Herbicide-Resistant Weed Struggle Unmatched in U.S. Agriculture
6 July 2021 In 2021, it’s resistant weeds that remain American agriculture’s Achilles’ heel.
“The evolution of herbicide-resistant weeds is one of the most significant developments in agriculture today,” says Dr. Aaron Hager, Extension Specialist, Weed Science, at the University of Illinois. “In terms of tough issues facing today’s farmers, this has to rank right at the top.”
This view is supported by a majority of U.S. ag retailers, who play a key advisory role for growers. In a weed survey conducted by Meister Media sister publication CropLife during the fall of 2020, 40% of respondents said that in their opinion, the state of weed control in agriculture was “volatile, as bad as I’ve ever seen it.” Another 37% said weed control remained volatile, “but was not as bad as it once was.” Only 16% of survey respondents said weed control was “getting better” than in prior years. The remaining 7% weren’t certain what the current state of weed control in agriculture was like.
In our interview on how retailers and growers should approach the 2021 season, Dane Bowers, U.S. Technical Product Lead for Herbicides at Syngenta, offered one of his take-home messages of the past several years: Managing resistance is less a technical problem than a human behavior problem.
“I think we have a pretty good idea on how to handle this from a technical standpoint. There are challenges — don’t get me wrong,” he acknowledges, “but we’re all creatures of habit. We tend to do the same thing if it works for us.”
As Dr. Bob Hartzler explained in his Iowa State University Integrated Pest Management blog: “The current approach to weed management in Iowa is at risk due to rapid expansion of herbicide-resistant weeds. In order to preserve the efficacy of herbicides two things must happen:
1) adoption of integrated weed management; 2) shifting the goal of weed management from protecting crop yields to minimizing the size of the weed seedbank. The first requires a shift in behavior, and the second a change in attitude.”
In addition to heavily promoting the use of residual herbicides, manufacturers and agronomists have advocated a zero-tolerance approach to weeds in the resistance fight. See just some weed escapes, but not too many? “That should be the canary in the coal mine,” Bowers advises. “Typically, in the first year of a resistance scenario, you don’t really think you have a problem, then it gets worse in year two. By year three, it’s a disaster. It’s really about getting ahead of it.”
Atop Bowers’ list of recommendations, and echoed by countless agronomists, is to: 1) Understand the unique challenges of any given farm, plus the driver weeds, and 2) Understand the need to start clean and stay clean. This means deploying a strong preemergence residual herbicide followed 14 to 21 days later by an overlapping residual, always incorporating multiple effective sites of action to reduce risk of resistant weeds going to seed.
Multi-Faceted Approaches
The use of multiple modes of action is integral to this conversation, given that the industry has had no luck in discovering any true blockbuster AIs since glyphosate.
For Certis USA, it is placing its bets on a new OMRI-listed, non-selective, broad-spectrum, fatty acid burndown herbicide, Homeplate, a capric and caprylic acid mixture.
“When you think about resistance, certain herbicides that work are single site and others have multi-site effectors. With Homeplate it’s multi-site, and with multi-site products they tend to get resistance built up much slower,” Scott Ockey, Western U.S. Field Development Manager.
“I was looking at the map earlier today and saw that the U.S. currently has 165 unique resistant weeds, and the next one behind that is Australia is 102. We’re leading the world in resistant weeds,” he explains, quoting data from the International Herbicide-Resistant Weed Database.
“Growers need new tools to fight resistance that are also flexible enough to be used on their crops, whether they are organic or conventional. Homeplate’s unique active ingredient is a major driver in its ability to kill weeds that have become resistant to other chemistries.”
Dr. Josie Hugie, Branded Technologies Data Manager, Wilbur-Ellis Co., points out that the U.S., where the dominant method for controlling weeds is with herbicides, has the most weed scientists of any nation, along with the largest number of resistance screening programs. There are perhaps many cases that simply have not been identified in less-resourced countries.
“Proper use and optimization of the herbicides available will continue to aid in combatting resistance with prevention of weed escapes and germination through residual herbicide break down. We use EFFICAX, for example, for improved deposition and retention of residual herbicides within the weeds’ germination layer.”
Herbicide resistance was also the key driver behind the launch of Belchim USA’s Tough 5EC. While it is not a multi-site product, it is unique in that it acts as an enhancer, targeting a unique pathway within the PSII group of herbicides. Labeled for corn, mint and chickpeas, Tough 5EC’s active ingredient, pyridate, had been overlooked for years, but was recently brought back to the U.S. by Belchim.
“We heard the concerns of growers across the United States, plagued by resistant and tough-to-kill weeds,” says Tom Wood GM for Belchim USA. The product enhances atrazine and is synergistic with HPPD chemistries, essentially filling in gaps in weed control. Belchim has three other products in its pipeline similarly targeted – to fill in gaps both in weed control and for products that are falling out of favor with the American grower, Wood adds.
“We’re not a multinational that has hundreds of thousands of research dollars available,” he adds, however: “Broadleaf weeds are developing more resistance, and we are at risk for losing chemistries like dicamba. You need someone to fill in the gap.”
Effective, Yet Troubled Solutions
In a January 2021 survey AMVAC conducted of 25 of its retail customers in 12 states, U.S. retailers expressed the greatest concern about waterhemp – not only to glyphosate but also due to that weed’s early-developing signs of resistance to HPPD inhibitors. Other resistant problem weeds identified by retailers are, in order: giant ragweed, marestail, Palmer amaranth, and kochia.
Today about 77% of U.S. corn acreage is treated with two-pass herbicide programs, according to AMVAC. While many growers are inclined to favor “one and done” programs, especially given their many tasks and time restraints, one-pass programs continue a decade-long decline which has accelerated in the last three to five years as weeds become more resistant to glyphosate and, increasingly, other herbicide groups.
Retailers said they have recommended both one-pass and two-pass programs with a higher-priced and lower-priced option for each. Add-on chemistries most often consist of atrazine, glyphosate, dicamba, and safened dicamba.
“As key advisers to farmers, ag retailers’ concern about resistant weeds points to an acute need for effective weed control options heading into the 2021 growing season,” notes Nathaniel Quinn, AMVAC Marketing Manager for Corn, Soybeans, and Sugar Beets. “For most corn farmers, multiple applications and a selection of several herbicides is likely to be their best course of action.”
Of course, most experts will say that weed resistance problems tie back to the U.S. ag’s overreliance on certain popular herbicides such as glyphosate – which has given weeds time to adapt over the years. Crop protection suppliers have, in turn, introduced new cropping systems based upon different/older chemistries to fight back, with both dicamba- and 2,4-D-resistant crops debuting into the market. According to industry watchers, these have helped control the spread of many glyphosate-resistant weeds since 2017, when they were launched.
The rollout of dicamba-resistant technologies hasn’t gone entirely smoothly, however. In the five years since dicamba-resistant crops began moving into crop fields across the country, off-target drift damage issues have dogged U.S. agriculture. This has led to numerous lawsuits and court battles.
In June 2020, troubles with the active came to a head when the Ninth Circuit Court of Appeals ruled that dicamba’s re-registration in October 2018 was invalid. It made applying dicamba formulations from three suppliers – Bayer Cropscience, BASF, and Corteva Agriscience – illegal across the U.S. One other formulation, Tavium from Syngenta, was not affected by the ruling.
Government regulators eventually stepped in and allowed existing stocks of dicamba to be applied throughout the summer 2020 months. And in October 2020, EPA re-registered new labels for dicamba formulations from Bayer and BASF for a five-year time period.
The re-registration with added restrictions to help reduce volatility and drift lends a sense of stability to dicamba, but concerns remain – and they are not limited to legal issues.
“It’s really a balance of how we are going to grow crops with the tools that we have, and how do we mitigate risk when the risk of losing acreage to weed species overcrowding because we can’t control them,” Wilbur-Ellis’ Hugie tells AgriBusiness Global. “Are you going to be able to harvest a crop, or do you have to worry about volatility or drift by using a tool that doesn’t have as much resistance? It’s very catch-22.”
Key words: “as much resistance.” In 2019, the first dicamba-resistant Palmer amaranth weeds were identified by Dr. Larry Steckel, Professor, Row Crop Weed Specialist at the University of Tennessee.
Steckel, on his UT blog, wrote that looking forward to 2021, a pre-applied residual that is effective on Palmer is now a necessity. Moreover, timely applications of Liberty must be used shortly after a dicamba application to remove escapes.
This is the fifth herbicide mode of action to which Palmer has become resistant since 1994 in Tennessee, Steckel noted. “If we take that 26-year period and divide by five modes of action the math would indicate that weed will evolve resistance to an effective herbicide in just 5.2 years of widespread use.”
Nonetheless, Scott Kay, Vice President of U.S. Crop, BASF Agricultural Solutions says, the “need for Engenia herbicide is greater than ever before due to increased weed resistance. When the weeds win, farmers see the impact to their livelihoods, harvests, and yields.” He adds, “Controlling resistant weeds is not only a physical challenge for farmers, it also can have a significant financial impact.”
It is estimated that certain resistant weed populations can reduce yields by 50% or more, Kay says. “This means that farmers planting dicamba-tolerant cotton and soybeans could potentially stand to lose more than $10 billion if they lost access to dicamba-based herbicides, like Engenia herbicide.”
The third company that had its original registration voided, Corteva, announced in late February that it was discontinuing its dicamba formulation FeXapan. Instead, the company is shifting its focus to its Enlist cropping system, which uses 2,4-D as its herbicide of choice.
According to Corteva company data, Enlist crops were planted on approximately 30 million acres of U.S. farmland during 2020, and the company expects this figure to increase by another 10% during the 2021 growing season.
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