Weeds resistant to herbicides are a way of life for farmers, one more concern to complicate an already complex production system.
But options exist not only to manage resistance but also to reduce the size of the weed seed bank.
“Resistance is here to stay,” said Adam Hixson, BASF technical service representative for Texas, Oklahoma, and New Mexico, during a recent media update on managing herbicide resistant weeds in the Southwest.
Adam Hixson, BASF technical service representative for Texas, Oklahoma, and New Mexico. (Photo by Shelley E. Huguley)
“We’ve heard the expression, ‘out with the old and in with the new,’” Hixson said. “I want to change that to ‘in with the old and in with the new.’”
Back to basics
Getting back to basics, he said, is crucial to managing herbicide resistant weeds. He called on Texas A&M AgriLife Professor and Extension Weed Specialist Pete Dotray to put the problem in perspective.
“According to the International Survey of Herbicide Resistant Weeds, we have eight resistant weed species in Texas,” Dotray said. “The first case of resistance in the state was noted 30 years ago, but in the last 10 years, glyphosate resistance has created a lot of concern.”
Dotray said Roundup resistant Palmer amaranth, also known as pigweed and carelessweed, was first identified on the Texas High Plains about 10 years ago, later than in some Mid-South and Southeastern states. He believes a key to that late arrival was that High Plains farmers never abandoned residual herbicides, especially the yellow herbicides like Treflan and Prowl.
Texas A&M AgriLife Professor and Extension Weed Specialist Pete Dotray (Photo by Shelley E.Huguley)
Overuse of Roundup, using the same chemistry over and over, and use of fewer herbicide and tillage inputs provided an open door for the increase in resistant weed populations, Dotray said. “Resistant weeds were likely already out there in extremely low numbers.”
Palmer amaranth resistance has complicated weed management, Hixson added. “We’ve seen multiple applications of glyphosate at labeled rates fail to control Palmer amaranth.”
He said remedies include manual control, such as hoeing, which is expensive and time-consuming. “Also, we’re always looking for that next ‘shiny object’ that will solve the problem.”
Shiny things have been scarce in recent years, however, so Hixson offers a different option. “We need to use what we have today, but use it in a more calculated, knowledge-based approach. We have to get back to the fundamentals of weed control.”
He and Dotray agree that successful weed control strategies do not focus solely on in-season herbicide applications. “Good weed management has to be a well-planned, year-round venture,” Hixson said.
Weed identification is a priority. “It’s important to identify the weeds and to understand fully the biology. Know when specific weed species are most vulnerable.”
He explained that Kochia, sometimes “a huge problem and resistant to several herbicides,” emerges early in the spring and typically has only one flush. An effective residual herbicide, applied at the right time, will take care of most Kochia issues.
Palmer amaranth, however, emerges from early in the season well into fall and requires a season-long management program.
Dotray said Palmer seed that emerge late in the season remain a threat to replenish the seed bank and create problems for the next crop year.
“We’ve looked at the abundance of seed one plant can produce,” he said. “Palmer that emerges early produces as many as 500,000 to 600,000 seed, maybe more, per plant. That’s a lot of seed. But a Palmer plant that emerges in August will still produce as many as 20,000 seed, also a lot. As late as September, emerging plants will produce 2,000 seed, and still hundreds by October. Even plants that emerge as late as November can produce some viable seeds.”
“Leaving just one plant,” Hixson said, "may add to the weed seed bank, a key factor for the next season. One seed per square inch represents more than 6 million seeds per acre.” So, next season’s weed control should start before this season ends.
Dotray said recent research shows a bit of good news about the longevity of Palmer seed. Studies have shown that some weed seed will retain viability for as long as 120 years.
“We had no good answer for how long Palmer seed remain viable, so five years ago we set up a test to see. We buried Palmer seed at various depths across the state.”
They uncover them at intervals, beginning at six months, again at 12 months, and yearly after that. Based on data from the first 48 months of the research, “Palmer seed viability begins to decline significantly after 12 months. Those findings were the same across all locations and at all depths. A second study initiated in 2018 has shown the same results so far,” Dotray said.
“The good news is that a farmer who does a good job of managing Palmer amaranth effectively with a systematic program can get them down to a manageable level in a short time.”
That system should include late applications to prevent escapes, he said.
Knowledge is key
Hixson said an effective weed management program also depends on knowing not only the weed species vulnerabilities but also the interactions of soils and chemistry.
He said using herbicides with multiple, effective modes of action should be a critical part of weed management
“But also understand the properties of the herbicides and how they respond to different conditions, including soil types and moisture. Soil leaching properties will affect herbicide efficacy,” he said. “Also, the more water soluble a product is, the deeper it will move into the soil profile. Less soluble usually means more soil binding.”
He said different soil types — changes in clay content, sand, organic matter level — all may affect herbicide activity.
He said in situations with good moisture, a product like Zidua could be the best option. “In dryland or subsurface drip irrigation conditions, Outlook would be ideal.”
They key is understanding the weed, the environment, and the herbicide properties, then using the proper material for the target weed under those specific conditions.
Timing and coverage
He added that application timing and coverage also matter.
“Also remember, the cottonseed trait package you plant determines the herbicides you can use.”
“Using residual herbicides, identifying weeds and understanding the difference in solubility and where a product fits best based on soil and moisture are critical to a systems approach to weed management,” Dotray added.
In response to a question about new dicamba labels, Hixson said BASF would not veer from the requirements established by the federal label in Texas, Oklahoma, and New Mexico
Dotray noted that the new registrations come with some significant changes, including bigger buffers, volatility reduction adjuvant requirements and application timing.
“Also, last year some states used 24-C exemptions to alter some regulations. So far this year, states that have applied for a 24-C have been denied.”
Hixson announced that BASF does have one “shiny object” in the pipeline, a new seed trait with tolerance to four herbicides –GLIXTP, pending regulatory approval. He anticipates introduction in 2023, with potentially more availability in 2024.
In the meantime, he said, “Old chemistry still has value.”