First suspected resistant pigweed found in Louisiana

Louisiana's Catahoula Parish is the latest site for suspected glyphosate-resistant pigweed in the Mid-South.

“The pigweed is in a sizable field and went through multiple applications of glyphosate,” says Sandy Stewart, LSU AgCenter cotton specialist. “There wasn't satisfactory control and there were some surviving plants from last year. Those are things that immediately send up red flags.”

There are as much as 20 to 40 acres where the pigweed is a problem. If it proves resistant after lab and hereditary tests, it will be the first confirmed in Louisiana.

Stewart was called to the field in early May. A 25-ounce application of Touchdown had already gone out and the pigweeds weren't dying as expected.

Strip applications of Roundup WeatherMax were made in the field at 33 ounces, 55 ounces, 77 ounces and 88 ounces. Although there was more activity as the rates increased, there were still surviving plants.

“When a 4X rate won't kill a weed — and some of these pigweeds are still plenty strong — something is obviously wrong.”

Even identifying the pigweed hasn't been easy. The genus, wide-ranging and diverse with over 70 types of pigweeds, is notoriously enthusiastic about swapping genetics.

“Pigweeds will hybridize,” says Scott Martin, Syngenta research and development scientist. “They interbreed easily and frequently. That may be why it's so difficult to identify this weed.”

Unfortunately, soil and plant collections haven't yielded enough for botanists to render a verdict on. For some reason, female plants are especially hard to come by.

At Syngenta's Vero Beach, Fla., research facility, “we're growing some seed in the greenhouse. Some are flowering. But to make the ID, we need female and male plants since it's apparently a dioecious species. We'll be traveling back to the field and gather more soil and plants for further study.”

The pigweed definitely isn't Palmer amaranth, says Stewart. There is a possibility it's tall waterhemp. If so, Louisiana would join Missouri as the only other Mid-South state with suspected glyphosate-resistant waterhemp.

Chuck Foresman isn't surprised at the slow identification. Precision is needed to make such identifications and sometimes that takes time, says the Syngenta manger of weed resistance strategies.

Foresman points to a tour he recently made of Lonoke, Ark., greenhouses where seed gathered from Newport, Ark.-area glyphosate-resistant Palmer amaranth was being grown (see

“The same mother plant had produced some 25 offspring. But the plants looked very different from each other. You'd never guess they were related.

“With that pigweed diversity comes larger opportunities for mutations to occur. That can mean resistance to various herbicides being imparted.”

It isn't just glyphosate that pigweeds are thwarting. “At the top of that list are the ALS herbicides. Some parts of the country have issues with PPO herbicides. This year, I've heard of numerous cases where glyphosate isn't working as well as it once did. That tells me this resistance problem is real and growing.”

Foresman says Louisiana isn't the only state to have found suspected resistance for the first time in 2007. “Other states are looking at pigweeds and are running appropriate studies to see if they're truly resistant. Resistance must be heritable and there must be field failure. In these cases, people are being diligent and making sure they have resistance and aren't overstating (the case).”

Stewart wants to know “if this is true resistance. Has there been a field failure coupled with proven resistance in the lab? In the field, there are so many things that can affect control. Just because weeds survive an application doesn't mean they're glyphosate-resistant.

“So they'll grow some in a greenhouse and document what rate of glyphosate it takes to kill the plant. Or, in the worst resistance cases, there isn't a fatal rate.”

After that, researchers can begin studying how to control the resistant weeds.

Martin says everyone is now aware of the resistance possibilities. With resistant marestail and the resistance situation in the Southeast, “we're keeping our eyes peeled for anything that looks out of place when we're walking fields.”

He says growers should focus on several things. “The main thing is to make sure to use no less than labeled rates. Consult the labels and make sure.

“And be alert in the field for any changes in susceptibility. Potential resistance should always be in the back of our minds.

“Alternate chemistries and add residual products when possible. Anytime multiple chemistries are used it helps forestall the advent of resistance.”

How did the suspect pigweeds quickly spread on so much acreage? One possibility is a flood. Pigweed plants make 300,000 to 400,000 seed per plant. The seed is tiny and black — “like spider eyes,” says Donnie Miller, LSU AgCenter weed specialist based at the Northeast Research Station in St. Joseph, La. “They're easily be carried by water. So the flood in the field is a plausible explanation for how it spread.”

Foresman says he was recently in a Minnesota field “where you could see exactly where the combine had gone through. The tailings kicked out the back end had a high concentration of resistant weed seed. Now, there are strips through the field that are uncontrolled. There are a lot of methods for this weed seed to move.”

Growers should pay attention to what their fields look like at harvest, suggests Foresman. “If there are weeds in the field, they need to pay careful attention for the sake of next year's weed management plan. There are things we need to pay attention to that we may not have been carefully monitoring in the past.”

It doesn't take much for pigweeds to get a toehold. A pigweed plant that's 4 inches tall can put out a seed head, says Miller. And plants from that seed will emerge the next year.

“It's important to watch sanitation on these sites,” says Foresman. “If you're growing a glyphosate-tolerant crop, use a residual herbicide. Just use it. Don't go through the anxiety and worry — just use it. That's a very good weed management practice.

“We have no new mode of action. It hasn't been found. The tools we have today must work for a long time. It can be done, but a management plan is needed and a residual is a good place to begin.”

The answer is…

Shortly before publication deadline, the Louisiana pigweed was identified as tall waterhemp. “The LSU botanist has been working with about 16 plants collected from the Catahoula Parish field,” says Miller. “Of those, only three turned out to be females — and they headed out a lot later than the others. The first plants I brought the botanist were males. To have a definite ID, he needed a female and seeds.

“We'll now try to confirm the resistance using required methods.”

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