Velvetleaf is an economically important weed in popcorn production fields in Nebraska. Many PRE herbicides in popcorn have limited residual activity or provide partial velvetleaf control. There are a limited number of herbicides applied POST in popcorn compared with field corn, necessitating the evaluation of POST herbicides for control of velvetleaf. The objectives of this study were to (1) evaluate the efficacy and crop safety of labeled POST herbicides for controlling velvetleaf that survived S-metolachlor/atrazine applied PRE and (2) determine the effect of velvetleaf height on POST herbicide efficacy, popcorn injury, and yield. Field experiments were conducted in 2018 and 2019 near Clay Center, Nebraska. The experiments were arranged in a split-plot design with four replications. The main plot treatments were velvetleaf height (≤15 cm and ≤30 cm) and subplot treatments included a no-POST herbicide control, and 11 POST herbicide programs. Fluthiacet-methyl, fluthiacet-methyl/mesotrione, carfentrazone-ethyl, dicamba, and dicamba/diflufenzopyr provided greater than 96% velvetleaf control 28 d after treatment (DAT), reduced velvetleaf density to fewer than 7 plants m−2, achieved 99% to 100% biomass reduction, and had no effect on popcorn yield. Herbicide programs tested in this study provided greater than 98% control of velvetleaf 28 DAT in 2019. Most POST herbicide programs in this study provided greater than 90% control of up to 15 cm and up to 30 cm velvetleaf and no differences between velvetleaf heights in density, biomass reduction, or popcorn yield were observed, except with topramezone and nicosulfuron/mesotrione 28 DAT in 2018. On the basis of contrast analysis, herbicide programs with fluthiacet-methyl or dicamba provided better control than herbicide programs without them at 28 DAT in 2018. It is concluded that POST herbicides are available for control of velvetleaf up to 30-cm tall in popcorn production fields.

Experiments were conducted to evaluate the effects of preemergence herbicides on the growth characteristics and competitiveness of sicklepod and common cocklebur to soybean. Alachlor was applied at 1.1 or 2.2 kg ai ha–1 and metribuzin at 0.2 or 0.4 kg ai ha–1 in glasshouse and field experiments. Alachlor at 2.2 kg ha–1 reduced sicklepod dry weight and leaf area 4 weeks after planting. Sicklepod water use was initially lowered by both rates of alachlor while soybean and common cocklebur water use was initially lowered by metribuzin at 0.4 kg ha–1. Sicklepod and common cocklebur fresh weights were reduced at soybean harvest by the high rates of alachlor and metribuzin. Soybean competing with sicklepod injured by 1.1 or 2.2 kg ha–1 of alachlor produced yields greater than soybean competing with untreated sicklepod. Soybean yields were not improved when metribuzin was used on common cocklebur.

Competitiveness of weeds that survive a PRE herbicide application (escaped weeds) might be altered because of herbicide injury. As a result, potential crop yield loss may be reduced. Field experiments were conducted at Ashland Bottoms, KS, in 2001 and 2002 and at Rossville, KS, in 2002. The objectives were to quantify corn growth and yield response to Palmer amaranth or velvetleaf competition, with or without isoxaflutole (0.03 kg ha−1) or flumetsulam (0.04 kg ha−1) application, and to determine seed production of Palmer amaranth or velvetleaf as affected by PRE herbicide. Palmer amaranth and velvetleaf densities ranged from 0 to 6 and 0 to 32 plants m−1 of corn row, respectively. At Ashland Bottoms in 2002, corn height at tasseling decreased with increasing Palmer amaranth (1.58 cm weed−1 m−1) and velvetleaf (1.32 cm weed−1 m−1) density when no herbicide was applied. With flumetsulam application, each increase in velvetleaf density reduced corn height by 0.4 cm. Escaped Palmer amaranth and velvetleaf were shorter than untreated plants at corn tasseling. At Rossville in 2002, Palmer amaranth that escaped isoxaflutole or flumetsulam application caused 13% corn yield loss (YL) at a density of 3 plants m−1. In contrast, corn YL from untreated Palmer amaranth at the same density was 30%. At Ashland Bottoms in 2002, velvetleaf that escaped flumetsulam caused 3% corn YL at a density of 3 plants m−1 compared with 38% YL caused by untreated velvetleaf at the same density. Seed production of Palmer amaranth was independent of density or herbicide treatment, whereas production of velvetleaf seed increased with density, with or without flumetsulam. The study showed that corn YL from both Palmer amaranth and velvetleaf that escaped a PRE herbicide was less than from untreated weeds, but seed production by escaped weeds was similar to that of untreated weeds.