Game changer: Palmer amaranth vs. Cornbelt farmers
1. Fields with Palmer amaranth populations should be the last fields harvested this fall and the last fields planted next spring.
2. Mark or flag areas where Palmer amaranth plants have produced seed. These areas should be intensively scouted the following season and an aggressive Palmer amaranth management plan implemented to prevent future seed production.
3. Do not mechanically harvest mature Palmer amaranth plants with crop harvesting equipment. Physically remove the plants immediately prior to harvest and either leave the plants in the field or place in a sturdy garden bag and remove the plants from the field. Bury or burn the bags in a burn barrel as soon as possible.
4. Fields in which Palmer amaranth seeds were produced should NOT be tilled during the fall or following spring. Leaving the seeds near the soil surface increases the opportunities for seed predation by various granivores.
To address the problems for next year, Hager says include soil residual herbicides at the full recommended us rates within 2 weeks of planting, followed by post emergent herbicides before the Palmer amaranth exceed 3 inches tall. And he says a successful long term program will require more than just herbicides. His colleagues at Purdue, Bill Johnson and Travis Legleiter, have produced an 11-page guide to Palmer amaranth which they say will produce 100,000 seeds per plant when competing with a crop, and a half million seeds per plant when not in competition with other plants.
The troubling part about Palmer amaranth is its resistance to herbicides. Johnson and Legleiter say, “Palmer populations have evolved resistance to multiple herbicide modes of action, including ALS inhibitors, triazines, HPPD inhibitors, dinitroanilines, and glyphosate. The majority of populations in the South are ALS-inhibitor- and glyphosate-resistant.” Their guide contains formulas for control in both corn and soybeans, but those methods must be carefully followed to achieve success. They say, “Palmer is a very aggressive and adaptive weed, and management programs that rely on a single mode of action (such as glyphosate as the only post herbicide) will typically be ineffective at completely controlling the weed.”
Johnson and Legleiter say the key to success is the use of residual herbicides. “Residual herbicides should be the foundation of all Palmer amaranth herbicide control programs in soybean. There are a variety of residual soybean herbicides that will control Palmer amaranth at its weakest point (emergence) and substantially reduce the number of plants requiring a post-emergence application. Using residual herbicides to manage Palmer will reduce the selection pressure of the few post-emergent herbicide options.”