Attacking glyphosate-resistant weeds with a premix herbicide combination with residual control makes sense across the South, where weed pressure is more intense than ever. Syngenta representatives focused on that strategy during a series of tours for ag retailers in central Louisiana recently.
“We think premix is the way to preserve that glyphosate chemistry,” says Jon Colburn, Syngenta’s South Delta district sales manager.
“Palmer pigweed is eating some folks alive. It’s moved into north Mississippi now and is causing major problems. We have to get growers thinking that the way to handle it is not just with two shots of glyphosate. Syngenta has a great portfolio of products to help them manage this issue,” says Don Porter, Syngenta technical brand manager for selective herbicides.
He points to Prefix as a herbicide premix for soybean providing effective residual control to fight resistant weeds. Prefix contains Dual and Reflex, two different modes of action, that will give up to five weeks of residual control.
“Prefix can also go out early postemergence. If you use Prefix and then tank mix with Touchdown, you’re pretty much home free. You can also use Prefix and go over-the-top with Flexstar GT, which is a premix of Flexstar with glyphosate. It was designed specifically to help manage glyphosate-resistant weeds. You’re still not going to be able to take out those trophy weeds with it. Four inches is the key height for resistant weeds,” Porter says.
Applied postemergence, Flexstar GT controls the same annual weeds glyphosate does. It also works on Palmer amaranth, waterhemp and ragweed, which have shown resistance to both glyphosate and ALS herbicides. In addition, Flexstar GT will control morningglory and velvetleaf, which tend to be tough ones for glyphosate alone.
“The thing to remember is that soybean weed control has changed. Syngenta has some nice solutions,” Porter says.
That goes for cotton, as well, says Shawn Potter, Syngenta brand manager of selective herbicides. “Weed management is a lot more complicated in cotton than it used to be. We’re now seeing Johnsongrass resistant to glyphosate, too,” he says.
Potter recommends starting with a clean crop, then overlapping residual herbicides with as many modes of action as possible.
“Reflex applied 14 days prior to planting will provide four to five weeks of residual control. Then come in with others with residual activity like Envoke and Sequence. By overlapping herbicides, you avoid having a gap when weeds can emerge. If resistant weeds do emerge and get some size on them, you don’t have a lot of options to clean them up,” Potter says.
Glyphosate-resistant weeds can also be attacked in corn and grain sorghum with Syngenta herbicides such as Halex GT, a postemergence herbicide with three modes of action; Lexar, which also has three modes of action; and Bicep. Lexar works best on silt loam soils and should not be used on coarse soils, says Keith Driggs, a Syngenta technical support representative based in Arkansas.
Rotating with corn is essential in areas where resistant weeds cause problems, says James Hadden, Syngenta technical support representative based in Georgia. “Corn is the crop that allows us to beat this thing down. It’s all about reducing the seed bank,” Hadden says.
“We have chemistries for corn like Callisto and atrazine that we can’t use in other crops. We can get more chemistries in that mix in corn.”
It is vital to stop glyphosate-resistant weeds before they are firmly established in fields. “If you let these weeds blow up on you, how many years are you going to fight them?” asks Chuck Foresman, Syngenta technical brand manager for weed resistance strategies.
He recommends not reducing herbicide rates, saying that increases the odds of developing resistance. Using different modes of herbicide action helps but it is now necessary to pay close attention to which ones are applied.
“Stacked resistance is the sucker punch,” Foresman says, citing Johnsongrass resistance to both Fusilade and glyphosate. “Weeds remember they’re resistant. Stacked resistance makes this very difficult and complex. Johnsongrass is an outcropper, like pigweed. You’re going to get some gene mixing and some moving with Johnsongrass.”
At this time, though, Palmer amaranth remains the biggest challenge for the largest number of Southern growers. “The thing with Palmer pigweed is that the pollen carries the resistance mechanism and it can spread a quarter mile or more a year. It’s going to keep walking. It’s going to stay on the move,” Foresman says.
Officially, at least, Louisiana has no herbicide-resistant weeds at this time, Foresman says. However, he thinks the state has resistant pigweed on 40,000 acres of its total 335,000 spread. Louisiana has 1.2 million total acres infested by Johnson grass, and Foresman believes as much as 64,000 acres have some herbicide resistance. He thinks 183,000 of the state’s 404,000 acres of horseweed are resistant to glyphosate, and 14,000 of Louisiana’s 112,000 acres of ryegrass show herbicide resistance.
“It’s important to rotate modes of action and to make sure you’re overlapping weed control with residual herbicides. Don’t let those weeds emerge. Once they emerge, it’s a crapshoot. So get those residuals down. Use full rates. Do whatever is necessary to apply at the right timing. It boils down to seed bank management. Rotate crops. Scout fields. Have a plan,” Foresman says.