Tankmixing herbicides has quickly become a necessary way to control new resistant weeds in cotton production. You might even call it a return to the days before 1996 when Bt cotton burst onto the scene with so much fanfare.
Suffice it to say that the environment has changed dramatically in the last five years. The experts can point to many factors contributing to the adoption of more tankmixing. But everyone seems to agree that the weed resistance epidemic – most notably resistant Palmer pigweed – is driving all decisions now.
As mentioned, tankmixing has been around for a long time, but it wasn’t deemed as essential during the days when Roundup Ready systems made farming so much simpler.
It’s not that farmers have forgotten how to match tankmix partners today. It’s rather an understanding that many factors can influence what herbicides to mix and when to apply them.
Nobody has a better appreciation for the situation than Bruce Kirksey, director of research at Agricenter International in Memphis, Tenn. Because the environment for crop production can be different at a research station, the approach to the problem can also be unusual.
“Our problem here at the Agricenter is that we don’t know exactly what crops we’ll have each year,” he says. “And sometimes we might have three or four different crops in a field.”
Challenge Of Crop Mixes
This uncertainty means that Kirksey and his crews can’t do an early residual burndown when it’s unclear what crops will be planted back into the field.
Plus, the Agricenter has alleys and other roads that can contribute to resistant pigweed outbreaks.
“This is going to be a challenging year for us on the research and production sides,” says Kirksey. “We have several weeds here at the Agricenter that are resistant.”
Kirksey says these weeds have become resistant to a certain herbicide mode of action that is specific in what it actually targets.
“I don’t think we’ve seen anything of this magnitude in a bigger area than what we have with pigweed,” he says. “A herbicide such as glyphosate attacks a broader spectrum, and it surprised everybody that weeds would develop resistance to it.”
Even with the different production environment of a research farm, Kirksey believes much can be learned this year with the ultimate beneficiary being the farmer.
Understand Weed Responses
Not only does a farmer need to understand the compatibility of two tankmix partners, but he must have an appreciation for how different weeds respond to herbicides.
For instance, some of the pigweeds in the South are different in terms of modes of action as compared to other regions. According to Kirksey, a producer might have ALS-resistant pigweed as well as glyphosate-resistant pigweed.
Because of these differences in the weeds, it’s possible that an ALS inhibitor tankmix partner won’t do any good against resistant pigweed.
It also pays to read labels on any herbicide that might be used in a tankmix.
“Most products will list the tankmix partners,” says Kirksey. “You definitely need to read that label more carefully than you have in the past.”
Kirksey also is a proponent of farmers talking to company representatives, Extension weed specialists, consultants and anyone else who can offer good information about how tankmix partners work.
One positive benefit of tankmixes today is that it gives farmers more choices as they evaluate strategies for controlling weed resistance.
“Many of the chemical companies are bringing out old products as well as some generics that are making a comeback,” says Kirksey.
And, while it’s beneficial to have additional choices for tankmixing, it’s also important to understand modes of action when switching to new technologies. Kirksey says it’s particularly important when moving to a Liberty or WideStrike system. A new mode of action always means understanding how new herbicides perform.
Timely applications of tankmixes are also critically important. If the spraying isn’t done early enough, young weeds can quickly grow too high to control – no matter what herbicide is used.
Other important factors must not be forgotten. When putting out a tankmix application on a field, a farmer or consultant needs to walk the turnrows to monitor weed outbreaks.
“In the early days, you could spray a field and come back in a couple of weeks,” he says. “Now you need to be there every week to see what is happening on those rows.”
Pitfalls Of Cultivation
Finally, Kirksey believes the potential increase in cultivation of acreage must be included in any tankmix strategy for controlling weed resistance. Although working the soil may kill some weeds, it’s possible that the soil will be disturbed, bringing more seeds to the surface. In a short time period, these seeds will create small pigweed outbreaks before planting.
This is where a timely application of a preemergent herbicide is necessary.
“The bottom line is that we have to be diligent and read those labels,” says Kirksey. “Don’t get into a guessing game about what will make good tankmix partners. Talk to as many people as you can. That can eliminate the guesswork.”
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Early Decisions Influence Success Of Tankmix Combination
Mississippi Extension cotton specialist Darrin Dodds has seen firsthand how tankmixing has become important in weed control.
“Tankmixing starts with your burndown,” he says. “If you didn’t have a residual as part of your early burndown, you barely had time to see pigweed coming up. You likely saw some very early pigweed emergence due to the warm weather this spring.”
The tankmix combinations available to producers are numerous, and Dodds says it’s sometimes difficult to sort through all of the options.
“The bottom line to remember is that there isn’t just one product that can do everything,” he says.
And while resistant pigweed is the major problem influencing all tankmix decisions, other weeds can be a threat to cotton production, such as henbit and horseweed.
The list of potential tankmix combinations can be long. Dodds has seen numerous effective combinations that perform well, including Roundup/2-4D and Roundup/dicamba – although at times these programs have required extra applications for complete control.
He also has seen effective results using Gramoxone and Reflex to partner with an existing herbicide.
“I’ve seen a Gramoxone/Cotoran combination do very well when used behind the planter,” says Dodds. “Don’t forget that the goal is to start the season as clean as possible.”
Is it possible that producers might try to use tankmix partners that aren’t compatible? Dodds doesn’t see this happen too often. He did observe some problems in 2011 when tankmixing with Gramoxone Inteon; but that formulation has since been discontinued.
“The biggest problem in picking a tankmix partner is making sure that it will do what you want it to do,” he says. “When you hear a farmer talk about how a herbicide didn’t work in a tankmix, there could have been other problems unrelated to the product – like lack of rainfall. If it doesn’t rain, the tankmixed herbicides won’t be activated. Such a scenario has nothing to do with the products.”
The positive news about any residual program is that cotton is a resilient crop capable of sustaining some injury while delivering good yields.
“That’s part of the deal when you’re trying to fight resistant pigweed,” says Dodds.