Scientists raise alarm on herbicide resistance

Three of the big producers of herbicides and the seeds genetically engineered to resist them continue to grapple with legal woes. But even if they overcome their legal challenges, they and their competitors may be losing the evolutionary battle against weeds that are rapidly developing resistances to whole groups of herbicides.

Dicamba intervention

BASF and Corteva both filed motions to intervene on June 12, nine days after the June 3 ruling by a three-judge panel of the Ninth Circuit Court of Appeals that vacated the Environmental Protection Agency registrations of three dicamba formulation. At the time, BASF said it wanted to intervene because of “the sudden and severe financial impact vacating the registration has had on farmers during this critical application time, when farmers now have less than a month to protect millions of acres under threat from resistant weeds.”

The three-judge panel subsequently concurred in permitting the EPA to allow the use of remaining stocks of the de-registered herbicides for the rest of this year’s growing season, lessening the urgency of immediate action, for this growing season at least.

On July 20, BASF, later joined by Bayer and Corteva, petitioned for an en banc review of the de-registration decision by a full 11-judge panel from the Ninth Circuit. “This request for ‘en banc’ review is necessary to correct errors by the panel in issuing a decision inconsistent with basic due process and administrative law principles,” according to the petition. On Aug. 17, the Ninth Circuit denied the petition.

BASF has been expanding its dicamba repertoire in recent years. In January 2019, BASF introduced five new cottonseed varieties, including new versions of its popular Fiber Max and Stoneville seeds, “and for the first time, two of the new BASF varieties contain dicamba-tolerant traits.”

In its petition, BASF said, “Without such dicamba-based products, it is estimated that farmers could lose up to $10 billion and $800 million annually in soybean and cotton yields, respectively.”

Roundup lawsuits continue

Meanwhile, Bayer lost a round in California on July 20 in one of three ongoing lawsuits over claims that its popular weed-killer Roundup, whose main ingredient is glyphosate, causes non-Hodgkins lymphoma. The decision by the California Court of Appeal kept alive the suit of school groundskeeper Dewayne Johnson, who won a 2018 jury trial blaming Roundup for causing his cancer. The jury’s $289.2 million award was followed by an even larger $2 billion award in a second trial, although it was later reduced to $78.5 million. The appeals court reduced the award still further, to a total of $20.4 million—$10.2 million for economic loss plus a matching amount for punitive damages. The appeals court judge rejected Bayer’s reliance on the EPA’s determination that glyphosate did not cause cancer, saying the court wasn’t bound by it.

Bayer settlement still being worked on

Johnson’s case is one of three that are not covered by a proposed $10.7 billion settlement that Bayer hoped would cover about 75% of outstanding lawsuits against glyphosate and dicamba products, as well as settling claims for PCB contamination dating back to the 1970s, before precursor company Monsanto stopped making them. The judge’s rejection of the EPA’s clean bill of health for glyphosate may make it harder for Bayer to settle the other two cases. Bayer is reworking some parts of that proposal after the California judge overseeing it indicated he was unlikely to approve it in its original form. Attorneys’ ads soliciting Roundup class-action plaintiffs continue to be heard on the radio.

Bayer seems confident that glyphosate and dicamba will be around in the near future; it just announced a new cotton seed genetically engineered to resist five types of commonly used pesticides. The idea is not to use them all at once, but to vary them to keep the weeds off-balance.

But some scientists are wondering whether the seed companies are already losing that evolutionary arms race. They warn that weeds are evolving new ways of resisting herbicides—ways that work against many types at once, not just one, so that it may not be possible to outwit or confuse them by switching up different herbicides and engineering resistance to more and more herbicides into seeds.

Bob Hartzler, a professor in agronomy at Iowa State University, and associate professor Prashant Jha, both of whom specialize in weed science, have published an annual herbicide guide for corn and soybean producers for years. In the 2020 guide, published in February, they warn that resistant weeds are evolving to resist herbicides in a new way.

Herbicides target susceptible weeds at a particular protein site, known as a receptor site or site of action. The weeds can resist by making slight changes to the site, making it harder for the herbicide to bind to it. But some weeds, like palmer amaranth and common waterhemp, have apparently evolved a new way to resist herbicides: by increasing their ability to metabolize them.

“The concern with MBR [metabolic-based resistance] is that an alteration in herbicide metabolism providing resistance to one herbicide group may provide cross-resistance to multiple herbicide groups,” said Hartzler and Jha. They add that “another complexity” of MBR is that it is “impossible to predict whether the changes in metabolism that provide resistance to one herbicide group will affect other, unrelated herbicides. Each situation will need to be evaluated individually to determine the extent of cross or multiple resistance.”

They conclude, “The potential for MBR to provide cross-resistance to multiple herbicide groups is a threat to our current production system. It is critical to develop herbicide programs that rely on multiple effective herbicide groups and provide full-season weed control, therefore minimizing weed seed production.”

But they add, “herbicides alone cannot win this battle. Production systems must be evaluated to determine what alternative strategies can be used to supplement herbicides. While strategies such as increased crop competitiveness via narrow-row spacing, planting cover crops, or harvest weed seed control methods

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do not provide the ‘big impact’ of herbicides, their contribution to reducing weed seed banks can make the difference between long-term success or failure in weed management, thereby preserving the utility of existing herbicide tools and new traits into the future.”

Such herbicide-free farming methods already have many adherents. It’s not either/or, according to Hartzler and Jha. “While herbicides will remain the primary tactic used to manage weeds for most growers, it is essential to evaluate opportunities to include non-chemical tactics into the production system. The suitability of these tactics varies widely among operations, but inclusion of any alternative strategy can greatly improve performance of herbicides and delay the onset of herbicide resistance.”

Micheal Owen, a plant and weed specialist now retired from Iowa State University after a distinguished 40-year career, told High Plains Journal, “I don’t see how things can continue as they have for the past 30 or 40 years. A lot of weeds have already evolved resistances to these herbicides. Herbicides will continue to be a component of weed management, but not as they have been used historically. They must be used in a more diversified weed-management program, with tillage and cover crops.” This means a return, in part, to older methods of weed control.

What’s not clear is whether and how export volumes of commodity crops like corn and soybeans can be sustained with the diversified weed-control methods being advocated. The crop practices now under question have favored larger and larger fields to achieve economies of scale. But mixing up weed-control strategies to hedge bets and reduce losses could mean splitting fields up into smaller plots with different strategies in each.

Owen has been working with an Israeli company whose researchers are exploring the use of irradiated pollen instead of chemical herbicides to control weed pests like water amaranth. Are such methods scalable? “Anything’s possible if states come up with enough money,” Owen said.

David Murray can be reached at [email protected].