The rain this spring has made for a frustrating year for many producers trying to get their soybean crop planted in a timely manner. On a positive note, there has been plenty of rainfall to activate the residual herbicides applied prior to planting, and many fields are clean. In some instances, however, the excessive rainfall has limited applications of these residual herbicides and, consequently, Palmer amaranth is now beginning to emerge in many fields.
Where Palmer amaranth has emerged, it is imperative to make a timely application of a postemergence herbicide. Most Palmer amaranth in Mississippi is resistant to both glyphosate (Roundup/Touchdown) and many ALS-inhibiting (Classic/Scepter) herbicides.
Postemergence herbicides such as Ultra Blazer, Cobra, Flexstar, and Prefix have been the most consistent performers on Palmer amaranth 2 to 3” in height. Prefix has the added benefit of providing additional residual activity as well. Liberty is also an excellent choice for Palmer amaranth control but may only be applied to Liberty Link soybean. Again, timing is critical with all of these herbicides.
The bigger Palmer amaranth gets the harder they are to control. Palmer amaranth has been shown to grow up to 1.5” per day and at this rate a 2” Palmer amaranth today could be too large to control in as little as two days with many of these postemergence herbicides. More information on the control of glyphosate- and ALS-resistant Palmer amaranth in soybean is available at http://msucares.com/pubs/infosheets_research/i1352.pdf