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 MARVELL ARKcotton producer Pace Hindsley left and Phillips County Extension agent Robert Goodson are hoping to control glyphosateresistant Palmer pigweed using a practice called zero tolerance

Zero tolerance for resistant pigweed

• Glyphosate-resistant Palmer pigweed threatens to propel weed control through a time-warp. • Solutions are emerging. One is zero tolerance. • Zero tolerance attacks the Achilles’ heel of Palmer pigweed, its seed. • Zero tolerance combines hand hoeing and supplemental herbicide application. • Solutions won’t be cheap.

From 1998 to 2008, weed control on Mid-South cotton farms was perhaps the easiest in the history of U.S. cotton production, thanks to Roundup Ready technology. The next few years, however, could be the hardest.

Glyphosate-resistant Palmer pigweed threatens to propel weed control through a time-warp — from a total post-emergence program to an era when hand choppers, residual herbicides, hooded sprayers and tillage were the tools of the weed control trade.

It’s the latter tool, tillage, which has attracted not only the attention of farmers, but conservation officials from coast to coast. Palmer pigweed not only threatens profits for growers, but tillage can threaten the long-established benefits of conservation, and perhaps place farmers dangerously close to compromising conservation plans.

But solutions are emerging. A recent tour organized by the Arkansas Association of Conservation Districts demonstrated some of the best management practices under consideration to address glyphosate-resistant Palmer pigweed in the Mid-South. The tour was part of a meeting of the National Association of Conservation Districts’ Herbicide Resistance Task Force.

The task force plans to outline recommendations on how to tame weed resistance in areas where it has occurred and prevent resistance where it has yet to happen.

One of the practices demonstrated on the tour was zero tolerance, a research project implemented by Arkansas weed scientist Ken Smith on two Mid-South cotton fields this season.

Zero tolerance attacks the Achilles’ heel of Palmer pigweed, its seed. According to studies conducted by University of Arkansas weed scientist Jason Norsworthy, the weed’s biggest weakness is the short life of its seed in the soil. In fact, 80 percent of Palmer pigweed seed die within one year in the soil. After 4.5 years, virtually none of the seed are viable.

Zero tolerance combines hand hoeing and supplemental herbicide application to ensure that pigweed doesn’t reach maturity and produce seed. Weed scientists believe if they can systematically reduce the seed bank, or the population of viable seed in the soil, they can slowly start to clean up heavily infested fields.

While the practice may make a field manageable again, there’s no way zero tolerance can completely eradicate the plant, or its seed.

 “It’s a numbers game,” Smith said. “These seeds produce such high numbers of seed, 200,000 plus seeds per plant. It doesn’t take very many escapes to build up a soil seed bank. That’s what’s happened to us.”

How many years?

Smith stopped short of saying how many years of zero tolerance it would take before a field could be returned to manageability again, “but the number of hours or days of hand weeding per acre is going to go down every year. If we ever let them get away from us and leave one out there, we can be in trouble again.”

It will require a change in perspective for cotton producers. “Thinking about how many pigweeds we can stand per row foot before we start losing yield is an antiquated mentality,” Smith said. “We need to start thinking about what we need to do today for next year.”

Pace Hindsley, who produces cotton around Marvell, Ark., is managing one of his cotton fields for zero tolerance, in cooperation with Smith. The field has been in continuous cotton for over 40 years, and has become infested in Palmer pigweed.

 The first step in zero tolerance for Hindsley begins after burndown with a preplant application of Reflex. After planting cotton, Hindsley applied Sequence and Dual over the top of cotton. When cotton started squaring, he cultivated the cotton one time, followed by an application of Valor and Roundup under a hooded sprayer.

Weed control since then has been in the hands of hired choppers. “If we have any escapes, we manually chop them down ourselves,” Hindsley said.

Hindsley figures the hands have worked a total of about 1.5 hours per acre on the zero tolerance field, but he has invested as much as $50 to $60 per acre in hand labor on other problem fields. Palmer pigweed chopping crews are loaded for bear, carrying heavy duty hoes, a machete or two, and a sharp shovel.

“This is the magnitude of the problem,” said Smith, holding up a giant Palmer pigweed plant which resembled a small tree with a 3-inch diameter base.

Constant battle

Robert Goodson, a county agent for the Arkansas Extension Service, says fighting them is a constant battle. “There are spots in the field where we chopped them, and we’d go back a second time, and if the roots from the chopped plant are on the ground, they’ll grow. I hate ’em.”

Hindsley spent close to $75 an acre on the zero tolerance field, compared to the $20 an acre he would spend on three shots of Roundup, prior to glyphosate-resistant pigweed. Unfortunately, those days are gone.

“Over the last 200 years, the easiest farming years have been the last 10 years to 12 years with Roundup control,” Hindsley said. “Now we’ve got this resistance, and it’s hard on us. It’s more expensive. It takes more labor, and we may have to rethink what we’re doing, including downsizing.”

Andrew Wargo, AACD president and farm manager for Baxter Land Co., in southeast Arkansas, says he has already seen a decline in rent values in pigweed infested fields. “And ag lenders are concerned that a producer farming an infested area is going to have more out-of-pocket costs and a smaller profit margin. In the last 12 to 14 months, we’ve seen some real awareness of that.”

In Georgia, where resistant pigweed first appeared in 2005, there have been amendments to conservation plans that allow some tillage used in conjunction with cover crops to control glyphosate-resistant pigweed. Weed scientist Jason Norsworthy is studying such a combination in research plots at the Lon Mann Cotton Research Station in Marianna, Ark. The research is paid in part by Monsanto.

In 2008, half a million glyphosate-resistant Palmer amaranth seed were placed in 22-foot square sections within cotton plots, each consisting of 8 rows, 300 feet in length. “We ran a disk over the field two times, then put in our tillage with and without a moldboard plow and with and without a cover crop of ryegrass,” Norsworthy said.

In 2008, the moldboard plow alone provided a 62 percent to 65 percent reduction in Palmer amaranth emergence, the same as the performance of the rye cover crop alone. Together, the two practices resulted in an 86 percent reduction in Palmer pigweed emergence. The moldboard plow was used only in the first year of the experiment.

“I’m not a big fan of a moldboard plow, due to erosion issues,” Norsworthy said, “but the data indicates if you use it on a very small acreage and put a cover crop back on top, it can be quite effective in suppressing pigweed emergence. The next step is to begin to overlay effective herbicides on top of that.”

The same practices in soybeans were much more effective, according to Norsworthy, who will tabulate findings after three growing seasons.

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