When EPA first registered the new dicamba formulations for over-the-top use on dicamba-tolerant crops in 2016, it was almost a case of déjà vu all over again, to quote the late baseball legend, Yogi Berra.
Twenty years before Monsanto had introduced the first glyphosate-tolerant cotton and soybean crops to similar fanfare. For several years the technology worked spectacularly, killing 2- and 3-feet tall Palmer amaranth, until it didn’t. Now it’s happening again.
“When the system first came out several years ago, we were getting just awesome control of Palmer amaranth with a half-pound of dicamba,” said Larry Steckel, professor of weed science at the University of Tennessee. “In 2019, we started getting numerous reports of folks disappointed with their Palmer amaranth control with the 1X rate of dicamba.
(Larry Steckel, University of Tennessee professor of weed science. Photo by Forrest Laws.)
“That got a bunch of us thinking that maybe we're starting to see a little bit of backsliding as far as the capability of the 1X rate of dicamba on Palmer amaranth,” said Steckel, who spoke at two of the stops on the first-ever online University of Tennessee Milan No-Till Field Day. (To watch any of the tours visit https://milannotill.tennessee.edu/research-tours/.)
The use of video allowed the speakers to show the levels of control of herbicides and other products at different growth stages at the Milan Research and Extension Center in Milan, Tenn., and other locations.
Steckel was speaking from a research plot that had been sprayed with 22 ounces of Xtendimax herbicide, which is the equivalent to a 1X rate of dicamba. The Environmental Protection Agency withdrew the registration for Xtendimax, Engenia and FeXapan following a ruling by the Ninth U.S. Circuit Court of Appeals earlier this summer.
“This is three weeks after application, and we applied them when the Palmer amaranth were 4 to 6 inches tall,” he said. “It also has a quart of Roundup PowerMax with it. You could see we've gotten control of a good many of them, but we've left a number of them, too.”
Steckel said the weeds in the plot aren’t growing as fast as they normally would if you look at the nearby check plot of pigweeds known to be susceptible to the recommended rate of the dicamba formulation.
“But just now they're starting to really get their feet back under them,” he said. “And a lot of these will not die in my experience from last year. They will also go on to produce seed.”
So how can growers manage dicamba-tolerant Palmer amaranth?
“We have to come back with some other kind of application, and there are a number of options here,” he said. “One would be Liberty in cotton, which would finish these off. Another would be a second application of dicamba.
“Barring that we have had some luck with Flexstar over the top of these, particularly if you catch them about a week after the dicamba application. But it looks like we’re starting to see kind of the slow progression of Palmer amaranth developing tolerance, increasing tolerance to dicamba.”
Steckel then moved to a plot that had been planted in Enlist soybeans. “In Enlist soybeans, the idea is to use a pre-emergence herbicide and then come back with your Enlist or your Liberty or a combination thereof,” he said. “This is an example of if you don't do that.
“So this had no pre. It just had one application of Enlist One on it at a quart rate, which is the 1X rate. And the thing I wanted to point out is we have some dead pigweed here, but we've got some that haven't died. This was applied on 6-inch tall Palmer amaranth. The Palmer amaranth that survived looked similar to what we see with the dicamba on the pigweed that survived.
“They're kind of laying down and it's checked up the growth, but they're not dead. And some of them indeed are putting their faces back to the sun and coming on.”
A lot of dicamba
Steckel said growers have used a lot of dicamba in his state on Palmer amaranth to manage it. “Again, it's done a very good job the first few years out of the gate. But in 2019, and the research we're seeing here today, we're not getting the control we saw in 2017 and 2018 at a half-pound rate of dicamba.”
The other option is the Enlist system and using 2,4-D or Enlist One. “But it looks like from what we see here, that in the ramp up in tolerance to dicamba the 2,4-D might be hitching along for the ride, as well.”