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Which weed next shows resistance?

For a long time weed scientists thought herbicide resistant weeds would never emerge as a problem. They were wrong. “We sat back and watched entomologists struggle with insects resistant (to insecticides) and thought nothing similar would happen on the weed side,” said Bob Scott, Arkansas Extension weed specialist at the annual Arkansas Soybean Research Conference in Brinkley, Ark., on Dec. 15. “We thought there weren't enough generations in a year, there were too many different herbicides and different biological systems that would prevent weed resistance.”

The idea of weed resistance was first presented back in the 1960s. “The guy who put it forward was kind of laughed out of the room. There was a general feeling it couldn't happen to us. But, lo and behold, in 1968, common groundsel showed up in orchard production. It was the first documented resistant weed.”

Since then, herbicide-resistant weeds have become a global problem. There are now over 300 unique species around the world known to be resistant to herbicides. These include 182 species of broadleaves and grasses.

There are currently 13 herbicide-resistant weeds listed in Arkansas, although Scott believes there are more waiting to be discovered. Currently, two of the 13 have been confirmed resistant to glyphosate.

“When you talk about weed resistance it's a numbers game. Judging by the attendance here, many folks are considering planting soybeans next season. Fertilizer prices being what they are, everyone is talking about rice acres dropping. So we've got over 3 million acres of soybeans in Arkansas and well over 90 percent of those are Roundup Ready. And we make more than one application of glyphosate per crop year. We're killing multiple flushes on millions of acres using the same chemistry.”

In addition, new Roundup Ready cotton varieties are being introduced “that can handle more, and later, applications of glyphosate. We're also seeing a switch in corn acres to Roundup Ready (varieties). And Roundup Ready rice has already been developed. It's just sitting on the shelf waiting for market acceptance.

“So you couple those facts with fall and burndown applications and it's not nice to fool Mother Nature. That's essentially what we're doing: there's a huge, unnatural selection pressure in the state. It's unprecedented and enormous.”

When putting such selection pressure on, resistance shouldn't surprise anyone. “For a long time, we didn't think we'd see this with glyphosate due to its mode of action being fairly complex. But it's showing up.”

The two glyphosate-resistant weeds confirmed in Arkansas are horseweed and common ragweed. “We're investigating some giant ragweed fields and are also concerned about some pigweed fields. Although I have no earth-shattering things to report on pigweed today, that doesn't mean we aren't looking.”

In some parts of Mississippi County, resistant horseweed has already resulted in movement from no-till cotton to other crops. Even so, Scott said he had “quite a few calls” on horseweed in soybeans in 2005.

“We've done some limited research on horseweed and need more. The programs we've looked at include Clarity and 2,4-D in the burndown, just like the cotton program. Synchrony and Python — both ALS herbicides — preplant have shown promise and pretty good activity. FirstRate post was the best of the worst we looked at. There wasn't a silver bullet post in a study we did in Crittenden County. So it'll be a program approach in soybeans.”

In 2004, Scott got a phone call from near Newport, Ark., on problem common ragweed. Traveling to the field, “we observed a lot of dead and injured plants. Studies showed it took about 8 quarts to kill that population — after the farmer had already treated. This led us to believe we had a resistant, or more tolerant, population.”

Nailing down resistance isn't all that quick. Before confirming resistance, weed scientists must go through a lengthy study process.

“You have to study tolerance, compare it to other populations of the same species and look at the heritability of that trait. In other words, are the suspect plants' offspring as resistant as the parents?”

Everyone wants to know what Arkansas' next resistant weed will be, said Scott. Most are very suspicious of pigweed as it's already been identified as resistant in Tennessee and Georgia.

“In Arkansas, when we say ‘pigweed’ we're mainly concerned with Palmer amaranth. One of its distinguishing characteristics is a long petiole attached to the leaf. That differentiates it from other pigweeds although pigweeds are sometimes hard to tell apart. They have no morals — they'll interbreed and create all kinds of hybrids and different looking plants.”

In the Georgia case of resistant pigweed, researchers sprayed Staple to clean it up. “In a lot of Arkansas counties, we've already got pigweeds that are resistant to Staple. (If glyphosate-resistant pigweeds emerge) that would eliminate two families of herbicide chemistry.”

How can glyphosate resistance be curbed? “We've got to rotate crops and chemistry. You've got to consider some conventional herbicides…If you want to prevent resistance use the herbicides at rates that will do some good. If you're tank-mixing 6 ounces of FlexStar with Roundup, you aren't putting enough FlexStar on — especially if it's being applied to 12-inch pigweeds. A full rate of FlexStar on 2-inch pigweeds will do the job.”

Even though pigweeds are the chief candidate for resistance, Scott is reticent to point towards them too emphatically. “Last year, everyone was worried about pigweeds and the calls starting coming in on giant ragweed. In fact, we had four fields of concern that were in Greene, Jefferson, Mississippi and Cross counties.

“We drove to the field in Greene County, near Paragould, and found the yellowing, stunting and partial control. Then, the plant recovered and grew out of it. That is the exact symptomology in the resistant common ragweed and horseweed.

“We haven't done all the studies on giant ragweed. So we don't know if we're looking at resistance yet. According to Monsanto, glyphosate has never been very good on this weed.”

Scott and colleagues did go back at the Paragould location and apply Roundup WeatherMax at 6-inch and 12-inch ragweed. “Then, we did a sequential of a 6-inch followed by a 12-inch treatment. This was a bare ground study with no crop.”

Twenty days later, they found the farmer was right: what he was doing — a single, 22-ounce application of Roundup WeatherMax — wasn't working.

“With a sequential treatment, we got about 90 percent control. In order to get that with a single shot, we had to go at least 44 ounces at 6 inches. If you wait until 12 inches, it doesn't work well.”

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