A 60-acre field in central Arkansas’ Prairie County has been confirmed with the Mid-South’s first incidence of barnyardgrass resistant to Newpath, propanil and Facet.

With the vast majority of the state’s rice expected to be planted in Clearfield varieties (which make use of Newpath), weed specialists say this is only the beginning.

Rapid adoption of the popular Clearfield technology by Mid-South producers (for more, see Clearfield) along with the fact that barnyardgrass is highly prone to evolve herbicide resistance means the discovery “wasn’t unexpected,” says Jason Norsworthy, weed scientist and associate professor at the University of Arkansas.

“But in the Mid-South — maybe even the United States — this is the first time I’m aware of a biotype having resistance to three modes of action in a single field.”

(For more on resistant barnyardgrass, see http://deltafarmpress.com/mag/farming_researchers_seek_treatments/index.html and http://deltafarmpress.com/mag/farming_top_rice_weed/index.html)

Ken Smith was hoping it would be a few more years before such Newpath resistance showed up. “When this spreads — and it will — we’ll have only Command, Ricestar-type products and Prowl left for controlling barnyardgrass,” says the Arkansas Extension weed specialist.

“Weed control in (the Prairie County) field is now severely limited. Growers need to know resistance can happen quickly and results in serious consequences.”

Last fall, called to a rice field west of Devall’s Bluff, Ark., by a worried consultant, Brent Griffen saw “several large areas of barnyardgrass although it wasn’t thick enough to hurt the rice or take it down. It was randomly scattered around the field.”

Griffen, Prairie County Extension staff chair, says what was “really eye-opening” was an area of the field where 15 ounces of Newpath (a normal rate is 4 ounces per acre) had been applied to the grass. “Even spraying those spots with that much Newpath didn’t kill it.”

Sufficiently alarmed, Griffen collected samples and sent them in for study.

Norsworthy — who oversees a weed resistance screening program in Fayetteville, Ark. — says the samples were resistant to Facet and propanil. That wasn’t a surprise because, in recent years, 48 percent of the barnyardgrass samples Norsworthy has screened have proven resistant to propanil, 28 percent are resistant to Facet and 21 percent are resistant to both.

Then Newpath seemed to fail in controlling the Prairie County sample.

“Any time something is new or odd we’ll go back and take a closer look,” says Norsworthy. “In this case, we sprayed the grass with eight rates of Newpath and the same with Grasp. We sprayed up to 32 ounces of Newpath, an 8X rate, on two-leaf grass.”

In early April, Norsworthy checked the plants he’d sprayed 21 days earlier. They were “barely bothered. We’d sprayed 20 plants with the highest rate of Newpath and killed zero. Not only that, but the untreated plants were about 18 to 20 inches tall. The treated plants were about 16 inches tall.”

To deal with grass weeds in rice, five major modes of action are available: propanil, Facet, Command, ALS herbicides (Grasp, Regiment, Newpath and Beyond) and some ACCase herbicides (Clincher, Ricestar).

“So, in this Prairie County field three of the five key pieces to control barnyardgrass are completely lost,” says Norsworthy. “For the last couple of years I knew a train wreck was coming. Well, the train crashed in this field and the farmer has lost the flexibility and numerous options to deal with barnyardgrass.”

Growers need to fully understand that once a herbicide mode of action is lost, it is gone for good. And once resistant grass is in a field, “it will always be there. Seed is in the soil and not all will emerge in a given year. Going with a herbicide program without Facet or propanil doesn’t mean that resistance will lessen in three or four years. From this day forward, those products will not be effective on barnyardgrass in that field. They will fail.”

Add Newpath resistance to the mix and the Prairie County producer is in a serious quandary.

“If he plants rice, what will he do to manage barnyardgrass? Well, Command, Prowl, Bolero and the ACCase herbicides (Clincher, Ricestar) are still available — but that’s it,” says Smith.

While the Prairie County field is so far unique, “I’m confident it isn’t the only one in the state with Newpath resistance,” says Norsworthy. “With the amount of Clearfield rice we’re planting and the amount of Newpath we’re applying, I’m confident we’ll see more.”

The weed scientists say growers must be “proactive” before Newpath-resistance becomes widespread and the Clearfield technology is no longer effective in managing barnyardgrass.

“Already,” says Norsworthy, “options are few — the last thing we need is even fewer.”

To read the unabridged version of this story see the April 23 issue of Delta Farm Press.

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