From Cotton Grower Magazine – June 2015
As June rolls around, many producers across the Cotton Belt will be thinking about making post-emergence applications. And a familiar foe, the pigweed, will be a primary target in many fields.
The preponderance of glyphosate resistance has made most growers keenly aware of the need to diversify their weed management programs. For many, one way to accomplish that goal is through tank mixing.
“If it’s Roundup Ready cotton, and you’re making a glyphosate application, you’re going to need either Dual or Warrant in the tank,” says Jason Norsworthy, Extension specialist with University of Arkansas crop, soil and environmental sciences.
Growers in the Natural State have adopted quickly to new technologies in an effort to diversify their management approaches.
“When we take a look, we’ve had a shift away from just straight Roundup Ready cotton to other technologies,” he says. “In Arkansas last year, about 75 percent of our cotton was GlyTol LibertyLink technology, which is going to give producers the opportunity to spray glyphosate as well as Liberty over the top of it.”
Bayer’s two-trait technology with full tolerance to glyphosate and Liberty was a popular choice in the Mid-South in 2014. Monsanto is introducing Bollgard II XtendFlex technology in 2015, which will also afford growers the ability to use Liberty and Roundup in the same cropping system.
Many growers who are new to the GlyTol LibertyLink and XtendFlex systems may be tempted to use Liberty and Roundup herbicides together in the tank, in place of a residual. According to Norsworthy, that may not always be a wise decision.
“What we’ve seen is that it works really well on small weeds,” Norsworthy says. “But at times, we can see some antagonism when we mix Roundup and Liberty together. If we’ve got some big weeds, those two herbicides can have some fits on some larger weeds when we start tank mixing them.”
For instance, Norsworthy says, if grass is a target in the field and it has grown large, it may be best to leave Liberty out of the tank mix. Similarly, if pigweed is a target and it has grown larger than three inches, it would be best to leave Roundup out of the tank and make multiple applications of Liberty about 10 days apart.
The main take-home message, in both instances, is that residual herbicides offer greater value in the tank, and timely applications are needed. Norsworthy says he often hears from producers who believe they should be able to pull residuals out of the post-emergence applications when tank mixing post-emergence products.
“What the residuals are doing is really helping to reduce the number of pigweeds, or just weeds in general, that we’re having to control with glyphosate or Liberty from a post-emergence standpoint,” Norsworthy says. “The more pressure we place on those herbicides, the more likely we’re going to have resistance.”
Rather than asking Liberty and glyphosate to do too much, Norsworthy encourages producers to think of alternative methods to remove troublesome weeds in the early summer months.
“If you’ve got some big escapes, by the time you get to mid-June, it’s not too early to think about putting a chopping crew in the field – if you’ve got big weeds you’re not going to be able to kill with Liberty or Roundup, that is,” Norsworthy says.