or about 10 percent. The tillage sequences performed are shown in Table 1. Stubble was either left standing or busted with a rotary flail mower. Equipment specifications along with depth and speed of tillage are presented in Table 2. All tillage was performed in a period of about 2 weeks in early June.

Residue Cover

Numerous soil, residue and equipment factors such as soil water content, soil texture, shear strength, slope, residue water content, tillage depth and tillage speed affect burial and distribution of residue during tillage.

Because of these factors, it is impossible to obtain uniform residue cover on a field or even on a plot. It is generally accepted that residue cover in a field ranges around some average value. A corollary to this is that two growers following similar tillage practices may obtain different residue covers depending on equipment, residue or tillage variables.

Three representative 5 .5-foot2 areas were selected per plot and framed with an aluminum border. These areas were photographed and the residue cover was measured by projecting the image on a 100-point grid and counting point-residue intersections. The residue covers for the different tillage operations are shown in Table 3, It should be understood that data in this table are relative estimates and are presented as a general guide. The data have been grouped by primary tillage treatments. Treatments 1 through 6 are moldboard plow, treatments 7 through 10 are disk and treatments 11 through 14 are chisel.

Table 1. Sequence of tillage operation (Wyesocki, OSU, Pendleton).

Table 2. Equipment and tillage specifications (Wysocki, OSU, Pendleton).

Table 3. Percent residue coverl achieved after various tillage operations (Wysocki, OSU, Pendleton).

Several comparisons can be made from these data. Observation of the three main groupings gives a relative measure of residue cover that can be achieved by different primary tillages. Differences in residue cover that result from busting stubble can be observed by comparing the various bust vs. standing stubble treatments. For example, compare treatments 1 and 2, 3 and 4, 5 and 6, 7 and 8, 9 and 10, and 11 and 14. In most cases there was little difference, however, comparison of treatments 7 and 8 show that busting stubble before disking may cause much greater burial of residue. A comparison of treatments 1 and 2 with treatments 3 and 4 shows the influence of plowing with and without trash boards. The intention of treatments 5 and 6 was to bury residue by plowing and then pull it back to the surface by chiseling, This was achieved to some degree and probably would have been more successful if chiseling could have been performed in a direction different than plowing.

Comparison of treatments 11, 12 and 13 shows the effect of different chisel shanks. Keep in mind that the shanks were varied only on the first tillage passes in each treatment. Sweeps were used for the second pass in each. A much greater difference might have been observed if the original shanks had been used on the second pass. In addition if faster tillage speeds had been used, twisted shanks might have buried more residue.

Summary

From a grower’s perspective there are several important points in this information:

1. Chisel or disk systems maintain sufficient residue at the surface to achieve 50 percent cover when initial residue levels are in the range of 7,500 to 8,500 pounds/acre.
2. Chisel and disk systems would probably achieve 30 percent cover when initial residue levels are in the range of 3,000 to 4,000 pounds/acre.
3. At initial residue levels of 7,500 to 8,500 pounds/acre, moldboard plow systems can achieve 30 percent cover provided special efforts are made to leave residue at the surface or secondary tillage returns some to the surface. Plowing more slowly with the trash boards removed or chiseling to plow depth after moldboarding are such practices. However, both require additional time and labor.
4. Residue cover alone does not tell the entire story. Both the size and distribution of residue are important. Small, well distributed pieces of straw present a different situation than large, poor distributed clumps. The percent residue cover could be the same in both cases, but a grain drill would perform differently in each case. Practices such as flailing or disking can be important to obtain small, well distributed residue.