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
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.
Photo 1a. Downy brome in the vegetative
Photo 1b. Magnified view of the collar
region showing characteristic open sheath, long fleshy ligule,
Photo 2a. Mature downy brome plant.
Photo 2b. Downy brome severely infesting
a field of winter wheat.
Photo 3a. Conservation tillage provides
greater soil cover during crop development than conventional
tillage and also prevents soil erosion.
Photo 3b. Conventional tillage.
Downy Brome Biology
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
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
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.
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.
Reducing the Downy
Brome Soil Seed Bank
Managing the soil seed bank is particularly effective since
the seeds are short-lived. The seed bank depletes more rapidly
than soil seed banks of weeds with longer seed dormancy such
as jointed goatgrass, mustards, or wild oat.
Facilitate Seed Germination.
Use management practices that increase germination of the soil
seedbank. Use a combine chaff spreader to evenly spread seed
(Photo 4a). Harrowing or other light tillage in dry stubble soon
after harvest increases seed-soil contact and germination when
fall moisture occurs. Tine harrows or skew treaders can improve
seed-soil contact with minimal residue disturbance. Use disks
with caution because of excessive surface residue burial and
pulverizing of soil structure. Research indicates a reduction
in the downy brome soil seed bank when light tillage is combined
with fall or spring control of downy brome plants (Figure 3).
Prevent Seed Production in Fallow.
Downy brome seeds form early in the spring. If wet weather delays
spring field operations, control downy brome with herbicides
before plants have headed to reduce seed production in fallow.
Where fall rains have stimulated germination and vigorous seedling
growth, apply nonselective herbicides in late fall to reduce
spring seed production. 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.
Minimize Seed Production in Crop.
Minimize seed production by optimizing crop competition and using
effective herbicides. Rapid establishment of a vigorous winter
wheat crop is a critical step in improving crop competitiveness.
Use competitive varieties adapted to your production area. Optimize
crop health by controlling other crop pests and providing adequate
plant nutrients for early crop root access. Avoid excessive amounts
of nitrogen and do not use surface top dress applications of
nitrogen in infested fields (Figure 4). Crop rotations which
include a spring crop allow both spring and fall tillage or herbicide
options to prevent seed production. A fall-seeded noncereal crop,
such as winter canola, allows the use of effective grass herbicides
which reduce downy brome seed production.
Intensive Tillage or Burning.
Use intensive tillage or burning to reduce the downy brome soil
seed bank in heavily infested fields where soil erosion potential
is low. Consider these options only when a spring crop is planned
(See box: Intensive Tillage and Stubble-Burn Considerations,
Prevent and Limit Infestations.
Clean tillage and harvest equipment after operating in infested
fields. Planting weed-free wheat seed is a good preventive practice
for all weeds. If patches are isolated in a field, consider special
harvest or tillage operations in these areas. Control downy brome
along field borders, fence lines and roadways to eliminate this
potential seed source. Control options might include planting
a permanent grass cover, timely mowing, and various herbicide
options throughout the rotation.
Photo 4a. Downy brome management begins
with harvest of the previous crop.
Photo 4b. Seed numbers are
greatly reduced with proper postharvest tillage.
Figure 3. Percentage of seed remaining in the
spring of 1995 and 1996 following various postharvest tillage
treatments. (Alex Ogg, USDA-ARS, Pullman, Wash.)
Figure 4. Winter wheat yield as affected by
fertilizer timing with and without the presence of downy brome.
(Dan Ball, Oregon State University, Pendleton, Ore.)
Crop Yield Losses
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
Figure 5. Percentage winter wheat yield
loss due to downy brome emerging 0 to 14 and 21 to 35 days after
winter wheat. (P.W. Stahlman and S.D. Miller, 1990. Weed Science
Control of Downy Brome
in Winter Wheat and Fallow
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
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
- Include as many cultural, mechanical, biological, and chemical
tools as you can in an integrated weed management strategy.
for a Downy Brome Infestation
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.
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.
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
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.
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
Deep band N fertilizer early in the fallow season rather than
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
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,
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.)
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,
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
- 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
- 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.
Managing Summer Fallow Before Winter
Use a nonselective herbicide to delay the primary tillage operation.
This increases residue retention, promotes water conservation,
and reduces soil compaction from tilling wet soils. Earliness
of a nonselective herbicide application is important to prevent
plants from forming a dense sod and setting seed. High populations
of larger plants also deplete soil water. Additionally, dust
interferes less with early spring-applied herbicides. An early
spring application of nonselective herbicides usually provides
better downy brome control than tillage alone when soil is moist,
temperature is cool, and rains are still relatively frequent.
Applications can be combined with timely tillage to further increase
control. However, delaying tillage and lengthening the spring
period of cool, moist conditions favors continued spring germination
of downy brome. Little germination of downy brome occurs after
early March in low precipitation wheat-fallow regions. The optimal
time to begin setting the dry mulch seed zone line varies with
location and weather conditions.
Consider residue management throughout the fallow season,
from harvest through planting. Minimizing tillage operations
increases residue retention, maintains surface roughness, and
minimizes soil water evaporation (Photos 5a, b). Research indicates
that 0.2 to 0.5 inches of soil water are lost per tillage operation
when moist soil is brought to the surface. Select tillage operations
which 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. If the dry mulch is still intact,
consider tilling or spraying only those areas in the field where
weed populations are sufficient to warrant control.
Managing Summer Fallow Before Spring
The practice of 18 months of fallow extending over two winters
before planting spring wheat should be used only on soils with
a depth greater than 40 inches. Shallower soils do not have adequate
capacity for storing two winters' worth of precipitation and
leaching of mobile nutrients such as nitrogen may occur. Consider
applying a residual soil-active herbicide in the fall for control
of winter annual grass weeds because no crop will be planted
for about 18 months.
Consider using season-long chemical fallow instead of the
traditional tillage system. Chemical fallow is the practice of
killing vegetation in the field during the fallow year either
with residual herbicides or, if necessary, applications of nonselective
herbicides. Tillage is not needed to maintain seed zone soil
water since winter wheat will not be planted. Northwest research
has shown that 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
has typically been lower seed zone water content under chemical
fallow, which is not important in this fallow-spring wheat sequence.
Chemical fallow retains more surface residue in order to comply
with conservation requirements in USDA programs. If winter runoff
on frozen soils commonly occurs in the area, consider chisel
or subsoiling on wide shank spacings in the fall before planting
Herbicides may be combined with tillage during chemical fallow.
Light tillage during the late summer of the fallow year may improve
soil seed contact of the downy brome seed remaining in the soil
and increase germination and soil seed bank depletion. Tillage
operations during the fallow year should retain enough surface
residue to prevent soil erosion.
If weather conditions permit, apply a nonselective herbicide
late in the fall before cropping. If a herbicide application
is possible, it can minimize sod formation by downy brome and
volunteer wheat, which interferes with spring crop establishment.
It also minimizes green bridge and buildup of root diseases on
Photos 5a. and 5b. Fallow tillage operations,
such as rod weeding, can add to air and water quality problems
through greater soil erosion.
and Stubble-Burn Considerations
Although moldboard plowing or burning of infested winter wheat
fields accelerate the depletion of the downy brome seed bank,
the effect will not be long-term if the field remains in a two-year
wheat-fallow rotation. Moreover, high potential for soil erosion
merits that these practices should be considered only following
winter wheat in the three-year WW-SC-F rotation within the Reclamation
Control Strategy. Longer rotations, such as WW-F-SW-F or WW-SC-SW-F,
realize little benefit from plowing or burning after winter wheat
to reduce the downy brome soil seed bank. Consider the following
issues to determine if plowing and burning should be part of
a Reclamation Control Strategy.
- Consider yield loss when deciding whether to moldboard plow.
Northwest research has shown that fall plowing increases evaporation
of overwinter precipitation and reduces overwinter soil water
storage by 1 to 2 inches compared to standing stubble (without
frozen soil runoff) or chiseled stubble. Spring wheat yields
increase about five bushels per acre for each inch of additional
- Delay plowing until postharvest harrowing or light tillage
and fall rains have stimulated downy brome germination.
- Plow furrow inversion rarely is complete, consequently, some
downy brome seeds are only shallowly buried and are able to emerge.
- Where soil water erosion is a concern, use conservation plowing
techniques which leave the soil rough and maintain more surface
- Do not plow where wind erosion is a problem.
- Destruction of downy brome seed in an infested wheat crop
by burning is estimated to be between 60% and 90% effective.
Some seeds on the soil surface will remain viable following stubble
burning because of an incomplete burn. 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
to produce a significant infestation.
- Timing of stubble burn influences effectiveness of seed destruction.
The effect of burning on the downy brome seed bank in a long-term
experiment has been studied at the Columbia Basin Agricultural
Research Center near Pendleton, Ore. 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, and 66% fewer than with
spring burning (Figure 7). While seed numbers were reduced, downy
brome had not been eliminated with this extreme use of burning.
About 14 viable downy brome seeds per square foot in the top
2 inches of soil remained following field 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 conserves
more overwinter precipitation, but is less effective in reducing
downy brome seed populations.
- Burning results in loss of residue nutrients. Nearly all
of the nitrogen and about half of the sulfur and phosphorus are
lost to the atmosphere. One ton of wheat residue contains 12
to 15 pounds of nitrogen, 3 to 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. Burning decreases soil organic
matter content, already low in much of the wheat fallow region,
which limits soil productivity and increases erosion.
- 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 infestations within
- Do not use burning on highly erodible land.
Figure 7. Downy brome seed numbers in the top
2" of soil following long-term fall burning, spring burning,
and no-burning treatments. (Dan Ball, Oregon State University,
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
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.
About the Authors
Joe Yenish, WSU Extension Weed Scientist, Pullman, Wash.;
Roger Veseth, WSU/UI Extension Conservation Tillage Specialist,
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,
Carol Mallory-Smith, OSU Weed Scientist, Corvallis, Ore.;
Don Wysocki, OSU Extension Soil Scientist, Pendleton,
Tom Gohlke, NRCS State Agronomist, Portland, Ore.
Information on the identification and ecology
of downy brome is found in "Downy Brome," extension
publication PNW474. Copies are available from county extension
offices in the Northwest.
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