How Much Surface Residue Is Enough?

Chapter 2 – Systems and Equipment, No. 7, Summer 1987

Roger Veseth

Northwest growers need to give special attention to maintaining an adequate level of crop residue on the soil surface at winter wheat seeding this fall. This is because the most severe soil erosion in this region typically occurs overwinter where winter wheat is seeded in a residue free, conventionally-tilled seedbed. Leaving a portion of the residue from the previous crop on the soil surface can effectively control erosion. Tillage implement selection and operation, crop rotation and other management options determine how much residue can be maintained on the surface.

Tillage and Rotation Impacts On Surface Residue

To help growers refine their tillage systems to maintain adequate surface residue, STEEP researcher Don MCCOO1 began a study in 1986 to determine the amount of residue remaining on the soil surface after different tillage implements. MCCOO1 is a USDA-ARS agricultural engineer at Washington State University in Pullman. The study is being conducted at the Integrated Pest Management (IPM) Research Site north of Pullman in cooperation with Frank Young, IPM project leader and USDAARS research agronomist at WSU, and other researchers.

The 1PM site covers 80 acres with large plots maintained with field-size equipment. The IPM study allows the evaluation of different implements under two crop rotations using conventional tillage and conservation tillage systems.

Two 3-year crop rotations on the IPM study are: (1)winter wheat-winter wheat-spring wheat, and (2) winter wheat-spring barley-spring peas. Under conventional tillage, the primary tillage implements are the moldboard plow after small grains and tandem disk after spring peas, Secondary tillage consists of two field cultivator operations before seeding with a standard double disk drill.

With minimum tillage the chisel is used for primary tillage. Implements and operations for secondary tillage are the same as for conventional tillage. Where winter wheat follows low-residue crops, a no-till drill is used without prior tillage.

Winter Wheat After Small Grains

In McCool’s study the winter wheat and spring wheat stubble was flailed after harvest in 1986 before tillage began. The flailing operation can significantly increase the amount of residue incorporated by primary tillage implements, such as the moldboard plow or offset disk, compared to residue incorporation with tillage in undisturbed cereal stubble. The reader needs to keep this in mind when comparing the results of this study with their on-farm operations.

Under conventional tillage, an 18-inch bottom mold board plow was used at a depth of 8 to 9 inches and 4.5 to 5.0 mph. The field cultivator, used in the following two operations, had 2-inch wide points on 12-inch spacings with a 3-bar tined harrow attached. The field cultivator was operated at a 6- to 7-inch depth, and the tillage operation was at a speed of 3.5 to 4.0 mph. Winter wheat was seeded in a 7-inch row spacing with a conventional John Deere double disk drill at 2 to 5 mph. A 5-bar spike-toothed harrow was attached to the drill.

Under minimum tillage, a chisel with 3.25-inch wide twisted shanks on 12-inch spacings was operated at an 8to 9-inch depth at 3.0 to 4.5 mph. Implements for secondary tillage and seeding were the same as for conventional tillage.

Although a no-till drill is not used after small grains in the 1PM study, it was added in this research project for comparison with the other tillage systems. The no-till drill is the USDA-III (yielder) with 5:15-inch paired-row spacings. The double disk openers place seed at 2 inches depth and fertilizer at 5 inches depth between the 5-inch rows.

Fig. 1 shows an example of the surface residue reductions of different tillage implements where winter wheat follows small grains under three tillage systems, The results are an average from winter wheat and spring wheat stubble. For comparison, a 30 percent surface cover (about 500 pounds/acre residue) is indicated on the figure. This is a nationally-accepted, generalguide as the minimum surface cover to qualify as a conservation tillage system, such as minimum or reduced tillage, or no-till.

The conventional tillage system beginning with the moldboard plow is the only tillage system which did not maintain at least a 30 percent surface cover. The two cultivator/ harrow operations did not reduce the surface residue level after the moldboard plow as much as after the chisel. This is because the cultivator brings to the surface about as much plow-buried residue as the cultivator/harrow operation buries.

Winter Wheat After Spring Peas

When winter wheat follows low-residue crops, such as spring peas or lentils, tillage system selection is critical for maintaining adequate surface cover, In the 1PM study, two tillage systems were compared with winter wheat after spring peas. Under conventional tillage, a tandem offset disk was used as the primary tillage implement instead of the moldboard plow. Disks were 20 inches in diameter and spaced 7.5 inches on center. Disk shafts were angled 72 degrees from the direction of travel and there was with a 36 degree angle between disk shafts. Secondary tillage and seeding operations were the same as for winter wheat after small grains. The no-till treatment was seeded with the USDA-III no-till drill.

Conventional tillage almost totally incorporated the surface residue with only 4 percent surface cover remaining after seeding (Fig. 2). The USDA-III also buried a considerable portion of the surface residue but still maintained more than the 30 percent surface cover needed for conservation tillage systems.

Residue Effectiveness

Researcher and producer experiences demonstrate that maintaining a portion of the residue from the previous crop on the soil surface can control soil erosion. But how much residue is needed to effectively control erosion? High amounts of residue are not needed and can make tillage operations and seeding difficult. Excess residue may also interfere with herbicide efficiency in weed control and increase the potential damage from other crop pests.

Through research over the past 10 years at the Palouse Conservation Field Station near Pullman, MCCOO1 and other researchers are developing a relationship between surface residue level and reduction in soil erosion in the Northwest (Fig. 3). Experimental plots at the Field Station are 76 feet long on a southeast aspect and have a 20 to 25 percent slope.

Under these experimental plot conditions, about 1,000 pounds/acre of surface residue (about 50 percent surface cover) reduced overwinter soil erosion by an average of 92 percent compared to soil erosion with no surface residue. Erosion protection increased sharply from O to 1,000 pounds/acre surface residue. With more than about

1,000 pounds/acre surface residue, there was only a small increase in soil erosion protection. The potential for soil water storage overwinter may increase with increasing surface residues, however.

McCool points out that the minimum amount of residue needed to effectively control erosion depends on slope length, slope steepness, soil texture, surface roughness, weather condition and other factors. For example, slopes longer than 76 feet and/or steeper than 20 percent may require more than 1,000 pounds/acre, whereas gentler slopes less than 20 percent may need only 500 to 1,000 pounds/acre. Surface roughness can substitute for a portion of the surface residue. The local Conservation District can assist producers in determining the amount of surface residue needed under their farming conditions.

Surface residue should not be an “all or nothing” situation. High surface residue levels can create production problems for the following crop. On the other hand, burying all the residue leaves the soil vulnerable to severe topsoil loss by erosion. Using up-to-date management technology, intermediate levels of residue can generally be maintained on the surface to control erosion and still maintain or improve production potential.