Considerations and Strategies for Selecting Crops and Rotations for Direct Seed Systems

 

David Huggins, Soil Scientist
Agricultural Research Service, U.S. Department of Agriculture
Washington State University
Pullman, WA 99164-6420

Introduction

Crop and tillage history, soil, climate, topography, pests, and farmers combine and interact so that every farm and field is unique-every year! Successful direct seed systems are derived from trying to understand every part of the farming system, applying scientific principles, and developing sound management strategies that integrate all important factors relevant for each farm and farm field. Crop rotation is a fundamental consideration in any farming system and has received much research attention. Still, the answer to "what is the best rotation for my farm" cannot be answered trivially, can be unique for every farm, field, and sub-field, and is subject to change over time in order to take advantage of new technology or information.

The phase-out of government price support programs, increase in world-wide agricultural competitiveness, and adoption of direct seed systems have stimulated a re-evaluation of crop rotation. Direct seeding impacts every part of the farm and represents a major change in environmental conditions (water, light, heat, nutrients, carbon dioxide, oxygen), and human resources (knowledge, skill, labor, equipment, financing) required for success. Crop rotations based on these fundamental changes have not been extensively developed for direct seed systems in the dryland cropping region of the Pacific Northwest.

My objectives are to:

  1. Describe major environmental changes that occur with a shift from intensive tillage to direct seed systems-emphasizing crop rotation implications.
  2. Present crop rotation principles that provide the scientific groundwork for designing direct seed rotations.
  3. Develop crop rotation strategies based on scientific principles.
  4. Provide some thoughts on future information and research needs.

Direct Seeding Impact on Environmental Resources

Conversion from intensive tillage to direct seeding has major effects on the farming environment that drives biological activity including crops, microbes and pests. The interactions and dynamics of water, light, heat, air, nutrients, and other elements cycling through soil and atmosphere are fundamentally affected. The consequences are that all life forms (including the farm family) are impacted, some favorably, some not, and the system evolves towards new communities of weeds, diseases, insects, and micro-flora and fauna. Intuitively, our crop rotations should also evolve to complement the new environment and the emerging biota that is supported.

Water

Water often limits crop production in the dryland regions of the Northwest. Direct seeding can improve water storage, maintain and improve water holding capacity, and influence the availability of water during periods that are critical for crop establishment and yield. Soil water storage is fundamental to the production systems of the region as 60 to 75 percent of annual precipitation occurs during November through March when crop water use is low. Under direct seeding, water storage is usually increased as surface residues trap and retain snow, enhance water infiltration, and reduce losses by evaporation and runoff. Many studies have been conducted throughout the Inland Northwest comparing residue management effects on stored water. Overwinter soil water storage with standing stubble generally ranges from 15 to 45 percent greater than a bare surface (some examples: Table 1). Ramig (USDA-ARS, Pendleton) found that fall plowing reduce stored soil moisture by 20 percent (about 2 inches) as compared to standing stubble over a ten year period on a Walla Walla silt loam (16 inch precipitation zone). In contrast, fall chisel plowing has resulted in greater overwinter soil water storage as compared to standing stubble when surface roughness decreases runoff on frozen soils.

Table 1. Wheat residue management effects on stored soil moisture.
Location Residue
treatment
(lb/ac)
Storage
(Inches)
Efficiency
(%)
 Lind, WA†  0 (burned)
4,500 (original)
10,000 (added)
 3.2
4.3
5.3
 48
66
81
 Pendleton, OR‡  Burned
Flailed
Standing
 6.8
8.7
9.0
 57
73
76
  Moro, OR§  Burned
Flailed
Standing
 4.5
5.7
6.0
 62
79
83

†Papendick (USDA-ARS, Pullman, WA); September, 1974 through March, 1975; 6.5 inches precipitation.
‡Ramig (USDA-ARS, Pendleton, OR); Aug. 1, 1980 through Feb. 28, 1981; 11.9 inches precipitation.
§Ramig (USDA-ARS, Pendleton, OR); Aug. 1, 1980 through Feb. 28, 1981; 7.2 inches precipitation.

The capacity of soil to store water is primarily dependent on soil texture and depth to root-restrictive layers and varies considerably across the region and within fields. For example, eroded knobs of clay or caliche can have low water holding capacity or root restrictive layers that limit water use, whereas bottomlands can have deep soil with large water holding capacities. Crop rotation and tillage options aimed at optimizing water use need to consider both water inputs as well as storage capacities. A method to evaluate differences in storage capacity on your farm would be to take soil samples to at least six feet (if possible) at various landscape positions following harvest of winter wheat. Note crop water use and rooting depth by examining water depletion for each foot of the sampled soil profile. A large increase in water at any particular depth signifies where the crop was no longer able to extract water. Useful soil water storage capacity would be limited to above this depth and will vary significantly across farm landscapes.

The availability of soil water during critical production periods may be as important as stored soil water. The establishment of fall and spring crops is dependent on adequate amounts of seed-zone water. Dry seed-bed conditions can limit the establishment of winter grain legumes (winter pea, winter lentil), winter grains (wheat, barley), or winter canola seeded into conventionally-tilled soil. Surface residues can conserve seed-zone water and benefit crop establishment (Table 2). Management of seed-zone water through surface residues may determine the regional adoption of these crops. For example, 12% soil moisture may be required for germination and establishment of winter canola (Murray, Univ. of Idaho crop physiologist). Data from Table 2 indicate that this requirement would have been met with no-till in early fall of 1988, but not until late fall under conventional tillage. Early fall establishment of winter canola is critical if development is to be sufficient for winter survival. Further research is needed to define critical soil temperature and moisture requirements for the establishment of fall seeded crops, particularly winter legumes and canola. Spring establishment of small, shallow seeded crops such as canola, mustard, linola and safflower may also benefit from improved seed-zone water under direct seed.

Table 2. Fall seed-zone soil water in no-till (NT) and conventionally-tilled (CT) plots.†

 Sample date  Soil depth
(Inches)

 
Residue treatment

 NT

 CT

(------Water % -----)
 LSD (0.05)
Sept. 21, 1988 0-6
7.8 a 8.4 a
 1.1
Sept. 24, 1988 0-3
14.1 a 9.7 b
 1.0
Oct. 9, 1988 0-3
12.3 a 8.4 b
 1.1
Nov. 11, 1988 0-6
28.9 a 28.3 a
 1.0
Plots located near Troy, ID; Huggins and Pan, 1991.

 

Greater water availability will only be beneficial if it can be used to increase yields or frequency of crop production. Rotation designs should consider crop management sequences that eliminate or limit summer fallow and efficiently use available water. For example, research in Colorado (summer rainfall area) has ranked dryland cropping systems in order of decreasing system precipitation use as wheat/corn > wheat/corn/millet > wheat/millet > wheat/corn/millet/fallow > wheat/corn/fallow > wheat/fallow (Farahani et al., 1998). In the dryland area of the Northwest greater crop use of available water could mean a shift from fallow to more continuous spring cropping, a shift from spring to winter crops, or a shift from winter annual crops to perennials. Symptoms of inefficient water use and a need for greater cropping intensity are runoff, ponding, water logging, soil erosion, leaching of water below the root zone, soil compaction, increases in soil-borne diseases, and increases in weed pressure (if the crop does not use resources mother nature will fill in the blank).

Heat

Crop rotation designs should consider temperature effects, as modified by residue characteristics and aspect, on crop establishment, pest pressure, and length of growing season (growing degree days). Direct seed systems effect temperatures of soil and air near the soil surface. In winter wheat stubble, maximum daily soil temperatures (2 inches) in the spring and fall were 1 to 5oC colder under direct seeding as compared to conventional tillage (Huggins, 1991). In contrast, daily minimum soil temperatures were often slightly warmer (1 to 2oC) under direct seed (Huggins, 1991). Reduced early growth of wheat in no-tillage has been associated with low soil temperatures in the surface 0.5 inch where wheat seedling meristematic regions (growing points) and zone of leaf extension are located (Aston and Fischer, 1986). Growth and development of grass family crops such as wheat, barley, corn, and millet have seedling growing points located beneath the soil surface and can be adversely affected by sub-optimal soil temperatures in the spring under direct seeding.

Air temperatures just above the soil surface (2 in) tend to be slightly colder in the fall, but warmer in the spring under direct seeding (Huggins, 1991). Leaf meristematic regions of broadleaf seedlings such as peas, lentils, mustard, canola, soybean and safflower are located above the soil surface and may benefit from warmer spring air temperatures under direct seed. For example, shoot biomass of winter lentils was not significantly different between no-tillage and conventional tillage in the fall, but was 32% greater in no-tillage as compared to conventional tillage in early spring (Huggins, 1991).

Crop residues can be managed through rotation (high versus low residue crops) or disturbance (harrowing, burning, etc.). Maximum daily soil temperatures (2 in, spring) averaged nearly 1oC warmer in Austrian winter pea as compared to winter wheat stubble (Huggins, 1991). Harrowing winter wheat and Austrian winter pea stubble increased maximum daily temperatures near the soil surface (0.5 in) by 30C, while soil temperatures at deeper depths (2 in) were not affected (Huggins, 1991).

Light

Surface residues can create a shaded environment that decreases the quantity of light reaching the soil surface and also changes the quality of light (Rickman et al., 1985; Huggins and Pan, 1991). In standing wheat stubble, photosynthetically active radiation (PAR) measured at noon decreased from 1200 umol m-1 s-1 at the top of the stubble (35 cm or 14 inches) to 100 umol m-1 s-1 at the soil surface (Huggins and Pan, 1991). Standing stubble also affected red/far red ratios which declined from 1.03 at 35 cm above the ground to about 0.8 at the soil surface. Reduced PAR could be expected to reduce seedling growth of C3 crops (peas, lentils, canola, wheat, barley) where photosynthesis light saturates at about 400 umol m-1 s-1; and probably have an even greater negative effect on the growth of seedling C4 crops (corn, millet,) as photosynthesis continues to increase with greater PAR in C4 plants. Rickman et al. (1985) reported reduced tillering and slower leaf development at PAR of 140 umol

m-1 s-1 both of which contributed to reduced wheat seedling biomass. A decrease in red/far red ratios can also decrease tillering in wheat and branching in broadleaves such as peas and lentils. Residue management to eliminate or reduce shading of seedlings may be warranted if light is limiting seedling development and growth.

Nutrients

Changes that occur in soil chemical physical, and biological characteristics under direct seeding effect nutrient availability, uptake, and utilization by crops. Cereal production in direct seed systems has benefitted from fertilizer placement as compared to broadcast applications. Advantages of fertilizer placement include: (1) increased seedling growth, fertilizer uptake, and potential yield; (2) reduced weed populations and increased crop competitiveness; (3) less nutrient tie-up from microbial decomposition of crop residues; and (4) potential benefits from disturbance in the seed zone reducing Rhizoctonia. The importance of fertilizer placement for cereal crops diminishes following legumes in high precipitation areas, however, deep placement of immobile nutrients such as P remains an important objective. If crops that produce large amounts of residue with high C/N ratios (wheat, barley, corn) are alternated with legumes (peas, lentils), deep band placement of N may be less critical. The greatest necessity for deep band placement occurs for small grain rotations in the low to intermediate precipitation zone. Increasing soil acidity continues to be a problem wherever ammoniacal fertilizers are used (anhydrous ammonia, aqua, urea). Soil pH tends to stratify over time in direct seed systems with lower pH occurring at the soil surface. Adjustments in pH could be made with surface applications of lime if economic sources were available. Soil pH under 5.5 will adversely affect legume yields (peas and lentils) and wheat yields will respond to lime applications when pH is below 5.3. Further research is needed to evaluate pH stratification effects-is low surface (0-2 inches) pH detrimental if subsurface pH is within tolerable limits? What are pH stratification effects on disease or weed pests?

Air

Increased soil compaction under direct seed can reduce the exchange of gases (CO2 and O2) between soil and atmosphere. Adequate O2 is required for plant root respiration and can become limiting if soil porosity is reduced and soil water content elevated. Conditions conducive to low soil aeration include: (1) soil compaction resulting from spring crop rotation emphasis (for example: winter wheat/spring barley/spring pea) in high precipitation zone; (2) poorly drained landscape positions with inadequate drainage/cropping intensity; and (3) the transition period: starting with initial conversion from tillage to direct seed system and before the development of soil macro-porosity resulting from remnant root channels and increased soil fauna and flora activity. The concept of least limiting water range (LLWR) has been developed to assess potential root growth and function from limiting soil physical factors: available water, soil aeration, and penetration resistance as a function of bulk density. Results from a fine-textured, poorly-drained soil in southern Minnesota indicate that penetrometer resistance and soil aeration were more important factors limiting root function in no-tillage, whereas available water limited root function in tilled systems (Betz et al., in review). These results indicate that soil compaction avoidance and accelerating the development of macro-porosity may be particularly important during the "transition period" of direct seeding and beyond.

Crop Rotation Principles

Greater crop yields, increased water and nutrient use efficiency, reduction of disease, weed, and insect problems, improved soil quality, greater biological diversity, wildlife enhancement, increased profitability and quality of life are all goals commonly associated with crop rotation. But determining the right rotation is a never ending process that evolves with greater understanding of the farming system, improvements in technology, and changes in social policies. A few over-riding principles tend to be repeated when crop rotation is discussed and I have attempted to summarize these as follows.

1. Match crop requirements with resource availability

Crops and crop varieties must be well adapted to the environment, otherwise crop competition and vitality and/or resource use declines and crops become more susceptible to pests and/or will promote environmental degradation. Resource availability changes with direct seeding and our traditional rotation designs need to be re-evaluated and adjusted to more closely match resource availability (including labor and equipment). Important crop characteristics include seasonal uptake pattern and amount of water and nutrients required; rooting depth; snow catching abilities; growing degree day requirements for reaching maturity and grain drying; temperature, water, light, and soil aeration needs for optimal germination, seedling establishment, and yield; crop residue production and decomposition; windows of opportunity and dates for critical field operations (seeding, pesticide application, harvesting, residue management); equipment needs; winter survivability (positive or negative!); pesticide options; and marketability/profitability need careful consideration--some examples are shown in Table 3. These characteristics need to be evaluated not only for individual crops but for the crop rotation as a whole-more on that later.

(click to view) Table 3. Matching crop characteristics with resource availability-some considerations for the dryland cropping areas of the Pacific Northwest.

Preliminary results of a STEEP supported cropping system study initiated in 1998 on the Palouse Conservation Field Station illustrate differences in seasonal water use and temperature requirements (Figs. 1 and 2). Cumulative growing degree days contrast the thermal requirements of cool season crops (wheat, mustard, canola; base temperature 3oC, optimum 30oC) versus warm season crops (corn, millet, dry beans; base temperature 10oC, optimum 30oC) that drive their vegetative development (Figure 1). Evident in Figure 2 is the more rapid use of water by wheat and mustard during mid to late June as compared to corn.

Consequences of poor crop adaptability to farm resources are decreased competitiveness (against weeds) for water, nutrients, and light, and greater susceptibility to disease and insect problems. Resources such as water and nutrients that are not used efficiently have the potential to cause environmental degradation such as soil erosion, sediment and nutrient (N and P) contamination of surface waters, excessive nutrient (N) leaching into ground waters, water logging of bottomland soils, and saline seep.

2. Use rotation as preventative medicine and design complementary crop sequences

Crop rotation is a fundamental tool of integrated pest and soil management. Crop rotation design in direct seed systems is particularly important for alleviating or avoiding yield constraints due to environmental conditions or pests. In the low precipitation zone, water rather than disease is usually the main yield-limiting factor and practices that optimize water use (crop rotation, residue management, weed control) have a major influence on yield. Crop rotation for disease and weed control is more important in intermediate and high precipitation zones where moisture is less yield-limiting. Pests which are most effectively controlled by crop rotation include soil-borne pathogens, weeds or insects with the inability to invade adjacent fields or areas; pests with a narrow host range; pests (insects, pathogens, or nematodes) that cannot survive long periods without a host; or rotations that provide the opportunity to interrupt weed, disease, or insect life cycles with different pest control measures (timing and use of different pesticides, green bridge control, crop competition, resistant crop varieties).

Crop rotation can be used to: (1) avoid soil compaction and equipment limitations, for example, shifting to a winter crop emphasis in the high precipitation zone; (2) alleviate adverse affects of stubble on spring crop establishment (including poor seed placement and soil-seed contact, equipment limitations, and cold temperature stresses) by planting spring crops into low residue crops such as peas or lentils; (3) improve seedbed moisture, increase winter survival, reduce winter injury, control soil erosion, and augment soil moisture storage through snow trapping by planting winter crops into high or moderate amounts of surface residue; (4) include legumes in rotation with cereals to reduce pest problems, enhance N fertility, and provide other beneficial 'rotation ' effects; (5) include mixtures of spring and winter crops, broadleaves and grasses to disrupt pest cycles and provide further opportunity to take control measures; (6) favor crops that are most profitable such as winter wheat.

Figure 1. Daily mean air temperatures and cumulative growing degree days (GDD) for cool season crop (spring wheat) versus warm season crop (corn) in 1998.

Figure 2. Soil water depletion (0-2 feet) for yellow mustard, hard red spring wheat, and corn during May and June of 1998.

3. Design rotations that build or maintain farm resources

Maintaining or improving farm resources such as soil quality over time are important objectives of crop rotation design. Raising the intensity and diversity of crop production to match available resources will help limit soil degradation processes (wind and water erosion, water logging, nutrient runoff and leaching) and improve soil organic matter, soil structure, water infiltration, and water holding capacity. Perennial crop production (grass, alfalfa) should be considered wherever feasible and rotated to different portions of the farm. Grass crops in particular can build and maintain high soil quality. Crop residue burning should be used sparingly, benefits of retaining stubble include maintaining soil organic matter levels, increasing soil water storage, recycling and providing plant nutrients, and preventing wind and water erosion. Representative values of nutrients in wheat stubble are 0.45% N, 0.15% P2O5, and 0.16% S. Every bushel of wheat produces about 100 lbs of straw, therefore a 100 bu/ac wheat yield would leave about 10,000 lbs/ac of straw with 45 lbs N/ac, 15 lbs P2O5/ac, and 16 lbs S/ac. Nearly all of the nitrogen and half of the sulfur and phosphorus are lost as a result of burning. Standing stubble also increases stored moisture by about 20% over burned residue. The extra moisture, about 2 inches, can translate into greater wheat or legume yields.

 

4. Design flexibility into crop rotation

Crop rotations should be designed so that plans can be adjusted to consider up to date information on crop prices, soil moisture storage, or other factors influencing crop decisions. This strategy may be particularly important in low precipitation zones where the choice to fallow or to spring crop may be based on threshold values of soil moisture storage (and predicted yields), crop prices, and acceptance of risk that occur during the winter prior to spring planting. Rotation flexibility can be decreased by the use of residual herbicides that may prevent the production of sensitive crops for years. Planting of winter crops with comparable spring crop counter-parts can be useful if poor stands are obtained in parts or all of fields due to pest problems, poor equipment performance, or low winter survival. Stands can be augmented by direct seeding of spring counter-parts of winter wheat, barley, oats, peas (soon), lentils (soon), and canola. Our regions ability to produce good winter and spring crops aids this flexibility.

5. Economics, Economics, Economics, Logistics, Logistics, Logistics

Direct seed crop rotations must be built around profitable crops and either include them more often in rotation or favorably augment their yield if produced less frequently. Equipment needs, balancing workload, farm programs, and quality of life are also important considerations.

 

Crop Rotation Strategies: Application of Principles, Identification of Research Needs

Beck (1998) suggested the use of native vegetation as an indicator of proper rotation intensity (water demand). Environments that support forest and tall/mixed grass areas with trees (high precipitation zone) are cooler and may have excess water at times, will support the most intensity-if growing season permits, and should use continuous high water use crops. Tall grass prairie areas with few trees (intermediate precipitation zone) may sometimes be too dry for high intensity rotations with high water use crops and have limited fallow. Mixed and short grass prairie areas (low precipitation zone) are usually too dry for intense rotations, use a small amount of fallow, and limit high water-use crops in rotation.

Low precipitation zone: Conversion to direct seeding may conserve enough water to intensify crop rotation. Currently winter wheat/fallow is the most commonly used rotation. Annual cropping of spring wheat (hard red or soft white) or barley may be feasible and certainly is justifiable where soil water holding capacity is limited to precipitation from one winter season (Wysocki et al., 1995). Decision rules for flexible cropping need to be developed and rotations that use alternative low moisture crops explored. Some rotation suggestions might be winter wheat/spring wheat/fallow; winter wheat/spring wheat/millet; and continuous spring wheat or spring barley. In continuous spring cereal rotations, direct seeding allows for earlier planting than under conventional tillage often in January or February. Early control of winter-germinated weeds and volunteer grains "the green bridge" is critical for Rhizoctonia root rot control; downy brome and jointed goatgrass are effectively controlled with spring cropping while Russian thistle will increase and may require herbicide applications or undercutting with sweeps or blades. Hard red spring wheat may have an economic advantage as high grain protein levels are easier to obtain in this water-stressed environment. Safflower and mustard may be alternative crops, however, fallow may be required following safflower as soil water depletion exceeds wheat. Drawbacks of continuous spring cropping include risk of crop failure and uneven spread of workload.

Intermediate precipitation zone: Common crop rotations consist of winter wheat/fallow and winter wheat/spring barley/fallow. Direct seeding may allow greater annual cropping and less fallow. Introduction of a broadleaf crop in rotation may be advantageous. Possible candidates include winter or spring lentils/peas, spring mustard or canola, linola, or safflower. Warm season grasses (corn and millet) should also be explored as possible rotation crops. Possible rotation sequences include:

winter wheat/spring barley/winter or spring grain legume (lentils or peas)

winter wheat/spring mustard or canola/winter or spring grain legume (dependent on fall seed-zone moisture)

winter wheat/winter or spring grain legume/spring mustard or canola

winter wheat/winter or spring grain legume/spring barley, wheat or corn/spring mustard or canola

Many combinations are possible and further research is needed to explore complementary rotation sequences and potential alternative crops. The use of perennials in rotation (grass, alfalfa, or perennial wheat in future) should be considered.

High precipitation zone: Farmers generally use rotations of winter wheat/spring pea or lentil, winter wheat/spring barley/spring pea or continuous cereal cropping. Intensification of high water use crops may be necessary under direct seed. Perennials should be an integral part of the farm; winter crops should be emphasized; spring crops carefully integrated into the rotation to provide pest control, but to avoid low temperature and excessive soil water conditions (and compaction); crop residues should be managed to augment fall seed-zone moisture, provide over-winter protection and snow trapping; rotation length should be at least three years and include grain legumes. Warm season crops may be temperature limited unless grown in favorable landscape positions. A possible rotation design is:

grass seed production/spring lentils/winter wheat/winter pea or lentil/spring grain, canola or mustard/winter wheat. This rotation has several important features including:

  1. intensification of high moisture-using crops including perennials;
  2. an emphasis on winter crop production with greater yield potentials;
  3. inclusion of winter grain legumes and spring crops to minimize pest problems and facilitate control measures;
  4. the management of crop residues and sequences to provide potential benefits to subsequent crops while minimizing potential adverse affects.
  5. control of soil erosion, building of soil quality, and limiting of soil and water degradation to maintain long-term productivity and environmental soundness.

Other rotation suggestions:

Further research and grower experience are needed to understand the pros and cons of these rotations.

Landscape considerations

Crop rotations are usually applied to fields and provide crop diversity through time. Spatial diversity of crop production across the landscape may also be warranted considering the diversity of environments that can occur in a single field. Managing spatial diversity effectively adds another dimension, or opportunity, for crop rotation strategies to develop environmentally sound and highly productive cropping systems.

Transition rotations

Transition rotations could be designed to reduce risks from pests, adverse soil properties (compaction, aeration, low temperature), and lack of human experience/knowledge that occurs during the transition phase of converting from intensive tillage to direct seed cropping systems. Potential crops could include:

  1. crops that promote soil aggregation, the development of soil macro-porosity, and the disruption of tillage pans such as perennials (grass seed, alfalfa) and tap-rooted crops such as canola and mustard;
  2. crops that effectively break pest cycles and improve nutrient relations such as grain legumes;
  3. crops providing sufficient rotation breaks to minimize pest problems; and
  4. crops that have been grown sufficiently to understand potential production problems.

Additional Research and Informational Needs

  1. Further development of agronomic zones, building on current delineations but expanding to include considerations of fall seed-zone moisture, precipitation distribution, modifying effects of topography, native vegetation, others?
  2. Evaluation of crop and cropping system water and nutrient use efficiency under direct seed.
  3. Development of crop rotations for the transition phase.
  4. Flex cropping and risk reduction assessments
  5. Profitability of alternative crops including yield thresholds
  6. Long-term cropping systems research in all precipitation zones

References

Aston, A.R. and R.A. Fischer. 1986. The effect of conventional cultivation, direct drilling and crop residues on soil temperatures during the early growth of wheat at Murrumbateman, New South Wales. Aust. J. Soil Res. 24:49-60.

Beck, D. 1998. Diverse no-till crop rotations in the Northern Great Plains. In: R. Veseth, (ed.) Northwest Direct Seed Intensive Cropping Conference Proceedings, Jan. 7-8, 1998, Pasco, WA.

Betz, C.L., R.R. Almaras, S.M. Copeland, and G.W. Randall. 19__. Least limiting water range: traffic and long-term tillage influences in a Webster soil. SSSAJ: In review.

Farahani, H.J., G.A. Peterson, and D.G. Westfall. 1998. Dryland cropping intensification: a fundamental solution to efficient use of precipitation. In: D. Sparks (ed.) Advances in Agronomy. Vol. 64. P. 197-223.

Huggins, D.R. 1991. Redesigning no-tillage cropping systems: alternatives for increasing productivity and nitrogen use efficiency. Ph.D. dissertation, Washington State Univ,. Pullman, WA.

Huggins, D.R., and W.L. Pan. 1991. Wheat stubble affects growth, survival and yield of winter grain legumes. SSSAJ. 55:823-829.

Rickman, R.W., B.L. Klepper and C.M. Peterson. 1985. Wheat seedling growth and development response to incident photosynthetically active radiation. Agron. J. 77:283-287.

Wysocki, D.J., P.E. Rasmussen, F.L. Young, R.J. Cook, D.L. Young, and R.I. Papendick. 1995. Achieving conservation compliance with residue farming in the low-precipitation zone. In: R.I. Papendick and W.C. Moldenhauer (eds.) Crop Management to Reduce Erosion and Improve Soil Quality, Northwest. USDA-ARS Conservation Research Report Number 40.