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PNW CONSERVATION
TILLAGE HANDBOOK SERIES
Chapter 2 - Systems and Equipment, No. 7, Summer 1987
How
Much Surface
Residue Is Enough?
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. Average amount of residue remaining on the soil surface and
percent surface cover after each tillage operation for winter wheat seeding
after winter wheat and spring wheat under three tillage systems at the
1PM Research Site near Pullman, WA, 1988 (McCOOL, USDA-ARS).
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, general
guide as the minimum surface cover to qualify as a conservation tillage
system, such as minimum or reduced tillage, or no-till.
Fig. 2. Average amount of residue remaining on the soil surface and
percent surface cover after each tillage operation for winter wheat seeding
after spring peas under conventional tillage and no-till systems at the
1PM Research Site near Pullman, WA, 1986 (McCOOL, USDA-ARS).
Fig. 3. Reduction
in soil erosion with surface residue compared to zero surface residue
at the Palouse Conservation Field Station near Pullman, WA (McCOOL USDA-ARS).
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.
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