On the way to becoming Arkansas’ Number One grass weed, barnyardgrass has tormented farmers while picking up resistance to a wide range of herbicides. And without agronomic changes the weed is likely to ratchet up the pressure on Mid-South crops.

Propanil resistance in Arkansas barnyardgrass was documented in the early 1990s. Shortly thereafter, Facet (Quinclorac) came in under a Section 18 and was widely adopted. By the late 1990s, weed scientists began to find Quinclorac resistance.

“What was interesting is that when we found Quinclorac resistance in barnyardgrass, it was also resistant to Propanil,” says Jason Norsworthy, University of Arkansas weed scientist.

The march towards resistance to multiple herbicides continued.

In the early 2000s, growers began to use Command (Clomazone). By 2007, the state had its first case of Clomazone-resistant barnyardgrass.

In 2009, Norsworthy confirmed the first case of ALS chemistry-resistant barnyardgrass -- products like Regiment, Grasp, Beyond, and Newpath in Clearfield rice.

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“Today, I’ve found seven populations within the state that are resistant to ALS herbicides. Two populations are resistant to Clomazone. We have numerous populations resistant to Propanil and Facet.

“Slightly more than 50 percent of the barnyardgrass samples that are sent to me for resistance testing are confirmed resistant to Propanil. In that same circumstance, 29 percent of barnyardgrass samples are resistant to Quinclorac.

“It’s also worth noting that 22 percent of all this barnyardgrass is resistant to both Facet and Propanil. So, many of our rice growers have completely lost two herbicide options.”

At the end of each growing season, growers, Extension agents and consultants send Norsworthy barnyardgrass samples for screening. “It’s intriguing that some of the fields haven’t had Facet or Propanil in them for almost a decade, now. However, they’re still resistant.

“So, once we lose a herbicide to resistance, it’s lost for the long haul. You can take it out of the system for a long time and it still won’t be effective when used again. Resistance in the barnyardgrass continues to persist.”

The resistance genes aren’t flushed out of the plant.

“What happens is there is no ‘fitness penalty’ for the plant having the resistance. As a result, once it goes into the soil seed bank, any offspring from the resistant barnyardgrass continues to carry the trait.”

And it isn’t just barnyardgrass.

“We’re seeing this with pigweed in cotton and soybeans. We’re seeing it with other weeds, as well.

“We’ve lost glyphosate on pigweed, something growers are well aware of. The same concept is playing out now in rice with barnyardgrass.”

The news isn’t good for the future, either.

“We still haven’t found a location where there’s Propanil, Facet, Command, and ALS resistance in the same field. But I think we’re quickly approaching the day when it will happen. Once it does, our growers will have very few options for controlling barnyardgrass in rice.”

Thankfully, there are other herbicides available.

Prowl, Bolero, RiceStar and Clincher – the latter two being ACCase, or Group 1, herbicides – “are basically the only other tools we’d have to manage barnyardgrass if we have a population that develops resistance to the four herbicides I mentioned earlier.

“When that happens it will be extremely challenging for growers to deal with barnyardgrass. It’s pretty easy for me to control it in small-plot research. I can easily flush a field, spray a herbicide in a timely manner and optimize its activity.”

But try putting Prowl, Bolero, RiceStar and Clincher across a large rice operation.

“You start looking at 1,000 or 2,000 acres of rice and it can take several days, if not a week, to get water across the fields much less deal with herbicides. That is not going to be a happy situation.”