Multiple herbicide resistance in barnyardgrass prompts red flag warning

• By Vicky Boyd,
Editor •

jason norsworthy, rice flatsedge
University of Arkansas Weed Scientist Jason Norsworthy said reduced rates of Loyant herbicide are very effective at controlling rice flatsedge, even those populations resistant to ALS herbicides. But resistance to Loyant has been found in at least one Arkansas population of barnyardgrass.

A grassy weed with resistance to multiple herbicides poses as big – if not a bigger – challenge to Arkansas rice growers than herbicide-resistant Palmer amaranth does to soybean and cotton producers.

That was the sobering outlook University of Arkansas Weed Scientist Jason Norsworthy presented about herbicide-resistant barnyardgrass during the recent University of Arkansas Rice Field Day in Stuttgart.

“The moral is all about overlapping residual herbicides,” he said. “We’re not using enough Bolero. We have to start integrating more herbicides into this system.”

In addition to weed management, the annual half-day event at the Rice Research and Experiment Station featured six other tour stops. Among the topics covered were variety and hybrid development, drones in agriculture, irrigation management, soil fertility, furrow-irrigated rice and soybean breeding.

A grassy ‘super weed’

The cause for Norsworthy’s red flag warning are scattered barnyardgrass populations resistant to propanil, Facet, Newpath and now – Loyant herbicide.

Each year, Norsworthy conducts screenings on weeds propagated from seeds sent in by county agents from herbicide failures. In 2018, he received barnyardgrass samples from several problem fields, including one identified as No. 169.

“We were surprised and concerned about what we saw in the greenhouse, but that’s in the greenhouse,” he said about No. 169.

Norsworthy then planted samples in open-field plots at Stuttgart from 15 different barnyardgrass populations submitted during 2018 as well as known herbicide-susceptible populations from previous years. Individual applications of propanil, Facet, Newpath and Loyant were made on four-leaf barnyardgrass.

Some of the populations were controlled by the individual herbicides, but No. 169 stood out.

“We didn’t do a lot with propanil,” Norsworthy said. “We had no response to Facet. We had no response to Newpath, and there was no response to Loyant. I’m not standing here telling you we have widespread resistance throughout the state to Loyant.

“I think it’s an herbicide we can still use on barnyardgrass, but there are some acres that we’ll not be able to control with propanil, Facet, Newpath or Loyant.”

Still on the list of effective barnyardgrass controls are Provisia and Clincher, he said.

As a result, Norsworthy recommended growers use Command (clomazone) on every acre as a preplant, and he’d also like to see more Bolero (thiobencarb) used as a residual.

Start clean

The goal is to take out the barnyardgrass when it’s easier to control before emergence.

“The moral of the story is if you’re after barnyardgrass, I’d recommend everyone try to kill it before it comes up,” he said. “It’s just like Palmer pigweed in soybeans or cotton because if it does come up, you’re going to have a very tough time trying to kill it.”

If you suspect herbicide resistance in an Arkansas field, contact your county agent to collect samples and submit them to Norsworthy for screening.

The deadline for submissions is Nov. 1, and he said he’d have results back to the county agents by late January or early February in time for growers to make preplant herbicide decisions.

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