Tests by the LSU AgCenter have confirmed herbicide-resistant pigweed at three locations in north Louisiana.

“We’ve joined the party,” said Daniel Stephenson, LSU AgCenter weed scientist who conducted the testing.

However, this party is no celebration but, rather, a meeting of the minds by LSU AgCenter scientists to figure out how to combat the problem. “We’re evaluating alternative weed control programs,” said Jim Griffin, LSU AgCenter weed scientist. “We’ll assist growers in planning control programs where weed control issues have occurred.”

Two of the resistant-weed fields were in Tensas Parish, and the third was in Franklin Parish. All three weed populations are resistant to the herbicide glyphosate, which includes Roundup as well as many other glyphosate products.

Roundup Ready soybeans were grown in one of the Tensas Parish fields, while Roundup Ready cotton was grown in the other two fields, according to Donnie Miller, LSU AgCenter weed scientist and director of the Northeast Research Station in St. Joseph, La.

Last year, Palmer amaranth (pigweed) from Concordia Parish and johnsongrass in Pointe Coupee Parish were confirmed as resistant to glyphosate.

Several states from Iowa to Georgia, including Arkansas and Mississippi, have had severe outbreaks of herbicide-resistant weeds, including pigweed, specifically Palmer amaranth.

Griffin said it’s not a surprise that the problem finally surfaced in Louisiana. “It was not a matter of if, but when.”

Griffin explained that herbicide resistance is the result of accelerated evolution. “The process begins with just a few plants with the genetic capacity to survive the herbicide treatment. It is believed that these plants, which occur naturally in the population at a very low level, are not a result of genetic mutation caused by the herbicide. These inherently resistant plants, when exposed to the same herbicide over several years, produce seed. Over time the population slowly shifts such that the resistant weeds become dominant. Since this process is slow, the producer may not notice the problem until large scale weed control failures occur.”

The pigweed from Concordia Parish survived amazingly high rates of glyphosate, Stephenson said. “To kill half the population would have required 54 times the normal rate.”

The Concordia Parish weed was found in 2009 in a field where Roundup Ready cotton had been grown in four consecutive years. In 2009, the farmer noticed pigweed that had not been killed by an aerial application of glyphosate. Samples of those plants were used to obtain seeds that were grown in a greenhouse and tested with varying rates of glyphosate.

The same protocol was used on johnsongrass that could not be killed with glyphosate alone. It was found in a field in Pointe Coupee Parish where Roundup Ready soybeans had been grown for 10 years.

Also in Concordia Parish, a cousin of Palmer amaranth, tall water hemp, is suspected of having herbicide resistance. Resistant tall water hemp has been confirmed in Mississippi by Mississippi State University.

In other states, including Arkansas and Mississippi, herbicide-resistant weeds have caused headaches for farmers, with reduced yields and harvest problems caused by the large weeds.

Palmer amaranth can grow up to an inch a day, expanding to a 4- to 6-inch diameter trunk that can damage harvest equipment. It thrives in heat that normally would suppress other weeds. Its pollen can move up to 600 meters, making neighboring fields vulnerable. One plant can produce up to 2 million seeds. Their small size makes it impossible to clean all seeds from farm equipment.

But weed experts say the problem in Louisiana is manageable.

“Louisiana is at a point where we can mitigate this,” Stephenson said. “We’ve got the opportunity to not be like our neighbors.”

He said growers in Concordia Parish are working together to fight the weed. They are using herbicides with residual action.

Stephenson said alternative herbicides, such as Valor, Dual, Reflex or Magnum, which have different modes of action to kill pigweed, can be used separately or with glyphosate.

Miller said chemical companies are offering rebates and other incentives for growers to use alternative herbicides.

Stephenson warned that using reduced rates of herbicides to save money is false economy.

During field scouting for insects and diseases, he said, farmers can destroy weeds to prevent seed production.

 

“Growing Liberty link soybeans where Ignite is used for weed control would also be a means to manage glyphosate-resistant weed problems,” Griffin said.

The Liberty Link gene is also available for other crops, including corn and cotton.

Griffin said crop rotation is another strategy, provided the rotational crop would be something other than one that relies on glyphosate for primary weed control. For example, he said, rice would be a good rotational choice.

Stephenson said addressing the problem will cost farmers more money, but it is less expensive than what farmers are facing in Arkansas. “It comes down to pay me now, or pay me later.”

The Palmer amaranth problem may be overshadowed by the herbicide-resistant johnsongrass. “With Palmer, I have bullets to control it, but with johnsongrass, I only have one bullet,” said Stephenson. “That is because in some areas of the state johnsongrass has already been shown to be resistant to herbicides that could be used as alternatives to glyphosate. That puts us in a real dilemma.” 

Chemical companies are working on new compounds to fight the weeds, but a new product is at least 10-12 years away. Weed scientists agree that glyphosate was the discovery of a lifetime, and nothing like it is on the horizon.

“There is no silver bullet coming,” said Stephenson.

Glyphosate was discovered by a team of researchers led by John E. Franz, a Monsanto chemist, in 1970. The herbicide kills plants by interfering with an enzyme required for growth. It is absorbed through its leaves and moves throughout the plant, and it is quickly broken down in the soil by bacteria.

Using herbicide-resistant crops has enabled farmers to lessen their environmental impact by reducing their reliance on plowing to kill weeds. Using minimal or no-till practices leaves soil more intact and reduces runoff from fields into waterways. It also means farmers don’t have to burn as much fuel to grow a crop, keeping food prices lower and helping farmers cut their expenses.