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Pigweed Hits North Alabama

Amanda Huber
Southeast Editor

Resistant Palmer Amaranth Can Spread Rapidly In A Field

Palmer amaranth is a fast-growing C4 summer annual weed that can grow up to 10 feet in height. One of the group known commonly as “pigweeds,” Palmer amaranth is a dioecious plant, which means that plants are either male only or female only, which produces an enormous amount of seed. Some plants are capable of producing more than 500,000 seeds.

Jason Ferrell, University of Florida Extension weed specialist, explains how to tell the difference in the male and female pigweed.

“Female flowers are prickly to the touch because of the presence of stiff bracts,” Ferrell says. “Male flowers are soft and will often release pollen if shaken.”

Know How To Identify The Weed

Ferrell says it is important to know the female flower from the male in case a population proves to be resistant to herbicide, and hand weeding is necessary.

“If Palmer amaranth in your field survives a glyphosate application, you must hand weed as quickly as possible to remove the female plants first, since they produce the seeds,” he says. “Each female plant is capable of producing 500,000 seeds, and if just a few plants survive one year, there will be thousands the next year.”

The days of being able to eliminate most all of the weed problems in their fields are over for Alabama producers, who had escaped the problems with glyphosate-resistant Palmer amaranth as found in their neighbor to the east.

Now begins a brand new era in weed management, and 2011 will be an important year in the battle against Palmer pigweed.

2011 A Critical Year

“We’ve been very lucky in Alabama as far as weed resistance,” says Charles Burmester, Extension agronomist with the Tennessee Valley Regional Research and Extension Center based in Belle Mina, Ala. “We haven’t had the major problems that a lot of other areas have had with Palmer amaranth.

“We have had some glyphosate-resistant horseweed, mainly in the northern part of Alabama, since early 2000, but we’ve been fortunate up to this point that we haven’t had that many problems,” Burmester says.

The agronomist says one reason they likely haven’t had problems before now is because Palmer amaranth just wasn’t a prominent weed species, especially in the northern part of the state where he is located.

“The first incident that was really confirmed was in the Barbour County area,” he says. “This year, we had 20 out of 63 counties that we have confirmed have resistant pigweed. That is a big jump in just one year.

“We’ve come a long way in one year as far as developing a problem across the state,” he adds. “We think that from all indicators, 2011 is going to be a very critical year for us, as far as Palmer pigweed.”

While the problems are still scattered and spotty in some fields, Burmester expects a great deal more pigweed this coming season and even the possibility of some fields that cannot be harvested.

Critical Management Points

Cotton Weed Control Program For Glyphosate-Resistant Palmer Amaranth

Maintain residual weed control from burndown through layby:

• Diuron + Gramoxone with crop oil concentrate (with Ignite if resistant horseweed) preplant foliar (PPF) 15 days ahead of planting.

• Reflex (w/Prowl, diuron or Staple) preemergence (PRE) behind the planter must use Reflex with PPF mix on fine textured soils of Tennessee Valley (and get 0.5 inch of rain before planting cotton). If this is not activated by rain or irrigation before cotton emerges, apply Staple before Palmer is two inches tall.

• Metolachlor (Dual Magnum, etc.) or Warrant with glyphosate (RRF cotton) or Ignite (LibertyLink cotton) early post (three- to four-leaf cotton).

• Layby Pro or diuron (add 2 oz Valor) + MSMA layby (These mixtures will kill a relatively large pigweed if the sprayer can cover it, but it must be kept off the cotton).

• These residual herbicides must be activated by timely rainfall or overhead irrigation in order to work. These products will add between $30 for Reflex, Staple, Dual or Valor and $40 (add Ignite burndown) per acre to your weed control costs above a system where two to three applications of glyphosate have worked in the past.

Source: Timely Information: Counties with Confirmed and/or Suspected Glyphosate-resistant Palmer amaranth in Alabama, which can be found at www.alabamaprecisionagonline.com.

With that in mind, he and other Extension agents in Alabama have spent much of the winter holding meetings and workshops to spread the word about management of herbicide-resistant Palmer amaranth.

“We still have a long way to go as far as education goes in Alabama,” Burmester says.

Some of the critical points for managing glyphosate-resistant Palmer amaranth are to kill all weeds prior to planting, especially the resistant weeds, using products that give residual control past planting.

Producers should maintain residual control by using soil-active herbicides throughout the season. Escaped pigweed should be hand rogued from fields. Maintain post-harvest pigweed control, especially in early harvested crops like corn, by mowing followed by an application of 2,4-D or Gramoxone plus 2,4-D.

Lastly, rotate crops and use herbicides with different modes of action in each crop.

Contact Amanda Huber at (352) 486-7006 or ahuber@onegrower.com.

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