By ANN HINCH
Assistant Editor

URBANA, Ill. — If kudzu is the weed that ate East Tennessee, Palmer amaranth – aka “Palmer” or “Palmer pigweed” – may be its counterpart in West Tennessee. And it’s moving fast, rather like waterhemp in Illinois.

For years, genetically modified soybeans and cotton enjoyed preferred growing conditions thanks to an inserted gene that rendered them immune to the herbicide glyphosate, most commonly associated with Monsanto’s Roundup. Farmers and other agronomists have noticed recently, though, that more of the weeds once vulnerable have developed a glyphosate resistance (GR) and are growing tall alongside cash crops designed to resist the chemical.

Nine years ago, marestail in Lauderdale County, Tenn., showed GR. Dr. Larry Steckel with the University of Tennessee-Jackson Department of Plant Sciences said herbicide resistance later showed up in Palmer, then giant ragweed and common ragweed, as well.

He told attendees at last week’s annual AgMasters Conference at the University of Illinois he’s had a difficult time convincing Tennessee growers that “the total post-emergence era is over.” The more they deal with Palmer, however, he said the more they’re becoming believers. Even PPO herbicides will not control Palmer taller than 4 inches, he said; Ignite won’t control it over 6 inches.

Growers of dry-land cotton have come to accept losing upwards of 25 percent of their crop to an Ignite-based system because it’s the only way to control the rapidly-growing weed. Besides, he said many had already gotten used to losing up to 20 percent of their cotton because of horseweed.

In Illinois, waterhemp has become perhaps the most problematic broadleaf weed in recent years. Steckel explained in Tennessee, Palmer chokes out waterhemp, so perhaps Midwest growers could learn from their Southern counterparts’ struggles with the weeds.

GR is showing up in Italian ryegrass and Johnsongrass growing in corn in Louisiana, Mississippi and Alabama. These haven’t made it to Tennessee, but Steckel looks for them to spread, citing how fast Palmer has taken over the western part of the state through wind patterns (it’s not plowing, since Tennessee is largely a no-till state, he explained) – considering it’s originally native to the Southwest desert.

“We ought to be growing it as a crop and not fighting it as a weed,” he quipped, showing a photo one farmer had taken of a Palmer stalk’s five-foot underground taproot. Palmer, he said, is clearly the lead weed in West Tennessee. (Row crops are not widely grown in East Tennessee, which has mountainous terrain.)

Palmer the pest
Steckel’s two-hour presentation included one tale after another of farmers he’s met fighting various battles – most losing – with resistant weeds, especially Palmer. He said it spread “unbelievably fast” across Southern states from just a few years ago and was detected in a Middle Tennessee field in 2009, and in Kentucky this year. Treating it with more Roundup only made it stronger.
“I wish I had a nickel for every time I’ve heard, ‘I thought I had one more year,’” he said of stricken growers, though he pointed out usually the first year of Palmer infestation is the worst, since growers are at least forewarned for later seasons.

“Until you have actually experienced this – it’s just devastating,” he added, describing one case in which a grower had to abandon over 200 acres in a field because the Palmer made it impossible to farm. In another, so much Roundup had to be applied that it damaged Roundup Ready soybeans more than the weeds choking them.

Steckel said male Palmer produces pollen and the female produces seed – and each makes “a lot.” One study showed the resistant enzyme is being passed through pollen, since 60 percent of the offspring of resistant male/susceptible female were resistant, compared to 20 percent from susceptible male/resistant female.

There are other herbicides, but like glyphosate, he’s afraid of Palmer and other weeds developing a high resistance. PPOs still have a fighting chance for now. Atrazine, a controversial chemical being re-reviewed by the U.S. Environmental Protection Agency, is effective, and Steckel said unlike glyphosate it does have a fitness penalty – if weeds develop resistance, one can stop using atrazine for a while and the resistance will slack off. GR apparently doesn’t go away.
There are chemicals that will treat Palmer as it grows, up to a certain point, but he said the key seems to be timely application. On growing-degree days, it can gain up to 2.5 inches in height.

“You’ve got hours, not days, to get that field sprayed,” he said. “You can’t spray it too soon.”

Other fighters

Last year, dealers sold out of LibertyLink soybean seed before Christmas, as it has resistance to Ignite, touted to control glyphosate-resistant weeds. Steckel said he even knows growers going back to old soybean varieties – as in nearly 30 years old, including a 1982 variety from an Ohio provider – so they can use other herbicides.

In Tennessee, he said Ignite has largely replaced glyphosate for treating cotton fields, and soy fields have largely gone to a fomesafen-based system. Some of the pre-emergence treatments Steckel favors for cotton, in varying combinations, include Boundary, Prefix, Valor, Envive, Authority and Valor XLT, though he warned the last two require a 12- to 18-month rotation waiting period on replanting cotton.

Post-emergence, he named Flexstar, Cobra and Blazer, which work best on Palmer between 1-4 inches high. He showed a photo of a field covered in Palmer, half of which was treated with Flexstar at 3-inch growth and the other, at the 6-inch stage – roughly two days’ difference in growth. The 3-inch had been killed, but the 6-inch had gotten even taller – “Makes all the difference in the world,” he said.

For soybeans, he favors Sharpen or dicamba plus glyphosate or Gramoxone Inteon for burndown 14-30 days before planting. For pre-emergence, he likes Valor and Dual MAGNUM, Prowl and Authorty MTZ; if that works but post-emergent beans need more treatment, he favors Dual MAGNUM and Flexstar.
If that doesn’t work for soybeans, he advises an early post-treatment of Flexstar, followed by Cobra, Ultra Blazer or Ignite before the weed reaches 4 inches.

He added that the upcoming Dow Herbicide Tolerance DHT trait will be able to tolerate 2,4-D applications. According to Dow AgroSciences, the corn seed could be available in 2012 and soybean, as early as 2013.

One more method Tennessee growers have re-instituted are chopping crews, which charge $50-$100 per acre to dig out large weeds like Palmer and haul them away. Between the cost for this, increased equipment, herbicide and application costs to fight GR weeds, as well as lost yield and acreage, Steckel estimates Tennessee growers spent and lost $49.4 million in soy and cotton this year.

Should waterhemp (or something harsher) spread similarly in Illinois in 2011 or after, he estimates similar types of losses for soybean growers upwards of $762.6 million a year – and as of February 2010, GR waterhemp has been confirmed in several states, including Illinois and Iowa.