Palmer Amaranth Control in Corn with HPPD Herbicides

Corn is a major component of crop production systems in Mississippi.  Corn has great economic potential, but it should also be utilized as part of rotational programs for managing glyphosate-resistant (GR) Palmer amaranth.  Corn is generally more competitive with weeds than cotton or soybean.  Also, there are herbicide modes of action labeled for corn that are not available in the other crops.  Using herbicides with different modes of action is an excellent resistance management tool.

The HPPD class of herbicide chemistry (Group 27), commonly referred to as “bleachers”, are labeled for application to corn, are effective for control of GR Palmer amaranth, but are not labeled for application to cotton or soybean.  Using HPPD inhibitors in corn reduces the pressure placed on herbicide families such as the PPOase inhibitors (Group 14; Flexstar, Reflex, Valor) and chloroacetamides (Group 15; Dual Magnum, Warrant), which are commonly used in cotton and/or soybean for GR Palmer amaranth control.  Two of the more popular HPPD herbicides are Callisto and Laudis.

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 glyphosate or Liberty 280 SL.  Rates for preemergence applications range from 5.3 to 7.7 fluid ounces per acre with the higher rates needed on fine-textured soils.  Calliso is normally applied as a component of the premixes Halex GT and Lexar rather than as a stand-alone treatment.  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.

Laudis is labeled only for postemergence use 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 and glyphosate or Liberty 280 SL.  Methylated seed oil surfactant should be added with Laudis.  Laudis is also a component of the premix Capreno.  Capreno may only be applied up to the V6 corn growth stage.  When Callisto, Halex GT, Laudis, or Capreno are mixed with atrazine, then application must be made before corn reaches 12 inches.

One negative of weed management in corn is that corn is harvested early 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 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 starts to dry down.  Then GR Palmer amaranth and morningglories start growing as sunlight begins to penetrate the corn canopy, and these weeds will eventually produce seed.  Application timings for HPPD herbicides are less restrictive than for atrazine, so a later application of Callisto or Laudis could lessen problems with these weeds post-harvest.

Controlling GR Palmer amaranth in corn with alternate herbicides could potentially reduce the amount of seed in the soil seed bank.  If the level of weed seed in the soil can be reduced with effective herbicides during the corn rotation year, then the density of GR Palmer amaranth to be dealt with in the cotton or soybean year should be reduced and more easily managed.