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Management of Palmer Amaranth, Resistant Kochia

Friday, March 28, 2014
filed under: Weeds

While the title of his talk was “Glyphosate-Resistant Weed Management in Sunflower,” the focus of Phil Stahlman’s presentation at the 2014 Sunflower Research Forum this winter was on two troublesome weed species: Palmer amaranth and kochia.

Stahlman, a veteran Kansas State University weed scientist based at the KSU Agricultural Research Center at Hays, provided an overview of the resistance issue in the central Great Plains, along with results of recent years’ research into control of key weeds in sunflower at the Hays and Colby stations of KSU.

The KSU weed scientist noted that Palmer amaranth has become consistently the most common weed in the central and southern Great Plains. In the 2013 National Sunflower Association Crop Survey, Palmer amaranth once again — as in the past several years — ranked as the most common weed in surveyed sunflower fields. As in 2011 and 2012, Palmer amaranth was found, in varying degrees, in 100% of the Kansas fields visited. The second most common weed in the 2013 Kansas survey fields was kochia.

Palmer amaranth also held that dubious number-one distinction, by far, in the Texas sunflower fields surveyed in the fall of 2013. Kochia was the most common broadleaf weed in surveyed South Dakota fields, as it was in 2012 and 2011.

Stahlman pointed out that in Kansas, moderate densities of Palmer amaranth can cause sunflower yield losses of 50% and even higher, though this weed species’ impact on crop yields generally is affected more by time of emergence rather than weed density.

Palmer amaranth is now present in roughly two-thirds of the lower 48 U.S. states. While not yet a problem in the Dakota/Minnesota sunflower production region, the Kansas weed scientist does expect Palmer amaranth to eventually become an issue there as well. It probably is the most robust and aggressive of all the pigweed species, he noted, and has largely displaced redroot pigweed in much of the central Great Plains.

Unfortunately, Palmer amaranth populations resistant to glyphosate were confirmed in Kansas as of 2012.

The kochia glyphosate resistance issue is more widespread, geographically. As of 2013, eight states and three Canadian provinces had confirmed the presence of resistant kochia populations. In some areas, such as western Kansas, the resistance has become quite prolific, and growers have had to transition to other means of kochia control, such as a mix of glyphosate + dicamba, the use of other post herbicides, utilization of pre-emergence herbicides and, in some cases, tillage.

A Manitoba report released in 2013 underscored the implications of kochia emergence timing for sunflower, Stahlman noted. The study, conducted by Derek Lewis and Robert Gulden, found that kochia emerging at the same time as sunflower reduced yields by as much as 76%. The 5% action threshold for early emerging kochia was four plants per square meter. The Manitobans also reported that kochia emerging after the four-leaf crop stage did not affect sunflower growth or yield.

So, what are the best avenues for controlling Palmer amaranth and kochia in sunflower, given the increasing presence of glyphosate resistance? That has been a core question addressed by Stahlman and his KSU colleagues since 2010.

Their research has included treatments with Zidua® (pyroxasulfone) alone and in combination with other herbicides. Zidua, a BASF product, works as a seedling growth inhibitor. It is currently registered in corn and soybeans, but registration on sunflower is still pending.

The 2012 and 2013 sunflower trial results at Hays and Colby reported upon by Stahlman at the Sunflower Research Forum consisted of the following:

• Palmer amaranth control as of 42 days after a dryland pre-emerge treatment at Hays, 2012.

• Palmer amaranth control as of 51 days after an irrigated pre-emerge treatment at Colby, 2012.

• Palmer amaranth control as of 29 days after an irrigated post-emerge treatment at Colby, 2012.

• Palmer amaranth control as of 58 days after a dryland post-emerge treatment at Colby, 2013.

• Palmer amaranth control as of 37 days after a dryland post-emerge treatment at Colby, 2013.

• Kochia and green foxtail control as of 58 days after a dryland treatment at Colby, 2013.

The results of the above-noted trials can be viewed on the National Sunflower Association website: But Stahlman’s summary of the research findings boiled down to the following points:

1. Good to excellent herbicidal control of major broadleaf weeds in sunflower — including herbicide-resistant biotypes — is possible.

2. Soil-applied herbicides’ effectiveness will be, not surprisingly, dependent upon timely rainfall or irrigation.

3. Mixtures of at least two active ingredients are most effective.

4. Herbicide cost is a deterrent to highly effective weed control in sunflower in the central Great Plains.

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