Downy brome (Bromus tectorum L.), commonly called cheatgrass (Photos 1a, b; 2a, b), has been a major grass weed in winter wheat in the Pacific Northwest for decades. Although it can be a problem regardless of tillage system, it is particularly troublesome in conservation tillage. When seeds remain on the surface or are buried shallowly, downy brome infestations develop rapidly without effective weed and crop management strategies.

Traditionally, farmers controlled dense infestations of downy brome by burying seed deeply using intensive tillage systems or burning stubble to destroy seeds in the surface residue. However, high rates of soil erosion often occur under such practices. National legislation, beginning with the 1985 Food Security Act and subsequent legislation in 1990 and 1995, requires that producers with highly erodible cropland effectively minimize soil erosion in order to participate in USDA programs. These laws have limited the use of intensive tillage and burning to manage downy brome infestations.

Recently, many producers changed their tillage and residue management systems to prevent excessive soil erosion (Photos 3a, b). Producers must develop strategies that minimize downy brome seed production and deplete seed populations already in the soil once they adopt conservation tillage.

This publication reviews aspects of downy brome biology as it relates to management and discusses key control strategies. Two management strategies are based on weed infestation levels: 1) Maintenance Control Strategies to control a light to moderate infestation resulting in low to moderate crop yield losses, and 2) Reclamation Control Strategies to reclaim land densely infested by downy brome, causing severe yield loss or crop failure.

Downy brome is a winter annual grass which germinates primarily in the fall. Although seed can be produced on plants that develop from spring germination, seed production is much more prolific on plants that develop from fall germination.

Seed Dormancy and Longevity in Soil.
After falling from the mother plant, downy brome seeds require a short after-ripening period. However, by the following fall there is usually very little seed dormancy and most seeds will germinate under favorable conditions. Some seeds will become dormant if they absorb moisture in the fall but lack adequate seed-soil contact and then dry out before germination can occur. This dormancy can last until the following fall and extend the life of that seed, thus contributing to a greater seed bank in the soil. Additionally, seeds in the aboveground crop or weed residue also will survive longer than those seeds in direct contact with the soil. Consequently, management practices that knock seed out of the residue or provide good seed-soil contact prior to the fall will help reduce persistence of downy brome seed in the soil.

Few downy brome seeds remain viable in the soil longer than two years. Consequently, minimizing seed production and stimulating germination of seeds are both effective strategies for rapidly reducing the downy brome soil seed bank (See box: Reducing the Downy Brome Soil Seed Bank, below).

Seed Germination and Emergence.
More than 95% of downy brome seeds in the top two inches of soil germinate during the first period of favorable moisture and temperature conditions after they are produced. Germination and emergence of downy brome typically occur after fall rains when soil temperature is just below 70°F, the optimal soil temperature for germination (Figure 1). However, if soil moisture is adequate, germination can occur at soil temperatures as low as 32°F.

Depth of seed burial greatly influences downy brome emergence (Figure 2). About 80% of the seedlings emerge from seeds located in the top 0.5 inches of soil. Maximum depth of emergence is about 2.5 inches.

Figure 1. Germination of downy brome seed as affected by temperature. (R.A. Buman and R.H. Abernethy, 1988. Journal of Range Management 41:35-39)

Figure 2. Emergence of downy brome in compacted and loose soils. (D.C. Thill and A.P. Appleby, 1979. Weed Science 27:625-630)

Growth and Development.
Downy brome exhibits rapid growth and development, making it highly competitive with winter wheat, particularly if emergence timing is close to that of wheat. The primary root system, which develops from the seed, grows throughout the fall and winter at soil temperatures just above freezing. Secondary or adventitious roots emerging from the plant crown usually are initiated in the fall or winter and are well developed before winter wheat resumes growth in the spring. The finely divided fibrous root system is highly efficient in exploiting soil moisture and nutrients. While roots can reach 3 to 4 feet deep, greater than 90% of the root mass is contained in the top 15 inches of soil.

By the time winter wheat dormancy ends, downy brome plants are often tillering or jointing. Heading usually occurs four weeks earlier than winter wheat. Seeds mature by late spring and most drop to the soil before wheat harvest. In fallow, rapid development of downy brome from heading through seed set makes it difficult to prevent seed production in the spring, particularly with prolonged wet conditions. Pollination occurs very quickly after heading and is difficult to recognize. Once pollination has occurred, at least some viable seed will be produced even if the plant is destroyed by shallow tillage. Additionally, under wet conditions, plants are often able to reestablish if roots remain in contact with soil. If downy brome has headed, some viable seed may still form even following application of a nonselective herbicide. However, using a herbicide with fast burndown may reduce seed production.

Seed Production.
Under favorable growth conditions, an infestation of downy brome can produce more than 500 pounds of seed per acre (1 pound of seed contains approximately 250,000 seeds). Up to 400 seeds are produced per plant. With this seed production potential and favorable conditions, downy brome populations can increase dramatically in just two to three years.

Losses due to downy brome infestations vary widely. Figure 5 shows the percentage yield loss of winter wheat with increasing density of downy brome plants. Winter wheat yield loss is influenced by both density of the downy brome stand and its emergence relative to the crop. Also, wheat varieties vary in their ability to compete with weeds. Economic losses are due to competition with the crop for nutrients, moisture, and other resources. While contamination of wheat harvest can result in a dockage assessment in severe infestations, downy brome seeds separate relatively easily from wheat during combining or screening. Moreover, with the exception of severe infestations, downy brome normally doesn't affect harvest efficiency.

While herbicides can provide fair to excellent control of downy brome, they are but one weed control tool and should be used as part of an integrated weed management strategy. Many herbicides require an adjustment in production practices for selective control of downy brome. Producers may need to consider special equipment needs such as incorporation tools or deep furrow drills when selecting herbicides. Some herbicides also may restrict future cropping or recropping options. Always refer to the herbicide labels and PNW Weed Control Handbook to determine which products or application rates of these products can be used legally.

Herbicide Resistance and Resistance Management.
Herbicide resistance is the ability of a weed biotype to survive a herbicide treatment to which the original population was susceptible. Typically, herbicide-resistant plants are present in a population at extremely small numbers. Repeated use of a herbicide selects for weed biotypes that are resistant to that herbicide. Once herbicide-resistant biotypes become common in a field, that herbicide is no longer effective. Moreover, persistence of weed seeds assures that a herbicide-resistant population will remain in a field for many years. Good field management practices can prevent development of large populations of herbicide-resistant weeds. Strategies to prevent such development include:

• Avoid year after year use of the same herbicide or herbicides with the same mode of action.
  
• Use herbicides with a short soil residual.
  
• Avoid repeated application of a herbicide in the same field within the same year.
  
• Use crop rotation to expand the number of herbicide options.
  
• Plant clean seed and use clean harvest and tillage equipment to prevent introduction of herbicide-resistant weeds into or between fields.
  
• Include as many cultural, mechanical, biological, and chemical tools as you can in an integrated weed management strategy.

First, determine the level of infestation in order to decide whether to pursue a Maintenance Control Strategy or a Reclamation Control Strategy. Consider a Reclamation Strategy when expected losses and control cost of downy brome in winter wheat exceed the difference between the net return of a spring crop compared to that of winter wheat. Historically, spring wheat yield is about 2/3 that of winter wheat in the Northwest. More recently, spring wheat yields have increased with improved varieties, more precise fertilizer placement, and conservation tillage practices which increase soil water storage. Research in the Northwest has shown that moderate infestations of downy brome emerging within 10 days of wheat emergence can reduce winter wheat yield by about 1/3.

Maintenance Control Strategies.
These focus on preventing a light weed population from developing into a dense infestation. Following are management options listed in chronological order, beginning with winter wheat harvest.

At Harvest
Uniformly distribute weed seeds, escaped grain, and residue to enhance seed-soil contact. Uniform distribution enhances seed germination, improves effectiveness of contact herbicides, and activity of soil-applied herbicides, reduces soilborne diseases associated with the roots of weeds and volunteer plants, and improves effectiveness of tillage. Chaff spreaders on combines can evenly distribute seed and residue (Photo 4a). Harrowing or other light tillage methods following harvest are less effective than chaff spreaders in distributing seed and residue; however, chaff spreaders or tillage also can spread localized downy brome infestations within fields. Therefore, consider harvesting or tilling infested patches in separate operations from the rest of the field.

Postharvest
Late summer or early fall is the best time to manage germination of the downy brome soil seed bank. Harrow or use other light tillage methods in dry stubble soon after harvest. Light tillage increases seed-soil contact, thus increasing germination when fall rains occur. Light tillage also provides greater seed-soil contact when a combine chaff spreading system is used. Tillage intensity will depend on the amount of crop residue. Tine harrows or skew treaders can improve seed-soil contact with minimal residue disturbance (Figure 3). Use disks with caution because they can bring about excessive surface residue burial and pulverizing of soil structure. Still, if they are set at lower angles and operated shallowly, disks can improve seed-soil contact without excessive residue burial.

Apply a nonselective herbicide late in the fall providing fall rains have stimulated good germination and growth of downy brome. Fall control of downy brome reduces seed set, particularly if weather conditions prevent early spring control. Fall control also reduces heavy sod formation by downy brome and volunteer wheat which can reduce the efficiency of spring-applied herbicides. Lower labeled herbicide rates are often more effective in the fall compared to the spring because the plants are smaller and winter stress aids in killing them. Efficiency of fall-applied herbicides depends on timely rains. Dry fall conditions often limit germination, emergence, and growth of plants, resulting in little impact of fall herbicide applications on the soil seed bank. Furthermore, drought stress and dust on the plants can impede the effectiveness of fall herbicide applications. Nonselective herbicides also can reduce seed production during the fallow year and thereby reduce the number and intensity of tillage operations. This results in greater storage of winter precipitation and reduced soil erosion.

Apply a soil-active herbicide in the fall prior to the fallow year in order to control winter annual grasses throughout the field, on field borders, or in localized infestations within the field. Soil-active herbicides can control volunteer wheat and winter annual grasses into the fallow year. Activity of some soil-active herbicides can be improved with fall tillage. Monitor treated fields in the spring to assess effectiveness of soil-active herbicides. However, spring cereal crops are usually not an option following fall application of most soil-active herbicides in fields originally planned for fallow.

Fertilizer Application
Test soil and apply only the recommended amount of nitrogen (N) fertilizer. Over-application can result in crop yield reduction due to downy brome competition.

Deep banding of N fertilizer improves winter wheat yield and crop competitiveness over downy brome compared to a broadcast application.

Deep band N fertilizer early in the fallow season rather than near planting.

Do not spring top dress N fertilizer in winter wheat fields with downy brome infestations. The crop rarely benefits from the nitrogen (Figure 4) and it will increase weed seed production and weed water use.

Be sure phosphorus (P) soil fertility is adequate and early primary roots can access P fertilizer. If P fertilizer is needed, a 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 in early seeded fields.

Seeding
Seed at a normal, optimal seeding date in your area for early wheat establishment. Most downy brome will emerge from seed within the top 0.5 inches of the surface (Figure 2) and will not germinate until moisture is sufficient. Seeding through a dry mulch layer into a moist seed zone with a deep furrow drill will allow wheat to emerge before downy brome and improve its competitive advantage. Northwest research has shown that wheat emerging three weeks prior to downy brome will prevent significant yield losses from light to moderate infestations (Figure 5). Yield loss increases dramatically when downy brome emerges within 7-10 days of wheat. Early winter wheat emergence is also important in achieving the differential growth between wheat and downy brome that improves efficiency and crop safety of postemergence herbicides.

Avoid excessively early seeding. It promotes yield-reducing disease and insect pests in a winter wheat-fallow system. These can include increased infestations of barley yellow dwarf, wheat streak mosaic, dryland foot rot, Cephalosporium stripe strawbreaker foot rot, stripe rust, Russian wheat aphid, and greenbug aphid, among others.

If rains occur just before anticipated 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 seeds may reduce the wheat's fall growth and competitiveness against downy brome. With later seeding in moist conditions, wheat and downy brome will emerge at the same time, resulting in greater weed competition (Figure 5) and reduced efficacy or increased crop injury from postemergence herbicides. Additionally, delaying seeding beyond the optimum seeding date will reduce yield potential. Delayed seeding may also result in seed zone soil drying which reduces wheat germination and emergence.

The potential crop injury of preplant incorporated (PPI) herbicides also increases as seeding date is delayed, because a shallower seeding depth is needed.

If downy brome infestation is severe, consider substituting a spring crop in place of winter wheat in the rotation. (See the Reclamation Control Strategies below for more information concerning when to substitute a spring crop for winter wheat.)

Herbicide Applications
Refer to the herbicide labels and PNW Weed Control Handbook regarding the use of herbicides for downy brome control in winter wheat. Spring planting in the event of a crop winterkill may be restricted with some PPI herbicides.

Localized use of herbicides can often be applied to field borders or other isolated infestations within fields.

The decision to use a PPI herbicide should be made as early as possible, ideally before the primary tillage operation. Early decisions allow adjustment in tillage which retains surface residue and offsets residue loss with herbicide incorporation.

Reclamation Control Strategies.
There are no quick fixes in solving a dense infestation of downy brome. However, reclamation strategies can greatly reduce downy brome infestations with 2 to 3 years' effort because the seeds germinate readily and are relatively short-lived in the soil. Combinations of management practices within and between years are most effective for reducing infestations.

Identify major production or environmental conditions that specifically contribute to severity of the downy brome infestation. Although several factors may be involved, knowing which management practices contribute to the problem helps focus reclamation efforts and maintenance strategies. Reduced winter wheat stands due to winterkill, disease, insects, drought, soil crusting, and late seeding or emergence will reduce crop competition and increase downy brome infestation. Downy brome competitiveness also increases with excessive amounts of N fertilizer or by N top dressing. First, evaluate crop production practices to determine which may be reducing crop competition or increasing weed competition. Then select management options based on the severity of the downy brome problem, conservation compliance requirements, soil depth, yield potential for winter wheat and rotation crops, and additional pest problems. Downy brome infestations usually are not uniform within fields. Identify areas of fields with dense infestations and intensify management in these areas before the infestation spreads throughout the field.

The most effective tool in a Reclamation Control Strategy is to include spring crops in the rotation (Figure 6). A minimum of two to three years out of winter wheat are needed to effectively deplete the downy brome soil seed bank. Additionally, including spring crops in a rotation so that winter wheat is not grown more than once in three years also reduces soilborne diseases of winter wheat, such as Cephalosporium stripe, strawbreaker foot rot, and root pathogens. Another rotation option could include a winter annual broadleaf crop such as canola for one of the two or three years out of winter wheat. This option allows the use of selective grass herbicides in the broadleaf crop. However, few herbicides provide 100% control of weeds and, if reclamation strategies are needed, it is best to remain out of winter annual crops for at least two years. Following winter wheat with a winter annual broadleaf crop and then two years of spring cropping effectively reduces the downy brome soil seed bank. The success of rotations that include recropping improves with greater annual precipitation. When adjusting tillage or rotation, check with the National Resource Conservation Service to adjust the farm conservation plan.

Figure 6. Downy brome population by crop year in two different rotations. (Frank Young, USDA-ARS, Pullman, Wash.)

Tillage and residue management options to control downy brome are more numerous when following winter wheat with a spring annual crop than with summer fallow. Fallow may be used in combination with spring crops to extend the number of years between winter wheat crops. Some prospective alternative crop/fallow rotations include winter wheat-fallow-spring wheat-fallow (WW-F-SW-F), winter wheat-spring barley or other spring crop-spring wheat-fallow (WW-SC-SW-F), or 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 is probably a better choice than a spring crop after winter wheat.

Consider the following information when a spring crop is included in a continuous cropping or crop fallow rotation to increase soil and moisture conservation and still maintain or increase productivity:

• Summer fallow prior to a spring crop means two winters' worth of precipitation will be stored in the soil. Be sure soils are deep enough to adequately hold this amount of moisture without excessive nutrient loss. Chemical fallow in this situation may allow for better soil conservation without the loss of soil moisture (See box: Fallow Management, p. 11).
  
  
• If downy brome seed is produced in the spring crop, consider postharvest harrowing or light tillage to facilitate seed-soil contact and increase germination of downy brome (See box: Reducing the Downy Brome Soil Seed Bank, above). Use a minimum level of tillage after harvest of the spring crop to retain as much residue as possible, since grain and straw production is expected to be less with spring crops than with winter wheat. Adjust tillage operations to maintain adequate soil residue cover.
  
  
• Use a minimum tillage or no-till system to plant spring crops early to reduce soil water loss. Availability of soil water is usually the most limiting factor in the production of spring crops.
  
  
• Apply a preplant nonselective herbicide in the spring at least two to three weeks before seeding spring crops (especially when direct seeding with a no-till drill) to avoid green bridge and root disease problems. Early spring herbicide applications also reduce sod formation by weeds and volunteer plants.

Intensive tillage, stubble burning, or herbicide applications may be used in a reclamation strategy (See box: Intensive Tillage and Stubble-Burn Considerations, p. 13). However, you can reduce downy brome infestations more if these practices are combined with rotations that include at least two years out of winter wheat. Truly integrated approaches to weed management will make it more difficult for weed bio-type populations to develop that resist any single form of control.

Downy brome is particularly well-adapted to the winter wheat-based cropping systems of the Inland Northwest. Moreover, it establishes most readily from seeds within 0.5 inches of the surface, making it an even greater problem under conservation tillage.

Downy brome seed is relatively short-lived in the soil. The soil seed bank can be rapidly reduced if no new seed is added. The key to managing moderate to severe infestations of downy brome is to get as much of the soil seed bank to germinate following the winter wheat crop and then using nonselective or residual herbicides to kill downy brome plants. This is most effectively done by spreading the seed evenly on the surface at harvest by means of a chaff spreader attached to the combine followed by postharvest, shallow tillage which mixes seed into the top 0.5 inches of the soil surface. Fall rains will then promote mass seed germination and the plants can be killed either with fall or spring nonselective herbicide applications or combinations of both.

In the most severe infestations of downy brome, rotating to a spring-seeded crop or some other rotation which takes the field out of winter cereal production for a minimum of three growing seasons is key. Spring crops provide selective and nonselective herbicide or tillage options that effectively control downy brome. Preventing downy brome growth for three seasons will very nearly deplete the soil seed bank. Once seed numbers are low, downy brome should be managed to minimize seed production and the severity of resulting infestations.

Introduced to the region around the year 1900, downy brome reached its present-day distribution about 1930. Over sixty years of intensive research has not yielded a "magic bullet" to solve the downy brome problem. However, the weed management toolkit includes numerous tactics that can be combined to minimize the severity of downy brome infestations while sustaining production of winter wheat and other marketable commodities.

Joe Yenish, WSU Extension Weed Scientist, Pullman, Wash.;
Roger Veseth, WSU/UI Extension Conservation Tillage Specialist, Moscow, Idaho;
Alex Ogg, USDA-ARS Plant Physiologist, Non-irrigated Weed Science Research Unit, Pullman, Wash.;
Donn Thill, UI Weed Scientist, Moscow, Idaho;
Dan Ball, OSU Weed Scientist, Pendleton, Ore.;
Frank Young, USDA-ARS Research Agronomist, Non-irrigated Weed Science Research Unit, Pullman, Wash.;
Eric Gallandt, WSU Weed Scientist, Pullman, Wash.;
Don Morishita, UI Extension Weed Scientist, Twin Falls, Idaho;
Carol Mallory-Smith, OSU Weed Scientist, Corvallis, Ore.;
Don Wysocki, OSU Extension Soil Scientist, Pendleton, Ore.; and
Tom Gohlke, NRCS State Agronomist, Portland, Ore.

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Issued by Washington State University Cooperative Extension, James J. Zuiches, director; Oregon State University Extension Service, Lyla Houglum, interim director; the University of Idaho Cooperative Extension System, LeRoy D. Luft, director; and the U.S. Department of Agriculture, in furtherance of the Acts of May 8 and June 30, 1914. 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. Published July 1998. $4.50 PNW0509