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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 3 - Residue Management, No. 18, July 1992


Tillage and Residue Management Strategies for Variable Cropland

Roger Veseth, Keith Saxon and Don McCool *

*Washington State University/University of Idaho Extension Conservation Tillage Specialist, Moscow, ID; and both USDA-ARS Agricultural Engineers, Pullman, WA.

Landscapes and soil properties can be highly variable within fields in the Northwest cropping region. Production limitations, yield potentials and the need for associated production inputs typically vary with changes in landscapes and soils. In the past, even variable fields have often been farmed with uniform production practices. In the 1990s, however, increasing environmental concerns and the need for improved production efficiency are demanding more precise farming of variable fields. Making adjustments in tillage, residue management and production inputs for precision farming of variable cropland can offer substantial opportunities to simultaneously improve production efficiency, profitability and resource protection.

Water is one of the most limiting crop production factors in much of the Northwest. Within fields, water is typically most limiting in areas with shallow soil, particularly on ridgetops and upper slopes, where crop residue production is low and soils are usually least conducive to water infiltration and storage. Management practices that improve water storage in these critical areas can significantly improve yield potential and profitability. Fortunately, saving water for crop production and controlling erosion generally go hand in hand. Variable tillage and residue management practices that focus on water storage also offer improved protection from soil loss by water and wind erosion, and associated pollution problems.

In contrast to ridgetops and upper slopes, the bottomland and lower slope areas have a lower erosion potential and often water is not the most limiting yield factor. Production problems due to excessive amounts of residue, more intensive weed and disease pressures, and wet soils can often be more yield-limiting than water availability. More intensive residue incorporation with tillage, or partial surface residue removal, might be more appropriate and profitable management practices in these areas.

The need for variable tillage and residue management systems within fields will vary across the region and for each grower and field. Following are examples of contrasting production goals that might be accomplished in the same field using variable tillage and residue management practices.

1. Increase water storage with surface residue and other practices in areas where water is most limiting.

2. Control soil erosion with surface residue and other practices on areas that are most vulnerable to erosion, sustain low residue production and benefit most from organic matter contributions to improve soil quality.

3. Accelerate soil drying and warming in wet areas that have excessive residue production and low erosion potential.

4. Reduce weed and disease problems with more intensive incorporation and/or removal of residue in areas where pest problems are more yield-limiting than water availability.

The greatest need for change in management strategies is to use tillage and residue management to increase water storage and erosion protection where most needed within fields. Intensive tillage practices for accelerating soil warming and drying, and reducing weed and disease problems have always been a part of conventional tillage systems.

Begin Residue Management at Harvest

Successful residue management systems begin at crop harvest — the first opportunity to begin processing and distributing residue. This becomes particularly important on upland field areas where more surface residue needs to be maintained with fewer tillage operations.

The need for combine attachments or modifications for uniform straw and chaff distribution depends on the tillage system planned for the next crop. With no-till and minimum tillage systems, uniform combine residue distribution can greatly improve crop establishment and yield potential. Even though combine residue distribution in high residue crops is a major concern, this can also be an important consideration when harvesting low residue producing crops, such as dry peas or lentils.

Principles of Water Storage and Erosion Protection

Water Storage Principles

About 60 to 75 percent of the region's annual precipitation usually occurs during November through March. Consequently, soil water storage during this period is essential to make effective use of the annual precipitation for crop production.

Grower experiences and numerous research trials in the Northwest have shown that retaining more crop residue on the soil surface increases capture of available precipitation where it falls, enhances infiltration into the soil, and reduces water loss by evaporation and runoff. There are several important hydrologic processes to consider for improving soil water storage in a variable tillage and residue management strategy. These include snow trapping, water infiltration, surface runoff and evaporation.

Snow Trapping - Where snow is an important part of the available water, maintaining at least a portion of the crop stubble standing overwinter cart retain more snow where it falls. This is particularly important on ridges and upper slopes that are normally blown free of snow and water is typically most limiting. This would also reduce the formation of large snow banks on the leeward side of ridges, which commonly reduce yields through delayed spring field operations, reduced nutrient availability or adverse effects on fall-seeded crops. Excessive surface runoff from snowbank melt and associated soil erosion are also concerns. Both the stubble and snow cover can reduce the depth and frequency of soil freezing, thus helping maintain water infiltration rates and reducing runoff potential on frozen soils. Similarly, winter crops are more protected from harsh winter weather, reducing winterkill and frost heave damage.

Infiltration and Runoff - Two important management options can affect the rate of water infiltration and reduce the potential for surface runoff overwinter. The first is to maintain a portion of the previous crop's residue on the soil surface. It helps protect the soil from the impact of raindrops that disperse soil aggregates and cause soil surface sealing. Crop residues slow water movement across the soil surface and allow more time for infiltration. As with snow cover, residue also reduces soil freezing.

The second management option is through tillage operations that roughen the surface, and fracture and loosen the soil. Surface roughness, like surface residue, helps slow water movement over the surface and allows more time for infiltration. In addition, it also has the advantage of increasing the proportion of large soil pore spaces (macroporosity), which can significantly increase water infiltration rates into the soil. This more rapid downward drainage within the soil reduces runoff and evaporation losses.

The importance of tillage to complement the effects of surface residue for increasing water infiltration and reducing runoff will depend on many factors, including the degree of soil compaction, soil texture, organic matter content, degree of soil aggregation, freezing depth and frequency of runoff events on frozen soils. These factors vary across field landscapes and locations and with field history. They need to be carefully evaluated when developing variable tillage and residue management systems.

Evaporation - Evaporative loss of water is relatively low over winter compared to that during the dry warm summers. Still, maintaining more crop residue on the soil surface has been shown to substantially increase overwinter soil water storage potential through reduced soil water evaporation and snow evaporation (sublimation). Furthermore, reduction of evaporative water loss is most critical in field areas where water is most limiting. Tillage operations and residue removal practices reduce surface residue cover and increase the potential for water loss from evaporation.

Water Storage Examples

It is often difficult to predict when one management practice may store more soil water than another practice because numerous soil, management and climatic factors can influence the results. However, by having a basic understanding of the principles that affect water storage — snow trapping, water infiltration into the soil, surface runoff and evaporation - growers can begin to evaluate alternative tillage and residue management strategies to improve soil water storage where it is most needed on variable cropland.

Several projects across the Inland Northwest have shown that overwinter water losses from a bare soil surface, where cereal stubble was removed or incorporated by tillage, can commonly reduce soil water storage by 1 to 2 inches or more compared to where the stubble was left standing. Fall chiseling or other tillage operations that leave a rough surface and retain much of the surface residue can increase overwinter water storage compared to undisturbed stubble if surface runoff occurs on frozen soils. Without surface runoff on frozen soil, or reduced infiltration potential due to soil compaction, tillage generally does not increase overwinter water storage over that stored with undisturbed surface residue alone.

Separately, surface residue and tillage can both influence soil water storage and runoff potential. Under some conditions, combinations of surface residue and tillage can increase soil water storage beyond the individual effects of either alone. This is particularly important when soils have low infiltration rates due to compaction or during heavy precipitation or rapid snowmelt on frozen soils.

Fig. 1 provides a good example of the complementary benefits of both surface residue and tillage in a 14-inch precipitation zone. The Paratill chisel (formerly called Paraplow) was used as the tillage implement. It leaves the soil surface and stubble relatively undisturbed while creating macroporosity to a depth of about 15 inches. A standard chisel with narrow straight points could provide a similar water infiltration potential, although evaporative water loss would be slightly higher because more soil is exposed.

Fig. 1. increased soil water storage from November through March, 1983-88, under standing, chopped and burned cereal stubble both with and without tillage 15 inches deep with a Paratill chisel (formerly Paraplow) 30 miles west of Colfax, WA (Saxton-ARS, Pullman).

In the 3-year study, 2 years of research were conducted on spring barley stubble and 1 year on winter wheat stubble. Use of the chisel in standing stubble provided 1.7 inches of additional water storage compared to untilled standing stubble. Chiseling after stubble burning increased water storage by 1.1 inches compared to the burned untilled treatment – the tillage effect alone. Chiseled standing stubble stored 2.8 inches more soil water than where the field was burned and then chiseled. Similarly, untilled standing stubble stored 2.2 inches more water than the untilled burned treatment — the residue effect alone. The largest difference was the combined effect of tillage and residue where the chiseled-standing stubble had 3.9 inches more overwinter water storage than the untilled, burned treatment. Water storage under chopped stubble was similar to that under standing stubble in both tillage situations. Tillage and residue management practices that increase overwinter water storage can significantly increase crop yield potential, particularly in the dryer regions and dryer portions of field landscapes.

Fig. 2 shows another example of the combined residue and tillage benefits in soil water storage and crop use. This research was conducted in a 20-inch precipitation zone near Pullman, WA. Minimum tillage systems, with the chisel as the primary tillage implement, increased water availability for winter wheat production after spring barley, spring peas and spring wheat compared to low-residue, moldboard plow-based systems. The additional water increased winter wheat yields approximately 3 to 15 bushels per acre.

Using surface residue and tillage to increase soil water storage offers the greatest benefits on ridgetops and steep slopes where water is most limiting. As an example, soil water storage and yields of winter wheat under minimum tillage and no-till systems were compared to conventional tillage on three slope positions near Colfax, WA (Table 1). As might be expected, minimum tillage and no-till both resulted in an increase in water storage on the mid-slope and ridgetop positions compared to the moldboard plow system. Little difference in water storage was found between the tillage systems on the lower slope. Yield responses to additional soil water on the mid-slope and ridgetop in the experiment were influenced by other production factors, and were not the typical 5 to 7 bushels per acre per inch of water.

Fig. 2. Comparison of winter wheat use of available water from April 1 to harvest under chisel-based and plow-based tillage systems after spring crops of barley, wheat and peas, 1988, Pullman, WA (Saxton-ARS, Pullman).

Soil Erosion

Tillage and residue management practices that focus on improving water storage generally are also the most beneficial for minimizing soil erosion. Many management options can help reduce the potential for soil erosion, including slope length reduction, cross slope or contour farming, crop canopy cover, surface residue, shallow incorporated residue and surface roughness from tillage practices.

Reducing the length of slope through the use of divided slope or field strip systems can be an effective component of conservation systems for variable cropland. Cross slope farming (farming operations performed along or near the contour of the slopes) also helps to reduce erosion potential and is a component of many farm conservation plans in this region. A growing fall-seeded crop can provide soil erosion control overwinter if adequate canopy cover develops before the critical winter erosion period. However, the dependability of crop canopy cover as a component of conservation systems is often limited by pest problems associated with earlier seeding dates, and by lack of seedzone soil water that delays germination and establishment.

Surface residue and surface roughness have been shown to be the two most effective management tools for erosion control and are important components of management strategies for variable cropland.

Surface Residue Cover - Research has shown that surface residue provides the greatest benefit for reducing soil erosion (Fig. 3). The surface cover factor on the vertical axis (from 0 to 1) provides an indication of the erosion potential with corresponding surface residue levels. The highest erosion potential would occur at a factor of 1, with no surface residue, and the lowest potential at 0, with 100 percent surface cover.

Even a relatively low surface residue cover can sharply reduce erosion potential. For example, increasing surface cover from 0 to 30 percent can reduce the surface cover factor (which approximates erosion potential) from 1 to about 0.2, an 80 percent reduction in erosion. In contrast, there is less increased erosion control benefit from each additional 10 percent increase in surface residue cover beyond about 40 to 50 percent cover. This testifies to the effectiveness of moderate, manageable levels of surface residue for erosion control.

Table 1. Increased soil water content in March and yield of winter wheat after spring wheat under no-till and chisel-based minimum tillage systems compared to low-residue, moldboard plow-based systems on three slope positions near Colfax, WA, 1980 (Schmick and Cochran-ARS, Pullman).

Slope

Position

Minimum Tillage No-till

Water Content

(inches/6 ft)

Yield

(bu/acre)

Water Content

(inches/6 ft)

Yield

(bu/acre)

Lower -0.6 -1 0.2 4
Mid-slope 2.1 2 3.7 10
Ridgetop 1.0 1 1.4 11

Surface Roughness - In addition to surface residue, surface roughness created by tillage operations can significantly improve soil and water management. Surface roughness slows surface water movement and increases the potential for infiltration into the soil, thereby reducing runoff and erosion potential. Chiseling, for example, creates an improved infiltration potential and slows runoff, while not incorporating an excessive amount of surface residue. Tillage roughness has been researched and classified to give proper credit in erosion prediction models that are being used by the Soil Conservation Service to assist growers in developing erosion control systems.

Fig. 4 shows the surface roughness factor (which also represents an approximation of the erosion potential) decreasing with increasing random roughness. Random roughness is a measure of the relative difference (standard deviation) between the average height and depth of the soil clods and the mean soil surface. For a reference, a random roughness of 1.5 to 2 inches would probably be about the highest that could be feasible after seeding with most double disk drills.

It is important to note that roughness is not nearly as effective for reducing erosion as is surface residue during the critical overwinter erosion period on seeded winter wheat. The effects of surface roughness for reducing erosion sharply decrease during the winter as the soil clods slowly break apart or "melt down, " reducing their effectiveness.

For example, a random roughness of 3 inches in the fall after the last tillage would provide a surface roughness factor of about 0.4, or a 60 percent reduction in erosion potential compared to a smooth surface. After 10 inches of rainfall over winter, the original random roughness of 3 inches would only provide a surface roughness factor of about 0.8, or a20percent reduction in erosion potential. This erosion reduction potential is equivalent to that of a l-inch random roughness beginning in the fall. In addition to the effects of precipitation itself, wetting-drying and freezing-thawing cycles also result in a gradual breakdown of soil clods.

Fig. 3. Surface cover effectiveness factor for erosion control in the Inland Northwest. The surface cover factor approximates the proportionate reduction in soil erosion from highest (factor of 1) to lowest (factor of 0) for each percentage increase in surface residue cover (McCool - ARS, Pullman, WA).

Achieving Tillage and Residue Objectives

Equipment Considerations

Tillage is the principal manipulator of residue. Almost any field operation, including seeding, will result in some residue incorporation. The primary tillage operation can often result in the most significant reduction in surface residue, and implement selection must be made to achieve the final residue level desired after seeding of the next crop. Inversion tillage implements, such as the moldboard plow and heavy disk, cause the most severe residue incorporation. However, with careful adjustment and use, they still can have application to conservation tillage systems, particularly in variable tillage and residue management systems. "Plowing uphill" (turning the plow furrow uphill) is also the only tillage operation that moves soil upslope. Tillage erosion from downhill plowing and other tillage operations over the years has significantly reduced topsoil depth on ridgetops and upper slopes.

Substantial improvements in water conservation and erosion control across variable cropland can be made with equipment that most growers already have. Frequently, minor changes in equipment selection, adjustment and operation are all that are needed. Some growers have chosen to make shop modifications of their present equipment. Others have opted for specialized commercial equipment for conservation tillage.

Some relatively new types of equipment, or at least those not considered standard equipment on most farms, may provide new management options to improve soil water storage, particularly for ridgetops and upper slope positions. An increasing number of types and brands of subsoiling or other deep tillage implements are on the market. Surface pitting implements, some also included subsoiling in the operation, are also becoming more common. There is growing interest in alternative practices for increasing water storage and reducing erosion in a variety of dryland cropping situations in this region. Their greatest advantage will probably be on field areas where water infiltration potential is limited by soil compaction or where runoff on frozen soils is a problem.

Fig. 4. Random Roughness Index (Inches) Surface roughness effectiveness factor for erosion control in the Inland Northwest. The surface roughness factor approximates the proportionate reduction in soil erosion from highest (factor of about 1) to lowest (factor of 0) for each incremental increase in random surface roughness after 0, 5 and 10 inches of precipitation (McCool - ARS, Pullman, WA).

Additional research is needed to help identify field conditions where these different implements are most beneficial and to develop management considerations for their use. On-farm tests with different subsoil tillage and surface pitting implements are currently underway as part of the STEEP II on-farm testing project and related projects in the Inland Northwest.

Residue Removal

Although tillage is the primary tool for managing residue, other practices might be part of the residue management strategy for variable cropland. Some of the excess residue produced on bottomland areas, where soil erosion potential is low, might occasionally be removed without significant detrimental effects. Where there are markets available, stubble could be clipped and baled. Combine residue-spreading attachments could also be disengaged or removed so that the straw and chaff rows are concentrated behind the combine for easier baling.

Burning has been used as a quick residue removal tool. Long-term repeated field burning along with tillage has been shown to be detrimental to soil productivity. However, burning might be a limited tool to occasionally manage excess residue and associated pest or production problems in wet bottomland areas. Care must be taken to avoid burning upland areas were water is more limiting to production, where erosion potential is higher and organic matter contributions are more critical to sustaining soil productivity. Increased public concern and strictness of air quality regulations must also be considered.

Scenarios of Variable Tillage

and Residue Management

Variable tillage and residue management approaches are increasingly being used within block farming systems on whole fields, as well as within divided slopes and field strips, and field divisions that specifically identify management units. The basic principles can also be adapted to much of the Northwest, spanning precipitation zones and topographic regions. However, the degree to which variable tillage and residue management within fields are needed and are possible will depend on each field and farm situation.

In some cases, the use of variable tillage and residue management practices within fields might apply only to the primary tillage operation, with all subsequent field operations the same across the field. In other situations, growers might maintain differences in practices up until seedbed preparation or planting of the following crop across the whole field. Finally, individual "management zones" could be continuously maintained within fields, such as with permanently divided slopes, field strips or other field divisions. The choice is up to the grower, with decisions based on differences in erosion and yield potentials, special production limitations, the particular layout and landscape of the field, ease of identi~ing the management units, travel distance between fields, available equipment and many other production considerations.

The final overwinter condition of the field is important for fall-seeded crops since this water storage period is critical to yield and erosion protection. Tillage and residue management decisions in the fall after harvest are also important for the next spring crop because they affect water storage and erosion potential overwinter and in the spring. Furthermore, they can affect water storage and erosion potential through the subsequent fall and winter when a fall-seeded crop will be planted. The following scenarios of variable tillage and residue management are presented to stimulate ideas for consideration and adaptation, and not necessarily for direct application.

Fig. 5. Possible scenario of fall tillage options on variable Northwest cropland after winter wheat going to spring peas, lentils or other spring crops.

Winter Wheat to Spring Crops

Where spring peas, lentils or other spring crops will be planted following winter wheat, growers might consider fall chiseling (or other non-inversion tillage operation) on the hilltops and upper slopes (Management Zone I of Fig. 5). In these areas, residue production is often low, soil erosion potential is high and yield is typically more limited by water availability than by pest problems. The remaining surface residue and rough, fractured soil after chiseling would effectively store overwinter precipitation, yet still allow early seedbed preparation and seeding. Depending on the type of chisel points, depth and spacing, speed of tillage and other factors, about 70 percent of the original residue cover would remain on the surface, a portion of which would be partially standing to aid in trapping snow.

On the wetter, lower slopes and bottomland areas (Management Zone II, Fig. 5), residue production from winter wheat is often significantly higher than in Zone 1, erosion potential is lower and yield is often more limited by pest problems and wet soil conditions than by water availability. More intensive tillage, possibly beginning with fall moldboard plowing, might help reduce excess residue levels, reduce problems from winter annual grass weeds and reduce the incidence of some cereal diseases that persist in the wheat straw, such as Cephalosporium stripe. Intensive fall tillage would also accelerate soil drying and warming in the spring to facilitate early seeding.

Another tillage and residue management approach for spring crops after winter wheat is shown in Fig. 6. This particular management strategy has been called the "Pennell System" after Roger Pennell, a Garfield, WA, farmer who has been developing it. Fall mulch tillage, with the chisel or cultivator and harrow, is used in Zone II after volunteer and weeds begin to grow. This tillage helps accelerate soil warming and drying in the spring, permitting an early seeding date. The stubble is left standing overwinter on Zone I to trap snow and store as much water as possible. This Zone extends low on dry south slopes but remains high on wet north slopes. A nonselective herbicide is applied in the fall to control volunteer grain and weeds. In the spring, a nonselective herbicide is applied to the whole field before direct seeding without prior tillage.

Fig. 6. Possible scenario of fall and spring management options on variable Northwest cropland after winter wheat going to spring grain or other spring crops.

Fig. 7. Possible scenario of fall tillage options on variable Northwest cropland after spring barley going to spring peas, lentils or other spring crops, or fallow.

Spring Barley to Spring Crops or Fallow

Spring barley produces much less crop residue than winter wheat and the residue decomposes faster. In a 3-year rotation, such as winter wheat-spring barley-spring pea/lentil or fallow, use of minimum tillage systems after spring barley is important in maintaining adequate surface residues to effectively store available water for the following crop and reduce erosion potential. Winter wheat planted on summer fallow after spring barley can be particularly vulnerable to soil erosion under intensive tillage systems. Growers should use every option to maintain as much barley residue through the fallow season and winter wheat seeding as possible. Maintaining part of the spring barley residue on the surface through a low residue spring crop and planting of the next winter wheat crop would also provide additional water storage and erosion control potential during that winter.

In areas particularly susceptible to erosion and water stress, like Zone I in Fig. 7, barley stubble could be left standing overwinter, possibly with fall subsoiling or surface pitting if there were problems with soil compaction or runoff events on frozen soils. Fall chiseling might be considered in Zone II, where the potential for barley residue production and winter annual grass weed problems are higher, and erosion potential is lower. Subsequent field operations could be the same on the whole field during the rest of the next cropping season.

Winter Wheat to Fallow

In dryer production regions under a crop-fallow rotation, the most critical erosion period associated with fallow is usually during the fail and winter after seeding winter wheat. However, there can be severe erosion during intense rain storms in the summer as well. Tillage and residue management practices in variable cropland should focus on the goal of maintaining adequate surface residue where needed, beginning after harvest at the start of the fallow year. To maintain more residue in the low-yielding, erosion-prone areas in Management Zone I (Fig. 8), wheat stubble could be left standing overwinter. Subsoiling, surface pitting or other tillage operations with minimal surface residue burial might help to further increase water retention and infiltration if there are problems with soil compaction or runoff on frozen soils (also see Fig. 6).

Fall chiseling might be considered in Zone II, where residue production is higher, erosion potential is lower and pest problems are possibly more limiting to yield than water availability. To help maintain more residue over the fallow period through winter wheat seeding, particularly on Zone 1, growers could consider early application of a nonselective herbicide as a substitute for early spring tillage operations, thus delaying the initial fallow tillage operation until later in the spring. The decision on when to begin fallow tillage would depend on soil texture, yearly weather conditions, weed problems and other factors that could affect seed zone water content at fall planting time.

Fig. 8. Possible scenario of fall and spring management options on variable Northwest cropland after winter wheat going to fallow in low precipitation zones.

Fig. 9. Possible scenario of fall management options on variable Northwest cropland after spring dry pea, lentils or other low residue producing crop going to winter wheat.

Low Residue Crops to Winter Wheat

When winter wheat is seeded after low residue producing crops such as spring dry peas or lentils, tillage and residue management decisions are critical to optimizing overwinter water storage and minimizing soil erosion. Direct seeding of winter wheat with no-till drills has worked effectively in some areas. Another option is a reduced tillage approach, commonly called" shank and seed, " where heavy duty, shank fertilizer applicators directly band fertilizer below the seeding depth without prior tillage, followed with a cultivator-rod weeder operation or nonselective herbicide before seeding. In some cases, the rougher, more porous surface created with shank and seed systems might reduce runoff and erosion more than with a smooth seedbed created with a double-disk no-till drill, under similar amounts of surface residue.

Under a shank and seed system on variable field landscapes, growers might consider direct-shanking of fertilizer on the entire field, then adjusting the next field operation according to the needs of specific management units (Fig.9). To maintain more surface residue on the erosion-prone, water-deficient areas of Zone I, a nonselective herbicide might be used to control volunteer and weeds before seeding. A field cultivator-rod weeder operation might be used on Zone II, where residue levels are higher and erosion potential is lower.

Conclusions

Differential tillage and residue management practices across field landscapes can optimize surface residue benefits where they are needed most, while minimizing associated production limitations where excess residue levels can be detrimental. The greatest benefit of surface residue and conservation tillage will be on field areas where water is most limiting to yield, and where soil erosion potential is greatest. Adapting tillage and residue management practices to variable cropland offers the potential for both improved profitability and resource protection.

Use of Trade Names

To simplify information, trade names have been used. Neither endorsement of named products is intended, nor criticism implied of similar products not mentioned.

     
 

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Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
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