2,4-D and Dicamba Reduce Control of Italian Ryegrass

Glyphosate-resistant (GR) Italian ryegrass has been documented in 32 counties across Mississippi.  Multiple resistance to glyphosate, ALS- and ACCase-inhibiting herbicides is also common in the Delta.  Only seven commercial herbicides are recommended by Mississippi State University for control of GR Italian ryegrass, and the only postemergence herbicide options are Select Max (clethodim) and paraquat.

While GR Italian ryegrass is a driver weed for winter burndown applications, it does not exist in a monoculture.  Cutleaf evening-primrose, curly dock, lesser swinecress, and horseweed (marestail) are common winter annual broadleaf weed species that are difficult to control with glyphosate alone.  It is well-documented that control from ACCase herbicides such as Select Max can be reduced when other herbicides are added to the spray tank.  However, because of the mix of weed species present in most Mississippi fields, herbicide applications targeting a single weed species are difficult to justify.  Therefore, burndown applications in Mississippi often consist of glyphosate plus an auxin herbicide (2,4-D or dicamba) plus Select Max targeting both GR Italian ryegrass and winter annual broadleaf weed species.

At the recent Southern Weed Science Society annual meeting in Birmingham, AL, MSU graduate student Garret Montgomery reported results of research evaluating combinations of Select Max with 2,4-D Ester and/or Clarity.  Control of GR Italian ryegrass was reduced with glyphosate plus Select Max at 8 and 12 oz/A when combined with 2,4-D Ester and/or Clarity.  The most effective control of GR Italian ryegrass was observed with the highest rate of Select Max (16 oz/A) plus glyphosate plus 2,4-D or Clarity.  2,4-D Ester, Clarity, or a combination of both was required to maximize control of winter annual broadleaf weed species such as henbit, shepherd’s-purse, and cutleaf evening-primrose.  Therefore, when herbicide combinations targeting GR Italian ryegrass and winter annual broadleaf weeds are needed, 2,4-D Ester and Clarity should be added to glyphosate and Select Max at the full use rate of 16 oz/A for optimum control of all species.

It should also be noted that a single herbicide application did not completely control GR Italian ryegrass and all winter annual broadleaf weed species.  Multiple applications of Select Max within a growing season are not recommended as the potential for ACCase resistance development increases.  When a sequential herbicide application is required, paraquat (4 pt/A of Gramoxone SL or 2.67 pt/A of 3-lb paraquat) should be applied 21 to 28 days following Select Max application.