Scientists waging war on weeds

Frances Gould, Schultz, Bruce  |  11/20/2013 3:04:35 AM

Drs. Jim Griffin and Daniel Stephenson, LSU AgCenter weed scientists, examine a Palmer amaranth plant. (Photo by Bruce Schultz)

Drs. Jim Griffin and Daniel Stephenson

Drs. Jim Griffin and Daniel Stephenson examine seed from a Palmer amaranth plant. Palmer amaranth seeds are tiny black dots in Griffin's hand. (Photos by Bruce Schultz)

LSU AgCenter scientists are waging war on weeds on several fronts as herbicide resistance becomes an increasing threat.

Dr. Daniel Stephenson, LSU AgCenter weed scientist at the Dean Lee Research Station, said herbicide-resistant Palmer amaranth is becoming more prevalent in northeast Louisiana and is suspected in other areas of the state. Resistant Palmer amaranth was first confirmed in Louisiana by Stephenson in 2010.

The 2011 drought has affected the efficacy of many herbicides. In drought situations, residual herbicides may not receive the moisture needed to be activated, or the efficacy of postemergence herbicides, like glyphosate, may be reduced because weeds are not actively growing or are too large when the herbicide is applied. If a weed like Palmer amaranth is drought stressed or tall when glyphosate is applied, it can be difficult to determine if the weed is glyphosate-resistant.

Stephenson said the best time to control Palmer amaranth is at emergence, which can be done with a soil-applied residual herbicide.

He said waterhemp, a cousin to Palmer amaranth, is becoming more of a problem in the Midsouth, but it isn’t as aggressive as Palmer amaranth.

Glyphosate-resistant Johnson grass has more potential to be damaging because it cannot be controlled with a soil-applied residual herbicide, Stephenson said, and it is spread easily through seeds or root segments.

Dr. Jim Griffin, LSU AgCenter weed scientist in the School of Plant Science, Environmental and Soil Sciences, said a farmer who had herbicide-resistant Johnson grass inadvertently moved the plant to other areas of a farm in Pointe Coupee Parish, probably by a combine that wasn’t cleaned before relocating.

Glyphosate-resistant Italian ryegrass is becoming more of a problem in Mississippi, where it has overtaken entire fields, and it is being found in Louisiana, Stephenson said.

Stephenson said farmers would be wise to spray herbicides or use tillage during the fall to prevent weeds from becoming more of a problem in late winter and early spring when burndown applications are made.

He said it is time to remember the practices used by farmers who grew crops before Roundup to learn how to mitigate herbicide-resistant weeds.

Last year, Stephenson said, Louisiana farmers had been invited to the herbicide-resistant party. "This year, we’ve sent in our RSVP," he said.

At the LSU AgCenter’s Northeast Research Station, Dr. Bill Williams, LSU AgCenter weed specialist, said, "Most of our work is centering around the development of weed-control programs to help prevent or delay development of herbicide-resistant weeds and to manage weeds that are already resistant."

To help farmers identify problem weeds in soybeans, Williams is working with Frankie Gould, LSU AgCenter communications director, on a field guide.

He said research has focused on weed control in three stages – at preplant, during the crop season and fall.

Williams said Italian ryegrass and henbit are a problem if farmers don’t treat early. "The bottom line is, there is nothing out there that will allow us to control Italian ryegrass and henbit after Feb. 1," Williams said.

Williams also said teaweed is becoming more prevalent across northeast Louisiana, partly due to regrowth of teaweed after corn harvest.

He said fall control of weeds focuses on stopping weed seed production. After that, cool-season weeds have to be addressed, Williams said.

Griffin said Louisiana farmers have not been adopting Liberty Link soybeans as quickly as Arkansas producers. "As long as Roundup works, they are going to continue with that," he said.

Several Liberty Link soybean varieties have proven high yields, and the glufosinate herbicide Liberty offers a good alternative to glyphosate, Griffin said, but Liberty is more expensive than glyphosate products. Also, Liberty is less effective than glyphosate on large weeds, and Liberty requires earlier application.

Griffin said several new herbicides show promise for controlling weeds that have become resistant to glyphosate. 

Dr. Donnie Miller LSU AgCenter weed scientist and research coordinator at the Northeast Research Station, said agency approval is pending for three soil-applied, pre-emergence herbicides that have good residual activity.

The chemical is pyroxasulfone, Miller said, and if approval is given, it will be sold in slight variations by three chemical companies.

He said BASF is planning to release the herbicide under the trade name Zidua.

It can be used directly on the soil at planting and post-emergence.

FMC is proposing to release the same herbicide under the name Anthem in tandem with Cadet to attack broadleaf weeds.

Valent is proposing a version with the name of Fierce, featuring a mixture with Valor. Both herbicides have good activity on broadleaf weeds and annual grasses, Miller said.

Miller said the chemicals offer farmers another alternative to fighting weeds.

"All three of them are showing good results," he said. "It will be another bullet in the gun."

Bruce Schultz

2010-11 funding for these projects:
$167,200 (soybean, corn/wheat and grain sorghum)

( This article was published in the 2011 Louisiana Soybean & Grain Research & Promotion Board Report )

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