U.S. Weed Scientists Take In-Field Look At Aussie Resistance Management Systems

Harrington seed destructor blasting chaff and pulvertized weed seeds behind combine.

Harrington seed destructor blasting chaff and pulvertized weed seeds behind combine.

U.S. producers thinking of adopting harvest weed seed control (HWSC) could take a few lessons from their counterparts Down Under, where HWSC has become an accepted weapon against resistant weeds.



But which of the Australian systems best fit with U.S. crops remains a key question.

Trying to sort through Aussie approaches was the main focus when several U.S. weed scientists toured Australia’s farming country and delved into a wide range of approaches that farmers there already have adopted.

The tour was organized by GROW (Get Rid of Weeds), a multi-regional “think tank” and educational group focusing on weed management and herbicide resistance. Cooperators include weed scientists, agronomists and other scientists with ag universities across the nation.

On the tour, U.S. weed scientists and researchers observed Australian farmers using some form of HWSC, including chaff lining, chaff tramlining and seed impact mills such as the Harrington Seed Destructor. A majority of Aussie producers now use at least one of those approaches.

Home-Grown, Farmer Developed Approaches

The way HWSC became entrenched in Australia is unique, according to Lauren Lazaro, assistant professor of weed science and agronomy at the Louisiana State University AgCenter.

“Australia growers saw a problem with herbicide resistance,” says Lazaro, a participant in the tour. “So, they started looking at HWSC on their own. Then they brought in Extension and industry to help them complete it. Those innovative growers were really the ones who launched a whole new integrated weed management tactic.”

It’s worth noting that HWSC does not replace the need for other management tools in the push to manage herbicide resistance, Lazaro stresses.

“They’re still using other chemical, mechanical and cultural weed management tactics, but HWSC is a tactic to help them drive down their soil seed bank.”

Will HWSC Methods Fit In U.S. Ag?

Part of the GROW tour’s objective was to determine if Australia’s HWSC technology has a place on U.S. farms. The group found certain things that might easily fit but also identified obstacles.

For HWSC approaches to work, “you have to make sure the weed seeds are in the chaff and not coming out in the straw,” explains Michael Flessner, Extension weed science specialist at Virginia Tech, who also was on the fact-finding trip.

Along with that, a high portion of weed seeds must still be on the plants when crop harvest starts. In the U.S., that may not be an issue.

U.S. weed scientists and researchers, who are also members of GROW (Get Rid of Weeds),tour group stand between two chaff lines during a recent tour of Australian harvest weed seed control tactics. Photo by Claudio Rubione, University of Delaware

“A lot of our problematic weeds have a high percentage of seed retention at harvest, which would make them good candidates for just about any form of HWSC, including the seed impact mills,” says Flessner. “There’s definitely a lot of potential there.”

The bad news is that not all combine headers or harvesters are created equal.

“On a corn header, we’re potentially losing a lot of weed seed at the header, so it’s not entering the combine,” Flessner said. Running an impact mill or tramlining chaff wouldn’t be as effective compared to results with, say, small grian.

It should be noted that a couple of Australian approaches – narrow windrow burning and directly baling straw – don’t require chaff separation, Flessner points out. We will delve into those systems later in this article.

Wheat And Soybeans First?

 “We think HWSC is going to work really well in wheat in the U.S. because it’s working in wheat in Australia,” says Steven Mirsky, research ecologist with USDA’s Agricultural Research Service in Maryland and lead weed scientist for GROW.

“Our big emphasis for HWSC going forward is to fine tune the U.S. soybean system and start experimenting with cotton to see if we also can find a fit with that crop, too,” Mirsky added. “We also will have to start working with equipment companies to adjust the headers to make it work for corn.”

Meanwhile, HWSC in a soybeans-corn rotation could benefit corn, according to Mirsky. “In a two-year rotation with corn and soybeans, corn would be the main beneficiary of HWSC tactics used in soybeans,” he says.

Here’s a closer look at how HWSC practices are used in in Australia and how they might be used in U.S. cropping systems:

Impact Mills

Integrated into combines, these mills mechanically destroy weed seeds contained in the chaff. This was the most common method of HWSC observed by the GROW group. For Australians, it’s easy and effective.



“It’s a one-pass system,” said Flessner. “There’s nothing to do after harvest. It kills a high percentage of the weed seeds, unlike chaff lining or chaff tramlining. Plus, you completely disperse nutrients in the crop residues.”

Can seed impact mills work in the U.S.? Initial costs and maintenance are significant, meaning the technology might be better suited economically for larger growers and/or farmers with more economic resources.

But that could change, according to Claudio Rubione, Extension associate in integrated weed management at the University of Delaware. At first, only one company manufactured the mills. Now, four brands offer the equipment, including one Canadian company. Competition, alone, might lower the upfront cost.

Also, one Australian manufacturer is offering a unit that it says will be cheaper to buy and maintain, plus its mill will fit into older combines, Rubione says.

In other words, whatever an impact mill costs today, it may be more affordable once more U.S. farmers ease into the system.

But with certain crops that are more common in the U.S., the mills might not be a strong option, tour participants learned.

Seed impact mills could present problems for Louisiana producers, according to Lazaro. “Soybeans in Louisiana don’t dry down as readily as wheat,” Lazaro points out. “With that added moisture content, impact mills can clog. The mills on the market now aren’t designed for that moisture level or the amount of material coming through it.”

On the other hand, “research in the Midwest shows that after the first fall freeze, impact mills work very well in soybeans,” Rubione says.

Flessner believes these challenges and other issues can be overcome in time.

“Australians went through a learning curve where the farmer, researcher and industry partnership came through with solutions,” Flessner observes. “That’s what we have to do in the U.S. Once we get seed impact mills operating in the U.S., that’s when we will really learn from them.”

Chaff Lining And Chaff Tramlining

With chaff lining and chaff tramlining, chaff material from the combine funnels into a narrow row behind the combine or onto dedicated wheel tracks or tram lines. These are common practices among Australian producers, the researchers found during the tour.

Compared to impact mills, these tactics aren’t as effective at eliminating weed-seed carryover, but the approaches cost less to implement and maintain.

Chaff lining controls weed seeds by condensing them, Flessner explains.

“Across the field, you’re collecting weed seeds in 100% of the field but putting them down on 5% of the field,” he specifies. “The chaff line is not good for seed germination, and if weed seeds do germinate, those seedlings face plenty of competition among the weeds in the line.”

With chaff tramlining, the chaff and weeds funnel into two rows behind the combine’s wheel tracks. With the chaff and seeds concentrated in an established wheel track, repeated trips over the field inhibits weed development.

Australian producers have seen a reduction in yield along dedicated wheel tracks due to this mechanical compaction. In addition, “Chaff lining and tramlining do require a spring burndown on those lines,” Lazaro says. “But it’s not necessarily a broadcast application, so less herbicide will be applied, which is saving time and overall cost.”

The Low-Cost Approach

Can chaff lining and chaff tramlining work in the U.S.? Modifications to the combine are fairly inexpensive and straightforward, according to Flessner.



Mirsky believes chaff lining could be a good practice for farmers who want to ease into HWSC. “It seems to have a lot of promise right now for our growers,” he adds.

Australians already made a significant transition into controlled-traffic farming, so it was easier for them to adopt chaff-line methods, Flessner points out. In the U.S., farmers who want to make chaff-line methods work will need to move into a tram system to gain the full benefit.

If they don’t adopt a controlled-traffic approach and lines shift around a little from year to year, are chaff line methods still worthwhile?

Flessner’s response: “Some of those weed seeds probably will still remain viable for two to three years, so if you don’t put the weed seeds right back on that same place, you could have two or three lines of weed seed out there rather than just one.”

Header size could also present a problem for Southeast producers using chaff lining, Flessner notes.

“We have a lot of 15-foot heads in the Southeast because that’s the right size for the job with our smaller fields and rolling topography. If you’re on a 15-foot header, you end up with twice as many chaff lines as you do with a 30-foot header (which are more common in Australia).”

Other Aussie Approaches – Pros And Cons

Other tactics such as narrow windrow burning, bale-direct and chaff carts could be tougher fits in some U.S. agricultural regions.

In narrow windrow burning, material coming out of the back of the combine funnels into windrows, which are then burned. It’s effectively kills seeds, but “the smoke and risk of a fire escaping into adjoining areas isn’t a good image for agriculture,” Flessner says. “You’re also burning off carbon when we’re trying to increase soil organic matter for better water-holding capacity and nutrient retention.”

Australians have moved away from narrow windrow burning for some of the same reasons, Australian scientists and farmers told the U.S. tour group. Most farmers there, in fact, use either chaff lining or impact mills.

In Australia, chaff carts collect the material from the combine, dump the material off field, then burn it. “We don’t see the tow-behind chaff carts being applicable in the U.S. because you’re adding another large piece of equipment to the operation,” Lazaro says.

In a bale-direct system, a combine tows a hay baler, which bales the chaff, straw and a significant number of weed seeds. Farmers typically sell those bales to feed lots or to other growers who raise livestock. How feasible that marketing sideline might be for a given farmer depends on the local feed markets. But U.S. producers would likely be unwilling to tow a large baler behind a combine, the U.S. researchers agreed.

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