by Emily Unglesbee
In keeping with the theme of 2020, a lot of things went wrong with weed control this year.
Between large weed seedbanks lurking on prevented planting acres, growing herbicide-resistance issues, weather complications, herbicide antagonism and a sudden court decision vacating dicamba labels, farmers have a lot of problems to choose from when reviewing the year.
Here’s a breakdown of each of these weedy challenges, along with recommendations on how to make sure 2021 weed control goes more smoothly.
- SEEDBANKS GOT RECORD DEPOSITS
The consequences of the record number of prevented planting acres in 2019 were on vivid display in many fields this year, particularly soybean fields. Where fields sat fallow and weeds went to seed last year, farmers battled unusually dense and persistent weed infestations this spring and summer, explained Bill Johnson, Purdue University Extension weed scientist.
“We had over one million prevent plant acres in Indiana alone last year, so a lot of waterhemp went to seed,” he noted. “If anyone doubted how much weed seed a waterhemp plant can produce, hopefully they understand now!”
In 2021, growers should target these heavily infested fields for extra management, including fall herbicide applications for winter annuals and full rates of overlapping residual herbicides for summer annuals, he said. In some cases, rotating from soybeans to corn might be best to rotate herbicide programs and more easily control herbicide-resistant broadleaves, he added.
- HERBICIDE RESISTANCE CONTINUES TO SPREAD
Weeds with resistance to one or more herbicides were another common culprit in this year’s weed control failures, said University of Tennessee Extension weed scientist Larry Steckel.
Steckel and his colleagues confirmed dicamba-resistant Palmer amaranth in his state earlier this year, and other types of resistance, such as glyphosate-resistance, PPO-resistance and HPPD-resistance have plagued soybean growers in recent years. Another troubling development has been the increase of glyphosate-resistant grasses in the southern U.S., such as junglerice, a type of barnyardgrass, Steckel said.
For growers farther north, herbicide resistance in waterhemp caused serious headaches this year, added Johnson. So far, the weed has developed resistance to seven different classes of herbicides overall, with some individual populations possessing five- and six-way herbicide resistance.
And although auxin resistance isn’t confirmed in waterhemp populations yet in Indiana, these types of herbicides are becoming less effective in some fields there, Johnson said. “A number of universities are screening waterhemp for resistance to dicamba and 2,4-D,” Johnson noted. “And we do have some populations where it takes higher rates of auxins to kill it.”
Growers facing dicamba-resistant Palmer amaranth populations should also keep in mind that 2,4-D postemergence applications in the Enlist platform may not be any more efficacious, Steckel added.
“Our fieldwork suggests that these dicamba-resistant populations are also tolerant to 2,4-D and we’re trying to confirm it in the greenhouse,” he said.
- METABOLIC RESISTANCE ON THE RISE
Another troubling development is the evolution of metabolic resistance in weeds, wherein weeds become capable of processing any kind of herbicide rapidly, thus dodging harm from it. The result is weeds that can survive multiple modes of action even before they encounter them in the field.
“I think we do have a little bit of metabolic resistance in Palmer amaranth in Tennessee that is cutting the shelf life of our residual herbicides,” said Steckel.
If growers find themselves struggling to control weeds like waterhemp with both corn and soybean herbicides, they should consider the possibility that they have metabolic resistance in their fields, Johnson warned.
As weeds evolve better ways to outsmart our chemical arsenals, farmers need to take concrete steps toward diversifying their weed control methods, Johnson added.
“We need to get people thinking seriously about adding another tool to their weed control toolbox,” he warned. “Whether it’s the cultivator or cover crops — to name two common ones — they need to think about this now.”
- HERBICIDE ANTAGONISM AT PLAY
Herbicides don’t always mix well, as some growers are discovering in this era of multi-herbicide-tolerant crop systems.
Specifically, auxin herbicides such as dicamba and 2,4-D can antagonize other chemicals in a tank mix, especially glyphosate and clethodim, leading to reduced grass control.
- WEATHER WOES
Some 2020 weed control failures can be chalked up to plain old weather problems, both Steckel and Johnson said. Large swaths of the country saw unusually dry conditions in June, which compromised both preemergence and postemergence herbicide applications.
“Some preemergence herbicides never really got activated,” Johnson explained. “And with the hot and dry weather later in June, the weeds that did come up were drought-stressed, so herbicide activity wasn’t optimal.”
- LEGAL ROADBLOCKS
On June 3, the U.S. Court of Appeals for the Ninth Circuit shocked the ag industry by vacating three over-the-top dicamba registrations in response to a lawsuit. Overnight, the legality of spraying dicamba was thrown into confusion, and it took five days for the EPA to issue a cancellation order permitting use of dicamba herbicides already in farmers’ possession.
“For folks who paid attention to the court order and quit spraying — and I think more obeyed it than didn’t — they weren’t able to clean their fields up,” Johnson said.
The fate of those three dicamba herbicides (XtendiMax, Engenia and FeXapan), as well as another soon-to-expire dicamba registration, Tavium, is still uncertain, as EPA weighs new registration requests from the companies for 2021.
Even if those herbicides get approved for use, farmers need to plan for the likelihood of continued state dicamba cutoff dates, which have been enacted in many states for multiple years, Johnson cautioned.
“Farmers need to face the reality of another year of dicamba restrictions,” he said. “We got a lot of calls asking for Indiana to extend their 2019 cutoff date — and it wasn’t happening. Be prepared for next year.”