Palmer amaranth (Amaranthus palmeri) is a competitive and challenging weed to manage in dry bean. Palmer amaranth can emerge season-long in dry bean, and is resistant to several herbicide modes of action in western Nebraska. While glyphosate-resistant Palmer amaranth is becoming more common in western Nebraska, ALS-resistant biotypes are more widespread. Most dry bean growers are using Raptor® plus Basagran®, or applying them together as the premixed product Varisto™, for post-crop emergence (POST) weed control. However, Basagran is naturally ineffective against pigweed species, and as Raptor is an ALS-inhibiting herbicide, none of these options are effective for controlling ALS-resistant Palmer amaranth.

Reflex® can be applied under sprinkler irrigation in western Nebraska and is the only effective POST option for controlling ALS-resistant Palmer amaranth. However, studies conducted at the Panhandle Research and Extension Center in Scottsbluff found Reflex to only be an effective POST option when following pre-crop emergence (PRE) herbicide programs.

With difficulty controlling emerged Palmer amaranth in dry bean, residual herbicides applied at or before planting are critically important. Research at the Panhandle REC comparing PRE and pre-plant incorporated (PPI) residual herbicide programs found that Eptam® + Sonalan® applied PPI or Prowl + Outlook applied PRE provided good early season control of Palmer amaranth (Figure 1). Eptam + Sonalan controlled Palmer amaranth later into the season than Prowl + Outlook.

Figure 1. Prowl + Outlook applied PRE (left), Eptam + Sonalan applied PPI (right). Both plots did not receive any POST herbicides.

• Raptor + Basagran POST programs are not effective at controlling ALS-resistant Palmer amaranth.
• Reflex is an effective POST control option. It is only labeled for use under sprinkler irrigation and must be applied at or before Palmer reaches four true leaves.
• Eptam + Sonalan applied PPI provided control of Palmer amaranth later into the season than Prowl + Outlook; however, both provided excellent control.

Residual herbicides are critically important in controlling Palmer amaranth in dry bean. If Palmer amaranth is the main species of concern, a PPI program of Eptam + Sonalan would be the best option. Consider using Reflex POST for sprinkler irrigated fields with emerged Palmer amaranth.