TILLAGE HANDBOOK SERIES
Chap. 5, No. 15, June 1994
Downy Brome under Conservation Tillage Systems in the Inland Northwest
Alex Ogg, Donn Thill, Dan Ball, Don Wysocki,
Tom Gohlke, and Harry Riehle*
- Downy brome (Bromus
tectorum L.), commonly called cheatgrass, has been a major grass
weed problem in winter wheat in this region for decades. Although it
is a problem regardless of tillage system, it can be particularly troublesome
in the crop-fallow rotation under tillage systems designed to retain
more crop residue on the soil surface. When the seeds remain on the
surface or are buried shallowly, there is a potential for rapid development
of downy brome infestations unless effective weed management strategies
- Traditional methods
of controlling dense infestations of downy brome have included burying
seed deeply by intensive tillage systems and burning to destroy seeds
in the surface residue. However, high rates of soil erosion have often
occurred during the subsequent fallow-winter wheat sequence when these
practices are used. National legislation, beginning with the 1985 Food
Security Act, requires producers with highly erodible cropland to effectively
minimize soil erosion in order to be eligible to participate in USDA
programs. These laws have limited the use of intensive tillage and burning
as downy brome management tools in this production region.
- Many producers
have changed their tillage and residue management systems in recent
years as part of their farm conservation plans. It is critical that
producers develop an effective management strategy to minimize downy
brome seed production and to deplete seed populations in the soil in
order to avoid dense infestations of downy brome after conversion to
conservation tillage systems. This publication addresses two management
strategy situations based on the level of weed infestation: 1) "Maintenance"
Control Strategies to control a light to moderate downy brome infestation
which caused minimal crop yield loss; and 2) "Reclamation"
Control Strategies to recover from a dense infestation of
downy brome which caused substantial yield loss, or may reduce future
yield potential to crop failure levels.
- More information
on the identification and ecology of downy brome is covered in "Downy
Brome," a new PNW Extension publication in the PNW Weed Series.
Copies are available through County Extension offices in the Northwest.
Characteristics Related to Competitiveness and Management
- Downy brome is
a winter annual grass which germinates primarily in the fall. Seeds
also can be produced from plants that developed from seeds germinating
in the spring, although seed production is much more prolific from fall-germinating
Conditions - About 95% or more of the downy brome seeds in the top
two inches of soil germinate with the first opportunity under favorable
moisture and temperature conditions. Seed germination and emergence
of most downy brome typically occur after fall rains when soil temperature
has cooled to less than 70F, optimum soil temperature for germination
of this weed. However, seed germination continues at cooler soil temperatures
down to 32F, if soil moisture is adequate.
- Dormancy -
Although newly produced downy brome seeds require a short after-ripening
period for maximum germination, there is usually very little seed dormancy
and most seeds germinate when favorable temperature and moisture conditions
exist. However, some seeds can become dormant if they have absorbed
moisture in the fall but are not in favorable seed-soil contact conditions
and dry out before germination can proceed. This dormancy can last until
the following fall, thus maintaining a greater seedbank in the soil.
In addition, seeds remaining in the above-ground residue will also survive
in the field for a longer time than when in contact with the soil. Consequently,
management practices that provide good seed-soil contact conditions
for fall germination can help reduce seed carryover.
- Emergence -
Depth of downy brome seed burial greatly influences emergence potential.
About 90% of the seedlings emerge from seeds in the top 0.5 inch of
soil. Maximum depth of emergence is about 2.5 inches. This is why seed
burial by moldboard plowing has been a traditional downy brome control
- Seed Longevity
- Few seeds remain viable in the soil for more than 2 years. Consequently,
minimizing seed production and stimulating germination of seeds in the
soil are effective strategies for downy brome control.
- Growth and
Development - Downy brome has a very rapid growth rate, making it
highly competitive with winter wheat, particularly if it emerges close
to the time of wheat emergence. The primary root system, which develops
from the seed, continues growing during the fall and winter at soil
temperatures just above freezing. Secondary or crown roots, which emerge
from the plant crown, usually are initiated in the fall or winter and
are well developed before winter wheat resumes growth in the spring.
Although rooting depth can reach 3 to 4 feet, over 90% of the total
root mass is usually concentrated in the top 15 inches. The finely-divided
fibrous root system is highly efficient in exploiting soil moisture
- By the time winter
wheat dormancy ends in the spring, downy brome plants are often in the
tillering or jointing stage. Heading usually occurs about 4 weeks ahead
of winter wheat. The rapid transition through the reproductive stages
can make it difficult to prevent downy brome seed production in the
spring of the fallow season under prolonged wet spring conditions. Pollination
occurs very quickly after heading and it is difficult to identify in
the field. If pollination has just occurred, at least some viable seed
will be produced if the plant is cut off by shallow tillage. In addition,
under wet spring conditions some plants are often able to reestablish
if any roots are in contact with moist soil. If downy brome has headed,
some viable seed may form following an application of a nonselective
herbicide. In this situation, use of a fast burndown herbicide may help
reduce seed production. Seeds generally mature by late spring and many
drop from the seed head before wheat harvest.
- Crop Impact
- Downy brome infestations of 10 to 50 plants per square foot have
reduced winter wheat yields by 40 to 92 percent, respectively, in the
Inland Northwest. Under favorable growth conditions, an infestation
of downy brome can produce over 500 pounds of seed per acre (1 pound
of seed contains approximately 250,000 seeds). As many as 400 seeds
can be produced per plant. With this seed production potential, downy
brome populations can increase dramatically in just 2 to 3 years under
favorable weather and production conditions.
- Long-term management
strategies for downy brome control should focus on the following three
- Use management
practices that facilitate germination of current year seed and seed
in the soil to deplete the downy brome "seedbank." This is
particularly effective since the seeds are relatively short-lived in
the soil. With weeds such as jointed goatgrass, mustards and wild oat,
that have longer seed dormancy in the soil, there are fewer management
options to speed depletion of the soil seedbank.
Seed Production in Fallow
- Because downy
brome seeds form early, it is important to control downy brome when
the plants are still small to reduce the potential of seed production
in fallow if wet weather delays spring field operations. In years when
fall rains stimulate seeds to germinate and seedlings to grow vigorously,
control of downy brome in late fall after harvest can further reduce
the potential of seed set the next spring. Soil active herbicides for
winter annual grasses can also be applied in the fall after harvest
to provide control through the spring of the fallow season. Use of nonselective
herbicides ahead of spring tillage can improve control, particularly
under wet conditions.
Seed Production in Crop
- Minimize seed
production in crop through optimizing crop competitiveness against the
weeds and effective use of herbicides as needed. Early vigorous establishment
of winter wheat is a critical step in improving crop competitiveness.
- Use competitive
varieties adapted to your production area. Optimize crop health by controlling
other crop pests as much as feasible and by providing adequate plant
nutrients available for early crop root access to increase crop competitiveness
against downy brome. Avoid excessive amounts of nitrogen and do not
use surface topdress applications of nitrogen in infested fields, both
of which can increase downy brome competitiveness and crop losses.
- Identify Infestation
Level and Select Management Strategy - Determine the level of infestation
and decide whether to pursue a Maintenance Control Strategy or a Reclamation
Control Strategy. A Reclamation Strategy would be considered when the
current or future loss from a downy brome infestation in winter wheat,
plus the cost of control measures, is greater than reduced returns from
a spring crop compared to winter wheat with the downy brome infestation.
Historically, spring wheats generally yield about 2/3 of the yield of
winter wheat in the Northwest, although spring wheat yield potential
has been increasing with newer varieties, improved fertilizer placement
and increased soil water storage under conservation tillage. Northwest
research has shown that moderate infestations of downy brome emerging
within about 10 days of wheat emergence can reduce yields by about 1/3.
- Use Crop Rotation
Options - A change in crop rotation is a critical part of Reclamation
Strategies, but can also be included in a preventative Maintenance Strategy
whenever possible. Include a spring crop to lengthen the wheat-fallow
rotation whenever possible, particularly when winter precipitation has
been above normal, thus increasing the potential for a successful spring
crop. Crop rotations with a spring crop allow both spring and fall tillage
or herbicide options to kill the weeds and deplete the seedbank in the
soil. A competitive fall-seeded non-cereal crop, such as winter rapeseed,
with the option of using a grass weed herbicide also would help reduce
problems from downy brome, and other weeds and diseases associated with
the wheat-fallow rotation.
- In most of the
wheat-fallow areas, use of longer rotations is limited by low annual
rainfall, lack of profitable alternative crops and commodity base acreage
requirement of farm programs. However, temporary changes in crop rotations
need to be evaluated in terms of increased winter wheat yield potential
with improved downy brome control. Furthermore, growers need to include
the additional benefits of a spring crop in rotation for improved control
of other weeds, such as jointed goatgrass, and soilborne diseases, such
as Cephalosporium stripe, compared to the continuous wheat-fallow rotation.
- Prevent and
Limit Infestations - Clean tillage equipment and combines after
operating in infested fields. Planting "weed-free" wheat seed
is always a good preventative weed control practice, although it is
less important for downy brome since most of these small light seeds
pass through the combine at harvest and are easily cleaned from wheat
seed. Make a special effort to control even light downy brome infestations
on field borders, fencelines and roadways to reduce downy brome encroachment
into the field. Control options might include planting permanent grass
cover, timely mowing, and various herbicide options through the rotation.
- Intensive Tillage
or Burning - In dense downy brome infestations, more intensive tillage
or burning could be used to reduce downy brome seed populations after
winter wheat harvest in infested areas of the field where soil erosion
potential is low. These options should only be considered when a spring
crop is planned and will be covered in more detail in the Reclamation
Control Strategies section.
- Maintenance Control
Strategies focus on preventing a light weed population from developing
into a dense infestation, as well as preventing the establishment of
a downy brome problem in fields. The following are some potential management
options listed in general chronological order, beginning with winter
1. Downy Brome
Control begins at Harvest
distribute weed seeds, volunteer grain and residue at harvest to
enhance seed-soil contact for greater seed germination, improve
effectiveness of contact herbicides, reduce tie-up of soil-active
herbicides, reduce the potential for soilborne diseases associated
with the roots of weeds and volunteer plants, and improve effectiveness
of tillage operations. Combine chaff spreading systems can effectively
distribute seed and crop residue for improved seed-soil contact.
Harrowing and other light tillage operations after harvest are less
effective than chaff spreaders in distributing seed concentrations.
These operations provide greater seed-soil contact benefits if a
combine chaff spreading system has been used. Keep in mind that
chaff spreaders, harrowing and other tillage operations can also
spread localized downy brome infestations within fields.
Mgmt. After Winter Wheat
is the best time to manage for improved germination of downy brome.
Use a harrow or other light tillage operation soon after harvest
when the stubble is still dry to increase downy brome seed-soil
contact and germination potential with the first fall moisture.
Tillage intensity will depend on the crop residue level. Tine harrows
or skew treaders can improve seed-soil contact with minimal residue
disturbance. Disks must be used with caution because of excessive
surface residue burial and pulverizing of soil structure. Still,
if they are set at lower angles and operated at a shallow depth,
disks can be used to effectively improve seed-soil contact without
excessive residue burial.
- Consider using
a nonselective herbicide late in the fall, if fall rains have stimulated
good germination and growth of downy brome. When feasible, fall
control further reduces the possibility of spring seed set if weather
conditions prevent early spring control. It also reduces the potential
of heavy sod formation by downy brome and volunteer wheat, which
can make control in the spring more difficult. Lower labeled rates
sometimes can be used in the fall compared to the spring because
the plants are smaller and winter stress can help kill them. However,
the option of fall application is highly dependent on timely rains.
Dry fall conditions often limit germination, emergence and growth,
thus restricting the fall spray option. Furthermore, drought stress
and the presence of dust on the plants also can limit the possibility
of a fall application and use of lower rates. Under conservation
tillage systems, nonselective herbicides provide an important tool
to reduce both seed production potential in fallow, and the number
and intensity of tillage operations needed, which can increase storage
of winter precipitation and reduce soil erosion.
- Consider using
a fall application of a soil-active herbicide to control winter
annual grasses on the entire field, on field borders or in localized
infestations in the field. Soil-active herbicides generally can
improve control of volunteer wheat and many winter annual grasses
through the early part of the following fallow season. Activity
of some soil-active herbicides is increased when a fall tillage
operation is used. Remember that spring cereals usually are not
an option following a fall application of most soil-active herbicides
in fields originally planned for fallow.
- If water runoff
on frozen soils is a common occurrence in the area, consider chiseling
in the fall across the slope at the typical frost depth to improve
storage of winter precipitation. A chisel or subsoiler with wide
shank spacings can reduce runoff potential with minimal residue
Management in the Winter, Spring and Summer (if a spring crop
is not planted)
using a nonselective herbicide to delay the primary tillage operation
as long as feasible to increase residue retention and water conservation,
and reduce soil compaction from tillage on wet soils. If a spring
application of a nonselective herbicide is planned, earliness is
important to prevent plants forming a dense sod and setting seed.
High populations of large plants also can significantly deplete
soil water. In addition, dust interferes less with the activity
of herbicides when applied during early spring. A nonselective herbicide
in early spring, with or without tillage, usually provides better
downy brome control than tillage alone when soil is moist, temperature
is cool, and rains are still relatively frequent. Lengthening the
early spring period of cool, moist conditions -- without drying
the surface soil with early tillage -- favors continued spring germination
of downy brome. Little germination of downy brome is believed to
occur after early March in the low precipitation wheat-fallow regions.
The optimal time to begin setting the dry mulch seedzone line will
vary with location and yearly weather conditions.
- Think residue
management during the entire fallow season, from harvest through
planting. Minimize the number of tillage operations to increase
residue retention, maintain surface roughness, and minimize soil
water evaporation -- about 0.2 to 0.5 inch per tillage operation
when moist soil is brought to the surface. Select and operate implements
to retain the most residue on the surface. Rod weed only when weed
populations and growth warrant, or when needed to reestablish the
dry mulch layer after a rain. Consider tilling or spraying only
those areas in the field where weed populations are sufficient to
warrant control if the dry mulch is still intact.
test and apply the recommended amount of nitrogen (N) fertilizer.
Over application can increase crop losses from downy brome competition.
- Deep banding
of N fertilizer generally improve winter wheat yield potential and
crop competitiveness over downy brome compared to broadcast application.
- If downy brome
is a problem, deep band N fertilizer early in the fallow season
rather than applying it near planting.
- Do not use
spring N topdress in winter wheat fields with downy brome infestations.
The crop will rarely benefit from the nitrogen and it can be greatly
increase weed seed production and weed water use.
- Be sure there
is adequate phosphorus (P) soil fertility and early primary root
access to P fertilizer. If P fertilizer is needed, deep band or
starter placement can stimulate wheat root growth and increase crop
competitiveness against downy brome. However, there is often less
crop response to P fertilizer with early seeding than with late
Incorporated (PPI) Herbicides
to the herbicide labels and PNW Weed Control Handbook regarding
the use of PPI herbicides for downy brome control in winter wheat,
particularly with deep furrow drills in the wheat-fallow regions.
Remember that spring planting after winterkill may be restricted
following some PPI herbicides for winter wheat.
- A PPI herbicide
might be considered to help control a potential downy brome infestation
in winter wheat when downy brome germination was poor the previous
fall and winter due to dry conditions, or some seed production occurred
during the fallow season. Site-specific use of PPI herbicides might
also be considered on field borders or other isolated infestations
- The decision
to use a PPI herbicide needs to be made as early in the fallow season
as possible, ideally before the primary tillage operation. Early
adjustments in tillage are needed for retention of additional surface
residue to offset residue loss with the herbicide incorporation
- If the weed
infestation potential is high enough, it may be better to change
to a Reclamation Strategy and plant a spring crop instead of winter
at a normal-"optimal" seeding date for the area to get
early wheat establishment. Seeding through a dry mulch layer into
a moist seedzone with a deep furrow drill can help the wheat crop
be more competitive against downy brome -- which emerges later after
fall rains. Northwest research has shown that a three week earlier
emergence of the wheat compared to downy brome often can prevent
significant yield losses from light to moderate downy brome infestations
(Fig. 1). Yield losses significantly increase when downy brome emerges
less than 7-10 days after wheat emergence. Earlier winter wheat
emergence can also be important in achieving the differential growth
stages between the wheat and downy brome needed for safe application
of some postemergence downy brome herbicides.
- Fig. 1.
Effect of the time of downy brome emergence after wheat emergence
- on wheat
yield loss potential under light to moderate infestations in the
Inland Northwest (Alex Ogg, USDA-ARS, Pullman, WA)
excessively early seeding dates because crop yields in a wheat-fallow
rotation can be significantly reduced by disease and insect pest
problems associated with earlier seeding. "Earlier than normal"
seeding of winter wheat on fallow can increase losses from barley
yellow dwarf, wheat streak mosaic, dryland foot rot, Cephalosporium
stripe, strawbreaker foot rot, stripe rust, Russian wheat aphid,
greenbug aphid and other insects and diseases.
- If it rains
before planting, delay seeding until downy brome emerges and control
the weed with a nonselective herbicide or tillage before seeding.
However, delaying the seeding date to wait for rain to germinate
downy brome may reduce the wheat's competitive advantage over downy
brome. With later seeding in moist soil conditions, wheat and downy
brome often emerge at the same time, increasing weed competitiveness
(Fig. 1) and preventing the differential growth stages needed for
application of some postemergence herbicides. In addition, delaying
seeding beyond the optimum seeding date for the area can directly
reduce yield potential. In dry years, delayed seeding may also result
in seed zone soil drying to the extent that it limits wheat germination
- The potential
for crop injury from some PPI herbicides could increase as seeding
date is delayed, because a shallower seeding depth is needed with
- Identify Reasons
for Severe Infestation - Try to identify the particular production
and/or environmental conditions which contributed most to the increased
infestation of downy brome. Although there are often several factors
involved, knowing the management options which contributed most to the
problem can help focus reclamation efforts and design future maintenance
strategies. Some common conditions known to increase losses from downy
brome include reduced crop competition because of winterkill, diseases,
insects, drought, soil crusting, and late seeding or emergence. Downy
brome competitiveness in infested fields can also be increased by excessive
application of N fertilizer or N topdressing.
- Develop a Multi-Year
Plan - There are no quick fixes in solving a dense infestation of
downy brome. However, downy brome infestations can be reduced significantly
in 2-3 years through a reclamation effort because the weed germinates
readily and seed is relatively short-lived in the soil. Combinations
of management practices are usually most effective for reducing infestations
- Consider Site-Specific
Management Approach - Downy brome infestations usually are not uniform
within fields. Growers should strive to reclaim small infestations in
fields before they become a whole-field problem.
Change a Key Reclamation Tool
- The primary component
in a Reclamation Control Strategy is a temporary change in crop rotation
with the inclusion of one or more spring crops. When downy brome infestations
are dense, a minimum of two to three years out of winter wheat are needed
to effectively deplete the downy brome seedbank in the soil. Select
management options based on the severity of the downy brome problem,
farm program and conservation compliance requirements, soil depth, yield
potential for winter wheat and rotation crops, additional pest problems
affected, and other factors.
- Other management
options such as more intensive tillage, weed seed destruction by burning,
and herbicide applications may also be considered in Reclamation Strategies,
but the most effective component is crop rotation.
- There can be other
pest management benefits of a rotation change. Temporarily lengthening
the crop rotation with spring crops so winter wheat is not grown more
than once in 3 years also can reduce crop losses from some soilborne
diseases of winter wheat, such as Cephalosporium stripe, strawbreaker
foot rot and several root diseases.
- The following
temporary rotation changes with various combinations of spring crops
will be described briefly:
Options for Reclamation Strategies
wheat-fallow-spring wheat-fallow (WW-F-SW-F)
wheat-spring barley/or other spring crop-spring wheat-fallow (WW-SC-SW-F)
wheat-spring barley/or other spring crop-fallow (WW-SC-F)
- In very low precipitation
areas or during drought cycles, the WW-F-SW-F rotation would probably
be a better choice than a spring crop after winter wheat. It also fits
most easily with farm program base acreage requirements since spring
wheat is planted and harvested during the normal program crop year for
winter wheat. However, an 18-month fallow period extending over two
winter seasons before spring wheat should only be used on soils that
have a depth of about 40 inches or more. Soils shallower than 40 inches
do not have adequate capacity for storing two winter seasons of precipitation
and mobile nutrients such as nitrate nitrogen may leach below the rooting
- In higher precipitation
areas and in wetter years, one or more spring crops should be grown
after an infested winter wheat crop for reclamation of a serious downy
brome problem. The success of rotation changes that include recropping
would likely improve with increasing levels of annual precipitation.
Farm program base acre restrictions may limit use of WW-SC-SW-F
and WW-SC-F rotation options in Reclamation Control Strategies
compared to WW-F-SW-F. Any time a rotation change is planned,
the producer should check with the local ASCS office to coordinate with
base allotment programs and with the SCS to adjust the farm conservation
plan. Producers have more tillage and residue management options for
downy brome control with a spring crop after winter wheat than with
- Both rotations
with three years out of winter wheat (WW-F-SW-F or WW-SB/SC-SW-F)
will result in similar depletion of the downy brome seedbank. They would
be more effective in fields with very dense infestations than the WW-SC-F
rotation with only two years out of winter wheat.
Considerations for Rotation Options in Reclamation Strategies
- The following
are some specific management considerations for rotation options in
Reclamation Strategies. They are listed in general chronological order.
Every production step is not included, but several considerations are
highlighted that may help improve downy brome control while optimizing
erosion protection and crop yield. Note that many
of the management considerations described under the Maintenance Control
Strategies section also apply to Reclamation Control Strategies. Please
refer to that section for additional information on related management
of an 18-month Fallow Before Spring Wheat (WW-F-SW-F)
harrowing or other light tillage with minimal residue disturbance
after winter wheat harvest to facilitate seed-soil contact and germination
of downy brome seeds.
- Consider applying
a soil-active herbicide in the fall for control of winter annual
grass weeds because no crop will be planted for about 18 month.
- Consider using
season-long chemical fallow. Remember that tillage is not needed
to maintain seed-zone soil water since winter wheat will not be
planted. Northwest research has shown that total soil water storage
over the summer fallow season is similar between chemical fallow
and fallow with various degrees of tillage intensity. The primary
difference in soil water storage commonly has been a lower seedzone
water content under chemical fallow, which is not important in this
fallow-spring wheat sequence. Chemical fallow would also retain
more surface residue in order to comply with conservation requirements
in USDA programs.
- If any tillage
is used during the fallow cycle before spring wheat, it should be
limited to retain adequate surface residue.
- Consider applying
a nonselective herbicide late in the fall if weather conditions
permit. If a herbicide application is possible, it can minimize
the potential of downy brome and volunteer wheat forming a dense
sod, which can interfere with spring crop establishment. It can
also minimize the buildup of root diseases on the roots of that
overwinter "green bridge."
- If overwinter
runoff on frozen soils commonly occurs in the area, consider chisel
or subsoiling on wide shank spacings in the fall before spring wheat.
a preplant nonselective herbicide is used in the spring, apply it
at least 2 to 3 weeks before seeding (especially under direct seeding
with a no-till drill) to avoid green bridge-root disease problems
associated with pathogens on roots of dying volunteer and grass
weeds. Early spring control also can reduce the potential of weeds
and volunteer plants forming a dense sod.
- Plant spring
crops early using a minimum tillage or no-till system to minimize
soil water loss. Water is usually the most important yield limiting
factor for spring crops in the wheat-fallow region.
Management after Spring Crops
downy brome seed was produced in the spring crop, consider postharvest
harrowing or light tillage to facilitate seed-soil contact and increase
germination of downy brome.
- To retain
as much spring crop residue as possible, use a minimum level of
tillage beginning after harvest and through the fallow year. (See
fallow management considerations under the Maintenance Control Strategies
Tillage and Residue Management Options for the Winter Wheat- Spring
- Although moldboard
plowing or burning of infested winter wheat fields can accelerate the
depletion of downy brome seed populations, they often do not provide
effective long-term control of dense downy brome infestations if the
field remains in a 2-year wheat-fallow rotation. Because of the high
potential for soil erosion in the subsequent fallow and winter wheat
crop, these practices should only be considered when a spring crop will
follow winter wheat in the 3-year WW-SC-F rotation as part of
a Reclamation Control Strategy. With longer rotations, such as WW-F-SW-F
or WW-SC- SW-F, there would be little additional downy brome
control benefit from plowing or burning after winter wheat. The following
are some management considerations for plowing and burning as part of
a Reclamation Control Strategy.
soil water loss potential of tillage options. Northwest research
has shown that fall plowing increases evaporation of overwinter
precipitation and often reduces overwinter soil water storage by
1 to 2 inches compared to overwinter standing stubble (without frozen
soil runoff) or chiseled stubble. The yield potential of the following
spring crop may be lower with fall plowing because of reduced water
storage. For example, spring wheat yields increase about five bushels
per acre for each inch of additional water.
- Delay plowing
until postharvest harrowing or light tillage and fall rains have
stimulated downy brome germination.
- Plow furrow
inversion is rarely complete. Consequently, some downy brome seeds
are only shallowly buried and are able to emerge.
- Where water
erosion is a concern, use conservation plowing techniques which
leave the soil rough and maintain more surface residue.
- Do not plow
where wind erosion is a problem.
of the seeds on the soil surface remain viable following stubble
burning because of incomplete burning on the surface. Destruction
of downy brome seed produced in an infested wheat crop is estimated
to range from 60 to 90%. Much of the current year's downy brome
seed is usually on the ground after wheat harvest.
- Burning has
no impact on downy brome seed in the soil. Even if most of the current
year's weed seed could be destroyed by burning after a heavy infestation,
sufficient seed may remain in the soil to produce a significant
- Timing of
stubble burning influences effectiveness of seed destruction. The
effect of burning on the downy brome seedbank in a long-term experiment
at the Columbia Basin Agricultural Research Center near Pendleton,
OR was recently studied by Dan Ball, OSU Weed Scientist. His research
shows that fall burning after each winter wheat crop in a stubble
mulch wheat-fallow rotation from 1931 through 1992 resulted in about
75% fewer downy brome seeds in the top 2 inches of soil than without
burning, and 66% fewer than with spring burning. Although seed number
was reduced significantly, the downy brome problem had not been
eliminated with this extreme use of burning. There were still about
14 viable downy brome seeds per square foot in the top 2 inches
- Consider soil
water loss potential from fall burning. Northwest research has shown
that fall burning, like fall plowing, reduces storage of overwinter
precipitation by 1-2 inches compared to standing or chiseled stubble.
Spring burning would conserve more overwinter precipitation, but
would be less effective in reducing downy brome seed populations.
burned areas should be fall chiseled or subsoiled on the contour
to minimize overwinter runoff and erosion.
- Limit burning
to spot treatment of dense infestation within fields.
- Burning should
not be used on highly erodible land.
- Burning results
in loss of nutrients in the residue. Nearly all of the nitrogen
and about half of the sulfur and phosphorus. A ton of wheat residue
contains about 12-15 pounds ofnitrogen, 3-4 pounds of phosphorus
and 5 pounds of sulfur. A 50 bushel per acre winter wheat crop produces
about 2.5 tons of residue per acre. Soil organic matter content
is extremely low in much of the wheat-fallow region. Burning further
decreases soil organic matter content, which influences a variety
of properties affecting soil productivity and erodibility.
Copies and Information
- Additional copies
of this PNW Conservation Tillage Handbook Series publication
are available through county extension and conservation district offices
in applicable dryland areas of the Pacific Northwest, or call Roger
Veseth at (208) 885-6386. The entire Handbook can be purchased
through county extension offices for $20, including postage, handling
and new Handbook Series publications.
- Pacific Northwest
Conservation Tillage Handbook Series publications are jointly produced
by University of Idaho Cooperative Extension System, Oregon State University
Extension Service and Washington State University Cooperative Extension.
Similar crops, climate, and topography create a natural geographic unit
that crosses state lines in this region. Joint writing, editing, and
production prevent duplication of effort, broaden the availability of
faculty, and substantially reduce costs for the participating states.
- For chemical
control recommendations, refer to product labels and the Pacific Northwest
Weed Control Handbook, an annually revised extension publication available
from the extension offices of the University of Idaho, Oregon State
University and Washington State University.
Extension programs and policies comply with federal and state laws and
regulations on nondiscrimination regarding race, color, gender, national
origin, religion, age, disability, and sexual orientation. The University
of Idaho Cooperative Extension System, Oregon State University Extension
Service and Washington State University Cooperative Extension are Equal
WSU/UI Extension Conservation Tillage Specialist, Moscow, ID;
Plant Physiologist, USDA-ARS Non-irrigated Weed Science Research Unit,
Weed Scientist, UI, Moscow, ID;
Weed Scientist, OSU, Pendleton, OR;
Extension Soil Scientist, OSU, Pendleton, OR;
State Agronomist, USDA-SCS, Boise, ID;
State Agronomist, USDA-SCS, Portland, OR;
State Agronomist, USDA-SCS, Spokane, WA
Web Page Created by Dr. Baird C. Miller, Dryland Cropping Systems Agronomist
State University Pullman, WA 99164-6420
August 16, 1995; Last Updated: August 16, 1995