Maybe they can resist chemicals, but they can’t resist being crushed — or at least not yet.
Harvest weed seed control is the latest development in the battle against weeds, with early adapters working with different designs and seeing success. The concept employs seed mills attached to the rear of the combine, gathering weed seeds during harvest and crushing them to prevent or reduce germination. Currently, there are four designs: the integrated Harrington Seed Destructor (iHSD), the Seed Terminator and the WeedHOG, all of which are Australian, and the Redekop Seed Control Unit, built in Saskatchewan.
In 2011, researchers with Agriculture and Agri-Food Canada (AAFC) created a vine crusher to help control European corn borer (ECB) in potatoes. Two metal rollers were attached to a motor and placed on the back of a potato harvester to gather and crush the vines, killing ECB larvae. But winged adults could fly to another field, so to optimize the value of a vine crusher, every farmer would have to have one.
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Ten years later, AAFC-Charlottetown researcher Andrew McKenzie-Gopsill re-examined the crusher and adapted it to gather weed seeds in potatoes and damage them to reduce germination. It worked: in 2021, he tested the unit, gathering seeds of lamb’s quarters, a common species on Prince Edward Island. The first set of seeds was fed through the crusher, collected and germinated in a petri dish. The second set was fed through the unit, gathered and germinated in soil in a greenhouse and the third was sent through with potato biomass and then grown out in greenhouse soil.
“When we did the petri plate under controlled conditions, we saw some of the lamb’s quarter seed, and even though there’s only half of the seed and it has either a hypocotyl or a radical, it germinated,” McKenzie-Gopsill says. “It was only in the petri plates but when we go to the field soil, those things that would potentially germinate when they only have a primordial shoot or root — they’ll die and they won’t emerge from the soil.”
Whether there’s a definite percentage of the seed that has to be damaged to prevent germination has yet to be determined. The results for field-based experiments using the vine crusher were unavailable early in 2023, but McKenzie-Gopsill liked what he saw last fall.
“We were able to harvest our potatoes and see material moving through the vine crusher, and it looked like it was doing what it was supposed to be doing. We collected some lamb’s quarters so we threshed those and we’re doing a germ test on those in the growth chamber. Then we collected soil cores to see if there’s any effect on the seed bank.”
One concern was whether they might be seed scarification that encouraged or increased germination. However, in testing in the greenhouse with the material that moved through the harvester, they had upwards of 90 per cent control with some of the larger seeds.
“If we increase germination following harvest, that’s good for a potato field, where we’re getting a small drawdown on the seed bank because any of those emerging annuals will be winterkilled out,” McKenzie-Gopsill says. “A week after we harvested, I went out to look at what we had and could see a bunch of common chickweed seedlings had emerged just in our rows that we’d harvested with the vine crusher.”
They found an increase in emergence and compared that to rows where the vine crusher wasn’t used, which had little or no weed emergence. There were nearly 200 chickweed seedlings per square metre at high density, but they should be winterkilled. He adds that chickweed can overwinter, but that’s usually with well-established plants.
As effective and inexpensive (at $10,000) a design as the vine crusher is, it’s best-suited to horticulture crops like potatoes, not at the back of a combine for corn or soybeans.
Test in the West
Corn, soy and wheat growers in Ontario and Quebec must manage resistant Canada fleabane and waterhemp, and in the West, growers are grappling with resistant wild oat and kochia.
Breanne Tidemann has been working with harvest weed control systems since 2014 when she started on her PhD and worked with a tow-behind version of the Harrington Seed Destructor, now available as a fully integrated system for combines. She’s also worked with preliminary prototypes of the Redekop Seed Control Unit.
“I’ve seen greater than 95 per cent of weed seed control of any of the weeds that go into the unit,” says Tidemann, a researcher at AAFC in Lacombe, Alta. “Anything that’s in the chaff and goes through the mill, it will control greater than 95 per cent of those weeds. The trick is actually getting it into the mill.”
Wild oat is one of Western Canada’s main problem weeds, and drops a significant number of its seeds before harvest, so the numbers are less effective. But of the 25 to 30 per cent seed retained, she’s still seeing 95 per cent control. Much of the efficacy in damaging the seed depends on the weed biology and stature, the accompanying crop and the number of modes of action to which the weeds are resistant.
“Weeds that are short to the ground and thistles or dandelion that float through the air are harder to get through the combine,” Tidemann says. “I originally thought wild oat has to be a lot easier to control in the mill than kochia because it’s such a small seed. I did a study with the Seed Destructor stationary, and I had kochia, green foxtail, volunteer canola, wild oat and I even put through peas, just as a large seed. With every single one, I had greater than 95 per cent control.”
Generally, seed size doesn’t seem to have as large an impact as the weed biology and when it’s maturing compared to the crop. Tidemann also tested volunteer canola seed, sieved it and separated it into five different thousand-kernel weights from 2.2 grams per thousand seeds to 5.8, and found a negligible increase in control with larger compared to smaller seed.
Can the weeds adapt?
While helpful, crushing, cutting or physical damage will not prevent weeds from evolving.
“I would never say that weeds can’t respond to any kind of weed control,” Tidemann says. “It doesn’t matter what it is, they will find a way, because it’s a selection pressure — it’s what evolution does.”
Researchers expect weeds to respond to damage by shifting their biology to drop their seeds earlier so they’re down before harvest, or to select for more prostrate forms making it harder to get into the combine. In Australia, where they’ve been using these control methods for 10 to 15 years, they’ve started seeing wild radish and annual ryegrass adapt.
“It’s why I would encourage using one before it becomes a last resort to manage resistant weeds,” Tidemann says. “If you have two effective (chemical) modes of action, it’s harder to develop resistance to either of them. With herbicides plus harvest weed seed control, it’ll be harder for weeds to develop resistance to both and become problematic. You can use it strategically to manage where those weed problems are.”
Tidemann says harvest weed seed control systems should be part of an integrated approach, with a combination of chemical, physical and cultural means increasing the chances of success.
Tidemann adds that manufacturers have been listening to growers and adjusting the units according to their needs. Since rocks can be a problem, some are mounting rock traps on the unit or magnetic strips to pick up bits of metal in the field.
The current integrated systems can run upwards of $100,000, but Tidemann says it’s the cost per acre that’s important. A grower she knows runs a Seed Terminator on four of his combines and factors his interest, financing, fuel and repairs as part of the operating costs.
“He has it broken down based on his 4,000 (owned) acres and it costs him $6.50 per acre. It started higher but based on what he’s spent and how many years he’s used it and the repairs, it’s down from $10 when he was first using it.”
