8 Ways to snuff weed seeds

Weed control changed forever in 1996, when Stine Seed Co. launched the industry's first Roundup Ready soybean varieties. The Roundup Ready technology — licensed by Monsanto, creator of Roundup herbicide — made weed control easy. A single pass of the herbicide killed every weed in a soybean field. Within a decade, weeds resisted glyphosate, the active ingredient in Roundup. Now, glyphosate resistance occurs in 38 weed species across 34 crops in 37 countries. Not surprisingly, weed control is no longer easy. "Easy is what got us into the problems we've got now," says Aaron Hager, University of Illinois Extension weed specialist. "We've created these problems ourselves because we want easy weed control. Nature is anything but easy."

Weed control may never be easy again, but that doesn't stop the search for alternative weed control methods. Below are eight ways farmers can keep seeds — and ultimately weeds — at bay in their fields.

1. Harvest Weed Seed Control Products

Stopping weed seeds from turning into pesky plants was the objective of Australian farmer Ray Harrington in the early 2000s when he built a tow-behind crop residue mill to pull behind a combine. Several companies have since refined that concept into mills that are bolted onto the combine's chaff distribution system.

The Harrington Seed Destructor, Redekop Seed Control Unit, and Zürn Seed Terminator all are considered chaff mills, incorporating some type of mill to pulverize weed seeds as they come out the chaff stream of a combine. From that first tow-behind unit, the three companies each have machines that integrate into the combine's chaff distribution system.

Redekop's system, avail-able since 2019, incorporates three stationary bars, nested with two rotating bars. Material drops into the bars, which pulverize and spread all crop residue as wide as the cutterhead. In tests, the company shows a 98% success rate for pulverizing weed seed.

That's the good news. The bad news is that means all seeds are not destroyed.

"The most important part is: we only can kill what goes in the machine," says Trevor Thiessen, president of Redekop Manufacturing.

Many small-seeded weeds such as Palmer amaranth and waterhemp tend to shatter before harvest. According to University of Illinois weed scientists, 72%, 92%, and 95% of waterhemp seeds remained on the plant at harvest in three years of research. That means a lot of seeds escape the harvest weed seed control devices, Thiessen says. "Let's say we lose one-third of the seeds to shatter before harvest. If the rest are making it into the mill, it is extremely effective," he ex-plains. "If we can eliminate those seeds from germinating, that's a win."

One challenge to seed-destroying mechanisms is their ability to work in different crops. Kevin Bradley, University of Missouri (MU) Extension weed scientist, recalls green soybean stems plugged the Seed Terminator during soybean harvest.

Also, the machines are expensive. Redekop's Seed Control Unit costs about $75,000 and demands more horsepower and uses more fuel than unequipped combines. Thiessen reckons running Redekop's system costs an additional $3 per acre, including upfront cost, depreciation, and fuel use.

"I think weed seed destruction is viable, just not economical yet," Bradley says. "The cost of the machine is the No. 1 impediment."

2. Electricity on the Go

Two manufacturers have developed machines that electrocute weeds on the go. The Weed Zapper Annihilator from Old School Manufacturing in Sedalia, Missouri, promises to kill weeds to the root in one pass. Meanwhile, the Swiss company Zasso has the XP and XPS weed electrocution machines for between-row weed control.

These products aren't quite ready for large acreage farms, Bradley says.

"In my opinion, they haven't gotten electrocution into a system that would be normally used by a conventional farmer," he explains. The largest boom available in the electrocution system is 40 feet and still requires a 200 hp. tractor to run the ma-chine's on-board generator. Plus, weeds must be taller than the crop canopy, which means users must wait until later in the season.

"That means those plants are old. Enough of the weed has progressed that they've had their effect on yield already," adds Hager. "What electrocution is doing is reducing seed production. That's a win. If you can preclude any seed production, you're not doing it for that year, but doing it as a benefit for years to come." Ideally, weed electrocution would occur when weeds are a foot tall or less to reduce competition with the cash crop.

"Can it work? Yes, but I think there's a little more to do to make it potentially useful for the average farmer," Bradley says.

3. Chaff Lining

Look Down Under for more weed control options. One of those is chaff lining, a weed seed control method used during harvest when farmers direct chaff out of the combine into a windrow. Ideally, farmers will drop chaff in the same narrow line year after year, says Peter Newman, agronomist with the Australian Herbicide Resistance Initiative. Because lines of chaff are in the same place year after year, weed control is concentrated in a band. Newman says weed seeds will either be suffocated and unable to emerge, or they'll rot after multiple years of the same process.

Chaff lining has many challenges. First, harvesters must cut low to the ground, capturing as much seed as possible. Also, combines may need a separator baffle to direct seeds into the chaff stream, according to research from the Grains Research and Development Corp. in Australia. Finally, the lines of chaff take longer to break down, although farmers who use the practice say the lines do not pose a problem for no-till seeders to plant through. Newman says chaff lining is a low-cost, do-it-yourself harvest weed seed control option that he estimates nearly a third of Australia's small grain farmers have adopted.

4. Burn Those Seeds

Take the chaff lining principle a step further and burn the windrow of chaff. This tactic of burning a narrow windrow was effective at reducing Palmer amaranth plant density by 73% and seeds in the soil seed bank by 62% in trials at the University of Arkansas.

While she was a student at Louisiana State University, Katie Patterson worked on a three-year trial of burning windrows following soybean harvest to determine the effect on weed seed presence and weed density. From 2018 to 2020, Patterson measured weed retention and density of Palmer amaranth, morningglory, and barnyardgrass. By itself, narrow windrow burning was 62% more effective at reducing the weed seed bank, compared to no weed control options. Narrow windrow burning — like all harvest weed control options — will not be effective on species that tend to shatter preharvest (like Palmer amaranth). However, narrow windrow burning plus effective pre- and postemergence herbicide programs offered the best weed control above and below the ground.

"Narrow windrow burning alone is not effective enough to control weed populations, and still allows seeds to go back into the soil seed bank," she notes.

5. Bury Seed Deep

While no-till or reduced tillage programs are popular, deep tillage is one way to remove weed seeds. In a seven-state research project funded by the United Soybean Board, deep tillage — at least 6 inches, preferably with a moldboard plow — buries weed seeds that are often found in the top 1 to 2 inches of the soil profile, says Hager.

Problem weeds such as Palmer amaranth and waterhemp have such small seeds that they won't emerge if they are plowed under and buried.

Here's the caveat: "You don't get a mulligan," Hager warns. "You get to do this one time, and one time only."

The top 2 inches or so of the soil profile is the zone where biological activity eventually degrades and decays the seed, he explains. If the soil is inverted, and seeds are buried 6 inches deep with a moldboard plow, they remain intact and viable for a long time. If that field is plowed again, seeds are brought back to the surface where they could — and probably will — emerge, he adds.

6. Waterfowl Foul-up

In 2014 to 2015, an MU research team learned that geese and ducks are effective transmitters of some of farmers' most har-rowing weeds.

Over two years, the team members recovered seed from 526 ducks and geese, finding up to 47 seed species in ducks and 11 species in geese; more than 35,000 seeds collected from the digestive systems of those waterfowl were planted and emerged. Most of these weeds contained resistance to many common herbicides: glyphosate, ALS chemistries, and more.

"Waterfowl consume a variety of weed seed and have the potential to spread weeds," said Jaime Farmer, an MU graduate student at the time. "Take the estimated 49 million ducks, each with an average of 18 pigweed seeds per duck. That's the potential of 882 million pigweed seeds being transported," adding that a duck feeding on Palmer amaranth in Mississippi can fly to Canada — and theoretically deposit a payload with viable herbicide-resistant weeds — in less than 40 hours.

7. Hoeing Crews

If a farmer can find one, hiring a crew of laborers to rogue crops may be the most effective means of control. Think of it: Palmer amaranth escapes in a soybean field can result in zero yield.

"People laugh at hoeing crews and migrant labor," Bradley says. "My opinion is people who are doing it have the right idea."

8. Diversity Is Best

When all else fails, effective weed control takes a diverse approach. All the solutions above won't work by them-selves, but some combination of them, in addition to use of effective herbicides, can provide control of problem weeds, Hager says.

"The more diversity that you bring into your weed management program, the better odds of success you will have," he says. "The more you want to simplify that, the more challenges you'll face in the future."