Fallow Systems for Semi-arid Eastern Oregon and Eastern Washington

Chapter 2 – Systems and Equipment, No. 6, Summer 1987

Don Wysocki

Maximizing the storage of water in soils is important for crop production and erosion control. In semiarid eastern Oregon and eastern Washington, the practice of summer fallowing is used to store winter precipitation and control weeds. An added benefit is the release of plant nutrients through mineralization of organic matter. Fallow is prepared and maintained by various tillage practices and/or herbicide applications. Because the objective of fallowing is to conserve and store water, the tillage practices and/or herbicides utilized should maximize water conservation and storage, but should also be cost efficient. Water storage will be maximized by those practices that permit the greatest amount of water to infiltrate during periods of precipitation and the least amount of water to evaporate or transpirate during dry periods. Selection of the types of tillage equipment and herbicides employed and the timing and frequency of tillage and herbicide applications are the management tools that can be adjusted to maximize water storage and minimize costs.

STEEP researchers Robert Ramig, USDA-ARS soil scientist, and Les Ekin, USDA-ARS research technician, at the Columbia Plateau Conservation Research Center at Pendleton, OR, have evaluated, over a 5-year period, six tillage and tillage-aid herbicide systems for fallow in a wheat-fallow rotation. The research was conducted about 12 miles west of Pendleton on a Ritzville soil with a maximum of 10 percent slope. The systems evaluated beginning with standing stubble after harvest were:

1. Let wheat stubble stand over winter and moldboard plow in March to a depth of 6 to 7 inches. Spring tooth and rodweed as necessary (4 to 5 times) to prevent weed growth.
2. Chisel wheat stubble in September to a depth of 14 to 15 inches with a chisel spacing of 24 inches. Spray Roundup in March to kill weed growth. Sweeprod (field cultivate with sweeps to a depth of 4 to 5 inches; rodweeder attachment on rear tool bar operated at depth of 3 to 4 inches) in mid-May to break capillary continuity between the surface and moist subsoil and to kill summer weed species, which emerged after spraying.
3. Disk wheat stubble in September to a depth of 4 or 5 inches. Spray Roundup and sweeprod as in system 2.
4. Let wheat stubble stand over winter and spray Round up in March. Sweeprod as in system 2. Spray seeded wheat with Sencor or Lexone in spring of crop year for control of downy brome.
5. Let wheat stubble stand over winter and sweeprod in March, Field cultivate and rodweed as necessary (4 or 5 times) to prevent weed growth. Spray Lexone or Sencor as in system 4.
6. Let wheat stubble stand over winter and spray Round up in March. Sweeprod as in system 2. No Lexone or Sencor used in seeded wheat.

All herbicides were applied according to labeled directions and rates. Wheat was seeded at a rate of 60 pounds/acre in 16-inch rows with a John Deere HZ drill about October 1. Hyslop wheat was grown 2 years, followed by 1 year of Fielder and then 2 years of Stephens. Broadleaf weeds in the wheat crop were controlled with an application of 2,4-D amine and Banvel D in late March or early April. Precipitation was measured with a standard U.S. Weather Bureau rain gauge. Water content of the soil was measured in l-foot increments from the surface to bedrock (soil depths ranged from 6 to 9 feet) using neutron moderation. Soil water measurements were made: (1) after wheat harvest, (2) about March 1 the following spring, (3) about November 1 (after seeding), (4) about March 1 of the crop year and (5) again after harvest.

Water Storage and Crop Yield

The precipitation during the five 19-month fallow seasons (from wheat harvest to March 1 of the crop year) averaged 18.0 inches. Water storage among all systems when averaged for the 5-year study was nearly the same (Table 1). Generally, on a single fallow season basis, fall chiseling (system 2) or fall disking (system 3) stored the least water, while spring plowing (system 1) or mid-May sweep rodding (systems 4 and 6) stored the most water. During one fallow season, fall chiseling (system 2) stored the greatest amount of water. This occurred as a consequence of frozen soil. Runoff water was unable to enter the soil except where chisel grooves penetrated below the frozen layer and permitted infiltration. Ramig points out that in areas where frozen soils are more prevalent than at this study site, fall chiseling may store greater amounts of water more frequently.

Table 1. 5-year average of soil stored precipitation and wheat yields for six different fallow systems (Ramig, USDA-ARS, Pendleton).

Average yields for the 5-year period are shown in Table 1. Highest yields resulted under systems 1 and 4 because of better control of downy brome by plowing and frequent tillage or application of Roundup on fallow and Lexone or Sencor on the wheat, Lowest yields occurred under fall disking (system 3), Soil treated by this system had lower yields probably because of the lack of standing stubble to trap snow or slow evaporative winds. Yields closely corresponded to the amount of stored water when downy brome was controlled.

Other observations made by Ramig and Ekin were that sweeprod tillage in mid-May incorporated viable downy brome seed into the soil, and it emerged with the seed wheat in the fall. Application of Sencor or Lexone in early March of the crop year selectively controlled the problem. The difference in yield between systems 4 and 6 results from competition by downy brome.

Sweeprodding in mid-May without prior application of Roundup (system 5) failed to adequately control downy brome even with the subsequent tillage and application of Sencor or Lexone during the crop year,

Water Use

Averages of 3 years of data on water use efficiency are presented in Table 2, Data for 2 of 5 years have been excluded because of drought and winter kill problems. The winter wheat crop removed nearly the same amount of water from all fallow systems. The amount of water used by the crop includes that extracted from the soil plus that which fell during the cropping season. This is shown in column 3 of Table 2. Water use efficiency is shown in column 5 of Table 2. Water use efficiency is computed by dividing the yield per acre by the total amount of water used. The fallow systems that produce the highest yields had the highest water use efficiencies.

Table 2. 3-year average of water use, wheat yield and water use efficiency for six different fallow systems (Ramig, USDA-ARS, Pendleton).

Table 3. Estimated plant residue remaining on the soil surface after seeding for six different fallow systems (Ramig, USDA-ARS, Pendleton).

Residue Cover

Crop residues that remain on the soil surface or partially incorporated enhance water infiltration and protect against soil erosion. Visual estimates of the percent of plant residues remaining on the soil surface after seeding are shown in Table 3. As might be expected, those fallow systems that included the least tillage had the most residue remaining on the surface after seeding.

Summary

Ramig draws the following conclusions from his study,

1. In the semiarid (less than 12-inch annual precipitation) areas of eastern Oregon and eastern Washington, fall chiseling or disking stubble stored less water than systems that allow the stubble to stand over winter.
2. Fall chiseling stored the most water during one cold winter when frequently frozen soil prevented infiltration of water in other systems.
3. Sweeprod systems (systems 4 and 6) required the fewest inputs, conserved the most water, produced the highest yields, had the highest water use efficiencies and left the most residue on the soil surface.
4. In a sweeprod system: wheat stubble is left standing over winter to trap snow and insulate the soil from freezing and evaporation; spring plant growth is killed with an early March application of Roundup; a single sweeprod tillage is performed in mid-May to early June to interrupt capillary loss of water from the soil and kill late emerging summer weeds; and 30 to 40 percent of residue remains on the soil surface after seeding. If rains of more than 0.30 inch occur, an additional rodweeding is required for supplemental weed control.

If carefully extrapolated, it is the author’s opinion that the results of this study have wide application because Ritzville soils occupy about 1.5 million acres in eastern Oregon and eastern Washington. The results of this study apply most closely to Ritzville soils, on landscapes of 10 percent slope or less, which do not remain frozen for extended periods during the winter. Timing of tillage operations and/or herbicide applications for fallow in some areas of Ritzville soils may vary slightly from those used in this study because of geographic differences or fluctuations in local weather patterns.

Use of Trade Names

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide. Always read and follow label instructions carefully. To simplify the information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.