Tillage and Stubble Management for Water Conservation

Chapter 3 — Residue Management, No. 17, July 1991

Don Wysocki and Roger Veseth

Water is the resource that most often limits crop production over the inland Pacific Northwest cereal producing region. Most management practices or decisions are directly or indirectly influenced by crop available water. Decisions on fertilizer application rates, seeding dates and seeding depth, type and depth of tillage practices, crop rotation, weed management and residue management are partially determined by available water or influence the amount of water stored. The development of rotation and tillage and residue management practices in both conventional and reduced tillage systems have focused on conserving plant available water.

One objective of reduced tillage farming systems is to control soil erosion by water management. Conservation tillage reduces erosion by slowing runoff and increasing infiltration. Because less water leaves the land, more is retained in the soil. One economic incentive for adopting conservation tillage is increased water storage. Throughout the history of crop production in the dryland PNW growers have recognized the relationship between soil erosion control and water conservation and importance of water conservation to crop production. Producers have the greatest influence on water management and water conservation through tillage and residue management practices.

Precipitation, surface wetting, surface pending, infiltration, percolation, water storage, runoff and evaporation are the physical processes of the water cycle. All of these processes take place at the surface of the soil. Growers can use tillage and residue management practices to partially control surface pending, infiltration, runoff and evaporation. Tillage and residue management are used to change the soil surface conditions for optimum water conservation.

The ideal surface conditions for water conservation change over the fallow-crop cycle. In the winter, surface conditions which favor infiltration and surface storage and oppose runoff are desirable. In the summer of the fallow period, conditions that minimize evaporations are desirable. Unfortunately the surface conditions for each are not the same. Those that favor infiltration and storage also favor evaporation.

Properly timed tillage is used to change soil conditions as the season changes from wet to dry for optimum water conservation. In addition to timing of tillage, growers need to assess depth and cost of tillage. Tillage should create the desired surface condition for the least cost and least soil disturbance. Growers need to assess not only timing of tillage, but also depth and cost.

When choosing tillage and residue management options growers should assess benefits of soil and water conservation in relation to weed and disease management. Tillage tools and residue handling techniques should be selected by comparing the benefits of improved soil and water conservation with weed and disease management needs and options. To make this comparison, growers need to know how much water is stored under different tillage and stubble management practices. To gather this data, Dr. Robert Rarnig, STEEP researcher and USDA-Agricultural Research Service Soil Scientist, and Les Ekin, Agricultural Research Technician, at Pendleton, OR, studied the effect of different tillage and stubble management practices on water conservation and storage in a winter wheat-fallow cropping system.

Study of Tillage and Stubble Management Treatments

Ramig compared water storage in six stubble and tillage management systems in fallow-winter wheat-fallow rotation (Table 1) at Moro and Pendleton, OR from 1978-84. Both studies were conducted on 6-foot deep Walla Walla silt loam soils on 3 to 5 percent slopes. Mean annual precipitation at Moro and Pendleton is 11.5 and 16.4 inches, respectively. All treatments received a glyphosate application in March of the fallow year. Autumn stubble treatments were performed in early September. Plots were sweep plowed 4 to 5 inches deep or moldboard plowed 6 to 7 inches deep in early March at Moro and early April at Pendleton. Rodweeding was performed as many times as necessary over the summer. Plots at Moro required one or two rodweedings, while plots at Pendleton required two or three rodweedings. Residue ground cover was not measured as part of this study, but estimated residue cover after seeding is presented to provide an indication of erosion protection. Straw production at Moro is 3,000 to 4,000 pounds/acre and at Pendleton is 7,000 to 9,000 pounds/acre. Winter wheat was seeded in late September or early October using an International Harvester deep furrow drill.

Table 1. Stubble and tillage treatment in winter wheat-fallow cropping system at Pendleton and Moro,OR, 1978-84.

Soil water content was measured monthly or biweekly during the growing season using the neutron probe. The experiment was conducted through five fallow-wheat cropping cycles. Based on soil water recharge or use, the 24-month fallow-wheat crop period was separated into four intervals: fallow winter, fallow summer, crop winter and crop summer. Soil water is recharged during the fallow winter, partially lost by evaporation during the fallow summer, again recharged during the crop winter, and depleted by the crop and lost by evaporation during the crop summer. Table 2 shows the duration of each interval at Moro and Pendleton.

Using data from Moro, let’s look at each one of these intervals more closely to see the influence of stubble and tillage treatments on water gain or loss.

Fallow Winter – This is a period of soil water replenishment. At the beginning of this period, Ramig indicates that the 6-foot soil profile is usually dry to the wilting point to a depth of 5 feet and contains a small amount of plant available water in the last foot. The dry soil is wetted as water is added by winter precipitation.

How much of the winter precipitation can be stored? Table 3 shows water storage at Moro as a percent of precipitation falling during the fallow-winter period for each stubble and tillage treatment for five fallow-wheat cycles. Several water storage trends can be observed. First, approximately 65 to 80 percent of the water that falls during this period is stored in the soil. Second, each crop cycle has different water storage efficiencies. Lastly, stubble and tillage management treatments influence differently in different years. The total precipitation over the period is presented at the bottom of each column.

The LSD value can be used to identify treatments that are statistically different. For instance, in the 1978-80 cycle NS means treatments have values that are not statistically different. The 1979-81 cycle has values that must vary by six or more to be different. Variation in water storage among treatments can be explained by the type of surface and weather conditions that existed during the fallow winter. Because the plots were on gentle slopes the differences in water storage are due mostly to evaporative losses and retention or loss of snow and not runoff.

Ramig reports that the most efficient water storage occurred during the 1980-82 and 1981-83 cycles. He notes that fall burning resulted in lower water storage than other treatments. Water storage during the fallow winter of the 1979-81 cycle was greatest where the stubble had been fall flailed, fall disked or fall chiseled. This season was colder than normal in January. Chiseling may have aided water infiltration into frozen soil and residue reduced the depth or duration of soil frost.

Table 2. Duration of fallow winter, fallow summer, crop winter and crop summer based on soil water gain or loss, at Moro and Pendleton, OR (Ramig, USDA-ARS, Pendleton).

Table 3. Percent of September-February fallow winter precipitation stored in soil under stubble and tillage treatment during fallow winter wheat rotation at Moro, OR (Ramig, USDA-ARS, Pendleton).

Ramig states that the 1982-84 fallow winter had above normal precipitation as snowfall, but winds blew much of the snow off of the flailed, burned and disked treatments resulting in lower water storage. In open, windy winters water conservation is increased by having residue cover, which acts as a barrier to evaporation and acts to trap snow. When runoff is more of a problem than in this study (sloping landscapes or in environments where soil frost is more prevalent) chiseling is likely to be a more effective water conservation practice.

Fallow Summer –The transition from fallow winter to fallow summer is marked by the change from water storage to water loss. During the fallow summer all precipitation that falls is usually lost by evaporation. An additional 1 to 1.5 inches of water stored during the fallow winter is also lost. Table 4 shows water storage at Moro as a percent of precipitation falling during the fallow summer for each stubble and tillage treatment over five fallow-wheat cycles. Differences in water loss are related to the type of soil surface conditions during the fallow summer. Precipitation that fell over the period is shown at the bottom of each column, along with the LSD value for interpreting statistical difference.

Ramig points out that the least amount of water was lost by the chisel treatment in the 1978-80 crop cycle. Water from a heavy, early-summer thunderstorm was trapped by chisel grooves not yet closed by secondary tillage. This water ran off other treatments.

Usually, the fall burned treatment lost the least amount of water. Ramig explains that this result occurred for two reasons. First, fall burn treatments stored less water and therefore evaporative losses were less. Second, when residue is absent secondary tillage produces a fine “dust mulch, ” which acts as a more effective evaporation barrier. During the fallow summer at Moro, expect to lose all water that falls (7.1 inches) plus another 20 percent or about 1.3 inches, for a total of 8.4 inches.

Crop Winter – As the season progresses from fallow summer to crop winter, precipitation is again stored as soil water. Table 5 shows water storage at Moro as a percent of precipitation falling during the crop-winter for each stubble and tillage treatment for five fallow wheat cycles. Crop winter precipitation and percent LSD values are shown at the bottom of each column. Wider variation in water storage was observed during this interval than previous intervals. Ramig speculates this variation is due to differences in soil surface conditions from secondary fallow tillage, seeding of winter wheat and open crop canopy during the crop winter.

Crop winter water storage average over all crop cycles for all tillage treatments was 54 percent. This is much lower than the average of 75 percent for the fallow winter. Ramig explains this difference by reduced water infiltration in the crop winter compared to the fallow winter and increased evaporation. The soil is wetter during the crop winter, which slows infiltration. Soil surface conditions are generally poorer for water infiltration because of pulverization of soil structure during fallow tillage. Freeze-thaw and wet-dry cycles also contribute to reduced water infiltration during the crop winter. Evaporation is greater during the crop winter because of less soil surface cover than during the fallow winter. The average crop winter water storage was highest in the fall chisel treatment. Rarnig indicates that deep chiseling can increase infiltration. Because water storage efficiency during the crop winter is low, Ramig believes water storage can be improved. He feels that we can achieve the same degree of storage as occurs in the fallow winter. Data presented from Moro for each of the three intervals serves to illustrate the water conservation principle for the PNW.

Table 4. Percent of March-October fallow summer precipitation stored in soil under stubble and tillage treatment during fallow winter wheat rotation at Moro, OR (Ramig, USDA-ARS, Pendleton).

Table 5. Percent of November-February crop winter precipitation stored in soil under stubble and tillage treatments during fallow winter wheat rotation at Moro, OR (Ramig, USDA-ARS, Pendleton).

Water Conservation Over Fallow-Crop Period

The water storage period for the fallow-wheat cropping sequence begins as fall rains wet the soil profile after wheat harvest and continues through spring of the crop year. Water is stored during the fallow winter, lost during the fallow summer and stored again during the crop winter. Water stored in the soil at spring of the crop year plus precipitation during the growing season is the water available for plant growth and crop production.

Tables 6 and 7 show water storage at Moro, and Pendleton, respectively, as a percent of precipitation falling during the 18-month storage period for each stubble and tillage treatment for five fallow-wheat cycles.

Ramig made the following observation at Moro: the average water storage ranged from 43 percent for fall chiseling to 38 percent for fall disking. In two of the five crop cycles fall chiseling stored the most water, fall disking or burning generally stored the least water, and the average water storage for all treatments and all fallow-wheat cycles was 40 percent.

At Pendleton he made these observations: during one in five crop cycles fall chiseling and standing wheat stubble each stored the most water, fall flailing and fall burning generally stored the least water, and water storage for the entire storage interval for all treatments and fallow-wheat cycles was 34 percent.

Summary

Water conservation is an important aspect of dryland farming in the PNW. Soil and water conservation are complementary practices. Growers perform tillage and residue management practices to keep water where it falls, to maximize stored soil water and minimize water loss from the soil profile.

Precipitation, surface wetting, surface pending, infiltration, water storage, runoff and evaporation are physical processes in water conservation, Water conservation practices are implemented to minimize or maximize the physical processes. Tillage and stubble management are the primary practices that growers use to conserve water. These practices also influence soil erosion, diseases, weeds, seed bed conditions and performance of seeding equipment. Developing tillage and stubble management practices to optimize water conservation and other factors is the goal of growers. Unpredictability in precipitation and weather conditions make the choice of tillage and stubble management difficult. The choice of practices would be easier if we could predict conditions over the storage period. Grow growers must now select practices based on average conditions (precipitation) or extreme conditions (frozen ground runoff events).

Table 6. Percent of 18-month precipitation stored by stubble and tillage treatment in fallow-winter wheat rotation at Moro, OR (Ramig, USDA-ARS, Pendleton).

Table 7. Percent of 18-month precipitation stored by stubble and tillage treatment in fallow-winter wheat rotation at Pendleton, OR (Ramig, USDA-ARS, Pendleton).

Data presented show little difference in water storage among the stubble and tillage practices when averaged over several crop cycles, however specific conditions in any one year can favor one set of practices over another. Practices which retain stubble over the fallow winter and minimize tillage during the fallow are least costly because fewer operations are involved. Chiseling in the fall effectively controlled winter runoff. Chiseling is an effective practice on sloping landscapes where runoff from frozen ground is expected. Secondary tillage in the spring must close the chisel marks to limit evaporation losses. In areas of shallow soil options such as annual cropping or fallow every third year need to be considered. In general a fallow winter wheat cropping system will store 70 percent of the water during the first winter and about 50 percent during the second winter. If storable water exceeds the storage capacity of the soil can this water be more effectively used without fallow? Water storage during the crop winter is only about 50 percent of the precipitation. Practices that encourage improved water efficiency during the crop winter should be considered. The choice of tillage tools and residue handling techniques is determined by comparing the benefits of better soil and water conservation with existing weed and disease pressures and selecting the best options available.