Malheur Experiment Station
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Information for Sustainable Agriculture
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RUSSIAN KNAPWEED CONTROL AS INFLUENCED BY HERBICIDES AND MOWING
Corey V. Ransom, Charles A. Rice, and Joey K. Ishida
Malheur Experiment Station
Oregon State University
Ontario, OR, 2003
Introduction
Russian knapweed is an invasive perennial weed
that thrives in rangeland and other non-cultivated sites. It forms
dense colonies that survive because of a large root system.
Fall-applied herbicides have been effective in controlling Russian
knapweed. Research on other perennial invasive weeds has shown that
mowing prior to herbicide application increases control. One possible
reason for increased control with mowing is that the removal of plant
biomass may allow more of the herbicide to reach the soil surface where
it is more likely to be taken up by the plant. This research
investigated the use of theBrown Brush MonitorTM
(Brown Manufacturing Corp., Ozark, AL) for mowing and herbicide application compared to
herbicides sprayed without mowing.
Methods
Trials were established at a Russian
knapweed infested site near the Snake River, south of Nyssa, Oregon to
evaluate Tordon (picloram) and Transline (clopyralid) applied alone or
following mowing with a Brown Brush Monitor. The Brown Brush Monitor
incorporates a mower and sprayer into one machine. It removes the
above-ground plant material, discharging it to the side of the machine,
and then sprays the herbicide from a boom mounted under the rear of the
mower deck. Spray-only applications were made with a CO2-pressurized
backpack sprayer calibrated to deliver 20 gal/acre at 30 psi. Plots
that were mowed and sprayed measured 15 ft wide by 30 ft long while
spray-only plots were 10 ft wide and 30 ft long. Treatments were
replicated four times in a factorial design with herbicide and mowing
completely randomized within each replication. Treatments with the
Brown Brush Monitor were made October 31, 2001 and spray-only
treatments were made on November 1, 2001. Russian knapweed response to
treatments was determined by visually evaluating control and measuring
shoot height and shoot density. Evaluations were made August 5, 2002
and June 26, 2003.
Results and Discussion
Herbicide applied alone or following mowing in
the fall of 2001 provided excellent control of Russian knapweed (Table
1). Visual control on August 5, 2002 was slightly higher when Transline
was applied following mowing compared to Transline alone. However, in
2003 there was no difference. Close to 2 years after treatment, both
herbicides, regardless of mowing, were still providing 93 percent or
greater Russian knapweed control. Herbicides with or without mowing and
mowing alone reduced Russian knapweed height compared to the untreated
control in 2002 and 2003. It is interesting that Russian knapweed
plants were shorter in the mowed treatment even 2 years after mowing.
This response might be related to soil moisture, competition for light,
or soil temperature differences where the biomass was removed by mowing
compared to the untreated plots. Mowing alone did not reduce Russian
knapweed density. Herbicide treatments with or without mowing reduced
Russian knapweed densities in both years. Russian knapweed densities
appeared to be stable across years in the mowed-only and untreated
plots. While only small differences in Russian knapweed control were
apparent among treatments, mowing appeared to enhance recovery of the
perennial grasses that were present. Grass populations were not uniform
enough to evaluate, but visible differences in grass growth were
apparent between mowed and unmowed plots.
Table 1. Russian knapweed control, height, and
density in response to herbicides applied alone or in combination with
mowing, south of Nyssa, OR. Malheur Experiment Station, Oregon State
University, Ontario, OR, 2003.
|
|
Russian knapweed† |
|
|
Control |
Height‡ |
Density |
Treatment* |
Rate |
8-5-02 |
6-26-03 |
8-5-02 |
6-26-03 |
8-5-02 |
6-26-03 |
|
lb ai/acre |
------------%---------- |
---------inches--------- |
--------no/ft2-------- |
Untreated |
-- |
0 c |
0 b |
20 a |
23 a |
11 a |
14 a |
Mowed |
-- |
0 c |
0 b |
14 b |
19 b |
11 a |
13 a |
Tordon |
0.5 |
93 a |
95 a |
11 c |
15 c |
0 b |
1 b |
Tordon + Mowing |
0.5 |
94 a |
96 a |
11 c |
14 c |
0 b |
1 b |
Transline |
0.38 |
87 b |
94 a |
10 c |
14 c |
0 b |
1 b |
Transline + Mowing |
0.38 |
91 a |
93 a |
14 b |
16 bc |
1 b |
1 b |
*Treatments that were mowed were applied on October 31, 2001 while spray-only treatments were applied November 1, 2001.
†Within columns, numbers followed by the same letter are not significantly different according to LSD at
(P = 0.05).
‡In plots where control was high, Russian knapweed height is based on the few plants that survived.
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For
additional information about the Malheur Agricultural Experiment
Station, please send an e-mail request to:
Dr. Clinton C. Shock
Clinton.Shock@oregonstate.edu
Malheur Agricultural Experiment
Station
595 Onion Avenue
Ontario, OR 97914
(541) 889-2174
FAX (541) 889-7831
Last updated
Wednesday July 7, 2004 .
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