Growers Continue to Battle the Weed Resistance Monster
March 8, 2020 To hear Chuck Farr tell it, he lives in the worst place known to man for weed resistance. And he may be right.
Farr, who operates MidSouth Ag Consultants in Crawfordsville, AR, is basically sitting on ground zero for multiple weed resistance. “I joke about it sometimes when I have two weed scientists doing research work 10 miles apart in fields we consult on,” he says.
But the resistance issues that growers face in Northeast Arkansas are no laughing matter.
“One thing’s for sure,” says Dr. Tommy Butts of the University of Arkansas. “There’s no easy button anymore. We have to throw everything we can at pigweed to get up to 98% control.”
Farr and Butts shared the challenges and frustrations of living with resistant weeds during the Cotton and Rice Conference, recently held in Memphis.
“An Absolute Nightmare”
“Every decision we make is based on weed resistance,” explains Farr. “I spend more time on weeds than anything else. I don’t know how we’re ever going to get rid of them. Our technologies have pretty much played out.
“If you’re in my part of the world in Arkansas, you have a hoe crew,” he adds. “I didn’t have one field in 2019 that didn’t have a hoe crew in it.”
Looking back, Farr attributes a heavy reliance on the Roundup Ready system in the late 1990s and early 2000s as one of the reasons for the current problem. Few residuals were used, and tankmixes were rare.
“We didn’t recognize the problem in time,” he says. “Plus, during that time, soybean weed control was only about $5-6 per acre.”
Today, the per acre cost for weed control has exploded to $100-125 or more, even with generic herbicide options. And that’s straining growers’ production budgets.
“We’re putting so much money into weed control, we’re often sacrificing on other inputs like fungicides, seed treatments and mix fertilizer,” notes Farr. “Pigweed is the most resilient weed I’ve ever seen. Hand weeding isn’t practical. We have to do something different.”
Dealing With an Increasing Problem
Butts explained that pigweed is now resistant to at least three classes of herbicides – PPO, ALS and glyphosate. That level of resistance is so widespread that any calls about pigweed are immediately assumed to be a three-way problem.
“Basically every row crop producing county in the state has PPO resistant pigweed,” he says. “The scary one to me is metolachlor resistance now confirmed in two locations in Northeast Arkansas. And we suspect, but have not confirmed, HPPD resistance. So far – knock on wood – that’s localized and not widespread. But it’s out there.”
He’s also closely watching reports of issues with grass control, including possible dicamba resistance in junglegrass in Tennessee.
“Resistance is going to increase,” states Butts. “So we have different things we need to rely on.”
- Cover crops of cereal rye have demonstrated effectiveness in helping suppress the first flushes of pigweed at planting, allowing growers a little more time to get in with timely post applications. “Cover crops won’t give you season-long control, but they can help,” says Butts.
- Deep tillage every 3-4 years can help manage pigweed seed viability, especially in areas with heavy infestations. “Most pigweed seed germinate in the top half inch of soil,” explains Butts. “Deep tillage can roll it into the ground and bury it. This way, we can eliminate about 95% of the seed’s viability. But if you deep till every year, you’re just bringing the seed back up again.”
- Adjust row spacing and seeding methods – and plant as early as possible – to get crops to early canopy.
- Sanitation is important. Keep equipment clean, get rid of the seeds and leave them in the worst fields. Don’t spread them around to other fields. “If we keep them in our worst fields, we can manage those differently from an economic and profitability perspective,” says Butts.
- Optimize application rates, paying close attention to spray equipment, nozzles and volumes.
- Adapt a zero-tolerance policy for resistant weeds, including chemical and mechanical management.
Managing with New Technologies
The application window for dicamba in Arkansas ends on May 25 – good timing for Southern Arkansas but too early in most years for post treatments in the northern parts of the state, notes Farr. There are no restrictions on use of the Enlist system in the state, but adoption of that technology has been regional to date.
“We’ve added Gramoxone or Direx to our burndown, then coming behind the planter with Brake, Cotoran, Gramoxone or dicamba when we can use it,” says Farr. “Then we come back over the top with Outlook or Warrant and two shots of a glyphosate/Liberty tankmix.
“Ninety-five percent control in my part of the world isn’t good enough anymore.”
Butts reported that current research is looking for options to use dicamba as a pre application, considering the May 25 cutoff. Early results have shown that using dicamba alone pre showed 60% control four weeks after application. A mix of dicamba and Brake showed 80% control in that same time period.
“The two products work opposite of each other, with helps manage risk,” explains Butts. “If we can get dicamba out early with no rain, we can get activation and control until it rains to activate Brake, which then kicks in with continued control.
“We have to get pigweeds before they get out of the ground and also be thinking 2-3 weeks out after our residual application,” he adds. “Last year with all the rain, Brake at a pint per acre did a heck of a job. And a combination of Brake and Cotoran, although expensive, produced greater than 95% control.”
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