Although Palmer amaranth is currently not widespread in most dry edible bean−producing states in the United States, it is widespread in Western Nebraska, a major dry edible bean−producing region. There is currently a lack of research on management and biology of Palmer amaranth within dry edible bean production. The objective of this study was to quantify the impact of season−long Palmer amaranth interference on yield of dry edible bean and seed production of Palmer amaranth. A field study was conducted in Scottsbluff, NE in 2020 and 2021. Palmer amaranth interference at densities of 0, 0.2, 0.3, 0.5, 1, and 2 plants m−1 row of dry edible bean was evaluated. Palmer amaranth interference reduced dry edible bean yield by 77% at 2 Palmer amaranth plants m−1 row compared to the weed−free control; and a 5% yield−reduction threshold was estimated to occur at 0.02 Palmer amaranth plants m−1 row. Yield reduction occurred primarily through a reduction in the number of pods plant−1 as Palmer amaranth density increased. Palmer amaranth plants produced 91,000 to 376,000 seeds plant−1 depending on densities, and as much as 140,000 seeds m−2. Study results will help farmers and other stakeholders estimate Palmer amaranth interference within their fields, and may help justify the economic cost of incorporating additional Palmer amaranth management practices.

In Western Nebraska, Palmer amaranth is becoming more prevalent where acetolactate synthase (ALS)−inhibitor−resistant biotypes are widespread. There are limited effective postemergence (POST) herbicides labeled for ALS−inhibitor−resistant Palmer amaranth control in dry edible bean. The objective of this study was to evaluate the efficacy of dimethenamid−P in a sequential preemergence (PRE) fb (followed by) POST program at two POST application timings, V1 and V3, for controlling ALS−inhibitor−resistant Palmer amaranth in dry edible bean. A field study was conducted in 2019, 2020, and 2021 in Scottsbluff, NE. PRE−alone programs of pendimethalin + dimethenamid−P provided inconsistent Palmer amaranth control. Dimethenamid−P applied POST following a PRE application of pendimethalin + dimethenamid−P provided effective Palmer amaranth control (>90%) at 4 WAV3 only at the V1 application timing in 2019. In 2020 and 2021 Dimethenamid−P applied POST at V1 and V3 following a PRE application of pendimethalin + dimethenamid−P provided 99% and 98% Palmer amaranth control at 4 WAV3 and 98% and 94% Palmer amaranth control at harvest, respectively. Palmer amaranth biomass was reduced 95−99% and 96−98% compared to the non−treated control when dimethenamid−P was applied POST at V1 and V3, respectively, following a PRE application of pendimethalin + dimethenamid−P in 2020 and 2021. Mixing dimethenamid−P with imazamox + bentazon POST performed similarly to the fomesafen−containing treatments and dimethenamid−P alone POST. Dimethenamid−P applied POST following a PRE application of pendimethalin + dimethenamid−P resulted in similar yield as the fomesafen−containing treatments. If fomesafen is not an option due to the crop rotation interval restriction, using dimethenamid−P in a sequential PRE fb POST program is the only effective alternative to control ALS−inhibitor−resistant Palmer amaranth in Nebraska. The use of dimethenamid−P in a sequential PRE fb POST program, alone or mixed with foliar−active herbicides should be considered by dry edible bean growers who are dealing with ALS−inhibitor−resistant Palmer amaranth.

The critical timing of weed removal (CTWR) is the point in crop development when weed control must be initiated to prevent crop yield loss due to weed competition. A field study was conducted in 2018 and 2020 near Scottsbluff, NE to determine how the use of preemergence herbicides impacts the CTWR in dry bean. The experiment was arranged as a split-plot, with herbicide treatment and weed removal timing as main and sub plot factors, respectively. Herbicide treatment consisted of no-preemergence, or pendimethalin (1070 g ai ha–1) + dimethenamid-P (790 g ai ha–1) applied preemergence. Sub-plot treatments included season-long weed-free, weed removal at: V1, V3, V6, R2, and R5 dry bean growth stages, and a season-long weedy control. A four-parameter logistic model was used to estimate the impact of time of weed removal, for all response variables including dry bean yield, dry bean plants m–1 row, pods plant–1, seeds pod–1, and seed weight. The CTWR based on 5% yield reduction was estimated to range from the V1 growth stage [(16 d after emergence (DAE)] to the R1 growth stage (39 DAE) in the no-PRE herbicide treatment. In the PRE-applied treatment, the CTWR began at the R2 growth stage (47 DAE). Dry bean plants m–1 row was reduced in the no-preemergence treatment when weed removal was delayed beyond the R2 growth stage in the 2020 field season. The use of preemergence-applied herbicides prevented a reduction in the number of pods plant–1 in 2020, and the number of seeds pod–1 in 2018 and 2020. In 2018, the number of pods plant–1 was reduced by 73% when no-preemergence was applied, compared to 26% in the preemergence-applied treatment. The use of preemergence-applied soil active herbicides in dry bean delayed the CTWR and preserved yield potential.