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DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION

Minimum and Delayed Conservation Tillage for Wheat–Fallow Farming

Dep. of Crop and Soil Sciences, Washington State Univ., P.O. Box B, Lind, WA 99341

* Corresponding author (schillw{at}wsu.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Maintaining crop residue, clods, and roughness on the soil surface during summer fallow is critical for wind erosion control in the low-precipitation (<300 mm annual) dryland wheat (Triticum aestivum L.) production region of the inland Pacific Northwest, USA. Conventional farming practices are intensive, involving eight or more tillage operations during the fallow cycle. My objective was to evaluate fallow conservation tillage management systems for soil water storage, residue retention, surface and subsurface soil cloddiness, surface roughness, wheat stand establishment, and grain yield during 6 yr at Lind, WA. The soil is a Shano silt loam (coarse-silty, mixed, superactive, mesic Xeric Haplocambids). Treatments were (i) conventional (tillage), (ii) minimum (herbicides and tillage), and (iii) delayed minimum (herbicides and delayed tillage). Averaged over years, precipitation storage efficiency in the soil was 51, 54, and 57% over winter, and 24, 26, and 26% at the end of the fallow cycle, for conventional tillage (CT), minimum tillage (MT), and delayed minimum tillage (DMT), respectively. Surface residue and surface clod mass were consistently reduced by 45% or more in CT compared with MT and DMT. There were no differences among treatments in seed-zone water content at time of sowing in September nor in grain yield in any year or when averaged across years. Results show that the long-term practice of minimum and delayed minimum tillage during fallow significantly increased surface residue and clod retention for erosion control with no adverse agronomic affects compared with conventional tillage.

Abbreviations: CT, conventional tillage • DMT, delayed minimum tillage • MT, minimum tillage • SE, precipitation storage efficiency

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
CONVENTIONAL TILLAGE practices in the semiarid (150–300 mm annual precipitation) wheat– summer fallow region of the inland Pacific Northwest often bury excessive quantities of residue and reduce soil cloddiness and surface roughness. Blowing dust from fallow, or newly sown winter wheat after fallow, causes recurrent soil loss and air quality problems (Papendick, 1998). Conservation tillage has been advocated for several decades to control soil erosion (Horning and Oveson, 1962), but many wheat growers are reluctant to change tried-and-proven conventional tillage practices. Growers in the low-precipitation region generally do not practice no-till summer fallow because of increased evaporative loss of seed-zone soil water during the dry summer months compared with tillage (Hammel et al., 1981; Schillinger and Bolton, 1993). In Washington State, less than 25% of crop production is with conservation tillage compared with a national average of 36% (Conservation Technology Information Center, 1997). With Shano and Ritzville (coarse-silty, mixed, mesic, Calciorthidic Haploxerolls) soils, which cover a wide geographic wheat–fallow area, clods are frequently pulverized during tillage (Schillinger and Papendick, 1997). By the end of the 13-mo-long fallow period, the surface soil is often powdery and lacks residue cover.

Development and adoption of agronomically feasible and more environment-friendly fallow management methods are needed. The objective of the study was to determine the long-term effects of conventional, minimum, and delayed minimum tillage systems during fallow on soil water storage, residue retention, surface and subsurface soil cloddiness, surface roughness, wheat stand establishment, and grain yield.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
A 6-yr tillage management study was conducted from August 1993 to July 1999 at the Washington State University Dryland Research Station at Lind, WA. Annual precipitation averages 244 mm. The Shano silt loam soil is more than 2 m deep with less than 2% slope. The experimental design was a randomized complete block with three tillage treatments replicated four times. Individual plots were 18 by 46 m, which allowed use of commercial-size farm equipment. Wheat and fallow phases of the study were present each year.

Tillage Treatments and Field Operations
The three tillage management treatments were (i) conventional tillage (CT)—standard frequency and timing of tillage operations using implements commonly utilized by growers, (ii) minimum tillage (MT)—standard frequency and timing of tillage operations, but herbicides were substituted for tillage when feasible and a noninversion undercutter V-sweep implement was used for primary spring tillage, and (iii) delayed minimum tillage (DMT)—similar to minimum tillage except primary spring tillage with the undercutter V-sweep was delayed until at least mid May. A complete list of field operations and timing for each treatment throughout the study are shown in Table 1.

Minimum tillage treatments were sprayed with a nonselective herbicide for postharvest control of Russian thistle instead of tillage in August of 1993, 1994, and 1995, but not in 1996, 1997, and 1998 when Russian thistle were not present (Table 1). In November, the plots were chiseled to depths of 25 to 40 cm with straight-point shanks spaced 120 cm apart (twice the shank spacing of conventional tillage). Chiseling was not conducted in 1996. A rotary shark's tooth subsoiler that caused little residue disturbance and created one 40-cm-deep pit with 4-L capacity every 0.7 m² was used in 1997 and 1998 in lieu of chiseling. Glyphosate was applied in late winter followed by primary tillage at 13-cm depth with the undercutter equipped with overlapping 80-cm-wide V-blades spaced 70 cm apart. A rolling harrow was attached behind the undercutter to break up large clods and fill air voids. The plots were rodweeded three times at 10-cm depth during late spring and summer and fertilized with aqua NH₃–N injected between every other row of the John Deere HZ deep furrow drill when sowing winter wheat in early September. Though sowing depth varied each year depending on soil water content, it was always the same for each treatment.

The delayed minimum tillage treatment was identical to the minimum tillage treatment except that (i) plots were not chiseled or rotary subsoiled in 1996, 1997, and 1998; (ii) primary spring tillage was delayed until mid May or early June; and (iii) only two rodweedings were conducted during late spring and summer (Table 1).

To alleviate rough areas and other problems associated with conducting all tillage operations in the same track, the rodweeder implement was pulled perpendicular or at an angle to plot direction over the entire experimental area whenever weeding was required in all three treatments. All treatments were sown at the same time. Winter wheat stand establishment failed in September 1994 due to insufficient seed zone water and all plots were resown to hard red spring wheat (cv. Butte 86) at 67 kg ha^-1 in 0.15-m rows with a double-disc drill in March 1995. In-crop broadleaf weeds were sprayed with 0.5 kg a.i. ha^-1 bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) applied in March (winter wheat) or April (spring wheat) during tillering stage of wheat growth.

Measurements
Water measurements in the 180-cm soil profile were made immediately after grain harvest in late July to early August (beginning of fallow), in March prior to primary tillage, and again in late August to early September just before sowing winter wheat. Soil volumetric water content in the 30- to 180-cm depth was measured in 15-cm increments by neutron attenuation. Volumetric water in the 0-to 30-cm depth was determined from two 15-cm core samples using gravimetric procedures (Gardner, 1986). In addition, volumetric water content in the seed zone was determined in 2-cm increments to a depth of 22 cm just before sowing winter wheat in early September of 1994, 1995, and 1996 using an incremental soil sampler (Pikul et al., 1979). All soil water measurements were obtained from three locations in each plot.

Surface soil cloddiness was determined at the end of the fallow cycle in 1994, 1995, 1996, and 1998 by measuring the diameter of individual soil clods within a 1-m-diam. sampling hoop randomly positioned at three locations in each plot. Wheel tracks were avoided. All clods with diameters 5 cm were sorted into 1-cm size increments, and the mass of each size group measured in the field with a battery-powered digital scale. Clod mass was not measured in 1997 because clod structure was dispersed during an intense rain shower. Subsurface soil cloddiness was measured by gently dry sieving 0.01 m³ of soil from the 0- to 10-cm tillage mulch layer through stacked 5-, 2.5-, and 1.2-cm² mesh screens. Clods not passing through each of the three mesh screens were then weighed. Subsurface clod measurements were obtained from the same 1-m-diam. area where surface clods had just been removed, therefore surface clods 5 cm in diameter were excluded from subsurface samples. Oriented roughness in all plots was measured soon after sowing in September using the chain method (Saleh, 1993).

Surface residue remaining from the previous crop cycle was measured several times throughout the fallow period by clipping and gathering all aboveground dry matter within a 1-m-diam. hoop. Three samples were always obtained from each plot. Wheat straw and dead Russian thistle plants were separated, placed in paper bags, and allowed to air dry in a low- humidity greenhouse before weighing.

Winter wheat stand establishment was determined by counting individual plants in 1-m row segments 21 d after sowing. Three row segments were selected and marked within each plot prior to emergence of wheat seedlings. Grain yield was determined in mid to late July by harvesting a 6.1-m swath through each 46-m-long plot with a commercial-size combine and auguring grain into a weigh wagon.

Precipitation was recorded at a standard U.S. Weather Bureau shelter located <1 km from the experiment site (Table 2). A frost depth tube (McCool and Molnau, 1984) was installed in undisturbed winter wheat stubble near the weather shelter and freeze–thaw status of the soil was recorded daily throughout the winter.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Surface Residue
Residue at the beginning of fallow (just after harvest) ranged from 2200 to 5700 kg ha^-1 during the 6-yr period, and was lowest after a spring wheat crop (1993-1994 and 1995–1996 fallow cycles, Table 3). Russian thistle produced more dry matter by the time of grain harvest than dry matter for the spring wheat crop. The important contribution of dead Russian thistle plants as residue during fallow after low crop production years in this study has been reported (Schillinger et al., 1999). By using herbicides instead of tillage for postharvest Russian thistle control, and noninversion undercutter V-sweeps for primary spring tillage, significantly greater quantities of surface residue were maintained with MT and DMT when compared with CT on all sampling dates (after the onset of tillage) during all years (Table 3). Retention of sufficient residue for government farm program compliance (>390 kg ha^-1) during fallow was not a problem, even with CT, when the previous winter wheat crop produced 4000 kg ha^-1 or more straw.

View this table: [in this window] [in a new window]   Table 3. Quantity of surface residue as affected by conventional, minimum, and delayed minimum tillage during the 13-mo-long fallow cycle. Values in parentheses for 1993–1994 and 1995–1996 fallow cycles are the percentage of total dry biomass comprised of dead Russian thistle plants.

View larger version (30K): [in this window] [in a new window]   Fig. 1. Mass of surface soil clods 5 cm in diameter or larger at time sowing in early September for 1994, 1995, 1996, 1998, and the 4-yr mean as affected by conventional, minimum, and delayed minimum tillage during fallow. Within years, means followed by a different letter are significantly different at P < 0.05. NS = nonsignificant.

View larger version (32K): [in this window] [in a new window]   Fig. 2. Subsurface (0–10 cm depth) soil clods in 1995, 1996, and 1998 as affected by conventional, minimum, and delayed minimum tillage during fallow. Within years, means for each subsurface clod size group followed by a different letter are significantly different at P < 0.05. NS = nonsignificant.

Soil Water Content
Figure 3 shows that there was little or no difference in soil water content among treatments immediately after grain harvest (beginning of fallow) during any year. Less than 15 cm of water remained in the 180 cm soil profile at the beginning of fallow, except in 1993 (Fig. 3a) when 53 mm of rain fell in July after wheat had reached physiological maturity and was no longer extracting soil water.

View larger version (62K): [in this window] [in a new window]   Fig. 3. Soil water content in the 180 cm soil profile during the 6-yr (1993–1999) study period at the beginning of each fallow cycle in early August, in early spring after over-winter soil recharge had occurred, and at the end of the fallow cycle in early September as affected by conventional, minimum, and delayed minimum tillage. Means for each sampling date followed by a different letter are significantly different at P < 0.05. Numbers above bars show storage efficiency (SE), i.e., the percentage of precipitation occurring during the fallow cycle which was stored in the soil. NS = nonsignificant.

During the 1996–1997 fallow cycle, when no fall chiseling was conducted in the MT and DMT treatments, several winter precipitation events occurred on frozen soil, or on thawed soil overlying a frozen layer, extending as deep as 22 cm (data not shown). The CT treatment stored significantly more water in the soil than MT or DMT (Fig. 3d). This was presumably due to frozen soil restricting water infiltration in nonchiseled soils, while channels in the chiseled soils extended below the depth of frost. The 1995–1996, 1997–1998, and 1998–1999 winters were relatively open and mild, and there were no over-winter SE differences among treatments (Fig. 3c, 3e, and 3f). Averaged across 6 yr, over-winter SE was 51, 54, and 57% for CT, MT, and DMT, respectively. These over-winter SE values are considerably higher (61, 62, and 65% for CT, MT, and DMT, respectively) if 1993–1994 is excluded from the data set.

Water remaining in the soil profile at the end of fallow cycle was always in the same relative ranking among treatments as measured over winter, that is, highest for MT and DMT in 1995 (Fig. 3b) and for CT in 1997 (Fig. 3d), but not different in the other years (Fig. 3a, 3c, and 3e). The 5-yr mean end-of-fallow SE was 24, 26, and 26% for CT, MT, and DMT, respectively.

There were no differences in seed zone water content among treatments in 1994, 1995, and 1996 (Fig. 4). Sowing of winter wheat was attempted in 1994, but the deep-furrow drill could penetrate no deeper than 20 cm, which was not adequate to reach the minimum soil water content (11 cm³ cm^-3, Fig. 4a) required for emergence from deep sowing conditions on silt loam soils (Lindstrom et al., 1976). Seed zone water content was adequate in 1995 (Fig. 4b) and 1996 (Fig. 4c) as well as in subsequent years (comparative measurements not taken) in all treatments. Many growers feel that it is necessary to create a fine dust mulch on the soil surface to retard evaporative soil water loss from fallow during the summer. These data, however, show that the greater mass of surface clods (Fig. 1) and subsurface clods (Fig. 2) in the MT and DMT treatments did not adversely affect seed zone water content compared with CT.

View larger version (28K): [in this window] [in a new window]   Fig. 4. Seed-zone soil water content at time of sowing in September 1994, 1995, and 1996 as affected by conventional, minimum, and delayed minimum tillage during fallow. There were no water differences among treatments at any depth (0–22 cm) in any year.

There were no differences in grain yield among treatments during any year or when averaged over 5 yr (Fig. 5). Due to generally favorable precipitation and growing conditions, grain yield averaged across treatments and years in this study was 3500 kg ha^-1 compared with the long-term (30 yr) average winter wheat grain yield at Lind of 2350 kg ha^-1. Janosky (1999), who conducted an economic analysis of this study for the 6-yr period, reported that MT and DMT slightly outperformed the CT system in terms of net returns over total costs.

View larger version (44K): [in this window] [in a new window]   Fig. 5. Yearly and 5-yr mean grain yield of wheat as affected by conventional, minimum, and delayed minimum tillage during the preceding fallow cycle. NS = nonsignificant.

	SUMMARY AND CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

2. On average, twice the surface soil clod mass and a rougher surface was achieved with MT and DMT compared with CT.

3. Chiseling in late fall with straight-point shanks benefitted over-winter precipitation SE in two out of six years when water runoff or snowmelt occurred on frozen soils, but had no effect during open winters when runoff was not a factor. Creating deep tillage channels at wide spacing in late fall with MT and DMT was equal to or greater for over-winter SE than with CT, where chisel shanks were more closely spaced and operated at a shallower depth. The best option to maximize over-winter water storage with minimal residue disturbance may be to create narrow pits about 40 cm deep with a long-tooth rotary subsoiler, with approximately one pit every 1 m².

4. Seed-zone water at the end of fallow (measured 3 yr) was not affected by tillage treatment. This suggests that finely divided soil particles within the tillage mulch may not be as important for retarding evaporative water loss during the summer as previously thought. Rather, creating of an abrupt break between the tilled and nontilled layer with primary spring tillage, which severs capillary channels from the subsoil to the surface, appears to be the dominant factor regulating over-summer evaporative water loss.

5. Delaying primary spring tillage until mid May and beyond had no adverse agronomic affects compared with MT and CT. The late winter application of a nonselective herbicide provided excellent control of downy brome (Bromus tectorum L.) and broadleaf weeds until at least 1 May in all years in the DMT treatment. Downy brome, the most problematic grass weed in the region, was well controlled in all tillage treatments during all years of the experiment.

6. Surface residue in excess of 2250 kg ha^-1 at time of sowing in MT and DMT treatments plugged the deep-furrow drill in 1997. This problem was easily remedied by blind sowing (with only the drill packer wheels) prior to actual sowing. Several implements, such as the coil packer, rotary hoe, and skew treader will bury, align, or otherwise cut straw to allow effective drill operation in heavy residue, but these implements also pulverize soil clods and, therefore, are not recommended in low-residue situations.

In conclusion, conventional tillage during fallow held no agronomic or economic (Janosky, 1999) advantages over MT or DMT in this 6-yr experiment. The CT system had distinct environmental disadvantages, especially when straw production from the preceding wheat crop was <3500 kg ha^-1. This research showed that, with judicious use of herbicides, tillage operations during fallow can be effectively reduced from eight (CT) to as few as three (DMT). If MT and DMT fallow management were widely practiced on Shano soils in the Columbia Plateau of eastern Washington, it is reasonable to expect a sharp reduction in wind erosion and suspended dust emissions, with associated benefit to air quality. Minimum tillage and delayed minimum tillage practices, as outlined in this paper, can be implemented by wheat growers with little or no hardship to their livelihood.

	ACKNOWLEDGMENTS

 
Appreciation is extended to Harry Schafer, Washington State University agricultural research technician; Bruce Sauer, farm manager of the Washington State University Dryland Research Station at Lind; and Steve Schofstoll, Washington State University technical assistant, for their excellent support. Funding for this study was provided by the Solutions to Environmental and Economic Problems (STEEP II) Program, the Columbia Plateau Wind Erosion/Air Quality project, and the Orville Vogel Wheat Fund.

Received for publication October 3, 2000.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 

• Conservation Technology Information Center. 1997. 1997 National crop residue management survey. p. 81. Cons. Tech. Info. Center, West Lafayette, IN.
• Gardner, W.H. 1986. Water content. p. 493–544. In A. Klute (ed.) Methods of soil analysis. Part 1. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison, WI.
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• Horning, T.R., and M.M Oveson. 1962. Stubble mulching in the Northwest. Agric. Info. Bull. 253. USDA-ARS and Oregon Agric. Exp. Station, Corvallis, OR.
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• Lindstrom, M.J., R.I. Papendick, and F.E. Koehler. 1976. A model to predict winter wheat emergence as affected by soil temperature, water potential, and depth of planting. Agron. J. 68:137–141.[Abstract/Free Full Text]
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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (9) Citing Articles via Google Scholar Google Scholar Articles by Schillinger, W. F. Search for Related Content PubMed Articles by Schillinger, W. F. GeoRef GeoRef Citation Agricola Articles by Schillinger, W. F. Related Collections Soil Conservation Other Soil Management Soil Erosion