With improved moisture conditions in much of Kansas this year, this is a good time to treat fields for perennial broadleaf weeds such as bur ragweed, bindweed, and Canada thistle.
Figure 1. Bur ragweed. Photo from Woollyleaf Bursage Biology and Control, K-State publication MF-2239.
Bur ragweed (also called woollyleaf bursage) is a perennial broadleaf weed, which is a significant problem on nearly 93,000 acres in the western half of the state. It is classified as a noxious weed in Kansas. It is adapted to low areas where water runoff collects in cultivated fields or in noncropland areas. The collection of water and the deep perennial root system, which can reach a depth of 15 feet, allow bur ragweed to survive extended periods of drought or harsh weather. These circumstances make it very difficult to control.
Bur ragweed is extremely competitive with crops, and can reduce grain yield by 100 percent in dry years. Even with irrigation, losses of 40 to 75 percent are common. Bur ragweed is more competitive with summer crops than with winter wheat because bur ragweed growth is minimal during much of the winter wheat life cycle. However, in dry years, bur ragweed will deplete soil moisture for fall-planted wheat and thereby reduce grain yield significantly.
Flower development begins in late July or early August. Seed contributes to the spread of bur ragweed and likely is a source of new infestations. New plants also arise from the vegetative buds, which develop on the root stocks, thus contributing to the spread of bur ragweed. Tillage also can redistribute vegetative buds, aiding the spread of bur ragweed.
Bur ragweed control is best when treated in late summer or fall, prior to a killing frost with Tordon tank mixed with dicamba or 2,4-D ester. Complete control of bur ragweed with a single treatment is unlikely. Control will not be as effective if the bur ragweed plants are under stress at the time of treatment. Bur ragweed is a difficult weed to control, and a single treatment application will usually not be sufficient.
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Table 1. Control of Bur Ragweed in Western Kansas with mid-September Treatments |
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Treatment |
Rate (lbs/acre) |
% Control 11 months after treatment (2-year average) |
|
Tordon + Banvel |
0.25 + 0.5 |
82 |
|
Tordon + 2,4-D LVE |
0.25 + 1 |
74 |
|
Roundup + Banvel |
1.5 + 0.5 |
16 |
|
Roundup + 2,4-D LVE |
1.5 + 1 |
27 |
Source: Woollyleaf Bursage Biology and Control, MF-2239 http://www.ksre.ksu.edu/bookstore/pubs/MF2239.pdf
For more information, see 2013 Chemical Weed Control for Field Crops, Rangeland, Pastures, and Noncropland, K-State publication SRP-1081, http://www.ksre.ksu.edu/bookstore/pubs/srp1081.pdf
Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist
cthomso@ksu.edu
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