Leaving the Back Door Open for Resistant Weeds
September 24, 2010 
It’s no secret that the scourge of glyphosate resistance is spreading. What started in South Georgia five short years ago now affects nearly every corner of the Southeast and Mid-South regions of the Cotton Belt.
What’s worse, say weed specialists, is that homegrown outbreaks of resistance occur every year, often in places thought to be safe from the epidemic.
“Prior to this year, I had not gotten a call about pigweed control failures with glyphosate south of Hwy. 82,” says Mississippi Extension Cotton Specialist Darrin Dodds. Running from Greenville east to Starkville and Columbus, Hwy. 82 cuts through the center of the state, effectively dividing it into northern and southern halves. “This year, I’ve gotten calls out of Holmes County, out of Humphreys County, and I’ve even gotten some calls off the Mississippi Gulf Coast where we grow cotton. It seems as if we’re just steadily spreading.”
Weed specialists in other parts of the Belt echo Dodds’ sentiment. 2010 has apparently been a banner year for the spread of glyphosate resistance.
Last year, Larry Steckel with the University of Tennessee Extension Service reported that resistance was at its worst along his state’s river counties. Just like his counterparts across the Mid-South and Southeast, he is now seeing a changing trend in the epidemic’s movement across the state.
“It really showed up in middle Tennessee this year, and north Alabama. Palmer pigweed had never been there before – no Palmer pigweed at all, let alone resistant Palmer pigweed. So it’s basically got all of our cotton acres now. Wherever we grow cotton, it’s there to some degree,” Steckel says.
Prime Conditions
What sets 2010 apart from other years when growers have been dealing with resistance have been the ideal growing conditions many have experienced this year. Farmers were able to plant early, and very few suffered setbacks during the first stages of the growing season.
Those factors – compounded by a mid-summer heat wave that saw temperatures linger in triple digits over much of the Belt – have made early harvests a reality across much of cotton-growing South. Those early harvests are a welcome sight for many in the Mid-South and Southeast.
“We got off to such a good start that I’m hoping we’re going to start picking early this year and get out a good crop before any hurricanes come,” Dodds says.
Still, he notes one major drawback of an early harvest. “If we do that, it will certainly leave a wider window than normal for weeds to come up after the crop is already out.”
Other specialists recognize this risk as well. As growers get out of the field on earlier dates this year, they increase the amount of time between harvest and the year’s first frost. In many cases, those days present prime growing conditions for the proliferation of Palmer amaranth – a weed that doesn’t need much help to take hold in a field.
“We’re going to be harvesting in August for a lot of the Mid-South and Tennessee. So from the time we harvest until the first frost, we can see a heck of a big population of pigweed blow up. So growers need to be mindful of that going into cotton next year,” Steckel says.
Striking Back
Every weed specialist will tell you there’s no silver bullet for dealing with glyphosate resistance. Dodds and Steckel agree that growers should take their own circumstances into consideration before making a decision about how to fight resistance during the post-harvest months.
Steckel says growers in Tennessee haven’t had much success with off-season cover crops because they don’t tend to put on the bulk necessary to drown out pigweed. Tillage, on the other hand, has been an effective tool in the area, especially in those fields that have been hit the hardest by glyphosate resistance.
“We had a lot of tillage going on. I know the National Resource Conservation Service is very concerned because some people are getting out of compliance due to the increased tillage. But as far as managing pigweed, it’s a good way to go,” Steckel says.
“Most of it probably germinates right on top of the ground, and if you bury it all and let it set a couple of years, most of that seed bank will be depleted, provided you can keep the rest of it from coming up. And that’s gong to be one avenue folks with real train-wreck fields need to look at.”
Steckel adds that in hilly areas a cover crop should be used after tillage to keep the soil from washing away. He also says reverting back to a no-till system in the years immediately following tillage is the best way to go.
“You don’t want to till it again because you’ll bring all those seeds right back up. You’ll get a pretty good mortality rate for one year, but you really need to wait about three years before you till it again.”
Dodds adds that a fall-applied residual herbicide can be a good tool against resistance depending on the species in question, although he’s quick to point out that this practice doesn’t work in every situation.
“A must for any residual herbicide program is to get the material applied prior to the emergence of the weed you are trying to control. In addition, an activating rainfall is necessary for maximum effectiveness. In the case of Palmer amaranth, the chance of each of these things happening prior to emergence may be slim. In addition, if you’re on some land that’s got some slope, you apply that residual and then you get a bunch of rain, you run the risk of seeing a little bit of wash that way just because there’s no ground cover there to protect it,” Dodds says.
Timing is also a key component. Dodds says that if a grower harvests later into the fall, an early frost could render an application unnecessary for Palmer amaranth.
“But if we back up and we’ve got two or three months where (pigweed) can get some size on them and put some seed head on and drop seed, then I think we certainly need to do something, whether that be some sort of tillage, some sort of herbicide application to take them out. The last thing we want to do is put seeds back in the seed bank during the growing season and put another flush back in after the growing season,” Dodds says.
Chemical Changeup
As the resistance issue worsens, specialists are seeing a greater adaptation of different chemistries to deal with the issue.
“A lot of growers have made wholesale changes. A lot of them moved from a glyphosate based system, to an Ignite based system this year. Coupling the ability to spray Ignite over the top with residual herbicides has worked,” says Steckel. “We’re just to the point now that it’s very difficult to grow cotton unless we can put Ignite over the top.”
Dryland acres are the most vulnerable in a Roundup system, because growers depend on a timely rain to get their herbicide activated.
“In the Roundup system, we had a number of growers stick with it like they’re supposed to. They put a pre- out, and it worked great if they got it activated. But if you don’t get it activated, you don’t have any options. We probably disked up about 2,000 acres of cotton this year, and replanted it to a Widestrike or Liberty Link cotton, because at least there was a post- option in Widestrike or Liberty Link,” says Steckel.
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