A number of calls have come in about Palmer amaranth in soybeans and what can be done. First of all, if the plants have been sprayed with glyphosate and they did not die, then they are likely to be glyphosate-resistant and spraying more glyphosate will not kill them. Products like Pursuit or Reflex will not control Palmer amaranth plants larger than 5 to 6 inches. Options are limited to Gramoxone, handweeding, or mowing.
After corn harvest, some fields may need to be mowed or sprayed with Gramoxone to prevent late-season seed production.
Late-season management of Palmer amaranth to reduce or prevent seed production is not easy or convenient, but it will pay dividends in the future. Removing the plants prior to harvest will reduce the spread of the seed within the field and from field to field. If a single plant can produce 500,000 seeds; spread over an acre that is 10 seeds per square foot. You can quickly see how eliminating seed production and preventing the spread of seed with equipment is a tremendous help.