Resistant weeds can hammer a farmer from pillar to post. The advance may be a steady creep, a maddening stutter-step, or a rolling charge. At harvest, after billions of dollars are spent on chemicals across the United States, combines roll over farmland cutting crops and weeds in tandem, scattering trillions of seeds back across fields as if by planting design. It’s a frustrating cycle that weed history repeats year after year and the soil seed bank gorges on a river of plenty.
However, Jason Norsworthy, weed scientist with the University of Arkansas, is attacking the soil seed bank with a no-prisoner policy: capture, burn and kill. Norsworthy is testing a new weapon in the resistant weed wars aimed directly at the seed bank reserve – narrow-windrow burning. The windrow burning technique captures and kills weed seed flowing through a combine, rather than allowing it to return to soil where a farmer eventually will contend with it again.
After capture, seed is killed by slow burning at high temperatures. A chute attached to the rear of the combine with disengaged spreaders funnels trash into a 30” wide windrow, and the concentrated biomass is then burned. In 2014, Norsworthy’s research in soybeans showed burns exceeding 400 degrees Celsius for six to seven minutes. “We purposely placed additional weed seed in the windrows for testing and noted 100% kill on johnsongrass, barnyardgrass, Palmer amaranth and pitted morning glory seed,” he says. “The weed seed degraded to ash, except for the morning glory seed, which essentially turned into tiny charcoal bricks.”
Norsworthy’s team made the chute from scratch, welding two pieces of square tubing to the combine axle. The cost of materials and welding was a negligible $200. The chute doesn’t impede harvest or efficiency, according to Norsworthy. During fall harvest of soybeans, heading into the wet season, he burns as close as possible behind the combine. Wheat harvest offers more latitude due to the approaching dry season, and it’s not necessary to burn immediately.
The windrow method is far less intense than a wheat field burn, which moves quickly and doesn’t kill weed seed in significant numbers. His team walks the concentrated rows with a propane torch, making sure the condensed lanes have opportunity to build heat slowly and maintain a high temperature. “We check our temperature associated with these burns to determine if we’re killing weed seed and later make sure temperature and the loss of weed seed viability correlate,” says second-year graduate assistant Jeremy Green. “The exact temperature needed is something we’re still testing.”
Narrow-windrow burning is relatively unknown in the U.S., but has been used in Australia over the past decade as part of the Harvest Weed Seed Control (HWSC) program pioneered by University of Western Australia weed scientists Michael Walsh and Stephen Powles. HWSC has proven highly efficient in Australia and relies on four control strategies: chaff carts, narrow-windrow burning, a bale-direct system, and the Harrington Seed Destructor. Each technique is a means of capturing or destroying weed seed, and ensuring it doesn’t return to the soil seed bank.
“In the U.S., there is an excellent opportunity to target Palmer amaranth. Probably the only positive attribute of this weed is its retention of almost all its seed production at crop maturity,” Walsh notes. “Narrow-windrow burning is the HWSC entry point for growers looking to target weed seed production during harvest. This system is cheap, easy to use and if windrow burning is implemented correctly there will be 99.9% control of weed seeds.”
In 2012, Norsworthy traveled to Australia and watched narrow-windrow burning for the first time in a wheat field. The technique is a complement to a weed control program and eases the expectancy of herbicides. “We haven’t gotten out and seen what other countries are doing to fight weeds. This is a technique that’s caught on in Australia in the last 10 years,” he says. “It’s not rocket science. Remove weed seed exiting the combine from the soil seed bank and ultimately lower risk of developing resistance to the next herbicides on the field.”
The windrow research is showing great promise after four years of trials. In combination with a herbicide program, Norsworthy believes burning will play a major role in the resistant weed fight. He wants to take effective herbicides and mix them with windrow burning or another part of HWSC as an integrated whole. “We’re never going to spray our way out of resistance. HWSC helps prevent the next herbicide-resistant weed from popping up,” he says.
Norsworthy has tested narrow-windrow burning in fields infested with glyphosate-resistant Palmer amaranth. In one location, he switched from Roundup Ready soybeans to LibertyLink soybeans, and supplemented the change with windrow burning. Over a three-year span, the soil seed bank dropped to a near zero level. “I’m talking about a field you couldn’t even put a combine in when we started three years earlier,” he describes. “We had an effective herbicide program backed up with narrow-windrow burning.”
As herbicide-resistant weeds flourish, producers pump more money into herbicide programs. Annually, herbicide-resistant weeds can increase soybean production costs as much as $50 per acre. Windrow burning offers big returns for minimal effort, while preserving herbicide options. “This can save farmers tremendous amounts of money over the long-term. No farmer will tell you resistance hasn’t cost them money,” Norsworthy notes.
Narrow-windrow burning is a fit for grain sorghum, soybeans and wheat. However, the process might not be as efficient in corn because less weed seed enter the combine compared to other crops. It also has potential for rice, but rice biomass is often moist, which is a block to high burning temperatures. Norsworthy is preparing a wheat project to test various herbicide programs on Italian ryegrass in conjunction with windrow burning.
How much longer will narrow-windrow burning be on the weed fight sidelines? Norsworthy predicts the process soon will be a common practice on U.S. farmland. Within five years he thinks combines will destroy weed seed inside the machinery with the Harrington Seed Destructor mechanism. “We don’t need to wait. Windrow burning offers an option now. In five years who knows what herbicides we’ll lose? There aren’t many effective ones left to lose anyway.”
Capture, burn and kill. Narrow-windrow burning is a tool to chase down weed escapes, the source for seed bank repopulation. “With resistant Palmer, we’ve found 99 percent of weed seed goes into the combine,” Norsworthy adds. “Why harvest and spread it back across your field? Get seed while you can by destroying it before it goes into the soil seed bank.”
What weed-fighting strategies have you used in your fields? What impact did you see? Let us know in the comments.