When it comes to controlling herbicide-resistant Palmer amaranth (pigweed) in soybeans, what you do behind the combine is just as important as what you do behind the planter.
That idea is very much borne out by Arkansas and Australian researchers in a 2016 article published in the journal Weed Science. In their study, the team members identified the top 3 practices most likely to reduce the number of pigweed seed that ultimately could be deposited in the soil’s seed bank. Of those practices:
- One is immediately available to any farmer.
- Another will require a bit of welding.
- The third option has mostly been an experimental approach in the U.S., but it shows great promise going forward.
Here are the 3 systems that made the most difference in how many pigweed seed are carried over to the next season, based on this multi-year evaluation.
#1. Grinding Weed Seed Into Dust.
This might be considered the “take no prisoners” approach.
An Australian-style chaff cart went the farthest toward reducing pigweed’s carryover seed volume. More importantly, the cart provided a proof-of-concept for what appears to be an even better mechanical approach waiting in the wings. More on that in a moment.
With the chaff cart system, the combine tows the cart and moves residue directly into the cart’s hopper. When the cart is full, it’s towed out of the field where the residue can be dumped and destroyed, typically by burning.
Jason Norsworthy, a University of Arkansas weed scientist and one of the study’s authors, quickly points out that including the cart in the study was more about proving the concept of seed removal than justifying its real-time use in fields.
“We really wanted to demonstrate that weed management will be easier on a long-term basis if you prevent weed seed from going into the soil seed bank to begin with. Plus, the efficacy of herbicides will ultimately improve as you further reduce the number of seeds left in the field.”
Removing weed seed with the cart simulated the effects of a soon-to-be-commercialized innovation called the Integrated Harrington Seed Destructor.
Essentially, the Destructor is a grinder mounted on the combine. Chaff and weed seed flow from the combine into the grinder’s cage mill, which spins at a hefty 1,500 revolutions per minute. In a very no nonsense way, the mill churns residue and seed into dust, then spreads it on the ground behind the combine. It boasts the added advantage of leaving all the residue on the field’s surface.
U.S. researchers determined that a tow-behind version destroyed 98% or more of the weed seed cycled through it.
The next version, which is part of the combine, could be commercially available as early as 2017, according to the manufacturer’s website. Cost of the unit has not been determined, but it should be considerably less than the price tag on the Aussie-built tow-behind grinding mill –around $200,000 (plus shipping from the other side of the planet).
A tow-behind version was invented by Western Australia farmer Ray Harrington, while the integrated combine-mounted version was developed by ag engineers at the University of South Australia. Commercialization will be through de Bruin Engineering.
Obviously, the results with these systems were dramatic.
“The Integrated Harrington Seed Destructor is really the future of fall weed seed management,” Norsworthy says. “Also, I think it’s going to be a lot more efficient than attaching an implement to the back of the combine.”
Norsworthy plans to test the on-combine unit in the 2016 harvest season.
#2. Cereal Rye Cover Cropping.
Virtually any farmer can do this.
Plenty of research already has been done on the capacity of cereal rye covers to reduce Palmer pigweed populations and seed carryover. The fact that this came in at number two in a diverse multi-year study underscores the system’s effectiveness.
The fall-planted cover crop’s large production of biomass and strong growth delays Palmer pigweed seed germination and suppresses growth.
“A rye cover can reduce Palmer amaranth emergence by 65% to 70%,” says Norsworthy. “That doesn’t eliminate the need for a pre-emergent herbicide, but it does take away some of the selection pressure that creates herbicide-resistant weeds.”
In 15 years of research, Norsworthy has established strong, healthy cereal rye stands, whether drilled or broadcast by air or ground. “I’m not married to any one strategy,” he adds. “There are a lot of ways to get an effective stand of rye.”
Norsworthy says that the optimum time to desiccate a rye crop is 14 to 21 days prior to planting. It’s also a relatively inexpensive tool. “The beauty of rye is that glyphosate is very effective in desiccating it. It’s not going to take a lot of inputs, especially if you do a good job of establishing the rye in the first place.”
If weeds are present when planting into rye, Norsworthy suggests applying paraquat with an effective pre-emergent herbicide behind the planter.
“After that, you treat it like a typical system. The beauty of cereal rye is that fewer weeds ultimately emerge. Also, your pre-emergent and post-emergent herbicides are going to perform better.”
#3. Narrow Windrow Burning.
Here’s where you’ll need to do a bit of welding.
In narrow windrow burning, the chaff exiting the combine is directed onto metal plates positioned at 45-degree angles. The plates act as a funnel, depositing the residue in a windrow about 30 inches wide. From there, it’s simply a matter of burning the windrows.
Narrow windrow burning destroyed 100% of the weed seed within the burned windrow, says Norsworthy. That’s not to say that some seed weren’t scattered between the windrows, but a significant portion of Palmer seed made it into the fire.
When the windrow is not burned – which might be the case under certain environmental conditions – more weed seed survived, “but you get a good bit of decay during the winter months, and we still saw a positive benefit,” Norsworthy says.
Adding the chute to a combine could cost around $200, according to Norsworthy, who already has received 40 to 50 requests on how to build one.
Also, A Word About In-Season Herbicide Programs.
Fall weed seed management “is not a cure-all,” Norsworthy stresses, “especially if you have extremely high populations of weed seed overwhelming the soil seed bank. It’s really about integrating an effective herbicide program with an effective fall management program.”
Herbicide programs that include both pre-emergent and post-emergent materials consistently outperformed post-emergent-only applications in the study, says Norsworthy. The study ran from 2010 to 2013, which gave researchers a variety of weather patterns for making those comparisons.
A sustainable in-season herbicide program, he adds, “begins with a sound pre-emergent program of at least one – if not two – effective modes of action at planting, followed by post-emergent applications, also with several effective modes of action.”
In his own region, many Midsouth producers have been shifting to LibertyLink soybeans to counter the advance of PPO-resistant Palmer pigweed. That means growers “only have one effective post-emergent option in the field” in those cases, Norsworthy says. “So that’s even more reason to focus on fall management strategies to minimize the soil seed bank.
“If we don’t have a season-long approach to weed seed management, the soil seed bank can increase quickly. One Palmer amaranth escape can turn into a loss of technology in 3 years.”
Editor’s Note: The authors of the paper that was published in the journal Weed Science include: Jason K. Norsworthy, Nicholas E. Korres and Michael J. Walsh, University of Arkansas, and Stephen B. Powles, University of Western Australia.