Special Publication 15-04
The Response of Creeping Wildrye
(Leymus triticoides) to Physical and
Chemical Mowing of Perennial
Pepperweed (Lepidium latifolium) and
Subsequent Herbicide Treatment
Brad Schultz, Extension Educator, Winnemucca, Nevada
Earl Creech, Extension Agronomist, Utah State University, Logan, Utah
Kent McAdoo, Extension Natural Resources Specialist, Elko, Nevada
Summary: A perennial pepperweed (Lepidium latifolium) community with a residual
component of creeping wildrye (Leymus triticoides) was mechanically or chemically mowed
and the regrowth treated with three different herbicides individually or as tank mixes.
Creeping wildrye that was mechanically mowed when at the boot stage tended to have less
cover and lower vigor than creeping wildrye in untreated plots. Plots mowed or retreated
with Roundup ProMax (glyphosate) had dramatic declines in creeping wildrye cover and
vigor. Plots chemically mowed with Reward (diquat), a contact herbicide, tended to have
greater creeping wildrye cover and vigor than untreated plots but less cover and lower vigor
than plots chemically mowed with 2,4-D. Regrowth treatments that used Telar
(chlorsulfuron) or 2,4-D generally had creeping wildrye cover and vigor greater than
untreated plots. The application of Telar alone (without treatment) had the greatest cover of
creeping wildrye (69 percent), which was over four times greater than in untreated plots (17
percent).
INTRODUCTION
Herbicide and/or other weed control treatments that reduce a plant community’s population of
perennial grasses can result in treated sites that are susceptible to rapid reinvasion by the same
and/or different weed species (Whitson and Koch 1998; Enloe et al., 2005; Lulow 2006). Mechanical
mowing of perennial pepperweed plants at the bud-to-early-flowering stage and retreating the
regrowth later that growing season (at the bud growth stage) with a systemic herbicide is an effective
control method in California (Renz and DiTomaso 2004 and 2006). Recent research in Nevada
(Schultz et al., 2014) found that Telar (chlorsulfuron) was equally effective, with or without a mowing
A partnership of Nevada counties; University of Nevada, Reno; and U.S. Department of Agriculture
treatment, and provided the best control of all herbicides tested. Chemical and mechanical mowing
treatments followed by the application of Roundup ProMax (glyphosate) and/or 2,4-D Ester typically
caused large declines in the cover of perennial pepperweed. However, the combination of no mowing
with regrowth herbicide treatment was as effective for controlling perennial pepperweed as was Telar
applied without a mowing treatment.
Telar may not be an appropriate herbicide to control perennial pepperweed for all possible situations.
In Nevada, mechanical mowing of perennial pepperweed stands is often not possible due to rough
microtopography and/or seasonal flooding. However, perennial pepperweed stands may be
”chemically mowed” (defined on Page 3, second bullet) and the regrowth treated later that growing
season. The response of creeping wildrye (Leymus triticoides), a common perennial grass in
Nevada’s grass-hay meadows, to the interactions of mechanical or chemical mowing treatments and
subsequent treatment of the regrowth is largely unknown. This study measured the response of
creeping wildrye canopy cover and vigor to physical and chemical mowing of a stand of perennial
pepperweed, herbicide application on the regrowth (including creeping wildrye), and the interaction of
mowing and herbicide regrowth treatment.
METHODS
The study site was located about 3 miles east of Elko, Nev., in the floodplain along the Humboldt
River, a perennial stream in this reach. Historically, the site was a seasonally flooded meadow of
predominately creeping wildrye (Soil Survey Staff 2015). Every treatment plot was within 200 feet of
the river, and in most years the study site receives flood irrigation from diversions during the spring
runoff. The proximity of the river results in a high water table, which provides access to water for the
roots of perennial pepperweed throughout the potential growing season, which is April to October.
The soil horizons ranged from silt loam to silty clay loam and were nonsaline to slightly saline (Soil
Survey Staff 2015). The entire study area had a long-established mature stand (i.e., high canopy
cover) of perennial pepperweed, with a residual understory of creeping wildrye (Figure 1).
Figure 1. The study area is to
the right of the mowed boundary
that outlined the perimeter of the
plots.
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Thirty treatment combinations were applied, including untreated plots (Tables 1 and 2). Treatment
combinations consisted of the following:
 A single mechanical mowing in July at the flowering growth stage of perennial pepperweed
followed by five herbicide treatments of the regrowth (including untreated plots) at the bud-to–
early-flowering growth stage.
 A single ”chemical mowing” treatment with a contact herbicide in July, followed by four
herbicide treatments to the regrowth, plus untreated plots. This treatment parallels chemical
mowing in that only the aboveground portion of the plant is affected. It has two differences from
mechanical mowing; 1) the treated part of the plant remains upright as standing dead litter and
2) the effect on individual plants is not uniform – some may receive the herbicide on only the
upper portion of the stems when the plant canopy has very high cover, whereas plants in open
areas may receive the active ingredient on almost all of their stems and leaves. Comparatively,
mechanically mowed plants will have all of their stems cut to nearly the same stubble height
(3 to 4 inches), and the cut stems are left lying across the top of the soil or plant stubble. This
allows any regrowth to emerge through and reside above the cut stems and leaves.
 ”Chemical mowing” with two systemic herbicides, singularly and as a tank mix, followed by
treatment of the regrowth with four herbicide treatments and untreated plots (Table 1). For ease
of discussion throughout this document, all plots that received an initial treatment with an
herbicide will be called “chemically mowed,” with the understanding that “mowing” with a
systemic herbicide affects both the aboveground stems and at least some of the root system
and its associated buds.
 All initial chemical and mechanical mowing treatments in July occurred on the same day and at
the same growth stage. All regrowth treatments occurred on the same day in September and
were at the same growth stage in each treatment plot.
Table 1. Initial and follow-up treatments to perennial pepperweed in 2009. All possible
combinations of initial and regrowth treatments resulted in 30 total treatment combinations.
Initial mowing treatments at the
bud-to-flowering stage
(July)
•
•
•
•
None (untreated)
Rotary mower
Reward (chemical mowing)
Roundup ProMax & 2,4-D Ester (chemical
mowing)
• Roundup ProMax (chemical mowing
• 2,4-D Ester (chemical mowing)
Regrowth herbicide treatment
at the bud-to-early-flowering
stage (September)
•
•
•
•
•
None (untreated)
2,4-D Ester
Telar
Roundup ProMax
Roundup ProMax & 2,4-D Ester
3
Table 2. Herbicide names, concentrations and rates applied for perennial pepperweed control.
Common
name
Systemic or
contact
Active ingredient
(ai) or acid
equivalent (ae)
concentration
Application rate
applied
Systemic
3.8 lb ae/gal
2.1 qt/ac
(2.0 lb ae/ac)
Telar
Systemic
75% ai/oz
1.0 oz/ac
(0.75 oz ai/ac)
Reward
Contact
2.0 lb ae/gal
2.0 qt/ac
(1.0 lb ae/ac)
Roundup
ProMax
Systemic
4.5 lb ae/gal
3.0 qt/ac
(3.375 lb ae/ac)
Product
name
2,4-D Ester
Chlorsulfuron
Diquat
Glyphosate
Each treatment combination was applied four times for 120 total treatment plots. All plots measured
10 feet by 30 feet. All physical and chemical mowing treatments occurred on July 21, 2009, when the
creeping wildrye was largely at the pre-boot growth stage, but with some plants displaying seedhead
emergence. The perennial pepperweed was at the bud-to-flowering growth stage. Most of the grass
stems (tillers) were not tall enough for their seedheads and upper leaves to reside above the
perennial pepperweed plants (Figure 2). Creeping wildrye phenology was variable between the boot
to full-seedhead-emergence growth stages. None of the emerged seed had become fully ripe.
Figure 2. The foreground shows
the stubble height common in sites
physically mowed to control
perennial pepperweed, and the
structure of untreated sites. Few
grass stems had emerged above
the perennial pepperweed flowers,
and few creeping wildrye
seedheads had emerged on most
grass plants.
Mechanical mowing was implemented with a rotary brush mower pulled behind an all-terrain vehicle.
Mowed vegetation was left on the plots where it fell. All herbicides were applied with a CO2pressurized backpack sprayer and a 10-foot boom with nozzles approximately 17.5 inches apart. The
chemical mowing treatments included the contact herbicide Reward (diquat), the nonselective
systemic herbicide Roundup ProMax (glyphosate) and the selective systemic herbicide 2,4-D-Ester.
All regrowth herbicide treatments occurred on Sept. 29, 2009, when the perennial pepperweed was at
4
the bud-to-early-flowering growth stages. There was little regrowth of the creeping wildrye after the
late July mowing treatments (Figure 3).
Figure 3. Regrowth of creeping
wildrye (bottom third of photo)
and perennial pepperweed
(upper two-thirds of photo) on
Sept. 29, 2009 on an area that
was physically mowed on July
21, 2009. Chemically mowed
plots occur on both sides of the
physically mowed plots.
Two response variables for creeping wildrye were measured: canopy cover and plant vigor. Canopy
cover was visually estimated in each treatment plot in two one meter square quadrats and averaged
for that plot. Vigor was estimated for each treatment plot using a scale of 0 (all plants dead) to 10 (tall,
robust plants with large abundant leaves and seedheads). Plots that received a vigor rating of 10
typically had plants with no observable injury from any herbicide and a large decline in perennial
pepperweed cover. The decline in competition from perennial pepperweed resulted in tall plants of
creeping wildrye with high leaf cover. All data were collected on July 16, 2010. For the creeping
wildrye, the seedheads had largely emerged, and for the perennial pepperweed, plants were in the
bud to very-early-flowering growth stage.
The means were calculated for each response variable for each treatment combination and analyzed
with a two-way analysis of variance to assess the respective effects of the mowing and regrowth
treatments and their interactions. When the analysis of variance test determined that the difference
between two or more means had a 90 percent or greater probability of being due to an applied
treatment, we used the Least Significant Difference mean separation test to determine which specific
treatments were different from one another because of treatments applied. All statistical tests used
the 10 percent probability level (P ≤ 0.10). That is, there was a 90 percent or greater chance that the
differences between two or more mean values were due to the treatments applied, and a
10 percent or less chance that the differences were due to some other factor.
RESULTS AND DISCUSSION
Creeping Wildrye Canopy Cover
There was nearly a 100 percent chance that the response of creeping wildrye was affected by
mowing treatment and the regrowth herbicide treatment (Table 3). There was a 98 percent chance
that interactions of mowing and regrowth herbicide treatments affected the canopy cover of creeping
wildrye (Table 3).
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Table 3. Analysis of variance table for creeping wildrye cover response to treatments (and treatment
interactions) applied to control perennial pepperweed.
Type of treatment
Mechanical and
chemical mowing
Herbicide on
regrowth
Mowing by herbicide
interaction
Degrees of
freedom
Probability that the means are different due to a factor
other than the treatment applied
5
0.00
4
0.00
20
0.02
Figure 4 shows the respective influence of each mowing treatment on creeping wildrye canopy cover.
Untreated plots had an average canopy cover of about 29 percent, which was nearly identical to plots
chemically mowed with Reward, a contact herbicide that only kills the aboveground portions of the
plant that comes in direct contact with the chemical. Untreated plots had about 5 percent less canopy
cover of creeping wildrye than did plots chemically mowed with 2,4-D Ester, a systemic herbicide.
There was at least a 10 percent chance the difference in the mean values among these three
treatments (P ≥ 0.10) was due to some factor other than the treatments applied.
Mechanically mowing perennial pepperweed (and creeping wildrye inadvertently) to a 3- to 4-inch
stubble height occurred at the boot-to-seedhead emergence growth stage of the wildrye. This
decreased creeping wildrye’s average canopy cover by about 10 percent compared to the untreated
and Reward plots, and by about 15 percent compared to plots chemically mowed with 2,4-D (Figure
4). There was at least a 10 percent chance (P ≥ 0.10) that the difference in the means between rotary
mowing, the untreated plots and the plots treated with Reward were due to some factor other than the
mowing treatment. The greater mean creeping wildrye cover for plots chemically mowed with
Figure 4. Effect of physical and chemical mowing treatments for perennial pepperweed control on the canopy
cover of creeping wildrye. Means (rows) followed by different letters indicate there is a 90 percent or greater
probability that the mean values are different due to the treatments applied.
a
Mowing treatment
Roundup ProMax
Roundup ProMax & 2,4-D Ester
a
b
Rotary mower
Ubntreated
bc
Reward
bc
c
2,4-D Ester
0
5
10
15
20
25
30
35
40
Creeping wildrye canopy cover (percent)
6
2,4-D Ester, as compared to the rotary mowed plots and plots mowed with Roundup ProMax, can be
attributed to the mowing technique with 90 percent or greater certainty (P ≤ 0.10). Chemical mowing
treatments that used the nonselective, systemic herbicide Roundup ProMax (glyphosate) reduced the
canopy cover of creeping wildrye to 3 percent or less. There is a 90 percent or greater chance (P ≤
0.10) that the decline in creeping wildrye canopy cover was due to glyphosate being the chemical
mowing treatment.
Figure 5 shows the response of creeping wildrye canopy cover to different regrowth herbicide
treatments. When the regrowth was not treated, creeping wildrye’s canopy cover averaged about
16 percent. Regrowth treated with Telar or 2,4-D had an average canopy cover that was over two
times greater than that of untreated plots, with slightly greater canopy cover on the plots treated with
2,4-D. It can be concluded with confidence (P ≤ 0.10) that the increase in wildrye canopy cover was
due to the various regrowth herbicide treatments applied. The large increase in creeping wildrye
cover after the Telar or 2,4-D treatments was probably due to two factors: 1) both herbicides resulted
in 95 to 100 percent reduction in the cover of perennial pepperweed (Schultz et al., 2014); thus, there
was much less competition from the weed; and 2) both herbicides had no obvious adverse effect on
the growth of creeping wildrye.
Regrowth herbicide treatment
Figure 5. The response of creeping wildrye canopy cover to various herbicide treatments applied to the
regrowth of perennial pepperweed in September (two months after initial mowing treatments in July).
Means (rows) followed by different letters indicate there is a 90percent or greater probability that the
mean values are different due to the treatments applied.
a
Roundup ProMax & 2,4-D Ester
ab
Roundup ProMax
None
b
Telar
c
c
2,4-D Ester
0
5
10
15
20
25
30
35
40
Creeping wildrye canopy cover (percent)
Regrowth herbicide treatments that included glyphosate (Roundup ProMax), either alone or with
2,4-D, had 9 to 12 percent less canopy cover from creeping wildrye than did the untreated plots
(Figure 5). For the Roundup ProMax with 2,4-D treatment, there was 90 percent or greater
confidence (P ≤ 0.10) that the large decline of creeping wildrye cover was due to the treatment. The
7
large decline in creeping wildrye was due to the Roundup ProMax component of the herbicide mix
and not the addition of 2,4-D to the tank mix. This conclusion is supported by the treatments that used
2,4-D as a stand-alone treatment; plots with these treatments had twice as much creeping wildrye
canopy cover as the untreated plots.
The combination of mowing and regrowth herbicide treatments resulted in large differences in the
mean canopy cover of creeping wildrye, ranging from 0 to almost 69 percent (Table 4). The difference
in mean canopy cover of creeping wildrye was due primarily to the treatment combinations applied
(P ≤ 0.10).
For untreated plots, the mean canopy cover of creeping wildrye was 16.9 percent (Table 4). Six
treatment combinations resulted in a mean canopy cover of creeping wildrye greater than
40.6 percent. The difference in mean canopy cover of creeping wildrye was due primarily to the
treatment combinations applied (P ≤ 0.10). The common threads among these six treatment
combinations were: 1) absence of glyphosate in the chemical mowing treatment and 2) the
application of either Telar or 2,4-D as a treatment of the regrowth in September. An additional five
treatment combinations had a mean creeping wildrye canopy cover greater than in the untreated
plots, but the increase in creeping wildrye from these treatment combinations could not be attributed
to the specific treatment combinations with a 90 percent or greater probability (P ≥ 0.10). Several
mechanisms probably interacted to facilitate an increase in creeping wildrye. At a minimum, there
was less competition for soil moisture and nutrients following the large decline in perennial
pepperweed Schultz et al., 2014), and little or no direct adverse effects from the active ingredients on
creeping wildrye.
Table 4. Mean canopy cover of creeping wildrye in response to interactions of mowing treatment and
application of an herbicide to the regrowth of perennial pepperweed. Treatment combinations (rows)
with different letters (mean separation) have a 90 percent or greater probability of being different due to
the treatments applied.
Mowing treatment
Regrowth herbicide
treatment
Mean canopy cover of
creeping wildrye
(percent)
Mean separation
Untreated
Telar
68.8
a
2,4-D Ester
2,4-D Ester
61.3
ab
Reward
2,4-D Ester
58.8
ab
Untreated
2,4-D Ester
52.8
abc
2,4-D Ester
Telar
42.0
bcd
Rotary mower
Telar
40.6
bcd
2,4-D Ester
None
39.4
Reward
Telar
32.3
Rotary mower
2,4-D Ester
27.5
bcde
cdef
defg
8
Mowing treatment
Regrowth herbicide
treatment
Mean canopy cover of
creeping wildrye
(percent)
Mean separation
Reward
Roundup ProMax
25.0
defgh
Rotary mower
None
22.5
defghi
Untreated
None
16.9
efghi
Reward
None
16.3
efghi
2,4-D Ester
Roundup ProMax
13.4
fghi
10.8
fghi
9.6
fghi
ghi
2,4-D Ester
Reward
Roundup ProMax & 2,4-D
Ester
Roundup ProMax & 2,4-D
Ester
Roundup ProMax & 2,4-D
Ester
2,4-D Ester
5.4
Roundup ProMax
2,4-D Ester
3.0
hi
Untreated
Roundup ProMax
2.9
hi
Roundup ProMax & 2,4-D
Ester
Telar
1.6
hi
Roundup ProMax
Telar
1.3
i
Roundup ProMax & 2,4-D
Ester
Roundup ProMax & 2,4-D
Ester
Roundup ProMax & 2,4-D
Ester
1.0
i
None
0.8
i
Roundup ProMax
Roundup ProMax & 2,4-D
Ester
0.6
i
Rotary mower
Roundup ProMax
0.5
i
Untreated
Roundup ProMax & 2,4-D
Ester
0.1
i
Roundup ProMax & 2,4-D
Ester
Roundup ProMax
0.1
i
Rotary mower
Roundup ProMax & 2,4-D
Ester
0.0
i
Roundup ProMax
None
0.0
i
Roundup ProMax
Roundup ProMax
0.0
i
The treatment combination of Reward/Roundup ProMax had a mean creeping wildrye canopy cover
of 25 percent, which was higher than expected given the use of glyphosate (Roundup ProMax) in the
regrowth treatment. The use of Reward, a contact herbicide, as the mowing agent probably explains
much of this outcome. Reward would have only killed the aboveground portion of the plants (grasses
and perennial pepperweed) with which it came into contact. Given the very high canopy cover of the
9
plant community (Figures 1-3) this would have left a large amount of standing dead material to
intercept much of the regrowth herbicide treatment that included glyphosate. Furthermore, at the time
of the September treatment of the regrowth, most of the creeping wildrye was in an inactive growth
state. These two factors would have reduced the amount of glyphosate that came in contact with
green photosynthetically active leaf material, the location where uptake occurs.
Eighteen treatment combinations had a mean creeping wildrye canopy cover less than in the
untreated plots (16.9 percent cover). Of these, 11 had creeping wildrye canopy cover of less than 2
percent. There was at least a 10 percent chance (P ≥ 0.10) that the decline in creeping wildrye
canopy cover, compared to the untreated plots, was due to some factor other than the treatments
applied. Therefore, treatment combinations that result in the near complete loss of creeping wildrye
would not be advised, regardless of the causal mechanism or their effect on perennial pepperweed.
Each of these 18 treatment combinations, except one (Reward mowing treatment with no regrowth
herbicide treatment’ 16.3 percent cover of creeping wildrye) used glyphosate (Roundup ProMax) as
part of the treatment combination.
Creeping Wildrye Vigor
The results for creeping wildrye vigor generally parallel those for canopy cover. There was close to a
100 percent probability (P ≤ 0.00)) that both mowing and herbicide regrowth treatments affected the
vigor of creeping wildrye (Table 5). There was a high probability (89 percent) that the interaction of
mowing and herbicide regrowth treatment affected plant vigor, but the effect of the interaction was not
as strong (probable) as either the mowing or regrowth treatment.
Table 5. Analysis of variance table for the vigor of creeping
wildrye in response to mowing treatments, regrowth herbicide
treatments and their interactions.
Source
Mechanical and
chemical mowing
Herbicide on regrowth
Mowing by herbicide
interaction
Degrees
Freedom
Probability
5
0.00
4
0.00
20
0.11
Compared to the untreated plots, mowing treatments that used a contact herbicide (Reward) or 2,4-D
Ester (stand-alone) resulted in greater vigor of creeping wildrye (Figure 6). There was a 90 percent or
greater chance (P ≤ 0.10) that the increase in vigor in plots treated with 2,4-D Ester was attributable
to that specific herbicide (Figure 6). This result occurs for two reasons: 1) the lack of adverse effects
that 2,4-D has on established perennial grasses (≥ 1 year old), particularly when they are treated at
the later growth stages (e.g., boot to seedhead emergence); and 2) high levels of control for perennial
pepperweed in 2,4-D-treated plots (Schultz et al., 2014). Mowing treatments that used the contact
herbicide Reward had a slightly lower, but not statistically different, vigor rating than mowing
treatments with 2,4-D. This probably occurred because Reward killed the upper portions of the
creeping wildrye during the boot-to-early-seedhead emergence growth stage, which is the critical
growth stage for the plant to store energy reserves it needs to overwinter and initiate growth the
10
following growing season. Some plants also may have used their existing energy reserves to initiate
regrowth following treatment, and subsequently failed to restore their energy reserves before the
drying soil in creeping wildrye’s root zone forced these plants to enter summer dormancy. Both
scenarios could have reduced creeping wildrye vigor the following growing season.
Figure 6. The effect of mowing treatment on the vigor of creeping wildrye. Means (rows) followed by
different letters indicate there is a 90 percent or greater probability that the mean values are different due
to the treatments applied. Note: Vigor was estimated for each treatment plot using a scale of 0 (all plants
dead) to 10 (tall, robust plants with large abundant leaves and seedheads).
Mowing treatment
Rounup ProMax
a
Rounup ProMax & 2,4-D Ester
a
Rotary mower
b
bc
Untreated
Reward
cd
2,4-D Ester
d
0
1
2
3
4
5
6
Vigor of creeping wildrye
7
8
Compared to the untreated plots, creeping wildrye’s decline in vigor from rotary mowing was small
and not statistically significant (P ≥ 0.10). However, there was a 90 percent or greater chance
(P ≤ 0.10) the difference in vigor between the mechanical mowing treatment and chemical mowing
treatments with Reward or 2,4-D Ester were due to the specific mowing techniques. Mechanical
mowing cut all plants down to a 3- to 4-inch stubble height. Contact herbicides only affect
photosynthetically active parts of the plant that come into direct contact with the active ingredient.
Given the tall height and high cover of the plant canopy (Figure 2), a large amount of the contact
herbicide was intercepted in the upper canopy and did not adversely affect the lower leaves and
stems of creeping wildrye, as did the mechanical mowing treatment at the boot growth stage.
When Roundup ProMax was used as an herbicide regrowth treatment on perennial pepperweed, the
vigor of creeping wildrye had a substantial decline (Figure 7). This response was undoubtedly due to
the herbicide’s nonselective nature. There was a 90 percent or greater chance (P ≤ 0.10) the decline
in creeping wildrye vigor from the glyphosate (Roundup ProMax) treatment was due to that active
ingredient. Regrowth herbicide treatments with either Telar or 2,4-D Ester had slightly greater
creeping wildrye vigor than the untreated plots, but there was a 10 percent or greater chance
(P ≥ 0.10) that the difference between the means was due to some factor other than the specific
herbicide treatment. Untreated plots had a mean creeping wildrye vigor rating of just over 5. Their
11
vigor rating was moderate because the creeping wildrye may have been intensively competing with
perennial pepperweed for sunlight, water and nutrients.
Regrowth herbicide treatment
Figure 7. Effect of herbicide treatment to the perennial pepperweed regrowth on the vigor of creeping
wildrye. Means (rows) followed by different letters indicate there is a 90 percent or greater probability
that the mean values are different due to the treatments applied. Note: Vigor was estimated for each
treatment plot using a scale of 0 (all plants dead) to 10 (tall, robust plants with large abundant leaves and
seedheads).
a
Roundup ProMax & 2,4-D Ester
a
Roundup ProMax
Untreated
b
Telar
b
b
2,4-D Ester
0
1
2
3
4
5
6
7
Vigor of creeping wildrye
Eight treatment combinations had a numerical vigor rating for creeping wildrye that was greater than
the untreated plots, and 20 treatment combinations had vigor ratings less than in the untreated plots,
but not all of these were statistically significant (Table 6). Only two treatment combinations - no
mowing followed by Telar and Reward followed by 2,4-D Ester - had vigor ratings greater than in the
untreated plots. This can be attributed to the specific treatment combinations with a 90 percent or
greater probability (P ≤ 0.10). A common thread among the eight treatment combinations mentioned
above was the absence of Roundup ProMax as part of the treatment. Another common feature (in
seven of the eight treatment applications) was the use of 2,4-D or Telar in the regrowth treatment.
Both herbicides had very good to excellent control of perennial pepperweed (Schultz et al., 2014) and
no adverse effect on creeping wildrye (Figures 5 and 7).The excellent control of perennial
pepperweed allowed the cover and vigor of creeping wildrye to increase, probably through both
increased tiller numbers and plant height (Figure 8) .
Twelve treatment combinations had mean vigor ratings of 3.0 or less (Table 6). There was a
90 percent or greater chance (P ≤ 0.10) that the vigor ratings between 11 of these treatment
combinations and the untreated plots was due to the specific mowing and regrowth herbicide
treatments applied. A common thread among these treatment combinations was the use of Roundup
ProMax as one or both parts of the treatment combination. Another 10 treatment combinations had
mean vigor ratings equal to or less than the untreated plots, but there was a 10 percent or greater
12
chance (P ≥ 0.10) that the difference in vigor between these plots and the untreated plots was due to
some factor other than the treatment combinations. A common, but not universal, thread among
these 10 treatment combinations was the use of Roundup ProMax (glyphosate) as a regrowth
treatment (common to six of the 10 treatment combinations). For these 10 treatment combinations, a
substantial portion of the active ingredient applied during the regrowth herbicide treatment would
have come into contact with elevated live and dead stalks of perennial pepperweed, dead standing
stalks of creeping wildrye (from the July chemical mowing treatment), and largely dormant stalks of
creeping wildrye. This occurred because the creeping wildrye plants had completed their annual
growth cycle (on the 2,4-D-treated and untreated plots) or had little photosynthetically active regrowth
following the rotary mowing treatment. The interception of the active ingredient before it could make
contact with the limited amount of photosynthetically active leaf material of creeping wildrye reduced
the potential negative effect from the Roundup ProMax component of the treatments.
Table 6. Vigor of creeping wildrye in response to the interaction of mowing treatments and herbicide
application to the regrowth of perennial pepperweed. Means with different letters have a 10 percent
or less (P ≤ 0.10) chance of being different from one another due to the treatments applied. Vigor
was estimated for each treatment plot using a scale of 0 (all plants dead) to 10 (tall, robust plants
with large abundant leaves and seedheads).
Regrowth herbicide
treatment
Mean vigor of
creeping wildrye
Telar
9.3
a
2,4-D Ester
9.0
a
2,4-D Ester
Untreated
8.8
ab
2,4-D Ester
2,4-D Ester
8.8
ab
Untreated
2,4-D Ester
7.8
abc
Rotary mower
Telar
7.8
abc
2,4-D Ester
Telar
7.8
abc
Reward
Untreated
6.5
abcd
Untreated
Untreated
6.0
bcde
Rotary mower
Roundup ProMax
6.0
bcde
2,4-D Ester
Roundup ProMax
5.5
cdef
Telar
5.5
cdef
2,4-D Ester
5.3
cdefg
Roundup ProMax & 2,4-D
Ester
5.3
cdefg
Untreated
5.0
cdefgh
Mowing treatment
Untreated
Reward
Reward
Rotary mower
Reward
Rotary mower
Mean separation
13
Mowing treatment
2,4-D Ester
Untreated
Roundup ProMax
Rotary mower
Roundup ProMax
Roundup ProMax & 2,4-D
Ester
Roundup ProMax & 2,4-D
Ester
Roundup ProMax & 2,4-D
Ester
Roundup ProMax
Roundup ProMax & 2,4-D
Ester
Rotary mower
Untreated
Roundup ProMax
Roundup ProMax & 2,4-D
Ester
Roundup ProMax
Regrowth herbicide
treatment
Roundup ProMax & 2,4-D
Ester
Mean vigor of
creeping wildrye
4.5
defghi
Roundup ProMax
3.5
efghij
Untreated
3.3
efghij
Roundup ProMax
3.0
efghijk
2,4-D Ester
3.0
fghijk
Roundup ProMax & 2,4-D
Ester
2.8
fghijk
Roundup ProMax
2.8
fghijk
2,4-D Ester
2.8
fghijk
Telar
2.5
ghijk
Untreated
2.3
hijk
2.3
hijk
1.8
ijk
1.5
jk
Telar
1.3
jk
Roundup ProMax
0.5
k
Roundup ProMax & 2,4-D
Ester
Roundup ProMax & 2,4-D
Ester
Roundup ProMax and 2,4-D
Ester
Mean separation
*
Figure 8. The growth of creeping wildrye
(center third of the picture) in a plot treated
with Telar the previous year. The yellow
plants in the bottom third of the picture are
perennial pepperweed plants that did not
receive any treatment the previous year but
were treated in the current year. There are
few observable creeping wildrye tillers in this
adjacent patch of perennial pepperweed.
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CONCLUSIONS
 Rotary mowing perennial pepperweed as part of a mowing-herbicide regrowth treatment
combination may decrease cover of creeping wildrye; however, the relatively large difference
between untreated and rotary mowed plots could not be confidently attributed to the treatment.
 Chemical mowing’ treatments with 2,4-D or Reward that were applied during the boot to earlyseedhead-emergence growth stage of creeping wildrye did not result in substantial increases in
grass cover.
 If an increase in existing perennial grasses is a desired treatment outcome, no treatment or
treatment combination for perennial pepperweed control should include a glyphosate-based
product. Treatments that routinely included glyphosate led to large reductions in creeping
wildrye cover and vigor.
 Treatments that include glyphosate should be limited to areas that can be reseeded to desired
perennial grasses or are intended to be managed as bare ground.
 Creeping wildrye responded equally to treatments with Telar (chlorsulfuron) or 2,4-D Ester.
Creeping wildrye appears to be very tolerant of these two products.
 Creeping wildrye had its greatest cover and strongest vigor when perennial pepperweed was
treated with Telar as a stand-alone treatment (i.e., no mowing) in July near peak flowering.
Plots that were either mechanically mowed or chemically mowed with 2,4-D Ester or Reward,
and had their regrowth treated with Telar, had substantially less creeping wildrye than in
unmowed Telar plots. This may reflect the potential adverse effect of physically mowing
creeping wildrye at the boot-to-early seedhead growth stage, and/or the interception of Telar by
the elevated dead stems of either creeping wildrye or perennial pepperweed following the
chemical mowing treatments. The researchers speculate that less active ingredient probably
made contact with the perennial pepperweed plants during the regrowth treatment due to
physical interference from standing dead material, which resulted in less control of the perennial
pepperweed and less of an increase in the creeping wildrye.
 The potential adverse effects of glyphosate are modified when the mowing treatment leaves a
large number of standing dead stems that intercept some of the glyphosate before it comes in
contact with the photosynthetically active portions of the creeping wildrye.
REFERENCES
Enloe, S., J.M. DiTomaso, S.B. Orloff, and D.J. Drake. 2005. Perennial grass establishment
integrated with clopyralid treatment for yellow starthistle management on annual range. Weed
Technology 19:94-101.
Lulow, M.E. 2006. Invasion by non-native annual grasses: the importance of species biomass,
composition, and time among California native grasses of the Central Valley. Restoration
Ecology14:26.
Renz, M.J. and J.M. DiTomaso. 2004. Mechanism for the enhanced effect of mowing followed by
glyphosate application to resprouts of perennial pepperweed (Lepidium latifolium). Weed Science
52:14-23.
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Renz, M.J. and J.M. DiTomaso. 2006. Early season mowing improves the effectiveness of
chlorsulfuron and glyphosate for control of perennial pepperweed (Lepidium latifolium). Weed
Technology 20:32-36.
Schultz, B., E. Creech, and K. McAdoo. 2014. The response of perennial pepperweed (Lepidium
latifolium) to physical and chemical mowing and herbicide treatment of the regrowth. University of
Nevada Cooperative Extension Special Publication 14-02. 19 p.
Soil Survey Staff, Natural Resources Conservation Service, United States Department of Agriculture.
Web Soil Survey. Available online at http://websoilsurvey.nrcs.usda.gov/. Accessed [04/30/2015].
Whitson, T.D. and D.W. Koch. 1998. Control of downy brome (Bromus tectorum) with herbicides and
perennial grass competition. Weed Technology 12:391–396.
Listing a commercial herbicide does not imply an endorsement by the authors, University of Nevada Cooperative Extension
or its personnel. Product names were used only for ease of reading, not endorsement. They should be selected for use based
upon the active ingredient and the specific bio-environmental situation to which it will be applied. Product labels change
often; therefore, applicators should always consult the current label prior to applying any herbicide.
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Copyright © 2015 University of Nevada Cooperative Extension
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