A Pacific Northwest Cooperative
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
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 growth stage.
Photo 1b. Magnified view
of the collar region showing characteristic open sheath,
long fleshy ligule, and pubescence.
Photo 2a. Mature downy
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
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
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.
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
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
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
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
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 efficiency.
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 38:224-228)
Herbicides for Control of Downy
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.
Management Strategies 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 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.
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, 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
- 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
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
Managing Summer Fallow Before
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
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 overwintering plants.
Photos 5a. and 5b. Fallow
tillage operations, such as rod weeding, can add to air
and water quality problems through greater soil erosion.
Intensive Tillage and
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 water.
- 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 residue.
- 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
- 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
- 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,
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, Idaho;
Carol Mallory-Smith, OSU Weed Scientist, Corvallis, Ore.;
Don Wysocki, OSU Extension Soil Scientist, Pendleton, Ore.;
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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