No seeds means no weeds

Harvest weed seed control systems pioneered in Australia nix weeds by destroying weed seeds.

Baseball Hall of Famer Wee Willie Keeler had a simple answer as to how he terrorized opposing teams when he played in the 1890s and early 1900s.

“Hit ’em where they ain’t.” 

Harvest weed seed control (HWSC) systems slightly spin Keeler’s hitting logic when it comes to managing weeds. 

“Hit ’em before they start.” 

HWSC systems developed in Australia destroy weed seeds before they germinate. “These harvest weed seed practices are driving weed numbers down,” says Shauna Stone, who farms with her son, Kent, near Kojonup in Western Australia. 

Farmer Shauna Stone wearing a black shirt with floral scarf
Shauna Stone
HWSC has reduced populations of rigid ryegrass – Australia’s version of waterhemp or Palmer amaranth – by 37% to 90% in Australian university trials. This weed that infests Australian small grains and pastures evolved resistance to 13 herbicide sites of action from 1982 to 2013. 

HWSC practices include combines that grind weed seed at harvest. Another strategy funnels harvest residue containing weed seed into narrow windrows at harvest. Farmers can then burn it, spray it, or leave it to rot. By reducing weed seed numbers, Australian farmers increase herbicide effectiveness on surviving weeds. 

“This is one thing people don’t like to hear, but we need to do something else besides [rely on] herbicides,” says Gordon Vail, product biology head for Syngenta. 

The Aussie Answer

No-till was a great route for Ray Harrington, Darkan, Western Australia, to take when he shifted from sheep farming to crop farming in 1994. That’s because it saves soil structure and moisture. A drawback, though, is that no-till systems largely hinge on herbicides to manage weeds. 

“Western Australia had the worst herbicide resistance cases in the world,” he says. “We farmers knew that herbicides failed. We just needed to break the cycle of weeds.” 

Harrington reasoned that the cycle could be broken by preventing seeds from germinating in the first place. He developed his “Big C Project” that targeted weed seed-containing harvest residue behind his combine.

After weighing various options, he chose to crush the weed seeds so they could not sprout. He used a cage mill that pulverized coal lumps into a black powder finer than talc.

“I thought that nothing can survive going through that, not even a weed seed,” he says. 

Harrington then met Stephen Powles, a University of Western Australia weed scientist, in the mid-1990s. Impressed with Harrington’s concept, Powles directed $20,000 of his research budget toward Harrington and a partner to build chaff-catching unit containing a mill pulled behind a combine. 

After experimenting with several techniques, they hit a sweet spot that destroyed 98% to 99% of weed seeds exiting the combine. This spurred creation of the Harrington Seed Destructor (HSD) and similar units that destroy weed seeds while combining.

Back in the USA

This caught the eye of the U.S. weed scientists who help farmers manage herbicide-resistant weeds. “In northeastern Arkansas, we have six-way [herbicide site of action]-resistant Palmer amaranth,” says Tom Barber, a University of Arkansas (U of A) Extension weed specialist. 

Barber and other weed scientists are testing several combine-integrated seed destruction units – including the HSD, the Seed Terminator, and the Redekop Seed Control Unit – that destroy seeds of late-emerging pigweeds like waterhemp and Palmer amaranth. 

“They vary in design, but the concept is similar,” says Kevin Bradley, University of Missouri (MU) Extension weed specialist. “Basically, these units run chaff and other harvest residue into a hammer mill or rolling cage mill that breaks weed seeds into smaller parts through a series of high revolutions.” 

Some models use two vertical mills to destroy the seed, while others use horizontal mills. Stationary bars or rings also catch the seed to further degrade it. Higher plant moisture, though, poses a hurdle for U.S. use. 

“It works great in cereals in Australia, because there’s not a lot of green material,” says Barber. “Soybeans in the U.S. are totally different, especially when cut in August and September when there’s a lot of green weed and soybean plant material. It is like mowing grass with dew. More moisture puts strain on the [weed-smashing] mill. The first time we tested the unit, we could only go 30 feet without clogging.”

Similar results occurred when Redekop Manufacturing engineers tested the firm’s Seed Control Unit (SCU) in 2016 and 2017. “It was a complete disaster,” says Trevor Thiessen, Redekop president. 

MU weed scientists tested another seed destructor unit, the Seed Terminator, that company officials say destroys weed seed via crushing, grinding, impacting, and shearing. Clogging also surfaced in MU tests. 

“Ryegrass in Australian wheat is one thing, but it’s not the same as the pigweeds in soybeans in our environment,” says Bradley.

Still, weed scientists and companies aren’t giving up. “We aren’t going to abandon these implements,” says Bradley. “We just need to make some more modifications to make them work in U.S. agriculture.” 

After 2017, Redekop officials redesigned the unit to meet green material challenges and have eliminated 98% to 99% of weed seed passing through the combine in company tests, says Thiessen.

”We found the mills were not designed to handle straw material,” he says. “So, we use rollers and baffles and a scraper system to keep them clean. A high volume of air also helps move it [straw and chaff material] through.”

To improve HSD performance, U of A weed scientists created an airflow funnel and adjusted the unit to better separate large chaff from small chaff, says Barber. To reduce plugging potential, HSD engineers have created a bypass function if excessively green spots are encountered. They are also designing mills to better manage high green material situations.

“It worked well as long as we combined after a killing frost,” says Barber. That’s good news for Upper Midwestern farmers who can harvest soybeans at this time. That’s not the case for mid-South farmers, though, who often harvest soybeans before frost. 

“It still has opportunity for us, but we have to tweak the system,” says Barber. Increasing combine size from the Class 7 unit used in U of A tests to Class 8 or Class 9 would also provide extra horsepower to better destroy weed seeds, Barber says. They’re also evaluating the use of desiccants to dry plant material before harvest. 

“Soybeans can be dry, but many times pigweeds are still green, especially before a frost,” says Barber.


This is not the end-all for weed management,” says Thiessen. Challenges include:

  • Weed seed presence at harvest. “There have to be seeds on the weeds to go through the combine,” says Thiessen. “If weed seed drops before harvest, or if the crop is harvested extremely high or if the weeds are below the combine head, these systems will not impact weed populations.” 
  • Weed seed shattering at the combine head. Weed seeds can later germinate if they dodge entering the combine by bouncing off the head. More work is needed on how much weed seed actually enters the combine, particularly if a desiccant is used, says Barber. 
  • Weed density. University of Illinois (U of I) weed scientists evaluated HSD performance over several waterhemp population densities. 

“Some densities were low, while others were a train wreck,” says Adam Davis, a U of I weed scientist. “The train wrecks were tough to get through, because there was more green material than the combine was used to handling. I would not recommend it as the first line of defense against weeds.” 

  • Early shattering weeds. Although this technology can effectively slice levels of late-emerging pigweeds, it doesn’t work well for earlier seed shedders like giant foxtail or velvetleaf, says Barber. 
  • Cost. In Australia, the HSD retails for around U.S. $60,000. Redekop has been selling its conversion kit for $70,000. “Once the North American market sees demand for it, our hope is the price will come down in the long run,” says Thiessen.  

Good Potential

Long-term, though, all agree these units have potential. This fall, John Deere will offer Redekop’s SCU in its S-Series combines as an option. John Deere had not released pricing at press time. 

AGCO has shared technical information with Seed Terminator officials as to how the system could adapt to AGCO combines. 

“If an emerging mechanical weed control solution can perform in a simple, long-lasting way, with a cost structure to give a farmer a one- to two-year payback, it would be a great solution that we’d hope to offer on harvesting equipment,” says Brad Arnold, senior vice president for global crop cycle and fuse connected services for AGCO.

“Making this work in the United States is a lot of trial and error,” says Meaghan Anderson, Iowa State University Extension field agronomist. “We have an advantage in the U.S. in that we have watched what they have done in Australia for 15 to 20 years. We can take the best of what works in our production system.” 

Resistance still lurks

Harvest weed seed control systems aren’t immune to weed resistance that’s plagued numerous herbicide sites of action.

“Waterhemp is awfully smart,” says Meaghan Anderson, an Iowa State University Extension field agronomist. “It could outsmart harvest weed seed control by dropping its seed before harvest.”

Still, a weed that sheds seed earlier also has less time to mature and grow, says Adam Davis, a University of Illinois weed scientist. This creates a smaller weed that’s easier to manage through herbicides and cultural practices like narrow rows. 

“Whatever we can do to make a weed less comfortable, the better,” says Davis. “That’s what integrated seed management is all about, dogging weeds all year long.”

Chaff Lining

Other harvest weed seed control (HWSC) techniques exist that cost less than combine-installed ones. 

Chaff lining, for example, concentrates chaff and other weed seed-containing residue that exits a combine into a narrow row. Michael Walsh, a University of Sydney weed scientist, says it costs Australian farmers $400 to $800 (approximately U.S. $285 to $575) to install a combine funneling system. 

Shauna Stone and her son Kent, who farm near Kojonup in Western Australia, built a combine funnel to channel chaff and harvest residue into narrow windrows for autumn burning. 

“We did that for several years, and it certainly helped,” says Stone. However, it concentrated nutrients into a 2- to 3-foot line that created uneven field crop growth. Autumn winds also made fire containment difficult. They’ve since shifted to a combine-integrated Harrington Seed Destructor. 

Burning isn’t recommended for U.S. farmers, due to issues with burning damp windrows and smoke, Walsh says. Instead, they can use chaff lines to rot weed seeds or kill them due to in-row competition. Spraying weeds in chaff lines uses less than 10% of the chemical normally applied across a field, Walsh adds.

“Chaff lines are particularly popular, as they are cheap and easy to implement,” says Stephen Powles, a weed scientist at the University of Western Australia. 

Farmers can also funnel chaff and harvest residue into tramlines that they use to spray and perform other field passes. Tramline traffic tramples weeds and slices seed sprouting potential, he says.  

“Harvest seed weed control is a success story in Australian grain cropping,” Powles says. “I have long advocated to my U.S. contacts that HWSC should be adopted in the U.S.”

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