Looking for a cheap, easy, mechanical solution for those problem weeds in your soybean fields? Weed scientists say you might try narrow windrow burning.

“We’ve been doing research into this going on five years,” says Jason Norsworthy, weed scientist at the University of Arkansas. “The windrow-behind-combine concept is very simple and came out of Australia. There, they’ve had to come up with alternatives for weed control – mainly ryegrass – because of the prevalence of resistant weeds.

“No one had ever tested this in soybean. About three years ago, I spent several weeks in Australia for a global resistance conference. I found that 65 to 70 percent of producers were using this windrow technique.

“You just have to build a chute that concentrates the material coming out the back of the combine. That material shoots out into a 30-inch windrow. That allows a very high heat load when you burn.”

Over the last several years, “we’ve tested this on pigweed seed, Palmer amaranth, where we placed it in the windrows and the fire has destroyed it. We’ve also done the same with johnsongrass, barnyardgrass, even morningglory. Morningglory seed is notoriously hard to destroy – once it gets into the soil it can remain viable for 50-plus years.” 

Seed bank

The key to weed management as a whole, says Norsworthy, “is to ensure you don’t have weed seed going back into the seed bank. A large percentage of these weeds hang onto their seed, enter the combine and exit the rear with the chaff and straw. Burning the windrows provided well in excess of 450 degrees. That temperature was maintained for several minutes and we had 100 percent kill of the weeds I mentioned earlier.”

That’s great news for Mid-South producers. “It’s a simple process they can use in light of the PPO-resistance and the fact we have to fewer and fewer chemical options to control weeds today. This can really help us get the soil seed bank down and take some selection pressure off the products we’re using – things like Dual, Zidua, metribuzin, and Liberty.

“Again, our research shows 100 percent kill for seed in the windrows. Obviously, some seed may shatter before it gets into the combine. Chances of that vary depending on the weed species. For Palmer amaranth, 99 percent of the seed goes into the combine.”

A three-year study done by Norsworthy and colleagues shows “using weed control programs – it didn’t matter if it was a poor program, moderately effective or very effective” were only boosted “once we introduced harvest weed seed control. By using something like narrow windrow burning, “we were able to further reduce the soil seed bank three-fold. So, by having a very effective herbicide control program in combination with narrow-windrow burning a farming operation will be in much better shape very quickly.”

Palmer amaranth is a relatively short-lived seed once it reaches the seed bank. Most of it germinates and emerges the following year. “Disrupt that supply to the seed bank along with narrow-windrow burning and we’ll have extremely low weed populations.

Burn ASAP

“You need to burn as soon as possible behind the combine. The windrows don’t need to be rained on. But it isn’t the end of the world if you can’t get it done. We were able to plant right back into fields where we hadn’t burned the windrows. Now, we didn’t get the benefit of the burn but sometimes you can’t beat a storm.”

The researchers are also looking at using the practice in corn. “In corn, there’s a lot less weed seed that goes into the combine simply because of the height header in corn versus soybean. But we’re looking at that, the possibilities of rotating corn and soybean while using the windrow burning.

“I think narrow-windrow burning really has a nice fit in the Mid-South. That’s especially true where there are PPO-resistant pigweeds and the soil seed banks are out of control. Right now, there are some fields out there where, despite what products you use, the seed numbers overpower everything. 

“If you look at herbicides, the effectiveness is contingent in part on how many weeds you’re asking a product to kill. With post-emergence herbicides, if I have one plant per square foot, I’m going to get much better spray coverage and the kill rate will be high. If I have 100 plants in a square foot, the likelihood of adequate coverage diminishes significantly and the kill rate will drop. Hopefully, narrow-windrow burning will actually help make our herbicides more effective and minimize the loss of them due to resistance.”

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How many producers are trying narrow windrow burning in the state?

“I don’t have a good handle on that. Forty or fifty came and looked at how it’s done during PPO-resistance plot tours earlier this summer. A handful have come and taken pictures of the chutes. We’re just now getting started with this. It isn’t a silver bullet but is a strategy that will really help us.

“Back in early to mid-August, the grower calls Tom Barber, Bob Scott, and I were getting involved ‘How do I build a chute? How do I hook a chute up to my combine? How do we make this work?’ I’m just not sure how many of them followed through with building chutes at this time.”

Integrated Seed Destructor

Also in the testing pipeline is an Integrated Seed Destructor.

“I’m in process of bringing an Integrated Seed Destructor in to Arkansas from Australia. We expect it to be here sometime in late October. My Australian counterpart is planning on arriving on Oct. 27 and we’ll begin to test its effectiveness.

“The system goes into the combine. There’s a lot of interest … and I like it because the seed destruction itself occurs within the combine. There’s no extra step associated with it. The initial seed destructor was a tow-behind unit (Harrington Seed Destructor) and was very expensive. All indications I’m getting is the current unit will be less than half the cost of the tow-behind.

“At this point, I think, it’s just a matter of showing it’s effective on the weeds we have here. Once that happens, the manufacturers will get the unit into some of the newer model combines.”