October 25, 2021
Larry Buss often says, "I haven't seen any weeds yet that are resistant to steel." Steel can refer to preplant tillage, cultivation — or in more recent cases, mechanical control of harvested weed seed.
Ever since Palmer amaranth was identified in Harrison County, Iowa, in 2013, Buss has been vigilant in doing his part to slow the spread of herbicide resistant weeds in Iowa and across the continent — spreading the word with national organizations like the Weed Science Society of America and Entomological Society of America, and at international events like the Manitoba Agronomists Conference and the Soil and Water Conservation Society's Annual Conference.
REDUCED WEED SEED BANK: A Redekop unit was tested on 500 acres of soybeans in Story County last year to kill waterhemp seed at harvest. "We had about 90% or more kill efficacy for waterhemp — and that was a multiple herbicide-resistant waterhemp population in soybean," says Prashant Jha, ISU associate professor and Extension weed specialist. (Photo by Prashant Jha)
These efforts grew with the launch of the Iowa Pest Resistance Management Program in 2017. Since then, Buss has collaborated with Iowa State University researchers, agronomists, landowners, crop consultants, ag lenders and commodity organizations to monitor the spread of resistance on farms in Harrison County, and test different practices and herbicide programs to control weeds like Palmer amaranth, waterhemp, marestail and giant ragweed.
Most recently, this involves harvest weed-seed techniques — more specifically, using a Redekop Seed Control Unit. Designed to be used with a John Deere combine, the Redekop unit uses high-impact mills to break the seed through physical destruction as it comes out of the back of the combine, killing the seed and preventing germination. According to Saskatoon, Saskatchewan-based Redekop, the unit can destroy as much as 98% or more of weed-seed germination during harvest. The unit also allows the operator to turn it on and off on the go.
"I have a few weeds at a field by Dunlap, so we're going to test it up there," Buss says. "Then, we are going to get a sample of the weed seed behind the combine to see if the unit helps with germination destruction."
Off to a good start
Iowa is one of the first states the unit has been tested in the U.S. — it was tested by Prashant Jha, an ISU associate professor and Extension weed specialist in 2020 at a farm in Story County.
Jha notes one Redekop unit was tested on 500 acres of soybeans in Story County last year, with promising results for controlling waterhemp seed at harvest. However, he notes the study is ongoing.
"We had about 90% or more kill efficacy for waterhemp — and that was a multiple-herbicide-resistant waterhemp population in soybean," Jha says. "We don't know what level of resistance those waterhemp plants had, but they had survived multiple applications, and that's why it made perfect sense to do some harvest weed-seed control. The same is true in Harrison County — they have populations resistant to Group 9 as well as Group 2 [herbicides] and most likely PPO and HPPD inhibitors."
This year, after running the seed destructor in soybean fields, Jha will monitor the changes in weed seed bank density over time by collecting soil core samples in the fall, and then counting weed emergence in the following spring.
"We will estimate how much of the initial weed-seed bank has emerged and how much has survived herbicide applications, and how many weeds are present at harvest and the weed-seed-kill efficacy of the Redekop seed unit," Jha says.
Of course, one of the big questions to be answered is: At what point does it become economically feasible to use harvest weed-seed control? Jha notes while the Redekop unit costs about $70,000, it will take time to determine how long it takes to pay for the machine by reducing the weed-seed bank.
PROBLEM WEEDS: Since 2017, growers, agronomists, Extension educators and other stakeholders in Harrison County have studied herbicide resistance in weeds as part of the Iowa Pest Resistance Management Program. This includes some key problem weeds in the area: Palmer amaranth (pictured), waterhemp, giant ragweed and marestail. (Photo by Bob Hartzler)
"It won't happen in one year, but we expect at least a 90% reduction in the seed bank," he says. "There will be some header/thresher loss — probably close to 25% to 30%. Kevin Bradley at the University of Missouri has seen close to 25% header loss, and some of the weed seeds are getting shattered. We had close to 30% to 33% header loss last year. It's not stand-alone, but we expect that, of the remaining 67% to 70% seed going inside the unit, 95% will be killed."
And there are other factors — like the potential savings on herbicide application costs in the future.
"There are millions of dollars right now going into managing herbicide resistance in corn and soybeans," Jha adds.
"If you calculate the cost of three applications in a season — burndown, pre- and postresidual — can we cut that cost by reducing the weed-seed bank in a three- to four- year time frame, and increase the longevity of the herbicide? More importantly, we are quickly running out of herbicide options because of multiple-herbicide-resistant waterhemp and Palmer amaranth populations," Jha says.
Larry Buss notes that for the time being, the best method for weed control is to keep them from competing with crops during the growing season, by controlling them upfront and preventing them from going to seed and expanding the weed-seed bank.
SPREADING THE WORD: Larry Buss speaks at a field day as part of a Weed Science Society of America and Entomological Society of America event in this 2019 photo. Buss notes that growers in Harrison County and across Iowa are getting the message that weeds must be controlled early on with a full rate and multiples modes of action. (Photo by Ethan Stoetzer)
"I'm not going to spend money on it yet, because I would prefer to invest it in a better sprayer or a more robust herbicide program. If herbicide resistance continues to get worse, we can use harvest weed-seed methods to significantly reduce the weed-seed bank, because it's going to wipe out the weed mechanically," he adds. "Weeds won't be resistant to steel, so you can kill it with preplant tillage, cultivation — or you kill the seed with the Redekop Seed Control Unit. But before we do that, I think farmers will look to control weeds upfront so they don't compete with the crop."
And, Buss notes the outreach efforts of the Pest Resistance Management Program are paying off — while herbicide resistance continues to be a challenge, people are aware it's a problem and are taking steps to slow its spread.
"I'm going to pat ourselves, in Harrison County and the Iowa Pest Resistance Management Program, on the back," he says. "Because I think we're getting the message out that we've got to control weeds early on with a full rate of herbicide, and multiples modes of action."
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