At the Southern Weed Science Society meeting in January, the first 10 presentations were about glyphosate-resistant (GR) Palmer amaranth.

There is likely no one in the Mid-South or Southeast not aware of the growing problems with this weed. It has evolved into a top priority for many farmers, consultants, distributors, industry reps and university researchers. As of last fall, GR Palmer amaranth was a problem in over 130 counties across the South.

Countless articles have been written about managing GR Palmer amaranth in the major Southern row crops over the last couple of years. University of Arkansas weed scientist Ken Smith discussed the LibertyLink technology in the Delta Farm Press .

LibertyLink is the only new herbicide-resistant crop technology we have to combat GR Palmer amaranth. Most other control strategies are designed around older herbicides, especially residuals, or cultural management practices such as tillage, row spacing, and crop rotation.

In Mississippi, corn has become a major component of our crop production systems. Production acres began a major shift to corn in 2007, and we should take advantage of corn’s economic potential and utilize corn as a part of our rotational programs for managing GR Palmer amaranth.

Corn is generally much more competitive with weeds than cotton or soybeans. Also, there are herbicide modes of action labeled for corn that are not available in other crops. Using herbicides with different modes of action is an excellent resistance management tool.

Controlling GR Palmer amaranth in corn with alternate herbicides could potentially reduce the amount of seed in the soil seed bank, which is the predominant source of the weeds. If the level of weed seed in the soil can be reduced with highly effective herbicides during the corn rotation year, then the density of GR Palmer amaranth that we have to deal with in the cotton or soybean year should be reduced and more easily managed.

University of Tennessee weed scientist Larry Steckel said in a Delta Farm Press article (Corn: resistant Palmer amaranth) that “rotating to a Roundup Ready corn hybrid and spraying glyphosate plus 2 quarts of atrazine was not going to cut it as a solid way to reduce the Palmer amaranth soil seed bank.” I completely agree with this statement. If you rotate the seed in your planter, but don’t rotate the herbicides you use, then the problems with glyphosate resistance will only continue to increase.

In a corn-cotton or corn-soybean rotation, we must exploit the herbicides we have available during the corn rotation year. There is a group of herbicides, commonly referred to as bleachers, labeled for application to corn that are extremely effective for control of GR Palmer amaranth. These herbicides are called bleachers because susceptible weeds turn white following a postemergence application. Three bleaching herbicides are Callisto, Laudis, and Impact.

Callisto has been around for several years and is marketed by Syngenta Crop Protection. Callisto may be applied preemergence to corn or postemergence up to the V8 corn growth stage. Most postemergence applications of Callisto are made at 3 fluid ounces per acre and tank-mixed with atrazine and/or glyphosate. Rates for preemergence applications range from 5 to 7.7 fluid ounces per acre with the higher rates needed on fine-textured soils.

Cotton and soybean may both be planted 12 months following one application of Callisto. But, if pre- and postemergence applications are made, then the rotation intervals increase to 18 months. The active ingredient in Callisto is also a component of the premixes Halex GT and Lexar. Halex GT also contains glyphosate and Dual II Magnum, while Lexar is a combination of Callisto, Dual II Magnum, and atrazine. These premixes both contain herbicides that provide residual control of GR Palmer amaranth, so they are good choices for attacking GR Palmer amaranth with multiple herbicide modes of action in a single application. Callisto alone is also very effective on crabgrass, morningglories, and prickly sida (or teaweed).

Laudis is a relatively new bleaching herbicide marketed by Bayer CropScience. It is labeled postemergence up to the V8 corn growth stage. The application rate for Laudis is 3 fluid ounces per acre, and as with Callisto, it is often tank-mixed with atrazine or glyphosate. Rotation intervals are slightly more lenient than for Callisto. Soybeans may be planted eight months and cotton 10 months following a Laudis application.

Last year was the first time I had worked with Laudis, so I do not know as much about it as Callisto. However, it performed well for me in 2009. In a trial I conducted last year with Daniel Stephenson, weed scientist at the LSU AgCenter, Laudis controlled Palmer amaranth and morningglory as effectively as atrazine. We also observed good control of johnsongrass when Laudis was tank-mixed with glyphosate or Ignite.

Impact is sold by AMVAC Chemical Corp. In contrast to Callisto or Laudis, Impact may be applied up to 45 days prior to corn harvest. Application rates for Impact range from 0.5 to 0.75 fluid ounce per acre, but a supplemental label is available for 2010 allowing Impact to be applied at up to 1 fluid ounce per acre. Cotton or soybeans may be planted nine months after an Impact application.

I have never worked with Impact in my corn plots at Stoneville, but after looking through some different state weed control guides, the consensus seems to be that Impact controls GR Palmer amaranth as well as Callisto or Laudis and may provide slightly better annual grass control, particularly on fall panicum, goosegrass and crabgrass.

One downfall of weed management in corn is that most years we harvest corn in late summer or early fall and weeds have an opportunity to grow and produce seed after harvest. Post-harvest weed seed production will contribute to the soil seed bank and cause problems the following year.

Some weeds will germinate and grow following harvest, but many of them, particularly pigweeds (GR Palmer amaranth) and morningglories, germinate in the crop after the residual control from atrazine plays out. The corn canopy keeps these weeds suppressed until the crop reaches physiological maturity and starts to dry down. Then the pigweeds and morningglories start growing as sunlight begins to penetrate the corn canopy, and these weeds will eventually produce seed.

Application timings for the bleaching herbicides are less restrictive than for atrazine, so a later application of Callisto, Laudis, or Impact should help prevent these weeds from causing problems post-harvest. They will also control late-emerging morningglories that can wrap up in corn stalks and interfere with harvest.

In areas with GR Palmer amaranth, rotating out of cotton or soybeans to corn and using the unique herbicides labeled for corn is an excellent strategy for long-term management of GR Palmer amaranth. Compared with corn, herbicide options for GR Palmer amaranth in cotton and soybeans are more limited and often less reliable.

Callisto, Laudis or Impact tank-mixed with glyphosate and atrazine should provide excellent broad-spectrum weed control while serving as a resistance management tool for GR Palmer amaranth.

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