“I’m sorry, I have no good news for you.” That’s how Jason Bond summed up the situation regarding herbicide-resistant weeds in his presentation at the annual meeting of the Mississippi Agricultural Consultants Association.
In addition to the ongoing problem with glyphosate-resistant Palmer amaranth, there was documentation in 2015 of the weed’s resistance to PPO herbicides, says Bond, who is research/Extension professor at the Delta Research and Extension Center at Stoneville, Miss.
“Last year was a rough one with Palmer amaranth,” he says. “We had thought we were doing pretty well with the resistance problem, but in some areas last summer we took a big step backward to the kind of problems we were seeing in 2007/08.”
After learning last year that University of Tennessee Row Crop Weed Specialist Larry Steckel had found a population of Palmer amaranth that was resistant to PPOs, Bond says, “We later sent several Palmer amaranth samples from Tunica, Coahoma, Bolivar, and Quitman counties to the University of Illinois for genetic testing. I was really worried, because in that part of the state it was a terrible year for weed control.” But, he says, all results came back negative.
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“After that, we took 100 samples from 50 different sites, in a north/south zig-zag along the Mississippi River from Issaquena County north to Tunica County, but mainly in Bolivar, Coahoma, and Tunica counties.
“These were then were grown in a greenhouse at the Stoneville station. We sprayed the resultant 2-inch Palmer amaranth with the equivalent of 1.5 pint of Reflex in 20 gallons of water. Of the 100 samples, 38 had survivors 21 days after treatment. Of that 38, there were 16 survivors that developed into plants that were at least as big as those that were untreated 21 days after treatment.
Average survival: 25 percent
“Larry Steckel noted that in Tennessee they were seeing an average Palmer amaranth survival after PPO treatment of about 25 percent, and ours were about in that range. What we know for sure,” Bond says, “is that in Bolivar, Coahoma, Tunica, and Sunflower counties we definitely had some plants that survived the 1.5 pint Reflex treatment. A couple of the Desoto County samples survived, but they didn’t develop into very aggressive plants.”
As with glyphosate resistance, he says, “If PPO resistance is identified in your county or in a county adjacent to yours, it is assumed all Palmer amaranth in that county and adjacent counties has the potential for PPO resistance.
“What I think we were seeing in some cases,” Bond says, “is not the first generation of resistance, but rather the second, or maybe even the third. I think this problem has been there, in that part of the state, for some time, and some of the poor control we’ve attributed to weather or other causes has actually been a larger resistance issue that was covered up by weather.”
Changes were made last fall to Mississippi recommendations for managing glyphosate/ALS-resistant Palmer amaranth in soybeans, he says. “We took the cotton program, which has been pretty successful, and applied those ideas to soybeans. We’ve had good luck with this kind of program in the past, before we knew exactly what we were dealing with in terms of resistance. We tried to factor in everything we could to make the best decision possible for managing this problem.”
Since glyphosate resistance became a problem, Bond says, Mississippi growers have, for the most part, chosen to stay with the Roundup Ready system, “and we’ve tried to make it work. In Tennessee, Arkansas, and other areas, many growers have moved to LibertyLink.”
Program for resistance management
Mississippi’s program for managing resistance centers on a preplant/preemergence/postemergence system, he says.
“The preplant application is basically to keep things clean from the time it goes out until planting, so you don’t end up with Palmer amaranth that are 24 inches tall that you can’t kill with anything but a shovel. Then we make preemergence and postemergence residual applications every 14-21 days to maintain a lethal dose.
“That’s the concept we’re going after. You can still get good control with metribuzin and Group 15 herbicides.”
Mississippi’s cotton recommendations are “basically unchanged,” Bond says, and include a preplant, preemergence, postemergence over the top, and post-directed applications.
In rice, Bond says, “We did a lot of work with Sharpen, and were pleased when we got a label for it.” But, it looks like we may be back to some kind of propanil-based treatment for small Palmer amaranth. Timing is critical, and where there is dense Palmer amaranth, we need to apply an early flood.”
Whatever the crop, he says, “It’s important to manage the weed seed bank.” A mature Palmer amaranth can have half a million or more seeds.
“The resistance problem will get worse,” Bond says. “Use all available tactics to combat Palmer amaranth: cover crops, variety selection, row spacing, planting date, hand weeding, etc. And monitor all herbicide applications closely for efficacy.”