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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chap. 5, No. 15, June 1994


Managing Downy Brome under Conservation Tillage Systems in the Inland Northwest Crop-Fallow Region

Roger Veseth, Alex Ogg, Donn Thill, Dan Ball, Don Wysocki,

Floyd Bailey, Tom Gohlke, and Harry Riehle*

Introduction

Downy brome (Bromus tectorum L.), commonly called cheatgrass, has been a major grass weed problem in winter wheat in this region for decades. Although it is a problem regardless of tillage system, it can be particularly troublesome in the crop-fallow rotation under tillage systems designed to retain more crop residue on the soil surface. When the seeds remain on the surface or are buried shallowly, there is a potential for rapid development of downy brome infestations unless effective weed management strategies are implemented.
Traditional methods of controlling dense infestations of downy brome have included burying seed deeply by intensive tillage systems and burning to destroy seeds in the surface residue. However, high rates of soil erosion have often occurred during the subsequent fallow-winter wheat sequence when these practices are used. National legislation, beginning with the 1985 Food Security Act, requires producers with highly erodible cropland to effectively minimize soil erosion in order to be eligible to participate in USDA programs. These laws have limited the use of intensive tillage and burning as downy brome management tools in this production region.
Many producers have changed their tillage and residue management systems in recent years as part of their farm conservation plans. It is critical that producers develop an effective management strategy to minimize downy brome seed production and to deplete seed populations in the soil in order to avoid dense infestations of downy brome after conversion to conservation tillage systems. This publication addresses two management strategy situations based on the level of weed infestation: 1) "Maintenance" Control Strategies to control a light to moderate downy brome infestation which caused minimal crop yield loss; and 2) "Reclamation" Control Strategies to recover from a dense infestation of downy brome which caused substantial yield loss, or may reduce future yield potential to crop failure levels.
More information on the identification and ecology of downy brome is covered in "Downy Brome," a new PNW Extension publication in the PNW Weed Series. Copies are available through County Extension offices in the Northwest.

Weed Characteristics Related to Competitiveness and Management

Downy brome is a winter annual grass which germinates primarily in the fall. Seeds also can be produced from plants that developed from seeds germinating in the spring, although seed production is much more prolific from fall-germinating plants.
Germination Conditions - About 95% or more of the downy brome seeds in the top two inches of soil germinate with the first opportunity under favorable moisture and temperature conditions. Seed germination and emergence of most downy brome typically occur after fall rains when soil temperature has cooled to less than 70F, optimum soil temperature for germination of this weed. However, seed germination continues at cooler soil temperatures down to 32F, if soil moisture is adequate.
Dormancy - Although newly produced downy brome seeds require a short after-ripening period for maximum germination, there is usually very little seed dormancy and most seeds germinate when favorable temperature and moisture conditions exist. However, some seeds can become dormant if they have absorbed moisture in the fall but are not in favorable seed-soil contact conditions and dry out before germination can proceed. This dormancy can last until the following fall, thus maintaining a greater seedbank in the soil. In addition, seeds remaining in the above-ground residue will also survive in the field for a longer time than when in contact with the soil. Consequently, management practices that provide good seed-soil contact conditions for fall germination can help reduce seed carryover.
Emergence - Depth of downy brome seed burial greatly influences emergence potential. About 90% of the seedlings emerge from seeds in the top 0.5 inch of soil. Maximum depth of emergence is about 2.5 inches. This is why seed burial by moldboard plowing has been a traditional downy brome control practice.
Seed Longevity - Few seeds remain viable in the soil for more than 2 years. Consequently, minimizing seed production and stimulating germination of seeds in the soil are effective strategies for downy brome control.
Growth and Development - Downy brome has a very rapid growth rate, making it highly competitive with winter wheat, particularly if it emerges close to the time of wheat emergence. The primary root system, which develops from the seed, continues growing during the fall and winter at soil temperatures just above freezing. Secondary or crown roots, which emerge from the plant crown, usually are initiated in the fall or winter and are well developed before winter wheat resumes growth in the spring. Although rooting depth can reach 3 to 4 feet, over 90% of the total root mass is usually concentrated in the top 15 inches. The finely-divided fibrous root system is highly efficient in exploiting soil moisture and nutrients.
By the time winter wheat dormancy ends in the spring, downy brome plants are often in the tillering or jointing stage. Heading usually occurs about 4 weeks ahead of winter wheat. The rapid transition through the reproductive stages can make it difficult to prevent downy brome seed production in the spring of the fallow season under prolonged wet spring conditions. Pollination occurs very quickly after heading and it is difficult to identify in the field. If pollination has just occurred, at least some viable seed will be produced if the plant is cut off by shallow tillage. In addition, under wet spring conditions some plants are often able to reestablish if any roots are in contact with moist soil. If downy brome has headed, some viable seed may form following an application of a nonselective herbicide. In this situation, use of a fast burndown herbicide may help reduce seed production. Seeds generally mature by late spring and many drop from the seed head before wheat harvest.
Crop Impact - Downy brome infestations of 10 to 50 plants per square foot have reduced winter wheat yields by 40 to 92 percent, respectively, in the Inland Northwest. Under favorable growth conditions, an infestation of downy brome can produce over 500 pounds of seed per acre (1 pound of seed contains approximately 250,000 seeds). As many as 400 seeds can be produced per plant. With this seed production potential, downy brome populations can increase dramatically in just 2 to 3 years under favorable weather and production conditions.

Focus on Key

Weed Management Goals

Long-term management strategies for downy brome control should focus on the following three areas.

Facilitate Seed Germination

Use management practices that facilitate germination of current year seed and seed in the soil to deplete the downy brome "seedbank." This is particularly effective since the seeds are relatively short-lived in the soil. With weeds such as jointed goatgrass, mustards and wild oat, that have longer seed dormancy in the soil, there are fewer management options to speed depletion of the soil seedbank.

Prevent Seed Production in Fallow

Because downy brome seeds form early, it is important to control downy brome when the plants are still small to reduce the potential of seed production in fallow if wet weather delays spring field operations. In years when fall rains stimulate seeds to germinate and seedlings to grow vigorously, control of downy brome in late fall after harvest can further reduce the potential of seed set the next spring. Soil active herbicides for winter annual grasses can also be applied in the fall after harvest to provide control through the spring of the fallow season. Use of nonselective herbicides ahead of spring tillage can improve control, particularly under wet conditions.

Minimize Seed Production in Crop

Minimize seed production in crop through optimizing crop competitiveness against the weeds and effective use of herbicides as needed. Early vigorous establishment of winter wheat is a critical step in improving crop competitiveness.
Use competitive varieties adapted to your production area. Optimize crop health by controlling other crop pests as much as feasible and by providing adequate plant nutrients available for early crop root access to increase crop competitiveness against downy brome. Avoid excessive amounts of nitrogen and do not use surface topdress applications of nitrogen in infested fields, both of which can increase downy brome competitiveness and crop losses.

General Considerations

Identify Infestation Level and Select Management Strategy - Determine the level of infestation and decide whether to pursue a Maintenance Control Strategy or a Reclamation Control Strategy. A Reclamation Strategy would be considered when the current or future loss from a downy brome infestation in winter wheat, plus the cost of control measures, is greater than reduced returns from a spring crop compared to winter wheat with the downy brome infestation. Historically, spring wheats generally yield about 2/3 of the yield of winter wheat in the Northwest, although spring wheat yield potential has been increasing with newer varieties, improved fertilizer placement and increased soil water storage under conservation tillage. Northwest research has shown that moderate infestations of downy brome emerging within about 10 days of wheat emergence can reduce yields by about 1/3.
Use Crop Rotation Options - A change in crop rotation is a critical part of Reclamation Strategies, but can also be included in a preventative Maintenance Strategy whenever possible. Include a spring crop to lengthen the wheat-fallow rotation whenever possible, particularly when winter precipitation has been above normal, thus increasing the potential for a successful spring crop. Crop rotations with a spring crop allow both spring and fall tillage or herbicide options to kill the weeds and deplete the seedbank in the soil. A competitive fall-seeded non-cereal crop, such as winter rapeseed, with the option of using a grass weed herbicide also would help reduce problems from downy brome, and other weeds and diseases associated with the wheat-fallow rotation.
In most of the wheat-fallow areas, use of longer rotations is limited by low annual rainfall, lack of profitable alternative crops and commodity base acreage requirement of farm programs. However, temporary changes in crop rotations need to be evaluated in terms of increased winter wheat yield potential with improved downy brome control. Furthermore, growers need to include the additional benefits of a spring crop in rotation for improved control of other weeds, such as jointed goatgrass, and soilborne diseases, such as Cephalosporium stripe, compared to the continuous wheat-fallow rotation.
Prevent and Limit Infestations - Clean tillage equipment and combines after operating in infested fields. Planting "weed-free" wheat seed is always a good preventative weed control practice, although it is less important for downy brome since most of these small light seeds pass through the combine at harvest and are easily cleaned from wheat seed. Make a special effort to control even light downy brome infestations on field borders, fencelines and roadways to reduce downy brome encroachment into the field. Control options might include planting permanent grass cover, timely mowing, and various herbicide options through the rotation.
Intensive Tillage or Burning - In dense downy brome infestations, more intensive tillage or burning could be used to reduce downy brome seed populations after winter wheat harvest in infested areas of the field where soil erosion potential is low. These options should only be considered when a spring crop is planned and will be covered in more detail in the Reclamation Control Strategies section.

Maintenance Control Strategies

Maintenance Control Strategies focus on preventing a light weed population from developing into a dense infestation, as well as preventing the establishment of a downy brome problem in fields. The following are some potential management options listed in general chronological order, beginning with winter wheat harvest.

1. Downy Brome Control begins at Harvest

  • Uniformly distribute weed seeds, volunteer grain and residue at harvest to enhance seed-soil contact for greater seed germination, improve effectiveness of contact herbicides, reduce tie-up of soil-active herbicides, reduce the potential for soilborne diseases associated with the roots of weeds and volunteer plants, and improve effectiveness of tillage operations. Combine chaff spreading systems can effectively distribute seed and crop residue for improved seed-soil contact. Harrowing and other light tillage operations after harvest are less effective than chaff spreaders in distributing seed concentrations. These operations provide greater seed-soil contact benefits if a combine chaff spreading system has been used. Keep in mind that chaff spreaders, harrowing and other tillage operations can also spread localized downy brome infestations within fields.

2. Postharvest Mgmt. After Winter Wheat

  • Fall is the best time to manage for improved germination of downy brome. Use a harrow or other light tillage operation soon after harvest when the stubble is still dry to increase downy brome seed-soil contact and germination potential with the first fall moisture. Tillage intensity will depend on the crop residue level. Tine harrows or skew treaders can improve seed-soil contact with minimal residue disturbance. Disks must be used with caution because of excessive surface residue burial and pulverizing of soil structure. Still, if they are set at lower angles and operated at a shallow depth, disks can be used to effectively improve seed-soil contact without excessive residue burial.
  • Consider using a nonselective herbicide late in the fall, if fall rains have stimulated good germination and growth of downy brome. When feasible, fall control further reduces the possibility of spring seed set if weather conditions prevent early spring control. It also reduces the potential of heavy sod formation by downy brome and volunteer wheat, which can make control in the spring more difficult. Lower labeled rates sometimes can be used in the fall compared to the spring because the plants are smaller and winter stress can help kill them. However, the option of fall application is highly dependent on timely rains. Dry fall conditions often limit germination, emergence and growth, thus restricting the fall spray option. Furthermore, drought stress and the presence of dust on the plants also can limit the possibility of a fall application and use of lower rates. Under conservation tillage systems, nonselective herbicides provide an important tool to reduce both seed production potential in fallow, and the number and intensity of tillage operations needed, which can increase storage of winter precipitation and reduce soil erosion.
  • Consider using a fall application of a soil-active herbicide to control winter annual grasses on the entire field, on field borders or in localized infestations in the field. Soil-active herbicides generally can improve control of volunteer wheat and many winter annual grasses through the early part of the following fallow season. Activity of some soil-active herbicides is increased when a fall tillage operation is used. Remember that spring cereals usually are not an option following a fall application of most soil-active herbicides in fields originally planned for fallow.
  • If water runoff on frozen soils is a common occurrence in the area, consider chiseling in the fall across the slope at the typical frost depth to improve storage of winter precipitation. A chisel or subsoiler with wide shank spacings can reduce runoff potential with minimal residue disturbance.

3. Fallow Management in the Winter, Spring and Summer (if a spring crop is not planted)

  • Consider using a nonselective herbicide to delay the primary tillage operation as long as feasible to increase residue retention and water conservation, and reduce soil compaction from tillage on wet soils. If a spring application of a nonselective herbicide is planned, earliness is important to prevent plants forming a dense sod and setting seed. High populations of large plants also can significantly deplete soil water. In addition, dust interferes less with the activity of herbicides when applied during early spring. A nonselective herbicide in early spring, with or without tillage, usually provides better downy brome control than tillage alone when soil is moist, temperature is cool, and rains are still relatively frequent. Lengthening the early spring period of cool, moist conditions -- without drying the surface soil with early tillage -- favors continued spring germination of downy brome. Little germination of downy brome is believed to occur after early March in the low precipitation wheat-fallow regions. The optimal time to begin setting the dry mulch seedzone line will vary with location and yearly weather conditions.
  • Think residue management during the entire fallow season, from harvest through planting. Minimize the number of tillage operations to increase residue retention, maintain surface roughness, and minimize soil water evaporation -- about 0.2 to 0.5 inch per tillage operation when moist soil is brought to the surface. Select and operate implements to retain the most residue on the surface. Rod weed only when weed populations and growth warrant, or when needed to reestablish the dry mulch layer after a rain. Consider tilling or spraying only those areas in the field where weed populations are sufficient to warrant control if the dry mulch is still intact.

4. Fertilizer Application

  • Soil test and apply the recommended amount of nitrogen (N) fertilizer. Over application can increase crop losses from downy brome competition.
  • Deep banding of N fertilizer generally improve winter wheat yield potential and crop competitiveness over downy brome compared to broadcast application.
  • If downy brome is a problem, deep band N fertilizer early in the fallow season rather than applying it near planting.
  • Do not use spring N topdress in winter wheat fields with downy brome infestations. The crop will rarely benefit from the nitrogen and it can be greatly increase weed seed production and weed water use.
  • Be sure there is adequate phosphorus (P) soil fertility and early primary root access to P fertilizer. If P fertilizer is needed, deep band or starter placement can stimulate wheat root growth and increase crop competitiveness against downy brome. However, there is often less crop response to P fertilizer with early seeding than with late seeding.

5. Preplant Incorporated (PPI) Herbicides

  • Refer to the herbicide labels and PNW Weed Control Handbook regarding the use of PPI herbicides for downy brome control in winter wheat, particularly with deep furrow drills in the wheat-fallow regions. Remember that spring planting after winterkill may be restricted following some PPI herbicides for winter wheat.
  • A PPI herbicide might be considered to help control a potential downy brome infestation in winter wheat when downy brome germination was poor the previous fall and winter due to dry conditions, or some seed production occurred during the fallow season. Site-specific use of PPI herbicides might also be considered on field borders or other isolated infestations within fields.
  • The decision to use a PPI herbicide needs to be made as early in the fallow season as possible, ideally before the primary tillage operation. Early adjustments in tillage are needed for retention of additional surface residue to offset residue loss with the herbicide incorporation operations.
  • If the weed infestation potential is high enough, it may be better to change to a Reclamation Strategy and plant a spring crop instead of winter wheat.

6. Seeding Considerations

  • Seed at a normal-"optimal" seeding date for the area to get early wheat establishment. Seeding through a dry mulch layer into a moist seedzone with a deep furrow drill can help the wheat crop be more competitive against downy brome -- which emerges later after fall rains. Northwest research has shown that a three week earlier emergence of the wheat compared to downy brome often can prevent significant yield losses from light to moderate downy brome infestations (Fig. 1). Yield losses significantly increase when downy brome emerges less than 7-10 days after wheat emergence. Earlier winter wheat emergence can also be important in achieving the differential growth stages between the wheat and downy brome needed for safe application of some postemergence downy brome herbicides.

Fig. 1. Effect of the time of downy brome emergence after wheat emergence
on wheat yield loss potential under light to moderate infestations in the Inland Northwest (Alex Ogg, USDA-ARS, Pullman, WA)
  • Avoid excessively early seeding dates because crop yields in a wheat-fallow rotation can be significantly reduced by disease and insect pest problems associated with earlier seeding. "Earlier than normal" seeding of winter wheat on fallow can increase losses from barley yellow dwarf, wheat streak mosaic, dryland foot rot, Cephalosporium stripe, strawbreaker foot rot, stripe rust, Russian wheat aphid, greenbug aphid and other insects and diseases.
  • If it rains before planting, delay seeding until downy brome emerges and control the weed with a nonselective herbicide or tillage before seeding. However, delaying the seeding date to wait for rain to germinate downy brome may reduce the wheat's competitive advantage over downy brome. With later seeding in moist soil conditions, wheat and downy brome often emerge at the same time, increasing weed competitiveness (Fig. 1) and preventing the differential growth stages needed for application of some postemergence herbicides. In addition, delaying seeding beyond the optimum seeding date for the area can directly reduce yield potential. In dry years, delayed seeding may also result in seed zone soil drying to the extent that it limits wheat germination and emergence.
  • The potential for crop injury from some PPI herbicides could increase as seeding date is delayed, because a shallower seeding depth is needed with later seeding.

Reclamation Control Strategies

General Considerations

Identify Reasons for Severe Infestation - Try to identify the particular production and/or environmental conditions which contributed most to the increased infestation of downy brome. Although there are often several factors involved, knowing the management options which contributed most to the problem can help focus reclamation efforts and design future maintenance strategies. Some common conditions known to increase losses from downy brome include reduced crop competition because of winterkill, diseases, insects, drought, soil crusting, and late seeding or emergence. Downy brome competitiveness in infested fields can also be increased by excessive application of N fertilizer or N topdressing.
Develop a Multi-Year Plan - There are no quick fixes in solving a dense infestation of downy brome. However, downy brome infestations can be reduced significantly in 2-3 years through a reclamation effort because the weed germinates readily and seed is relatively short-lived in the soil. Combinations of management practices are usually most effective for reducing infestations in fields.
Consider Site-Specific Management Approach - Downy brome infestations usually are not uniform within fields. Growers should strive to reclaim small infestations in fields before they become a whole-field problem.

Rotation Change a Key Reclamation Tool

The primary component in a Reclamation Control Strategy is a temporary change in crop rotation with the inclusion of one or more spring crops. When downy brome infestations are dense, a minimum of two to three years out of winter wheat are needed to effectively deplete the downy brome seedbank in the soil. Select management options based on the severity of the downy brome problem, farm program and conservation compliance requirements, soil depth, yield potential for winter wheat and rotation crops, additional pest problems affected, and other factors.
Other management options such as more intensive tillage, weed seed destruction by burning, and herbicide applications may also be considered in Reclamation Strategies, but the most effective component is crop rotation.
There can be other pest management benefits of a rotation change. Temporarily lengthening the crop rotation with spring crops so winter wheat is not grown more than once in 3 years also can reduce crop losses from some soilborne diseases of winter wheat, such as Cephalosporium stripe, strawbreaker foot rot and several root diseases.
The following temporary rotation changes with various combinations of spring crops will be described briefly:
  1. Rotation Options for Reclamation Strategies
  2. Winter wheat-fallow-spring wheat-fallow (WW-F-SW-F)
  3. Winter wheat-spring barley/or other spring crop-spring wheat-fallow (WW-SC-SW-F)
  4. Winter wheat-spring barley/or other spring crop-fallow (WW-SC-F)
In very low precipitation areas or during drought cycles, the WW-F-SW-F rotation would probably be a better choice than a spring crop after winter wheat. It also fits most easily with farm program base acreage requirements since spring wheat is planted and harvested during the normal program crop year for winter wheat. However, an 18-month fallow period extending over two winter seasons before spring wheat should only be used on soils that have a depth of about 40 inches or more. Soils shallower than 40 inches do not have adequate capacity for storing two winter seasons of precipitation and mobile nutrients such as nitrate nitrogen may leach below the rooting zone.
In higher precipitation areas and in wetter years, one or more spring crops should be grown after an infested winter wheat crop for reclamation of a serious downy brome problem. The success of rotation changes that include recropping would likely improve with increasing levels of annual precipitation. Farm program base acre restrictions may limit use of WW-SC-SW-F and WW-SC-F rotation options in Reclamation Control Strategies compared to WW-F-SW-F. Any time a rotation change is planned, the producer should check with the local ASCS office to coordinate with base allotment programs and with the SCS to adjust the farm conservation plan. Producers have more tillage and residue management options for downy brome control with a spring crop after winter wheat than with summer fallow.
Both rotations with three years out of winter wheat (WW-F-SW-F or WW-SB/SC-SW-F) will result in similar depletion of the downy brome seedbank. They would be more effective in fields with very dense infestations than the WW-SC-F rotation with only two years out of winter wheat.

Special Considerations for Rotation Options in Reclamation Strategies

The following are some specific management considerations for rotation options in Reclamation Strategies. They are listed in general chronological order. Every production step is not included, but several considerations are highlighted that may help improve downy brome control while optimizing erosion protection and crop yield. Note that many of the management considerations described under the Maintenance Control Strategies section also apply to Reclamation Control Strategies. Please refer to that section for additional information on related management decisions.

1. Management of an 18-month Fallow Before Spring Wheat (WW-F-SW-F)

  • Consider harrowing or other light tillage with minimal residue disturbance after winter wheat harvest to facilitate seed-soil contact and germination of downy brome seeds.
  • Consider applying a soil-active herbicide in the fall for control of winter annual grass weeds because no crop will be planted for about 18 month.
  • Consider using season-long chemical fallow. Remember that tillage is not needed to maintain seed-zone soil water since winter wheat will not be planted. Northwest research has shown that total soil water storage over the summer fallow season is similar between chemical fallow and fallow with various degrees of tillage intensity. The primary difference in soil water storage commonly has been a lower seedzone water content under chemical fallow, which is not important in this fallow-spring wheat sequence. Chemical fallow would also retain more surface residue in order to comply with conservation requirements in USDA programs.
  • If any tillage is used during the fallow cycle before spring wheat, it should be limited to retain adequate surface residue.
  • Consider applying a nonselective herbicide late in the fall if weather conditions permit. If a herbicide application is possible, it can minimize the potential of downy brome and volunteer wheat forming a dense sod, which can interfere with spring crop establishment. It can also minimize the buildup of root diseases on the roots of that overwinter "green bridge."
  • If overwinter runoff on frozen soils commonly occurs in the area, consider chisel or subsoiling on wide shank spacings in the fall before spring wheat.

2. Spring Crop Establishment

  • If a preplant nonselective herbicide is used in the spring, apply it at least 2 to 3 weeks before seeding (especially under direct seeding with a no-till drill) to avoid green bridge-root disease problems associated with pathogens on roots of dying volunteer and grass weeds. Early spring control also can reduce the potential of weeds and volunteer plants forming a dense sod.
  • Plant spring crops early using a minimum tillage or no-till system to minimize soil water loss. Water is usually the most important yield limiting factor for spring crops in the wheat-fallow region.

3. Fallow Management after Spring Crops

  • If downy brome seed was produced in the spring crop, consider postharvest harrowing or light tillage to facilitate seed-soil contact and increase germination of downy brome.
  • To retain as much spring crop residue as possible, use a minimum level of tillage beginning after harvest and through the fallow year. (See fallow management considerations under the Maintenance Control Strategies section.)

Alternative Tillage and Residue Management Options for the Winter Wheat- Spring Crop Sequence

Although moldboard plowing or burning of infested winter wheat fields can accelerate the depletion of downy brome seed populations, they often do not provide effective long-term control of dense downy brome infestations if the field remains in a 2-year wheat-fallow rotation. Because of the high potential for soil erosion in the subsequent fallow and winter wheat crop, these practices should only be considered when a spring crop will follow winter wheat in the 3-year WW-SC-F rotation as part of a Reclamation Control Strategy. With longer rotations, such as WW-F-SW-F or WW-SC- SW-F, there would be little additional downy brome control benefit from plowing or burning after winter wheat. The following are some management considerations for plowing and burning as part of a Reclamation Control Strategy.

1. Moldboard plowing

  • Consider soil water loss potential of tillage options. Northwest research has shown that fall plowing increases evaporation of overwinter precipitation and often reduces overwinter soil water storage by 1 to 2 inches compared to overwinter standing stubble (without frozen soil runoff) or chiseled stubble. The yield potential of the following spring crop may be lower with fall plowing because of reduced water storage. For example, spring wheat yields increase about five bushels per acre for each inch of additional water.
  • Delay plowing until postharvest harrowing or light tillage and fall rains have stimulated downy brome germination.
  • Plow furrow inversion is rarely complete. Consequently, some downy brome seeds are only shallowly buried and are able to emerge.
  • Where water erosion is a concern, use conservation plowing techniques which leave the soil rough and maintain more surface residue.
  • Do not plow where wind erosion is a problem.

2. Stubble Burning

  • Some of the seeds on the soil surface remain viable following stubble burning because of incomplete burning on the surface. Destruction of downy brome seed produced in an infested wheat crop is estimated to range from 60 to 90%. Much of the current year's downy brome seed is usually on the ground after wheat harvest.
  • Burning has no impact on downy brome seed in the soil. Even if most of the current year's weed seed could be destroyed by burning after a heavy infestation, sufficient seed may remain in the soil to produce a significant infestation.
  • Timing of stubble burning influences effectiveness of seed destruction. The effect of burning on the downy brome seedbank in a long-term experiment at the Columbia Basin Agricultural Research Center near Pendleton, OR was recently studied by Dan Ball, OSU Weed Scientist. His research shows that fall burning after each winter wheat crop in a stubble mulch wheat-fallow rotation from 1931 through 1992 resulted in about 75% fewer downy brome seeds in the top 2 inches of soil than without burning, and 66% fewer than with spring burning. Although seed number was reduced significantly, the downy brome problem had not been eliminated with this extreme use of burning. There were still about 14 viable downy brome seeds per square foot in the top 2 inches of soil.
  • Consider soil water loss potential from fall burning. Northwest research has shown that fall burning, like fall plowing, reduces storage of overwinter precipitation by 1-2 inches compared to standing or chiseled stubble. Spring burning would conserve more overwinter precipitation, but would be less effective in reducing downy brome seed populations.
  • Fall burned areas should be fall chiseled or subsoiled on the contour to minimize overwinter runoff and erosion.
  • Limit burning to spot treatment of dense infestation within fields.
  • Burning should not be used on highly erodible land.
  • Burning results in loss of nutrients in the residue. Nearly all of the nitrogen and about half of the sulfur and phosphorus. A ton of wheat residue contains about 12-15 pounds ofnitrogen, 3-4 pounds of phosphorus and 5 pounds of sulfur. A 50 bushel per acre winter wheat crop produces about 2.5 tons of residue per acre. Soil organic matter content is extremely low in much of the wheat-fallow region. Burning further decreases soil organic matter content, which influences a variety of properties affecting soil productivity and erodibility.

Additional Copies and Information

Additional copies of this PNW Conservation Tillage Handbook Series publication are available through county extension and conservation district offices in applicable dryland areas of the Pacific Northwest, or call Roger Veseth at (208) 885-6386. The entire Handbook can be purchased through county extension offices for $20, including postage, handling and new Handbook Series publications.
Pacific Northwest Conservation Tillage Handbook Series publications are jointly produced by University of Idaho Cooperative Extension System, Oregon State University Extension Service and Washington State University Cooperative Extension. Similar crops, climate, and topography create a natural geographic unit that crosses state lines in this region. Joint writing, editing, and production prevent duplication of effort, broaden the availability of faculty, and substantially reduce costs for the participating states.
For chemical control recommendations, refer to product labels and the Pacific Northwest Weed Control Handbook, an annually revised extension publication available from the extension offices of the University of Idaho, Oregon State University and Washington State University.
Cooperative Extension programs and policies comply with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. The University of Idaho Cooperative Extension System, Oregon State University Extension Service and Washington State University Cooperative Extension are Equal Opportunity Employers.
Roger Veseth, WSU/UI Extension Conservation Tillage Specialist, Moscow, ID;

Alex Ogg, Plant Physiologist, USDA-ARS Non-irrigated Weed Science Research Unit, Pullman, WA;

Donn Thill, Weed Scientist, UI, Moscow, ID;

Dan Ball, Weed Scientist, OSU, Pendleton, OR;

Don Wysocki, Extension Soil Scientist, OSU, Pendleton, OR;

Floyd Bailey, State Agronomist, USDA-SCS, Boise, ID;

Tom Gohlke, State Agronomist, USDA-SCS, Portland, OR;

Harry Riehle, State Agronomist, USDA-SCS, Spokane, WA


World Wide Web Page Created by Dr. Baird C. Miller, Dryland Cropping Systems Agronomist
Washington State University Pullman, WA 99164-6420
email: millerbc@wsu.edu
Created: August 16, 1995; Last Updated: August 16, 1995
     
 

Contact us: Hans Kok, (208)885-5971 | Accessibility | Copyright | Policies | WebStats | STEEP Acknowledgement
Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
Redesigned by Leila Styer, CAHE Computer Resource Unit; Maintained by Debbie Marsh, Dept. of Crop & Soil Sciences, WSU