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PNW STEEP II EXTENSION
CONSERVATION FARMING UPDATE - December 1995


Getting Ahead of Downy Brome

Roger Veseth, WSU/UI Conservation Tillage Specialist

Downy brome (Bromus tectorum L.), commonly called cheatgrass, has been a major grass weed in winter wheat in the Inland Pacific Northwest for decades. The 1995 crop year was no exception for much of the region.

Although downy brome can be a problem in any tillage system, it can be particularly troublesome in two-year rotations under tillage systems designed to retain more crop residue on the soil surface. When the seeds remain on the surface or are shallowly buried, there is a potential for rapid development of downy brome infestations unless effective weed management strategies are implemented. It is critical that producers develop strategies to minimize downy brome seed production and deplete seed populations in the soil in order to avoid dense infestations of downy brome after conversion to conservation tillage systems.

Initial Considerations

Identify Infestation Level and Select Management Strategy - A maintenance control strategy can be used to control light downy brome infestations which cause minimal crop yield loss. A reclamation control strategy includes a change in crop rotation to recover from a dense infestation of downy brome which caused substantial yield loss, or may reduce future yield potential to near crop failure levels. Consider a reclamation strategy when the current or future profit margin from winter wheat with a dense downy brome infestation -- plus the cost of control measures -- is less than the profit margin from a spring crop. Inland Northwest research has shown that moderate infestations of downy brome emerging within about 10 days after winter wheat emergence can reduce yields by 30% or more. Spring wheat in this region has generally yielded about 60-70% of the yield of winter wheat -- without downy brome infestations. Spring wheat yield potential has been increasing, however, with new varieties, improved fertilizer placement technology, early control of volunteer grain and weeds, and increased soil water storage under conservation tillage.

Use Crop Rotation Options - A change in crop rotation is the critical part of a reclamation strategy, but can also be included in a preventative maintenance strategy. Include a spring crop to lengthen a two-year rotation whenever possible, particularly when winter precipitation has been above normal, thus increasing the potential for a successful spring crop. Spring cropping allow both spring and fall tillage or herbicide options to kill the weeds and deplete the soil seedbank. A competitive fall-seeded noncereal crop, such as winter rapeseed or canola, with the option of a grass weed herbicide, also would help reduce problems from downy brome and other weeds and diseases associated with the two-year 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 or wheat legume rotations.

Prevent and Limit Infestations - Clean tillage equipment and combines after operating in infested fields. Make a special effort to control even light downy brome infestations on field borders, fence lines and roadways to reduce downy brome encroachment into the field.

Plowing or Burning - With heavy downy brome infestations, moldboard plowing or burning in the fall after winter wheat could help accelerate the reduction of the downy brome seedbank. Since about 90% of the downy brome seedlings emerge from seeds in the top inch of soil, seed burial with moldboard plowing can significantly reduce seed survival if they are not brought back near the surface with subsequent tillage. While burning has no impact on downy brome seed in the soil, destruction of above-ground seed generally ranges from 60 to 90%. However, since seed burial or burning in seldom complete, these practices alone do not provide effective long-term control of dense downy brome infestations if the field remains in a two-year winter wheat rotation. In addition, on highly erodible land these options should only be considered when a spring crop is planned because of the higher soil erosion potential the subsequent year in fallow and the next winter wheat crop. Growers also need to consider soil water loss potential -- commonly 1-2 inches less water is stored overwinter with a bare soil surface compared to soil with residue cover.

Focus on Key Weed Management Goals

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

1) Facilitate Seed Germination During Fall and Winter

Use management practices during and after winter wheat harvest that facilitate germination of current year seed and seed in the soil during the fall and winter to deplete the downy brome "seedbank." This is an effective management approach because the seeds are relatively short-lived in the soil, in contrast to other weeds such as jointed goatgrass, mustards and wild oat, that have long seed dormancy in the soil.

Combine chaff spreading systems can effectively distribute weed and volunteer seeds and crop residue for improved germination potential. Chaff spreaders are more effective in distributing seed concentrations than postharvest harrowing and other light tillage operations. However, harrowing or light tillage postharvest can improve seed-soil contact for germination during the fall and winter. Keep in mind that chaff spreaders, harrowing and other tillage operations can also spread localized downy brome infestations within fields.

If overwinter water runoff on frozen soils is a common problem in the area, consider chiseling across the slope in the fall at the typical frost depth to improve storage of winter precipitation. A chisel with wide shank spacings or a subsoiler can reduce runoff potential with minimal residue disturbance.

2) Prevent Seed Production in Fallow or Alternate Crop Years

Consider applying a nonselective herbicide late in the fall, if fall rains have stimulated good germination and growth of downy brome. 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.

If fallow is planned, consider a fall-applied, 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 provide improved control of downy brome through the early part of the fallow season.

Because downy brome produces seed early in the spring, it is important to control plants when still small to reduce the potential of seed production. Nonselective herbicides used ahead of spring tillage can improve control, particularly under wet conditions. They provide an important tool to reduce both seed production potential between winter wheat crops, and the number and intensity of tillage operations needed, which can increase storage of winter precipitation and reduce soil erosion.

3) Minimize Seed Production and Yield loss in Crop

Minimize seed production by optimizing crop competitiveness and effective use of herbicides. Early vigorous establishment of winter wheat is a critical step in improving crop competitiveness. Northwest research shows that there is minimal winter wheat yield loss from light to moderate downy brome infestations if the wheat emerges about 3 weeks ahead of downy brome emergence. Yield losses significantly increase when downy brome emerges less than 7-10 days after winter wheat.

Use competitive varieties adapted to your production area. Optimize crop health by controlling insect pests and diseases as much as feasible and by providing an adequate supply of plant nutrients for early crop root access. 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.

Additional Information

This article was developed from excerpts from the new Pacific Northwest Extension publication titled "Managing Downy Brome under Conservation Tillage Systems in the Crop-Fallow Region of the Pacific Northwest," Pacific Northwest Conservation Tillage Handbook Series No. 15, Chapter 5. The publication provides an overview of downy brome characteristics related to competitiveness and management opportunities. Two management strategies based on the level of weed infestation are presented in a detailed chronological approach to production options.

Copies are available free through county extension and conservation district offices in applicable dryland areas of the Pacific Northwest, or call Roger Veseth at (208) 885-6386. The complete Handbook can be purchased through county extension offices for $20, including postage, handling and new Handbook Series publications. More details on the identification and ecology of the weed are available in the new color PNW Extension weed series publication PNW 474 "Downy Brome,"available for $.50 through county extension offices.

     
 

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