Cotton Damage from POST Applied Residual Herbicides

2011 is quickly shaping up to be the year in which nothing comes easy.  Weather conditions are continuing to cause issues with the 2011 cotton crop.  Scattered hail damage from storms over the past couple of days has been reported from the north Delta as well as extreme northeast Mississippi.  In addition, over this past weekend and early last week, high winds and blowing soil particles caused additional crop injury in the upper Delta as well as in northeast Mississippi.  Given the lateness of the crop, extended periods of dry weather, and the challenges growers have faced thus far, nearly everyone is already hoping for a fall similar to that of 2010.  Although this year has proven to be extremely challenging, nearly 60% of the Mississippi is rated as being good to excellent.

The development and spread of glyphosate-resistant weed species, particularly Palmer amaranth, has forced nearly all growers to at least consider the use of residual herbicides.  Once the crop has emerged, the options for broadcast applied residual materials becomes fairly short.  Metolachlor (Stalwart, Parallel, etc.), s-metolachlor (Dual Magnum, Medal, Cinch, etc.) or acetochlor (Warrant) are usually the products considered for residual pigweed control.  Generally, these products are applied with glyphosate (Sequence) to Roundup Ready Flex cotton or Ignite on Liberty Link cotton in order to gain broad spectrum weed control as well as residual control of problematic weed species in a single sprayer pass.  However, it is important to keep in mind that crop injury is possible when mixing these residual materials with glyphosate or Ignite.  The photos below were provided by Mr. Charlies Stokes, Area Agronomy Agent in northeast Mississippi, and show the type of crop injury that can occur when applying residual products with glyphosate or Ignite.  The damage shown in these photos occurred following an application of Roundup + Warrant to Flex cotton; however, this type of injury can also occur when metolachlor or s-metolachlor is mixed with glyphosate or Ignite as well.  Damage usually appears as somewhat round areas (in some cases these areas run together to form larger areas of damage) of necrotic tissue on leaves that were present at the time of the herbicide application.  While this injury is unsightly and can cause some sleepless nights, the plant will usually grow out of the damage with minimal impact upon yield.  Given good growing conditions, injury generally becomes less pronounced in the 2 – 3 weeks following application.  Although we have gotten into the mindset that any crop injury is unacceptable, we must weigh the tradeoff between potential crop injury and the benefits of residual weed control.  In most cases, some crop injury can be traded for adequate control of pigweed and other species.  Given alternative glyphosate-resistant pigweed control methods in Flex cotton (plowing, chopping crews, hooded sprayers, etc.), some crop injury may become the rule rather than the exception.