When glyphosate-resistant Palmer amaranth pigweed swelled from rare to everywhere in west Tennessee in June 2008, University of Tennessee weed Larry Steckel was as surprised as anyone.

“It floored me. We found it over such a large geography. We received phone call after phone call for two months.”

Steckel, speaking at Cotton Incorporated’s Crop Management Seminar in Tunica, Miss., says resistant Palmer pigweed is now infesting 10 counties in Tennessee, two counties in Mississippi, 16 counties in Arkansas “and they’re fairly confident they have it in Louisiana as well.”

The level of resistance has increased as well, according to Steckel. “In 2007, we were seeing resistance at 44 ounces of glyphosate, but we could get control at 88 ounces. That all changed in 2008. We went well north of 152 ounces of glyphosate without seeing any stunting.”

Steckel’s first call on a glyphosate-resistant pigweed in 2008 was for a 148-acre cotton field in Lauderdale County, Tenn. There were two spots in the field, roughly seven rows wide and 120 feet long. In between the two spots were scattered resistant plants.

Ten days after application of 120 ounces of glyphosate, there was little effect on the pigweed in the field, Steckel said.

The field had been in corn in the previous year, “which concerns me. Our recommendation for resistant weeds is to rotate crops so we can hit them with a different herbicide. The grower did that. The problem is that between the time we cut our corn in August and when we get our first frost, we can have a heck of a population built up. We have to manage it not only in the corn, but after we harvest the corn.”

If resistant pigweed is in a small isolated spot in the field, one solution may be to take out the cotton with the weeds, an action taken on the aforementioned field with an application of Gramoxone and Caporal.

“While we’re in the early stages, we could just nip it in the bud, take them out, call it a loss (for the cotton), watch the field and monitor the situation,” Steckel said.

Steckel said that fortunately, there were only a couple of cases where resistant pigweed “took the whole field. But to me, more of that is going to come in the future.”

One of those cases occurred in a soybean field in Shelby County, Tenn., where resistant pigweed was apparently spread by a combine during 2007 soybean harvest. “The field had been in continuous Roundup Ready soybeans. The farmer is going to have to do something different, maybe go to LibertyLink soybeans.”

On a cotton field in Shelby County, weed scientists put Envoke over-the-top on cotton, which had no impact on the pigweeds. “So not only was it glyphosate-tolerant, it was ALS-tolerant as well.”

As for recommendations, Steckel says he drew heavily from North Carolina State University weed scientist Alan York and Stanley Culpepper, University of Georgia Extension weed scientist “where most of the glyphosate-resistant Palmer pigweed problem is now.”

On the cotton field, Culpepper suggested a salvage treatment of Valor, MSMA and Direx run through the hoods. After that, “at least we could see the rows,” Steckel said. “It did not take out the pigweeds in the row. By defoliation, a lot of them were big, put a lot of seed on and started lodging the row. The treatment was over $60 an acre and we still lost a lot of yield due to competition.

“This is what we’re going to see in a lot of fields next year in the Mid-South. There are a lot of spots that farmers ignored. They harvested through those spots and we’re going to see a lot of that seed spread.”

One control measure is the time-honored tradition of hand labor and/or chopping crews, notes Steckel. “We have had consultants pulling up weeds, and they talked to their farmers about doing the same.”

To manage resistance on your farm, Steckel suggests “going with early pre-emergence and early post-emergence herbicides and combinations of those. We’re not going to get by with one pre-emergence herbicide, especially if we know we have resistance.”

Chemical options for cotton include an application of 2 ounces of Valor or a pint of Reflex 30 days to 14 days before planting. “The pint of Reflex is currently labeled only in Arkansas, but Syngenta is working on a full label. Southeast growers can put out a pre-emergence application of Reflex, but we can’t do that safely here because of our soils.”

Steckel says growers with resistant pigweed can also go with a pre-emergence application of either Caporal, Direx, Cotoran or Prowl, followed by an early post application of Dual and glyphosate. Layby options include either Valor, Reflex, Caporal, Suprend, Layby Pro or Direx plus MSMA.

Steckel added that growers should get used to seeing cotton injury from herbicides as they move away from a glyphosate-only program.

Give some thought to crop rotation, too, Steckel says. One possibility is wheat double-cropped with soybeans. Burndown with Gramoxone Inteon after wheat harvest combined with a pre-emergence herbicide.

Rotating to corn can expand herbicide options to products such as atrazine, Degree, dicamba, Callisto, etc. But corn may not always be the best option for managing resistance in west Tennessee, Steckel noted. “There is a reason why we don’t plant corn on a lot of these acres, particularly with fertility prices and seed costs. If it rains, we can yield 150 bushels, but it doesn’t we could get only 70 bushels. So it’s not a good option on our non-irrigated acres.”

On the other hand, growers should explore grain sorghum as a rotational partner, Steckel said. “We do have some herbicide options for resistant pigweed.”

Steckel says some growers in west Tennessee who’ve had resistant Palmer pigweed have had success with LibertyLink cotton. “But they jumped on the pigweed when it was small. The difference between the size that Ignite can control Palmer pigweed, which is 4 inches tall, and the size where it’s too big to control, might take place over 72 hours. As big as a lot of our growers have gotten, that’s going to be a challenge.”

According to anecdotal information gathered by weed scientists, glyphosate-resistant Palmer pigweed is an immediate threat on about a million acres cross the Southeast and Mid-South.

Steckel says another pigweed species, common waterhemp, is starting to emerge in Mid-South fields. “We’re seeing it in Arkansas and in Lauderdale County, Tenn. It’s muddying up the situation. Farmers say they have pigweed not dying from glyphosate, but it doesn’t look like Palmer. To tell you the truth, it’s hard to tell them apart.”

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