A Familiar Foe
March 19, 2010 
It’s been a year since many growers in the Mid-South got their first taste of a serious glyphosate-resistance infestation.
In the first months of 2009, many growers in states like Tennessee, Arkansas and Mississippi weren’t prepared to deal with resistance management. But even those who took the threat seriously weren’t able to be proactive, as weed experts across the Mid-South had urged.
“I can remember last year we had growers who took the pigweed issue seriously, and they put Valor on 21 days ahead of planting, or put Reflex on 14 days ahead of planting,” says Larry Steckel, Row Crop Weed Specialist with the University of Tennessee Extension Service. “The problem was that they turned out to be 45 days ahead of planting. With all the rain, they just couldn’t get in the field to plant. So those applications were just a waste of money.”
The result was a banner year for glyphosate-resistant Palmer amaranth. Counties on Tennessee’s western border have some degree of infestation in just about every field. Counties located just inside that have almost 50% infestation, according to Steckel. And the problem seems to be moving further east from the Mississippi River.
One thing’s for sure following Palmer amaranth’s trail of destruction in 2009; Resistance won’t sneak up on anyone this year. Growers in the Mid-South have a different attitude.
“They’re actually frightened. With the phone calls I get, I don’t know how else to explain it. And they probably need to be,” Steckel says.
Round Two
There’s no guaranteeing that a wet spring won’t throw a kink in the plans of Mid-South growers again this season. Still, most weed experts agree that a pre-plant residual is the best way to kickoff a resistance management program.
Steckel says that he is advising growers to utilize a PPO because much of the acreage in West Tennessee will be no-till or limited-till. Many growers have had success with products like Valor or Reflex as their first pre-plant application.
Valor calls for a longer window, with ideal application coming 21 days before planting. With Reflex, an application can be made from 21 to 14 days before planting. And growers should be aggressive soon after emergence.
“You want to come back with Cotoran right behind the planter, or Dual in that first application on one-leaf cotton,” says Steckel. “And the idea is, as Valor or Reflex is playing out, you put one of those two on and they pick up the slack. And it gives you an overlapping residual.”
The notion of overlapping residuals in a resistance-management program is catching on throughout the Mid-South and Southeast.
“If we’re talking about pigweeds, your best strategy is to overlap your residual, like the roof on a house,” says Frank Carey, Field Marketing Development Specialist with Valent. “If you want to keep the water out, you don’t butt them up edge-to-edge, you overlap them.”
In this system, it’s important for the grower to be comfortable with predicting the length of control his herbicide will give him. An overlapping application should be made a week before the first runs out to ensure that the pigweed never sees the light of day.
“Once these pigweeds come up, this Palmer can grow fast – about an inch a day, in season. I mean it grows fast.
“And if it gets over 3 inches tall, your control options are about gone. So the first of your herbicide labels are going to say 3 inches or less, some of them are going to say 6 inches. But at an inch a day, that’s not very long. You’ve got to be very timely in getting out there. That’s why you’ve got to use residuals,” says Carey.
Weather Proof
Still, as growers discovered in 2009, even the best laid resistance-management plans can get thrown off course by the weather.
Steckel believes growers in Tennessee are in a unique position because they typically have less irrigation than their neighboring states in the Mid-South. While growers who spray under irrigation in Mississippi can guarantee that they’ll have their residuals activated, many growers in Tennessee can make no such claim.
That’s why Steckel recommends growers in his state leave themselves an insurance policy of sorts.
“I really think if you have severe pigweed infestation going forward, you’ve got to have a variety you can put Ignite on and use it as a backstop. Because otherwise we’re sunk. If that pre doesn’t work, you’re done,” he says.
Steckel believes Ignite is a valuable piece of a resistance-management program, even if early residuals are able to be effective. In his recommended program, a shot of Ignite could hold Palmer off after a second residual application breaks. “Then you can probably go in underneath. You get enough height separation between the cotton and any weeds that emerge that you can run a hood or a post-direct rig and feel pretty good about them,” he says.
Carey says that Valor is a viable option with a hooded sprayer, although he echoes Extension weed scientists across the Southeast in urging growers to diversify their management programs.
“A lot of people want to use their Reflex at pre-plant instead of the Valor, which is fine if you do that and then come back with Valor at layby. You just don’t want to be 100 percent reliant on the same type chemistry. Valor and Reflex are kissing cousins, so you don’t want to just use those two,” says Carey.
“You want to have some Dual in there, you want to have diuron or some Direx in there, maybe a Caporol. You’ve got a tool box full of options, you want to use as many of them as you can.” ■
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