Corn acres have been rising steadily in the Mid-South and Southeast states. Higher grain prices have played a major role, certainly, but a less-talked-about reason has been glyphosate-resistant Palmer amaranth.

Controlling glyphosate-resistant amaranth or pigweed with different herbicides in corn can be a good strategy, but it is not without risks. And farmers need to be careful not to compound the resistance-fighting effort by using the same mode of action they’ve been using in other crops.

The problem isn’t confined to one part of the southeastern states or specific locales within them. The reliance on a limited number of postemergence herbicides greatly streamlined the management of weeds in the Southeast. But farmers are paying for it in increased headaches and cost.

“Herbicide-resistant weeds have spread very quickly in North Carolina,” said Alan York, emeritus professor of weed science at North Carolina State University. “The northern Piedmont is now the only segment of the state that has not been affected by resistant Palmer amaranth. In the southern Piedmont and the Coastal Plain, it is everywhere.”

How did producers get to this point? Glyphosate “offered so many advantages and worked so well for so long,” says York. “Even if we could have foreseen what would happen, I am not sure we would have done much different.”

York says farmers are spending $35 to $40 more per acre for herbicides compared to five years ago. “It is a problem that is here to stay. And we need to learn the lesson that it could happen again.”

Just rotating crops is not necessarily going to address the problem. Switching from Roundup Ready cotton or soybeans to Roundup Ready corn may simply increase the selection process for glyphosate.

A good resistance management strategy, York said, requires crop rotation along with use of multiple modes of herbicide action, use of residual herbicides, rotation of herbicide modes of action and integration of crop management strategies that reduce dependence on herbicides.

Larry Steckel, Extension weed scientist with the University of Tennessee, says he and fellow weed specialists noticed an uptick in the number of corn weed control calls they were receiving last spring.

The reasons for the increase were threefold. “First, there were by far more corn acres this year for weed control problems to be an issue; second, much of the corn was planted into fields with intense Palmer amaranth seed banks; and third, many of these fields did not receive an activating rainfall for pre-applied herbicides.

Controlling weeds in corn, either continuous corn or corn rotated with other crops, may require some rethinking of standard practices, such as ending weed control efforts at lay-by.

A lot of pigweed comes up late in the season, Steckel notes. “You might be able to destroy a lot of weed seed before first frost. Managing weeds after harvest may mean you have less to manage next year.”

Fall tillage may be helpful, when appropriate. (Tennessee is, after all, a leading no-till state.) “You can spray Gramoxone and a residual herbicide, or Valor or Metribuzin if the land is going into soybeans. You might spray Princep if the land is going into corn.”

Hand removal is an option but it may not be viable if your field is past the point where a lot has gone to seed. “But if you get to it while the pigweed is still flowering, the results can be good,” he said.

Drought conditions added to the problems of Tennessee corn growers. “All these corn fields on rotated land increased the likelihood of herbicide carryover injuring establishing corn seedlings. This was not a concern a few years ago when glyphosate, which has no soil activity, was pretty much all that was used.

“Now that herbicides like fomesafen (Reflex, Flexstar) are used on literally every soybean acre and many cotton acres it has to be factored in to planting decisions. Corn can be injured if it is planted within 10 months of a fomesafen application. The late-planted cotton and soybeans in 2011 forced later application dates for fomesafen. That, and early-planted corn this year, often resulted in only a nine months waiting period between a fomesafen application in 2011 and corn planting in 2012.”

Fomesafen carryover is not that common even with less than a 10-month waiting time, as wet falls will quickly degrade the herbicide. However, the fall of 2011 was dry and allowed the fomesafen to persist in a number of cases. Fortunately, most of the affected corn was able to escape injury.

Many corn acres were planted into fields with intense Palmer amaranth seed banks, he noted. So even if the corn herbicides provided 99 percent control, there was still considerable Palmer amaranth present in these fields.

“Unfortunately, in Tennessee this year, good pigweed control from a pre application in corn was more the exception than the rule,” said Steckel. “Many areas received only a few tenths of rain from late March when the corn was planted until early May.”

Though it was too dry in many fields to activate herbicides it was not too dry for Palmer amaranth to emerge.

Trying to manage such escapes was made even more complex due to a windy April leaving growers with a very small window to spray. In many areas, growers had only a handful of hours to be able to spray safely. By the time many growers could try to clean up escaped pigweed the corn was 18 inches to 28 inches tall and the Palmer was 8 inches to 24 inches in height.

“This led many of us to dive well into the labels to see which herbicides with good pigweed control could be applied to fairly large corn,” he noted. “There are a number of herbicides that can be applied to tall corn, but it typically takes two herbicides with good activity on Palmer to provide adequate control of large Palmer plants.”

Meanwhile, atrazine, which is a primary corn herbicide for Palmer amaranth, is not labeled to be used when corn is taller than 12 inches.

Fortunately, there are other herbicides besides atrazine that can be effective on large Palmer in large corn. Those include Halex GT, Capreno, Realm Q, Status, Callisto, Laudis, Impact/Armezon and Liberty (on LibertyLink hybrids).

“From a resistance management standpoint, I am a little hesitant recommending Liberty in corn when we are relying so much on it in soybean and cotton. That is why I am glad to see some new options in taller corn.”

Specialists in other states are developing additional recommendations for taming glyphosate-resistant Palmer amaranth and other problem weeds in corn.

Eric Prostko, University of Georgia Extension weed specialist, says multiple herbicide chemistries will be required to lessen selection pressure and delay the occurrence of herbicide resistance in a field. But other control strategies must also be included:

  • Tillage with a moldboard plow: While it may be of concern to no-till proponents, burying Palmer amaranth seed at least four-inches deep in the soil will provide approximately 50 percent control of Palmer amaranth.
  • Extreme rye cover crops: “A well-managed rye cover crop, planted in the fall, terminated at its maximum height, and rolled will form a dense mat of residue which will provide approximately 60 percent to 90 percent control of Palmer amaranth,” he said.
  • Decreased row widths: Decreasing row widths results in faster canopy closure and shade formation. Palmer amaranth seed requires light for germination.
  • Hand-weeding: Manual removal of escaped female Palmer amaranth plants before seed production will prevent further seed-rain into a field.
  • Mechanical cultivation: “No weeds are steel-resistant, but an aggressive combination of non-chemical and chemical control tactics will be required to manage herbicide-resistant Palmer amaranth,” said Prostko.