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Use of OUST® Herbicide to Control
Cheatgrass in the Northern Great Basin
Mike Pellant
Julie Kaltenecker
Steven Jirik
Abstract-The herbicide OUS'f® (sulfometuron methyl) is being
used on rangelands in the northern Great Basin to control cheatgrass
(Bromus tectorum) for 1 to 2 years, thereby improving the success of
rehabilitation projects. Experimental data indicate that OUS'f®
provides more effective cheatgrass control than either burning or
disking. OUS'f® was applied to a 100-acre cheatgrass-infested
seeding near Mountain Home, Idaho, where it reduced cheatgrass
density by 91 percent compared to an adjacent untreated control.
Remnant perennial grasses were more vigorous in the OUS'f®
treatment. OUS'f® can be used to effectively control cheatgrass for
up to 2 years at a cost of$25-$28 per acre (herbicide and application
costs), using either ground or aerial application.
The Problem: Exotic Annual Grass
Invasion on Western Rangelands _
The introduction oftwo exotic annual grasses, cheatgrass
(Bromus tectorum) and medusahead wildrye (Taeniatherum
caput-medusae ssp. asperum), onto rangelands of the western United States was undoubtedly one of the most significant ecological events in North American history (Peters
and Bunting 1994). Anthropogenic disturbances associated
with settlement and growth of the human population, expansion of transportation systems and agriculture created
openings in native plant communities for invasion and
eventual dominance of exotic species (Mack 1981). Exotic
annual grass accumulation in the understories of native
shrub communities creates a continuum offine, combustible
fuels (Billings 1994), resulting in dramatic increases in
wildfire frequency and size (Pellant 1990; Whisenant 1990).
These exotic species have adapted to a broader range of
habitats, including salt-desert shrub and pinyon-juniper
(Pinus-Juniperus) communities, resulting in the conversion
of diverse natural landscapes to fire-maintained annual
grass rangelands (Billings 1994; Monsen 1994; Sparks and
others 1990).
In: Monsen, Stephen B.; Stevens, Richard, comps. 1999. Proceedings:
ecology and management of pinyon-juniper communities within the Interior
West; 1997 September 15-18; Provo, UT. Proc. RMRS-P-9. Ogden, UT: U.s.
Department of Agriculture, Forest Service, Rocky Mountain Research
Station.
Mike Pellant is Rangeland Ecologist, U.S. Department of the Interior,
Bureau of Land Management, Idaho State Office, Boise, ID 83709. Julie
Kaltenecker is Research Assistant, Boise State University and U.S. Department of the Interior, Bureau of Land Management, Idaho State Office.
Steven Jirik is Rangeland Management Specialist, U.S. Department of the
Interior, Bureau of Land Management, Lower Snake River District, Boise, ID
83705.
322
The establishment and spread of cheatgrass and the
subsequent wildfires have had serious impacts on wildlife
habitat and populations. Habitat loss has resulted in population declines for shrubland-obligate wildlife, ranging from
breeding birds and rodents to wintering ungulates (Knick
and Rotenberry 1995; McAdoo and Klebenow 1978). Food
web dynamics are modified by loss of forage for herbivorous
species and breeding habitat for both prey and predators.
Herbivores exert additional pressure on the remaining native plant community resulting in depletion of forage and
browse (McAdoo and Klebenow 1978). Livestock can be
similarly affected by loss of forage following wildfire.
The costs associated with wildfire suppression and rehabilitation are also growing as wildfires increase due to the
pervasiveness of flammable annual grasses in the Great
Basin (Pellant 1990). Additional concerns arise as more
people move to urban-wildland interfaces and wildfire threats
to private property increase. The issues surrounding the
impacts of cheatgrass and associated wildfires will continue
to grow unless effective and economical control treatments
are applied on selected cheatgrass rangelands.
Traditional Cheatgrass Control
Methods
Traditional methods to control cheatgrass-livestock
grazing, burning, mechanical (disking or plowing), or broadspectrum herbicides-have all been used with varying degrees of success. Results are highly dependent on timing of
treatment application with regards to cheatgrass phenology, soil moisture, and pre- and post-treatment climatic
conditions (Hull and Holmgren 1964). Livestock grazing
prior to seedripe can reduce biomass and seed production,
however heavy use will only partially suppress cheatgrass
while resulting in negative impacts to perennial grasses
(Young and Tipton 1990) and biological soil crusts (Beymer
and Klopatek 1992; Brotherson and others 1983). Burning
prior to seed maturity can significantly reduce cheatgrass
density (Pechanec and Hull 1945; Stark and others 1946;
Stewart and Hull 1949), although results are variable due to
the failure of fire to deplete the soil seed reserve and the
enhanced seed production of post-fire cheatgrass plants.
Plowing or disking can be effective if done before seedripe,
and if the existing seed bank is buried at least 2.5 inches
(Hulbert 1955), however treatment of steep or rocky areas
may be impractical.
Herbicide use on public lands was limited until 1991 when
the Final Environmental Impact Statement for Vegetation
Treatment on BLM Lands in Thirteen Western States
USDA Forest Service Proceedings RMRS-P-9. 1999
Table 1-Seedbed preparation treatments applied to degraded rangeland in the 1992 Elko
Cheatgrass Suppression Study.
Treatment type
Description
Acres treated
Mechanical
Disk plow with double offset disk after germination
with at least 1 inch of growth but prior to dough stage.
3.9
Burn
Controlled burn prior to cheatgrass seedripe.
4.5
OUST® herbicide
Spring application at 1.0 oz/acre prior to seedripe.
2.5
Control
No treatment.
2.0
(USDI 1991) was approved, allowing the use of 21 herbicides to control cheatgrass and other weeds. Factors limiting
the use of herbicides include chemical and application costs,
selectivity, interception and inactivation of the chemical by
surface litter, and weed seed longevity (Ogg 1994). Perceptions of the public about herbicides and their impacts on
wildlife can also negatively influence the application of
herbicides on public lands.
OUST® (Sulfometuron Methyl)
OUS'f® is a water-dispersible herbicide approved for use
on rangelands in the 1991 Record of Decision for the Vegetation Treatment EIS (USDI 1991). OUS'f® functions as both
a pre- and post-emergent herbicide through inhibition ofthe
enzyme acetolactate synthase (ALS), which catalyzes the
production of three amino acids: leucine, isoleucine, and
valine (Keeler and others 1993; Boutsalis and Powles 1995).
The ultimate result of ALS inhibition is death ofmeristematic tissue and eventual plant mortality (Keeler and others
1993; Hofgen and others 1995). OUS'f® has low toxicity and
does not accumulate in animal tissue, nor does it persist in
the environment for extended periods (DuPont 1996). The
half-life ranges from 20 to 100 days, depending on soil
chemistry, temperature and moisture. If applied in the fall,
OUS'f® requires rainfall to move into the soil where it kills
germinating annual grasses. Spring-applied OUS'f® is absorbed by the roots and foliage of growing plants causing
target plant mortality within 4 to 6 weeks. OUSTID should be
applied prior to flowering to prevent viable seed production.
OUST® affects actively growing tissue, thus it is effective
in controlling rapidly growing annual plants. New spring
growth ofperennial plants may be stunted by 0 USTID but are
generally not killed since they have established roots that
penetrate below the level of the herbicide movement.
Project Summaries
1992 Cheatgrass Suppression Study,
Elko, NV
treated with one of four seedbed treatments: burning, mechanical, herbicide, and control (table 1). The entire area
was drilled with a perennial plant seed mixture using a notill Amazon drill in October 1992. Frequency data were
collected in July 1993 and June 1994 using 10 nested plot
frequency frames on five 50-ft transects in each treatment.
OUS~ proved to be the most effective treatment for
reducing cheatgrass, followed closely by burning (fig. 1).
Both the OUS'f® and burning treatments maintained
cheatgrass at lower frequencies than in the control in 1994
(the second growing season). The disking treatment provided moderate cheatgrass control in 1993, however
cheatgrass frequencies in the disked treatment exceeded
even the control in 1994. Disking with an off-set plow rarely
buries all cheatgrass seed and creates a disturbed seedbed
that is ideal for post-treatment establishment of cheatgrass
from the seedbank.
No data on seeding success are presented here because the
seed mixture was mistakenly applied at roughly one-half of
the planned application rate. Seeded plant establishment
was very spotty. The only species in the seed mixture that is
present in more than a trace quantity today is 'Immigrant'
forage kochia (Kochia prostrata), a perennial half-shrub
imported from the steppes of Eurasia.
100
80
~
g
~
t:r
f!!
u.
60
40
20
0
An experimental project was initiated in May 1992 near
Elko, Nevada to compare cheatgrass control techniques on
degraded rangeland prior to drilling with a perennial plant
seed mixture. Strips within a 13 acre study area were
USDA Forest Service Proceedings RMRS-P-9. 1999
Control
Burn
Disk
OUST
Figure 1-1992 Cheatgrass Suppression Study,
Elko, NV. Frequency of cheatgrass recorded in
0.0625 m 2 frames.
323
1994 OUST® Pilot Project, Mountain Home, 10
In April 1994, OUS~ was applied to a portion of a
marginally established vegetative fuelbreak ("greenstrip")
(Pellant 1990) near Mountain Home, Idaho. The site was
originally plowed and seeded with a disk chain (Pellant
1988) to a perennial grass and forb mixture in fall 1987. In
1989, the original seeding was determined to be a failure
due to an extended drought. To remedy this condition, a
burn treatment was applied in September 1989, followed
by drill-seeding with crested wheatgrass at 8 lb/acre in
November 1989. This seeding was not successful due to
cheatgrass competition and unfavorable climatic conditions
after the seed application. Part of the site reburned in
July 1993, providing an opportunity to test the use of
OUS~ on 100 acres. OUS~ was applied at 1 ozlacre in
April 1994 when cheatgrass was actively growing and prior
to the boot stage. In September 1994, a multi-species seed
mix comprised of approximately 50 percent crested wheatgrass was applied to the OUS~-treated area with a rangeland drill at 10.5Ib/acre. An unburned, untreated portion of
the greenstrip was monitored as a control.
Density of cheatgrass and crested wheatgrass plants in
each area was determined along five 50-ft transects with
10 plots per transect. Plot size was 0.0625 m 2for cheatgrass
and 0.25 m 2 for crested wheatgrass. Densities were converted to number ofplants/m2 for comparison oftreatments.
Cheatgrass density was dramatically reduced by the
OUS~ treatment compared to the control in 1995, 1996,
and 1997 (figs. 2 and 3). Although cheatgrass increased in
1996, densities remained below 100 plants/m 2. Density of an
exotic annual forb, bur buttercup (Ranunculus testiculatus),
increased considerably from 1995 to 1996. This was probably due to a large, residual seedbank in the soil combined
with decreased competition from cheatgrass. In 1997, bur
buttercup density was substantially lower in the OUS~­
treated area compared to the control. The mechanism for
this decline could be competition from the seeded crested
wheatgrass combined with climatic influences.
The effects of reduced cheatgrass competition on seeded
species is demonstrated by the success of the crested
Figure 3-1994 OUST® Pilot Project, Mountain Home,
10. Control of cheatgrass (left side of photograph) by
OUST® application in April 1994. The photograph was
taken in June 1995.
wheatgrass. Treatment with OUS~released existing crested
wheatgrass plants from cheatgrass competition, resulting in
improved plant vigor ofthe remnant perennial grasses in the
treated area compared to the control in 1995. The perennial
plants in the treatment area had greater biomass and
number of seedstalks compared to similar plants in the
control. They were also a darker green, possibly indicating
more efficient use of available nitrogen where cheatgrass
competition was reduced.
Treatment with OUS~ also promoted the establishment
of crested wheatgrass in the first growing season. An average of17 crested wheatgrass seedlings/m2 established in the
seeded treatment during the 1996 growing season (fig. 4).
Crested wheatgrass densities were considerably lower in
1997 due to seedling mortality, but were still twice as great
in the treatment area compared to the control.
30
25
Bur Buttercup
Cheatgrass
20
700
Includes
17 seedlings/m2
~
600
.~
15
Q)
Q
500
10
~
400
.~
<Il
0
300
200
Control
OUST
100
Control
OUST
Control
OUST
Figure 2-1994 OUST® Pilot Project, Mountain Home,
10. Density (plants/m 2) of cheatgrass and bur buttercup in control and OUST®-treated areas, 1995-1997.
324
Figure 4-1994 OUST® Pilot Project, Mountain
Home, 10. Density (plants/m2) of crested wheatgrass in control and OUST®-treated areas, 19951997. The OUST® treatment was seeded in
September 1994; the control was not seeded in
1994.
USDA Forest Service Proceedings RMRS-P-9. 1999
Table 2-Comparison of cost and efficiency of ground and aerial application of
OUST® herbicide.
Herbicide cost ($)
Application cost ($)
Average acres treated/day
Ground
10.50/oz
11.00-15.00/acre
50-100 (60 ft boom)
Operational Use of OUST®
In 1996, the Bruneau Resource Area in the BLM's Lower
Snake River District began using OUS'f®to control cheatgrass
in large post-wildfire rehabilitation projects in the Snake
River Birds of Prey National Conservation Area. Cheatgrass
and wildfires are two of the greatest management concerns
in this area which supports North America's largest concentration of nesting raptors (Kochert and Pellant 1986). Once
cheatgrass density is reduced byOUS'f®, rehabilitation with
perennial vegetation, especially shrubs, is conducted to
reestablish habitat for raptor prey species and groundnesting birds, and provide livestock forage.
Ground and aerial application methods are being evaluated to determine the most practical and cost-effective
OUSTl"D application procedures. While herbicide and application costs are similar for ground and aerial application
(table 2), trade-offs exist with regards to the amount of
rangeland that can be treated per day and the impacts of
label restrictions for each application method (DuPont
1996, 1997). Aerial application is recommended for large
tracts ofland and treatment of rocky sites that might cause
application equipment damage. However, small tracts with
adjacent cropland or dwellings are better treated with
ground application, where no buffer is required as opposed
to the 200 ft "no treatment" buffer required for all aerial
applications.
There are a number of label restrictions associated with
the use ofOUS'J'® on non-crop lands. Application ofOUS'f®
requires equipment dedicated for non-crop use. Application
equipment cannot be used on cropland once it is used to
apply OUSTl"D. Livestock use of treated rangeland cannot
occur within 1 year of application. Because of the potential
persistence of the herbicide in soil, seeding into the treatment area should be delayed for one growing season. Both
aerial and ground application require wind speeds between
3 and 10 mph. OUSTl"D cannot be applied on frozen ground.
OUSTl"D can be applied in late fall or early spring to control
cheatgrass. Initial observations indicate that fall application ofOUS'J'® provides better cheatgrass control than spring
application since both fall- and spring-germinated cheatgrass plants are killed. However, viability tests on cheatgrass seeds and results from previous projects (for example,
the 1994 OUS'f® Pilot Project) indicate that spring treatments may be effective if application is done in advance of
seed ripening.
Most native perennial plants are not adversely impacted
by an OUSTl"D application of 1.0 ozlacre, however a temporary chlorosis and stunted growth is often observed on
treated areas. Sandberg bluegrass (Poa secunda) has been
observed to suffer some mortality following spring OUS'J'®
application at 1 ozlacre. Viability of seed produced by native
plants may be reduced in treated areas, affecting potential
USDA Forest Service Proceedings RMRS-P-9. 1999
Aerial
10.50/oz
8.00-18.00/acre
1500-2000 (45 ft boom)
recruitment during the first growing season following
OUST® application. Monitoring studies established on
treated areas will provide more quantitative information on
the effects of OUS'J'® on native vegetation.
Ongoing Studies
Initial results indicate that OUS'J'® is an effective tool for
cheatgrass control over a 1 to 2 year period, allowing release
of native and seeded perennial species from cheatgrass
competition. Studies are currently being conducted by the
USDA Forest Service, Rocky Mountain Research Station,
Boise State University, and the USDI Bureau of Land
Management to determine: 1) minimum application rate
that provides effective cheatgrass control; 2) effectiveness of
fall versus spring application; 3) effects of OUS'f® on survival and reproduction of native plants and microbiotic
crusts; 4) effects of OUS'f® on germinating (seeded) perennial plants; and 5) effectiveness of OUS'J'® in controlling
medusahead wildrye. Results from these studies will provide managers with better guidelines on the use of OUS'J'®
in the management and rehabilitation of cheatgrassinfested rangelands.
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USDA Forest Service Proceedings RMRS-P-9. 1999