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Taking a Bite Out of Palmer Amaranth

Monday, November 1, 2010
filed under: Weeds

here are lots of prolific weeds out there. But when sunflower growers in the High Plains are asked, “Who is king of the weeds?” Palmer amaranth often comes out on top.

This pigweed species is very competitive and continues to germinate throughout the production season when conditions are right. It grows aggressively and will usually tower above the sunflower canopy, according to Kansas State University-Hays weed scientist Phil Stahlman.

During a fall 2010 tour of western Kansas, Stahlman found that most sunflower fields had at least some population of Palmer amaranth. His research has clearly shown that this weed can cut sunflower yields in half. National Sunflower Association field surveys found Palmer amaranth to be the most common weed species found in commercial fields in Kansas and Texas.

Michael Herrmann is one grower who has had success in controlling Palmer amaranth this year. Located in south central Kansas, about 40 miles east of Dodge City and 60 miles south of Hays, Herrmann is a dryland no-till producer. The Edwards County grower agrees that Palmer amaranth is his most consistent weed issue in sunflower. He uses rotation and a variety of herbicides to control the pesky weed. The rotation includes two years of winter wheat, followed by two years of milo and then the sunflower.

Herrmann’s milo weed control program includes atrazine at various rates (depending on the field’s weed history), plus a few other labeled combinations with the atrazine. He is a stickler on burndown. His fall and spring burndown consists of a combination of glyphosate, 2,4-D and Banvel, which gives him good broadleaf control. On fields that will be planted to sunflower, Herrmann times his spring burndown for late March/early April.

Herrmann uses a combination of Roundup (Gly Star® Plus), Spartan® Charge and Dual Magnum® for season-long weed control in sunflower. He likes to apply this “cocktail” seven to 10 days before planting to give time for rainfall to incorporate the Spartan and Dual. Rates include 32 oz of Gly Star Plus, 1.33 pints of Dual and 5 oz of Spartan Charge plus an AMS adjuvant.

He is happy to relate that his sunflower fields are clean, with the exception of a few Palmer amaranth plants on the field edges. Asked what aspect of this combination is giving him the Palmer amaranth control, Herrmann believes it is the high rate of Spartan Charge, which includes the active ingredient in Aim herbicide. Dual controls his grasses and does have broadleaf activity as well. Palmer amaranth is listed on the Dual label as a weed controlled by the herbicide.

With this combination, Herrmann does not have to come back postemergent to control grasses. A 2010 sunflower field was very clean — with the exception of several narrow bands up and down the field. “I realized later that one of my sprayer nozzles had been plugged. The Palmer was definitely evident, as well as a host of other weeds. It was quite a demonstration for me,” Herrmann says.

Herrmann spent $30.84 per acre on this herbicide combination. “It is likely on the high side for weed control in sunflower, but it pays off in my rotation,” he remarks.

The Kansas producer plants winter wheat back into the sunflower stubble or winter barley (planted in February) if it gets too late for wheat. “Depending on my wheat stand, I will topdress the wheat and use Finesse® to take out volunteer sunflower and any other broadleaf weeds that might have emerged,” he explains. “Other than that, my wheat fields are clean. So my sunflower herbicide investment pays me dividends in my next small grains crop because I really have clean fields.”

Herrmann plans to stay with the 5-oz rate of Spartan next year. “I have learned that cutting rates may save money up front, but it has always cost me in the end,” he observes. As to any sunflower seedling damage with high Spartan rates, Herrmann says he did have a piece of ground that received a heavy rain before emergence, and the plants were set back for awhile. “They grew out of it but I likely had a slight yield loss,” he reports. (When selecting a Spartan rate, the manufacturer, FMC, strongly recommends soil testing for organic matter, soil type and soil pH to avoid plant injury. (See the article titled “Spartan: The Workhorse Herbicide” in the February 2010 issue of The Sunflower.)

Phil Stahlman agrees that a Spartan + Dual combination can be quite effective with proper activation — especially at the rates Herrmann is using. “Rainfall or irrigation (minimum of 3/4” on medium and fine-textured soils) before weeds germinate is needed for optimum performance of soil-applied herbicides,” Stahlman advises. “Lesser amounts may not fully activate the herbicides on heavier soils.”

The KSU weed scientist cautions growers to pay particular attention to soil texture, pH and organic matter content when deciding on rates of soil-applied herbicides, especially Spartan products. “As soil pH increases above about 7.2 and organic matter drops below about 2%, the greater the risk of crop injury unless Spartan use rate is adjusted according to label recommendations,” Stahlman explains.

Michael Herrmann’s sunflower yield goal is a minimum of 1,500 lbs/ac. He usually does better than that, but points out that drought and heat can get pretty intense in his part of Kansas.

What does he like about sunflower in his rotation? Herrmann answers without hesitation. “Sunflower is tough and can withstand weather variances. That is why I plant sunflower instead of corn. Corn just doesn’t work consistently for me,” explains this veteran Kansas producer. — Larry Kleingartner
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