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No quick cures for glyphosate-resistant weeds

For five years, glyphosate-resistant horseweed has moved swiftly across farmers’ fields. Despite the work of many great researchers there remain no quick remedies to the troublesome weed.

Mostly a no-till problem, the history of resistant horseweed (or marestail) is interesting. “The resistant type was first discovered in Delaware in 2000,” said Andy Kendig, Missouri Extension weed specialist at the annual Delta Center field day outside Portageville, Mo., on Aug. 31. “In 2001, it was found in western Tennessee. In 2002, it was found in Missouri and Arkansas.

“What’s scary is exactly a year after discovery it was already widespread in Delaware. The same pattern was seen in Tennessee. The first time I started seeing it while driving around Missouri was in 2003. The last couple of years, phone calls to me on this weed have been heavy.”

There are good treatments available to deal with resistant horseweed but none are standalone cures. Through research findings on the weed, Kendig has come to several conclusions. Among them:

Fall herbicides aren’t recommended

In a field with horseweed infestation ranging from 75 to nearly 150 plants per square yard — “a bad infestation” — none of Kendig’s treatments did much good.

In January, after November treatments, “we found less horseweed but control was still a long way from adequate. Our best treatment was 25 plants per square yard. That’s not good enough for a grower. Some of our March and April applications did a bit better job but with them, the presence of a residual herbicide didn’t help.”

Basically, the fall herbicides aren’t lasting long enough to do much good. In January, the November treatments may look clean. “But in March, the weeds cut loose. By planting time, we had a serious horseweed problem again.”

A later date for burndowns?

Kendig sprayed 2,4-D and Clarity on a twice-a-month schedule from Feb. 15 through May.

“Basically, after a March 15 burndown with either 2,4-D or Clarity, horseweed germination was greatly reduced but still wasn’t zero.”

This leads to a difficult question: does Kendig recommend producers wait until after March 15 to burndown?

“The limitation is the 2,4-D has a one-month preplant interval. If you get up to March 21, you’ll be forced to wait to April 21 to plant cotton. Depending on the rate used, Clarity has a two- or three-week preplant interval. This is definitely something for producers to think about, though.”

Before making any decision, Kendig cautioned the data is from one year only.

“One thing that scares me is every year is different. A slightly different weather pattern could mean the ‘magic date’ will be April 1 instead of March 15.”

FirstRate, Envoke, Ignite

Kendig is frequently asked how to deal with resistant horseweed in soybeans. “Fortunately, for soybeans, the herbicide FirstRate has been pretty good. There were some failures when the horseweed was large or drought-stressed. But FirstRate is the standard recommendation and mostly does a good job in soybeans.”

In cotton, Kendig and colleagues have tested Envoke several times. “Once it didn’t work very well on knee-high horseweed. Last year, we tried Envoke again and it looked better. However, the second time around, we targeted horseweed that was only 8 inches tall. We need some further testing.”

The Liberty Link system with Ignite has also garnered a lot of attention. Ignite as a burndown treatment has been somewhat inconsistent in Kendig’s research.

“But there’s no question when growing Liberty Link cotton, once temperatures warm up Ignite is an option for control of horseweed.”

Gramoxone mixtures

Gramoxone alone releases horseweed. However, Gramoxone tank-mixtures with Caparol, Cotoran and Direx can provide upwards of 75 percent control. Other mixes have also shown promise.

“Something else we may also consider is a good disk and hipper. That may be the answer for some of our horseweed problem.”

Tossing glyphosate not the answer

To prevent resistance from developing, Kendig has tried to remove glyphosate from burndown treatments. It hasn’t worked.

“Glyphosate controls too many weeds to throw it out entirely. For resistance prevention, think in terms of Band-Aid chemistry. There could be potential in beans and cotton to replace a glyphosate application with a pre-emerge treatment.

“But I caution folks against saying, ‘I’m taking glyphosate out of my burndown.’ If you do that, you’re giving up weed control.”

Resistant Palmer amaranth

In late July, a “probable” case of glyphosate-resistant pigweed (Palmer amaranth) was confirmed in central Georgia. The weed is suspected to be confined to several locations. Across the nation, ears of weed scientists pricked up.

“On Palmer amaranth, the list of things you can add in Roundup Ready cotton is short. Prowl pre-emerge or incorporated Treflan/Prowl is an option, but incorporation isn’t very popular.”

As a pre-emerge herbicide, Prowl has been “rather inconsistent. It depends on a good, activating rainfall. There’s no question in my mind that incorporating the yellow herbicide does a very good job. However, I also recognize that using it requires a lot of time and diesel fuel. Because of that hardly anyone is doing it now.”

Now that resistant pigweed has likely been found, “my views have changed a bit. Is it time to put a half rate of Cotoran back in behind the press wheel?”

If the resistant pigweed spreads, Kendig said, lay-by chemistry will play a major role in control.

“(Roundup Ready) Flex cotton should be widely available in about five years. A big concern weed guys have is hoods and lay-by equipment will go to fence rows. I hate to hear that and feel it’s a bad situation.

“Our lay-by chemistry is key in managing palmer amaranth. We’ve done a lot of work with all the lay-by materials. Some are a little better than others. But the big thing with these materials is getting them sprayed on time. The Flex cotton, ideally, will allow us always to get an ideal height differential.”

What about Zorial?

“It’s a good, soil-applied treatment, and we’ve tested it extensively. But it’s hard to find at agriculture chemical dealerships now that growers have essentially stopped using pre-emergence herbicides.”


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