Glyphosate-tolerant pigweed confirmed in West Tennessee

Glyphosate-tolerant Palmer pigweed has been found in west Tennessee’s Lauderdale and Crockett counties. The announcement comes on the heels of a similar finding in Georgia pigweed earlier this summer.

“The fields were in continuous, Roundup Ready cotton for many years — at least from the late 1990s on,” Larry Steckel, Tennessee Extension weed scientist, said Sept. 23. “Roundup was the primary weed control on them although there have been some post-directed chemistries on them as well.”

Were rates and sprayings properly applied?

“To my knowledge, correct, full-label rates were used. I’m very familiar with the farmers involved. They’re very good at growing crops and don’t cut rates. I’m confident this wasn’t human error.

“Nowadays, we’re putting Roundup on everything. It’s led to unprecedented selection pressure. We were bound to find genes that could handle the chemistry.”

Called to the fields in 2004, Steckel said it was immediately evident something wasn’t right. “The way it looked — live pigweeds side-by-side with dead pigweeds at the same height — raised a red flag with me. When I checked the fields, pigweed was all that wasn’t being controlled. My first thought was, ‘Well, this could be the real deal.’”

There were plenty of pigweed in both fields. However, that alone didn’t cause Steckel much worry. “Western Tennessee is covered up with Palmer pigweed. It isn’t uncommon to see fields with a bunch of it. I get called to a lot of fields on suspicious weeds. After investigating, most of the time the escapes are due to rain after application, surfactant issues or something else. But none of that applied here.”

This past spring, Steckel and colleagues decided to put out a number of trials: two in the questionable fields and two placed randomly in the counties. Normally, Palmer pigweed less than 6 inches tall can be “smoked” with a half rate of glyphosate, said Steckel.

“So in these tests, we looked at a half-rate, a full rate, a double rate and a 4X rate. At the two random sites, we got complete control on everything with the low rates.”

In the two suspect fields that wasn’t the case. “At the half-rate of Roundup WeatherMax, control was around 50 percent. At the full rate (22 ounces), control was around 80 percent. At the 44-ounce rate, we still had some escapes. At the 4X rate (88 ounces), everything was killed.”

Tom Mueller coordinated greenhouse and laboratory studies of the tolerant pigweed populations. “In some ways the Palmer pigweed appears to be similar to glyphosate-tolerant horseweed (marestail),” said Mueller in a press release. “All the treated Palmer pigweed plants look the same for two or three days after application; they all wilt and turn yellow. However, at about four days after spraying, the tolerant plants stop wilting and start new growth from lateral buds. Our preliminary laboratory analysis indicates the mechanism of action, or how the plant tolerates the glyphosate, appears to be the same in the Palmer pigweed and in the glyphosate-tolerant horseweed.”

In light of the test results, what are Steckel’s recommendations?

“First, producers need to get more chemistry in the tank, more modes of action. And that’s been already been happening.

“I just did an informal survey of some retailers and, in the last year, they believe around 90 percent of our cotton had a pre-emerge (herbicide) put on. Primarily, the reason for that was control of glyphosate-resistant horseweed.

“Dual over-the-top of cotton postemergence will be a terrific tool. We’ll be preaching that.

“Most importantly, Roundup rates shouldn’t be cut. Producers must use the full rate and get good coverage.”

Could the finding impact no-till acres?

“With glyphosate-resistant horseweed we’ve already seen a reduction in no-till acres. However, as successful as we’ve been with using pre-emerge herbicides, I think we’ll see no-till acres rebound — especially when you consider the cost of diesel. Even with this new threat, I see that happening.”

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