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Scorched Earth Attack on Weeds

Narrow windrow burning robs the seed bank

Harry Stephens is scorching earth to save money and boost yield. The east-central Arkansas producer spent several hundred dollars to treat 3,000 acres of soybeans with innovative weed prevention through a few bars of iron, plywood and propane bottles. Narrow windrow burning, a weed control method pioneered in Australia, might offer producers a reduction in soil seed bank population and a drop in herbicide expense.

Palmer amaranth waits to storm Stephen’s farmland each year, bursting from the ground in early spring. “We get some pigweed as big around as a man’s leg,” he says. Two years back, miscommunication at planting left 35 acres with no pre-emergence herbicide and the oversight cost Stephens $100 per acre in subsequent ineffective treatments, because once trailing, there is no way to catch up with the speed and ferocity of Palmer. Get four or five days behind Palmer and the chemical solution is over; the only answer is cold steel.

“Pigweed packs an incredible financial punch in this area,” notes Robert Goodson, Phillips County Extension agent. “I know some farmers spending $80 per acre just in herbicide, before application costs or anything else. That’s extreme, but that’s what some folks have to pay. We pay attention to anything that helps reduce pigweed seed.”

After billions of dollars are spent on herbicides annually, combines cut crops and weeds as a matched pair at harvest, scattering trillions of seeds back into fields to choke out subsequent crops. It’s a maddening cycle as weed history repeats each year and the soil seed bank gets fat on a full larder.

Searching for more weapons in the resistant weed war, Stephens heard about the narrow windrow pioneering efforts of Jason Norsworthy, weed scientist with the University of Arkansas. Norsworthy first observed the technique in Australia as part of the Harvest Weed Seed Control (HWSC) program pioneered by University of Western Australia weed scientists Michael Walsh and Stephen Powles. 

“My expenses fighting pigweed get bigger every year, and narrow windrow burning is a promising way to fight weed seed,” Stephens says. A chute attached to a combine funnels trash into a 30"-wide windrow. The concentrated biomass builds heat slowly as the burn degrades weed seed to ash, rather than allowing it to hide in the seed bank and rage across fields again. Temperatures in the windrow often exceed 450°F for several minutes, Norsworthy adds. “Narrow-windrow burning destroys all weed seed within the chaff,” he says. “It is a highly effective means of managing the soil seed bank, especially resistant pigweed.”

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Norsworthy has tested narrow windrow burning in fields with glyphosate-resistant Palmer amaranth. At one location, he switched from Roundup Ready soybeans to LibertyLink soybeans and supplemented the change with narrow 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 says. “We had an effective herbicide program backed up with narrow windrow burning.”

The potential of a simple capture, burn and kill approach to weed seed caught Stephens’ attention and he next visited Crittenden County Extension agent Russ Parker to get a bare bones look at a windrow chute. Measurements in hand, Stephens took the specs to his sons, Kimbrough and Harry Jr., who built two chutes in a single afternoon with a few bars of 20" metal and a sheet of plywood. “The whole setup cost a few hundred dollars,” Kimbrough says. “All I had to do was take the spreader bars off the combines.”

During cutting, the chutes don’t impede combine speed or performance. After combining, Stephens drives along the windrows in a truck with a propane torch on a trigger, or Kimbrough and Harry Jr. ride the windrows on four-wheelers, with mounted propane torches wide open. 

The smoldering heat last for days, turning the biomass to a blanket of ash. At first glance, the result is a series of long black stripes. In practice, the payoff is a direct assault on the mother lode source of weed seed. The narrow windrow method is far less intense than a corn or wheat field burn, which moves too quickly to kill weed seed in significant numbers. Stephens says the windrow burns are contained. 

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Goodson recently analyzed a single Palmer specimen from Stephens’ ground and estimates it contained 1.8 million seeds, an astounding amount of resupply for the seed bank. Goodson spends up to 60% of his time fighting Palmer in Phillips County and believes narrow windrow burning might become common practice on many farms. 

After harvest, Goodson flagged and recorded the locations of particularly heavy 2016 Palmer infestations. He’ll use those test point locations during the 2017 and 2018 crop seasons as a gauge to measure weed reduction. 

Resistant Italian ryegrass is also a mounting problem on Stephens’ land, and Goodson predicts windrow burning will help improve control of ryegrass as well 

“This is about long-term reduction of the seed bank, complementary to an herbicide program,” Goodson notes.

The logic of allowing billions of weed seeds to mix with soil after harvest is turned upright by narrow windrow burning, and Stephens hopes for strong results: “I’m doing this to save dollars and increase yield over time.”

“When I drive to states in the north, the farther I go the less pigweed I see. Those farmers may soon be in a position to consider windrow burning because they’re only a decade behind us,” he says. “I’d do anything to get back those 10 years of weed resistance.” 

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