Weed resistance issues increasing

This year, there are some 1.4 million acres of rice in Arkansas. Given the new chemistries available, researchers say it’s surprising that growers continue to find more fields of propanil-resistant and Facet-resistant barnyardgrass.

“We’ve got Newpath, Regiment, Ricestar, Clincher — new herbicides with new modes of action — but due to the wide weed spectrum that propanil and Facet have, we continue to rely on them quite a bit,” said Bob Scott, Arkansas Extension weed specialist, at the recent fields day at the Rice Research Station in Crowley, La.

So far, Louisiana has dodged much of the resistance problem developing in Arkansas. Scott encouraged Louisiana producers to do everything they can to keep it that way.

Almost half the Arkansas rice acreage is in Clearfield varieties. “We expect Clearfield will be in over half the acres next year. That tells me there’s a half million acres of herbicide resistance pressure on ALS chemistry.”

Growers often apply two shots of Newpath at 4 ounces before applying Beyond. There’s “a tremendous selection pressure on that particular chemistry, and we’re concerned about the development of an ALS-resistant barnyardgrass in that production system.”

One thing many Arkansas growers are doing — “and we’re encouraging it,” said Scott — is incorporating one more mode of action, whether Command or Facet early. Another option is “using a product like Duet in a tank-mix to pick up the broadleaves that Newpath misses. So, on a lot of rice acres, we’re doing some resistance management.”

Since 2002, when Scott began work in his current position, Arkansas has developed — or discovered — five glyphosate-resistant weeds. The first was horseweed that is believed to have originated in western Tennessee no-till cotton. The horseweed has recently spread through eastern Arkansas.

“Now, if a grower calls complaining about horseweed after a burndown program, I assume it’s resistant and we recommend a quart of 2,4-D, minimum, and preferably about 8 ounces of dicamba. Horseweed has changed the way we farm both cotton and soybeans. In a post-emergence situation with horseweed, options are scarce.”

After resistant horseweed, resistant common ragweed and giant ragweed were found. The state now has at least seven or eight populations of confirmed resistant giant ragweed.

“Those are at least 1X higher in tolerance to glyphosate. But while (the resistant ragweeds) interest me as a weed scientist, they probably aren’t as agronomically important. There are alternative methods of control — tillage and other herbicides will do the job.”

The next glyphosate-resistant weed found was Palmer amaranth. When glyphosate was introduced into the state, “it was badly needed due to ALS-resistant pigweed. Products like Scepter and Classic had pretty much selected out all the pigweeds we could kill and we were left with the Palmer amaranth.

“So, we already have ALS-resistant Palmer amaranth in Arkansas. Now, we’re beginning to have a lot of fields turn up with glyphosate resistance. I’m aware of at least 40 fields that received a full rate of Flexstar in an effort to clean up pigweeds after 22 ounces of Roundup WeatherMax killed everything except the soybeans and the Palmer amaranth.

“We’re right on the cusp of having a tremendous problem with this weed in both cotton and soybeans.”

Scott and research colleagues are looking at residuals for pigweed control and post-emergence salvage options — “and, unfortunately, there aren’t any. We’re also evaluating new technologies like LibertyLink soybeans, dicamba-stacked soybeans and STS soybeans. Of course, one problem with the STS soybeans is we’ve already got ALS-resistant pigweed.”

The most recent glyphosate-resistant weed discovered in Arkansas is johnsongrass. Also confirmed in north Mississippi, the weed is already a problem in Chile and other South American countries.

“The johnsongrass will tolerate up to about 44 ounces of Roundup WeatherMax, so it isn’t completely resistant. However, it will effectively tolerate a field rate.”

Louisiana may already have resistant/tolerant johnsongrass. Researcher Daniel Stephenson recently moved from Arkansas to work for the LSU AgCenter. Shortly after he arrived, Stephenson called Scott “and said he wanted to send up five populations of suspicious johnsongrass to compare to the resistant Arkansas populations. We’re currently in the process of doing that.”

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