It isn’t easy trying to control Palmer amaranth or pigweed in row crops in some parts of the Mid-South these days.

Compared to the late 1990s when farmers could spray a pint of Roundup a couple of times, growers in some areas are finding the currently available herbicide tools aren’t working as well as they did even a year or two ago.

The new reality – increasing resistance to dicamba and 2,4-D, cross resistance to the auxin herbicides and some residuals and the potential for metabolic resistance in a few weed species – is leading weed scientists to ask if we are approaching the end of an era in herbicides.

“One of our growers in Tennessee said: ‘I don’t try to grow cotton anymore – I try not to grow pigweed,” said Dr. Larry Steckel, professor of weed science at the University of Tennessee’s West Tennessee Research and Education Center in Jackson. “That’s his mindset going into raising his crop.”

Steckel was asked about the reasons for such a turnaround in a grower’s thinking on the Crop Doctors Podcast, which is broadcast weekly and hosted by Drs. Tom Allen, plant pathologist, and Jason Bond, weed scientist with the Mississippi State University Delta Research and Extension Center at Stoneville, Miss.

Resistance discussion

That included an update on the initial reports of the development of resistance in Palmer amaranth to dicamba in west Tennessee the trio discussed in a Crop Doctors podcast last year.

“We’ve done a lot of greenhouse dicamba screens this year on Tennessee pigweed populations, and we clearly have some populations that have upped their game,” said Steckel. “We’re seeing some living through pretty high dicamba rates in the greenhouse, moving toward resistance. So it’s really gotten worse.

“Some of it is cross resistance to 2,4-D; some of it is and some isn’t. In some of these populations, there’s a little bit of metabolic resistance, similar to what they’re seeing in Arkansas with Dual and Warrant. The number of days of residual you get from those is being reduced. It’s not zero, but it’s not the three-plus weeks like you would expect.”

Steckel was a relatively new weed scientist at the University of Tennessee when reports of glyphosate resistance in Palmer amaranth begin popping up, first in Georgia and then in the Mid-South states in the early 2000s. Bond became an Extension weed scientist at Mississippi State a short time later.

“What I remember with glyphosate and Palmer amaranth down here, particularly after two or three years of dealing with the problem, is you would see two things,” said Bond. “One would be uniform 50 to 70 percent control at seven days. A bunch of the resistant ones would regrow and some would die.

“Then you might go five miles down the road and spray and the control would be zero. You would have to look for something else, another weed in the field, to know you had even sprayed.”

Tank mixes

Thus far, similar problems with the dicamba and 2-4,D formulations appear to be occurring primarily in west Tennessee and to a lesser extent in Arkansas, according to Steckel. Glyphosate resistance in Palmer amaranth may have played a role in situations where farmers have continued to tank mix glyphosate and dicamba.

“The glyphosate was just no help at all (on Palmer amaranth),” said Steckel, referring to when dicamba- and 2,4-D-resistant crops were first labeled. “So dicamba had to do all the heavy lifting. People went back to the old habits of not using residuals and all that kind of stuff.

“Now we’re going back to using residuals, but the horse is out of the barn. I don’t know where we’re going here on herbicides. I don’t know any new one that will bail us out. We’re overlapping residuals, and maybe the HPPD trait will help some. There’s resistance to that in Palmer in some states so that may be a short-lived answer.”

Allen asked about the need for additional resource investment in surveys to determine how widespread the resistance issue continues to be when growers are seeing control failures?

“I think we have to monitor it now to try to find any port in the storm,” said Steckel. “If we can get partial control out of any herbicide, it has to be thought of as a tool. We have to get a handle on what’s working and what isn’t and formulate a plan around using those herbicides.”

Nearing the end

Steckel referred to a speech by Dr. Patrick J. Tranel, a crops researcher at the University of Illinois, who said farmers and weed scientists may be nearing the end of an era for herbicides for weed control.

“Tranel is one of the smarter metabolic herbicide researchers on the planet,” said Steckel, “And from where I’m standing, especially with pigweed, I couldn’t argue with him. We definitely need newer herbicides, but that metabolic resistance issue makes you wonder if those will be all that effective for very long?”

(Metabolic resistance describes the defense mechanism plants have developed over time to protect them from toxic substances. In some cases, weeds can develop enzymes that can convert an active ingredient into metabolites that don’t kill the plant.)

“These enzymes can detoxify herbicides and not just one herbicide like we’ve been used to,” Steckel noted. “It’s not full-blown resistance on a lot of them; it’s just kind of incremental resistance. It just adds up over time, and it it’s broad spectrum. It can be Dual, it can be dicamba, it could be Roundup. If they’re all in play, that’s the real concern we’ve got going forward.”

“And that’s different than what a lot of people associate with herbicide resistance,” said Bond. “You can kill glyphosate-resistant Palmer with Liberty, for example. That’s not metabolic resistance. Metabolic resistance is often described as just Pandora’s box. Once you open it up, you’re going to have challenges with a variety of different products.”

The Crop Doctors Podcast is made available through the Mississippi Crop Situation Blog which is updated on a regular basis and has a Saturday email alerting subscribers to the newly posted content by the Mississippi State University Extension Service. To listen to this podcast, visit http://extension.msstate.edu/shows/mississippi-crop-situation.