ALS-resistant Palmer amaranth is difficult to control in dry beans. Currently labeled preemergence and preplant-incorporated herbicides are effective for controlling ALS-resistant Palmer amaranth. However, a postemergence herbicide is generally needed to provide season-long control.

ALS-resistant Palmer amaranth is resistant to Pursuit and Raptor, while Basagran is generally ineffective for controlling pigweed species. Varisto, a prepackaged mixture of Raptor and Basagran, also is ineffective. Reflex is an effective herbicide for Palmer amaranth control, but because of label restrictions, many dry bean farmers are unable to apply Reflex within their rotation.

If dry bean growers are managing Palmer amaranth and are unable to apply Reflex, a post-applied group 15 herbicide (such as Outlook) may be the best alternative. Group 15 herbicides will not control already-emerged weeds, but they will control Palmer amaranth as it germinates.

Group 15 herbicides applied sequentially can provide overlapping residual activity and supplement weed control achieved by postemergence herbicides. Outlook currently is the only labeled group 15 herbicide for postemergence application in dry beans.

Post-application timing

Field experiments conducted at the Panhandle Research and Extension Center in Scottsbluff, Neb., in 2019 and 2020 demonstrated that sequential applications of Outlook can provide similar control of Palmer amaranth as Reflex applied postemergence.

In these studies, all herbicide programs contained a preemergence application of Prowl H20 (32 fluid ounces per acre) plus Outlook (13 fluid ounces per acre). Postemergence herbicides differed among herbicide programs and application timing (see table).

Outlook applied at the first trifoliate (V1) provided season-long control of Palmer amaranth in both 2019 and 2020. When Outlook was applied at the third trifoliate (V3), season-long weed control was achieved in 2020, but not in 2019.

Palmer amaranth emergence occurs throughout the season, generally from early May to late August. Consequently, weed control programs targeting Palmer amaranth need to provide season-long weed control, or weed control from planting through canopy closure.

Weather conditions in 2019 likely led to a faster breakdown of residual activity from applied preemergence herbicides compared to 2020, resulting in Palmer amaranth emerging after the V1 growth stage.

Consequently, Palmer amaranth plants were not controlled when Outlook was applied alone at V3 in 2019. While in 2020, the sequential application of Outlook at V1 or V3 prevented Palmer amaranth emergence through the R1 stage, when the crop canopy was large enough to close rows and suppress weed emergence.

Take-home messages

Preemergence herbicide longevity can be variable from year to year, along with the timing of Palmer amaranth emergence, depending on weather and other factors.

Outlook applied at V1 in 2019 and 2020 provided excellent Palmer amaranth control, while Outlook applied at V3 was unable to suppress Palmer amaranth in 2019.

Application timing is critical for sequential applications of Outlook; however, the longevity of soil applied herbicides and the timing of Palmer amaranth emergence is highly variable and difficult to predict.

Lawrence is a Nebraska Extension integrated weed management specialist, and Teo is a graduate research assistant at the University of Nebraska-Lincoln.

Source: UNL CropWatch, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.