Top rice weed gains strength

In 2002, college degree in hand, the young farmer returned home to Cord, Ark., ready to take over the family land that his father and grandfather had worked previously. As in many Arkansas rice fields, he soon found barnyardgrass to be a major headache.

But after several years of battling the weed that seemed to be growing increasingly resilient, the young farmer couldn't recall his elders having done the same. Sufficiently alarmed, last fall he began calling weed specialists that might provide control answers.

“The operation had been in a rice/soybean rotation although, in some years, they grew rice following rice,” says Jason Norsworthy, research weed scientist and assistant professor at the University of Arkansas. “They'd used Command, like every other rice grower in the state — although at 1 pint per acre, about as high an amount as can be used with their soil type.”

After the problem continued to worsen, the farmer collected samples and sent them to Norsworthy for screening. The first greenhouse work on the grass was done last winter.

When the initial screening with Command — at a label rate of 0.8-pint pre — had little effect on the weed, Norsworthy had difficulty believing the results. “Was this for real?”

But results from the second screening were the same. After that, “we did dose-response work to see how the tolerance/resistance level differed from known susceptible populations. That's where we look at various rates on both known susceptible plants and the potentially resistant biotype. A 2X rate on the susceptible plants produced a clean situation. On the plants of interest, we still had healthy barnyardgrass at 4X rates — and that pointed to resistance.

“Researchers must be careful in claiming resistance in weeds. But I'm confident that's the case with this biotype.”

Around 14 days after treatment, the curve showed a 2.4-fold level of resistance to Command in the barnyardgrass versus the susceptible populations.

What are the chances of the 2.4 resistance level increasing? “We don't have a handle on that. If we take the surviving plants and screen them again, there is potential the resistance would increase. But that's speculation.”

The real importance in the discovery is “control failures at a labeled rate under field conditions. A 0.8-pint rate is recommended for a silt loam soil.”

One complication is Command rates shift with soil types. More of the product can be used on a heavier clay soil.

“But if we try that on a silt loam, we'll kill the rice.

“There's a very narrow margin of selectivity in rice to Command. If we begin increasing the rate and there are still survivors, we can't increase the field rates even further. The rice will be dead, so that isn't an option.”

This coming winter, Norsworthy will be searching for herbicides that can be used at planting, pre-emergence, to control the resistant biotype.

“We're also preparing to do work on post-emergence applications. If a guy has Command failures, what can he do?”

The problem with post-emergence — and it appears that's what happened on the farm where the biotype in question was found — is Command has usually already been applied. When the pre application fails, “you come back out a month after planting, ready to make the post-emergence application. But rather than having one- or two-leaf barnyardgrass, you have six-leaf that's tillered.”

With the rice herbicides available, it's very difficult to control large barnyardgrass consistently. And the farmer that found this problem weed faced exactly that. “When he sent the samples in he said, ‘Absolutely nothing works. Help me.’”

Curious to know how far the resistant weed might have spread, Norsworthy and colleagues have pulled barnyardgrass samples in a 3- to 4-mile radius from the problem field.

“The resistance was found about a mile from the Black River, in an area of about 10,000 acres of row crops worked by a handful of farmers. The location is on the edge of the Delta.”

One factor to consider: the area is river-bottom that floods regularly. Since that's happening, “it means this resistance has at least had the opportunity to move into other areas. Barnyardgrass seed will easily move via water — sometimes very long distances. We're certainly hoping the resistance is restricted to the one field.”

Unfortunately, Norsworthy did find more suspicious barnyardgrass in the area. One area farmer had sprayed Command that appeared to have failed. An adjacent field also had large barnyardgrass, but Facet was effective in controlling it.

“It looks like Facet will be an option early. Again, though, it's often very difficult to control large barnyardgrass with it. It's hit-and-miss in those situations.”

Not interested in a repeat of past wars with barnyardgrass, the young farmer planted all Clearfield rice this year. “So he's spraying two applications of Newpath, and he's done a fabulous job of controlling barnyardgrass.”

But the popularity of Clearfield varieties may ultimately be their downfall, warns Norsworthy. “Honestly, going with Clearfield repeatedly is a concern. More and more growers are using Clearfield — sometimes continuously with no rotation. A survey last year showed that happening.”

By making two applications of Newpath followed by Beyond (another ALS inhibitor), “essentially you're using three herbicide applications of the same mode of action. And, especially in a continuous rice system, the ALS chemistry is extremely prone to resistance.”

If current Clearfield adoption trends and spraying continues, “I'm afraid it will be a short time before we find ALS-resistant barnyardgrass. If that happens, we'd have propanil-resistant, Command-resistant, Facet-resistant and Newpath-resistant barnyardgrass. We are pushing our herbicide arsenal to one small corner of the warehouse. And we can't afford that — barnyardgrass is, by far, the most troublesome weed for Arkansas rice production.”

Was finding Command-resistant barnyardgrass a surprise for Norsworthy? “Actually it was a bit of a surprise because there isn't a lot of resistance to this mode of action worldwide. There are only two or three other documented cases of resistance to the chemistry.

“This mode of action hasn't been used elsewhere as much. There are 1.5 million acres of rice in Arkansas and 93 percent of that is sprayed with Command. That's been the case for a decade, or so. So there was potential for resistance to develop. Any time you use a specific herbicide year after year after year, you're setting up resistance.”

The number of fields being sprayed with Command may begin to decline since growers are adopting Clearfield technology more. Command isn't as widely used in Clearfield varieties.

“It's interesting that in 1992 we had propanil-resistant barnyardgrass develop. In 1998, Facet-resistant barnyardgrass was confirmed.

“There isn't a lot of resistance to Facet around the world except in barnyardgrass. That product came on the market in 1992. So, in a matter of only six years, barnyardgrass developed resistance. It seems Command is about on the same track.”

Is it possible the barnyardgrass has evolved resistance(s) to herbicides other than Command? “Yes, and that's another big reason we're doing further testing this fall looking at various herbicides pre-emergence… Prowl, Newpath, Bolero (delayed pre-) and other herbicides to see if there's any additional resistance.

“In the initial screening conducted in the greenhouse, we did see failure with propanil on the suspect population. That study needs to be repeated a couple more times before making any definite claims.

“There is a lot of genetic diversity within barnyardgrass and, as a result, it has a lot of potential to develop herbicide resistance. The big question is what compound it'll become resistant to next.”

Barnyardgrass generally emerges with rice and is similar in appearance. It mimics rice in its morphological characteristics and is highly competitive with the crop.

Most barnyardgrass plants produce about 100,000 seed. “So, you can take one resistant plant and, with decent germination, next year there can 25,000 to 50,000 plants you won't control with a given herbicide.

“We're very interested to know how it spreads and we want to minimize it. Propanil-resistant barnyardgrass has spread throughout the state since 1992. Facet resistance is in most rice-producing counties now. I'm hoping we can do a better job of managing this Command resistance.”

New recommendations to control the grass will be ready prior to next growing season.

“Much will be based on what we find in the greenhouse this winter. We plan on studying this in an isolated spot where rice isn't grown. Combine the field condition studies and greenhouse work and we'll have solid, updated recommendations before next season.”

Currently, Norsworthy is also checking over 20 other populations of barnyardgrass collected around the state for resistance to a variety of herbicides.

“We ask county Extension agents to help us with this. If anyone reports herbicide failure in the field, we ask them to collect seed heads along with a field herbicide history. With those, we're hoping to confirm the failure was mechanical or environmental and not resistance.”

Since last year, Norsworthy has screened provided samples against most rice herbicides including Clincher, Facet, Command, Newpath, propanil and glyphosate. Last year, of the 21 samples sent in, about half were propanil-resistant and half were Facet-resistant.

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