A number of calls have come in about options to control large Palmer in full-season soybeans. The beans are 30 to 36 inches tall and the Palmer amaranth plants are poking through the canopy. In most cases the field has been sprayed with glyphosate. There are no good options. Late-season Palmer amaranth management needs to focus on stopping seed production, and chemical choices are likely to be ineffective.
The Palmer is probably glyphosate-resistant (since sprayed once already and surviving) and a number of the populations are resistant to Group 2. This leaves the contact herbicides, PPO’s (Group 14). With less than 20% of the weed above the soybean canopy, you will not do much more than burn some of the upper leaves and the Palmer plants will likely continue to grow taller. Likewise, the soybean canopy will be compromised by the herbicide, so you will lose the effect of the soybean shading. Also keep in mind the Reflex application needs to be made before bloom, and the replant to corn next season is ten months; Ultra Blazer has a 50 pre-harvest timing; and Cobra has 45 day pre-harvest interval.
Allowing the plants to remain in the field will mean more seeds back into the seedbank and more headaches next year. Also, if the plants remain in the field and then they move through the combine, those seeds are moved over a much larger area and in many cases, moved to another field. So my advice is not let them produce seed. This means hand pulling the plants, and if the plants have seedheads on them, remove the plants from the field. The longer the plants remain in the field the more seeds they will produce.
