Narrow-windrow burning destroys soil seed bank
Resistant weeds can hammer a farmer pillar to post. The advance might be a steady creep, a maddening stutter-step or a rolling charge. At harvest, after billions of dollars are spent on chemicals across the U.S., combines cut crops and weeds in tandem, scattering trillions of seeds back across fields as if by planting design. It’s a frustrating cycle as weed history repeats year after year and the soil seed bank gorges on a river of plenty.
Jason Norsworthy, weed scientist with the University of Arkansas, is attacking the soil seed bank with a no-prisoner policy: capture, burn and kill. He’s 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 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 spreaders disengaged) funnels trash into a 30"-wide windrow. The concentrated biomass is then burned. In 2014, Norsworthy’s research in soybeans showed burns exceeding 400°C 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 morningglory seed,” he says. “The weed seed degraded to ash, except for the morningglory 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 $200. The chute doesn’t impede harvest or efficiency, Norsworthy says. 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 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 Jeremy Green, a second-year graduate assistant. “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 for the past decade. The Harvest Weed Seed Control (HWSC) program was 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 2012, Norsworthy traveled to Australia and watched narrow-windrow burning for the first time in a wheat field. The technique complements a weed control program and eases the expectancy of herbicides.
“We haven’t got out to see what other countries are doing to fight weeds,” Norsworthy says. “It’s not rocket science: Remove weed seed exiting the combine from the soil seed bank and lower risk of developing resistance to the next herbicides on the field.”
Palmer amaranth seed gathers in a dry irrigation ditch just after harvest. Herbicide-resistant weeds can increase soybean production costs as much as $50 per acre.
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 field, he switched from Roundup Ready to LibertyLink soybeans and supplemented the change with windrow burning. In 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, farmers 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 say 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 enters the combine compared to other crops. It also has potential for rice, but rice biomass is often moist, 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 will soon be a common practice in the U.S. Within five years, he thinks combines will destroy weed seed inside the machine with the Harrington Seed Destructor mechanism.
“We don’t need to wait. Windrow burning offers an option now. In five years, who knows which herbicides we’ll lose? There aren’t many effective ones left to lose anyway,” he adds.
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% of weed seed goes into the combine,” Norsworthy says. “Why harvest and spread it back on the field? Get seed while you can and destroy it before it goes into the soil seed bank.”