Home   News

As Arkansas Growers Struggle With Increasingly Resistant Weeds, State Weighs Labeling

The Arkansas State Plant Board is scheduled to hold a public hearing Nov. 21, during which it will consider changes to regulations now governing the use of pesticides containing dicamba and 2,4-D.
The review comes as Arkansas row crop farmers struggle with herbicide-resistant Palmer amaranth – better known as pigweed -- in both soybeans and cotton, and as soybean growers become increasingly wary of the potential for dicamba drift damage as neighboring growers adopt new dicamba-tolerant bean varieties.
In December 2014, well ahead of federal Environmental Protection Agency action, the Arkansas State Plant Board approved regulations for the use of two crop technologies, Dow Agroscience’s Enlist Duo, a formulation of glyphosate and 2,4-D choline, as well as Monsanto’s M1691 — now known as Xtend — a dicamba herbicide for use in cotton and soybeans. (See: Xtend is similar to another herbicide, Clarity, which is labeled for use in corn.
For producers, weed pressure isn’t the only factor driving crop variety selection. Farmers also have to consider who their end customers might be. China and the European Union both approved import of Xtend soybeans this year; China in February and the EU in July. U.S. growers planted Xtend beans in 2016, even though the federal Environmental Protection Agency had not yet registered the dicamba formulation meant to work with these seeds. When the federal Department of Agriculture announced deregulation of the dicamba-tolerant trait in late 2014, Xtend cotton seed was made available to growers for the 2015 season.
Enlist technology has not yet received export approval.
Building resistance
Pigweed and other broadleaf weeds had become resistant to Monsanto’s Roundup system, which had dominated corn, soybeans and cotton in Arkansas since its introduction in the late 1990s.
Tom Barber, professor of weed science for the Crop, Soil and Environmental Science Department at the University of Arkansas System Division of Agriculture, said Roundup was seen as a blessing when it was introduced in the state.
“When we got Roundup, it was a phenomenal herbicide — probably the best one we’re ever going to have,” Barber said.
“Roundup made weed control easy — and that was the problem. It made it easy, so all we did was spray Roundup — we didn’t alternate modes of action,” Barber said. “I guess it was 2005 when we found the first glyphosate-resistant Palmer amaranth. Since then, the list just got longer and longer for glyphosate-resistant weeds.
“When you talk about resistance, and did we see it coming — there were a lot of people out there — one company in particular — who said it could never happen — but I don’t think the weed scientists ever felt that way,” Barber said.
Barber and other weed scientists said the tendency of growers to rely on a single mode of action, even when effective, has contributed to increasingly multi-resistant weeds.
Overuse leads to resistance
“Overuse of any tactic will lead to resistance,” said Jason Norsworthy, professor of weed science and Elms Farming Chair for the Division of Agriculture. “It has nothing to do with the age of the herbicide, old or new — we can take a newer herbicide, and repeatedly use that product over and over, and it breaks rather quickly.
“Many of the herbicides we use today to manage pigweed resistance are some of the oldest chemistries we have out there today,” he said.
Bob Scott, professor of weed science for the Division of Agriculture, said that by mid-2015, growers and agronomists were reporting pigweed that was also resistant to PPO herbicides such as Valor and Flexstar. PPO, or protoporphyrinogen oxidase inhibitors, affect chlorophyll production and cause plant cell wall degradation in targeted plants. 
“When we started discovering PPO-resistant pigweed, which was already glyphosate-resistant, and to other herbicides in some cases, we knew we were now dealing with a multi-resistant pigweed,” Scott said. “We’re really down to Liberty, and LibertyLink soybeans. It’s sort of our last post-emergence option for fighting pigweed. And this is a big problem for us, because again — if we just continue to use Liberty without any other options, we’ll have Liberty resistance before we know it.”
The effort to stay ahead of, and slow the development of, herbicide resistance in pigweed and other weeds by both growers and seed technology manufacturers has led some to frustrating ends. Some farmers were eager to give the new Xtend products a try, despite lack of EPA registration of the companion herbicide. Some growers took matters into their own hands and allegations of off-label applications flourished.  
The Arkansas State Plant Board received a few complaints in 2015 for off-target movement for off-label dicamba applications on cotton, but by July 2016, the board reported receiving about two dozen official complaints of dicamba drift injury in soybeans. The Missouri Department of Agriculture reported receiving more than 100 such complaints in the state’s Bootheel alone. In October, a Missouri farmer was charged with first-degree murder, with the victim being one of several farmers who filed complaints over crop injury due to off-label spraying.
Nov. 21 meeting
On Nov. 9, the EPA registered a label for Xtendimax™ with Vapor Grip™ Technology, a Monsanto dicamba formulation that is intended to control weeds in dicamba-tolerant cotton and soybeans, and is said have to lower volatility. Whether or not Arkansas will register a label for its use in the state is one of the agenda items on the plant board’s Nov. 21 meeting.
Norsworthy and other scientists said the main impediment to getting a green light from the State Plant Board is that Monsanto hasn’t allowed researchers at the Division of Agriculture or any research university in the nation to conduct volatility testing on the new dicamba formulation.
“No one within the state has conducted any non-biased research nor has had the opportunity to test that formulation and validate whether it’s truly an improvement,” Norsworthy said. In Arkansas, typically, other researchers are allowed to test the products to evaluate manufacturer claims and make recommendation on its use. Engenia, another improved dicamba formulation that is believed to soon receive registration by the EPA, has been evaluated by Norsworthy for four years.
“Monsanto’s XtendimaxTM formulation with Vapor GripTM has never been tested by a university for reduction in volatility,” Barber said. “So there is no independent third-party data out there that say it’s better — it’s just Monsanto saying it’s better.”
Kevin Bradley, an associate professor in the Division of Plant Sciences at the University of Missouri said he and his fellow UM researchers were surprised that Monsanto had chosen not to allow them to test for volatility.
“No one at any university has been allowed to test volatility of this new Vapor Grip formulation from Monsanto,” Bradley said. “It’s kind of unprecedented, really. I don’t know how many times in our past that something this significant for farmers has come close to market with no testing from university scientists.”
Bradley and other researchers said they had been given the opportunity to test the formulation’s efficacy for weed control, but that the research agreements with Monsanto did not allow for volatility.
“Monsanto owns the technology,” he said. “We have to sign a lot of documents — there’s no freedom of movement when you’re testing someone else’s proprietary technology. You can get in a lot of hot water in a lot of different ways if you were to go off and do testing that hasn’t been agreed upon.
“Everyone has reservations,” Bradley said.

Trending Video


Video: Mesonet

SWes Lee, OSU Extension Mesonet agricultural coordinator, discusses soil moisture levels and how the latest storms impacted them.