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Keeping the PA* on PA**

Friday, February 1, 2013
filed under: Weeds

Producers in Kansas and adjacent High Plains states have, for several years, viewed Palmer amaranth as one of the most – if not the most – troublesome weeds in sunflower. The annual National Sunflower Crop Survey reported Palmer amaranth in 18% of surveyed fields as of 2005; by 2009 the level had risen to 90%. As of 2011 and 2012, all of the Kansas fields visited in the annual crop survey contained populations of this pigweed species.

As of 2012, the concern quotient now has been ratcheted upward with the confirmation of glyphosate-resistant Palmer amaranth in the Sunflower State. Kansas State University weed scientists collected waterhemp and Palmer amaranth seeds in the fall of 2011 from various soybean and cotton fields in the eastern and south central districts of the state. Greenhouse tests indicated resistance among a number of the waterhemp populations as well as in two Palmer amaranth populations from south central Kansas.

Additional Palmer amaranth seeds, collected in the fall of 2012, are now being evaluated as well, with preliminary results suggesting degrees of resistance in populations from near Wichita (Sedgwick County) and Great Bend (Barton County)

“Palmer amaranth is an extremely competitive weed, and the development of glyphosate resistance means it will require an effective integrated weed management program to achieve acceptable control,” notes KSU weed scientist Dallas Peterson. “Continuing to rely only on glyphosate for weed control will only speed up the development of glyphosate-resistant weeds and diminish its effectiveness.”

Glyphosate-resistant Palmer amaranth is already a serious problem in the southeastern United States, Peterson adds, and has dramatically impacted weed control programs there and even cropping systems.

Phil Stahlman is well-versed in the challenges presented to sunflower growers by Palmer amaranth — and was so even prior to the discovery of glyphosate-resistant populations. The Kansas State University weed scientist, who is based at KSU’s Agricultural Research Center at Hays, has studied this weed for a number of years and been instrumental in the development of control recommendations. He focused on Palmer amaranth in a presentation at the recent 2013 National Sunflower Association Research Forum.

This weed species, which now is present in roughly two-thirds of the lower 48 U.S. states, continues to slowly progress northward and westward. While not yet in Minnesota or the Dakotas, Stahlman expects it to eventually show up there as well. “Palmer amaranth is perhaps the most robust and aggressive of all the pigweed species, and it has largely displaced redroot pigweed in much of the central Great Plains,” he says. Among its telltale characteristics is terminal inflorescence: a single plant can produce hundreds of thousands of seeds in a single year. As a diecious species, there are both female and male Palmer amaranth plants. The males produce the pollen; the females the seed.

Palmer amaranth thrives in hot, dry conditions and can grow up to several inches in a single day if conditions are ideal. It doesn’t like shade and hence often grows taller than whatever crop — including sunflower — is in the field. In Kansas, sunflower yield losses of more than 50% have occurred at even moderate densities of Palmer amaranth, Stahlman reports. (Just eight to 10 plants per square meter have cut soybean yields by nearly 80%.)

It sounds ominous. But there are tools available to control this weed in sunflower. Stahlman has researched Palmer amaranth management for a number of years and, most recently, conducted a series of trials with financial support from the Kansas Sunflower Commission and the National Sunflower Association.

In a 2010 pre-emergence trial conducted at both Hays and Colby, the KSU weed scientist looked at control of Palmer amaranth and several other weeds (tumble pigweed, kochia, puncturevine and green foxtail) with Spartan®, Dual Magnum®, Broadaxe® and Prowl® H2O. With Spartan at the 2.6-oz rate, Palmer amaranth control came out at 83%; with Spartan at 3.2 oz, it was actually slighter lower. Dual Magnum provided 78 and 93% control, respectively, at the 12.1- and 15.1-oz rates. Broadaxe at 14.6 and 18.3 oz gave 89 and 100% control, respectively. Prowl H2O recorded 84% control at 48 oz.

A 2011 trial, conducted at Hays and Manhattan (a higher-rainfall location) focused mainly on mid-season Palmer amaranth control with Broadaxe applied at 21 days prior to planting and also pre-emergence. Rates were 14, 18 and 22 oz. Control was quite strong at Hays with both application timings. At Manhattan, it was very good at pre-emergence but not strong with the preplant treatment.

Another 2011 trial, conducted at Hays and Colby, looked at Palmer amaranth control with pre-emergence applications of Spartan and Zidua® combinations. At 49 days after planting, the addition of Zidua at 1.7 oz to a 2-oz Spartan increased Palmer amaranth control by about 9%; the addition of 1.7 oz of Zidua to a 4-oz Spartan level hiked control by 20%. Control was slightly better when 3.4 oz of Zidua was mixed with those two rates of Spartan, resulting in virtually 100% control when the Zidua rate was 6.7 oz.

The 2012 growing season at Colby was, like across much of the High Plains, very dry. A pre-emergence trial produced extreme variability, with no treatment outstanding. The best results were from a mix of 4 oz of Spartan and 3.4 oz of Zidua. Zidua is not currently labeled for sunflower, but the process toward getting that approval is progressing.

The take-home message on Palmer amaranth control that Stahlman delivered to the Sunflower Research Forum audience boiled down to these points:

• Single active ingredient herbicides generally have not been as effective as herbicide mixtures, though most producers have been sticking with single modes of action up to this point.

• Broadaxe has provided good to excellent control of several key weeds, including Palmer amaranth (though it is weak on puncturevine). (Broadaxe consists of a combination of the active ingredients in Spartan and Dual Magnum.)

• Zidua is promising. However, its use rate may be limited by cost. More evaluation of early preplant applications of this herbicide is needed, according to Stahlman.

— Don Lilleboe
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