Waterhemp has evolved resistance to seven herbicide modes of action in the United States and to five in Canada, which limits weed control options for producers. The objective of this research was to quantify the level and duration of residual control of multiple herbicide-resistant (MHR) waterhemp with five Group 15 herbicides (acetochlor, dimethenamid-p, flufenacet, pyroxasulfone, and S-metolachlor) applied preemergence in a non-crop area. Four field trials were conducted over a 2-yr period (2021, 2022) in southwestern Ontario, Canada. By 4 wk after application (WAA) 91% of waterhemp had emerged in the nontreated control area. The numerical control of waterhemp with all Group 15 herbicides, with the exception of pyroxasulfone, was greatest at 4 WAA, then control declined. Flufenacet provided the lowest waterhemp control; dimethenamid-p and S-metolachlor provided intermediate control, and acetochlor and pyroxasulfone provided the highest control. Waterhemp control with pyroxasulfone peaked at 6 WAA with 99% and declined to 77% at 12 WAA. Flufenacet (low and high rates) was predicted to reduce waterhemp emergence by 50% for 42 to 44 d after application (DAA). Dimethenamid-p, S-metolachlor, and acetochlor (both formulations and three rates) were predicted to reduce waterhemp emergence by 80% for 36, 43, and 33 to 51 DAA, respectively; in contrast, pyroxasulfone was predicted to reduce waterhemp emergence by 80% for 82 DAA. This study concludes that of the Group 15 herbicides evaluated, flufenacet provides the lowest and shortest residual control of waterhemp, and pyroxasulfone provides the highest and longest residual control of waterhemp.

The development of glufosinate-resistant soybean cultivars has created opportunities for use of glufosinate applied postemergence for weed control. Four field experiments were conducted in 2021 and 2022 to ascertain the effect of glufosinate rate and the addition of ammonium sulfate on annual weed control in glyphosate/glufosinate/2,4-D–resistant soybean. An increased glufosinate rate of 500 from 300 g ai ha−1 improved control of common ragweed, common lambsquarters, redroot pigweed, and foxtail species and resulted in decreased density and dry biomass of common lambsquarters and foxtail species. The addition of ammonium sulfate to glufosinate increased control of common lambsquarters, 2 and 8 wk after application (WAA), and of foxtail species, 2, 4, and 8 WAA, but did not improve control of common ragweed and redroot pigweed. Increasing the dose of glufosinate from 300 to 500 g ai ha−1 improves control of common ragweed, redroot pigweed, common lambsquarters, and foxtail species; however, the benefit of the addition of ammonium sulfate to glufosinate is weed species-specific.

Tolpyralate is commonly mixed with atrazine for improved control of common annual weed species in corn production systems in the United States and Canada. Weed control efficacy with this mixture is enhanced with the addition of methylated seed oil (MSO) Concentrate®; however, there is little information on the efficacy of tolpyralate + atrazine with other proprietary adjuvants. Therefore, four trials were conducted at field research sites in Ontario, Canada, to evaluate the efficacy of tolpyralate + atrazine when applied with six different commercially available adjuvants on four annual broadleaf and two annual grass weed species in corn. The adjuvants evaluated were MSO Concentrate®, Agral® 90, Assist® Oil Concentrate, Carrier®, LI 700®, and Merge®. A treatment of tolpyralate + atrazine applied with no adjuvant was also included in the study. For the control of velvetleaf and wild mustard, the adjuvants evaluated with tolpyralate + atrazine did not improve control. At 8 wk after application (WAA), the use of Agral® 90, Assist® Oil Concentrate, Carrier®, MSO Concentrate®, or Merge® with tolpyralate + atrazine provided similar or greater control of common ragweed than tolpyralate + atrazine applied with LI 700®. At 8 WAA, the adjuvants performed similarly with tolpyralate + atrazine for the control of common lambsquarters; however, LI 700® was the only adjuvant that did not improve control compared to tolpyralate + atrazine applied without an adjuvant. At 8 WAA, MSO Concentrate®, Carrier®, and Merge® improved control of barnyardgrass and foxtail species with tolpyralate + atrazine to a similar or greater level than Assist® Oil Concentrate, Agral® 90, and LI 700®. Overall, MSO Concentrate®, Carrier®, or Merge® should be added to tolpyralate + atrazine for control of the myriad of weed species interfering with corn production.

Velvetleaf is an economically important weed in agronomic crops in Nebraska and the United States. Dicamba applied alone usually does not provide complete velvetleaf control, particularly when velvetleaf is taller than 15 cm. The objectives of this experiment were to evaluate the interaction of dicamba, fluthiacet-methyl, and glyphosate applied alone or in a mixture in two- or three-way combinations for velvetleaf control in dicamba/glyphosate–resistant (DGR) soybean and to evaluate whether velvetleaf height (≤12 cm or ≤20 cm) at the time of herbicide application influences herbicide efficacy, velvetleaf density, biomass, and soybean yield. Field experiments were conducted near Clay Center, NE in 2019 and 2020. The experiment was arranged in a split-plot with velvetleaf height (≤12 cm or ≤20 cm) as the main plot treatment and herbicides as subplot treatment. Fluthiacet provided ≥94% velvetleaf control 28 d after treatment (DAT) and ≥96% biomass reduction regardless of application rate or velvetleaf height. Velvetleaf control was 31% to 74% at 28 DAT when dicamba or glyphosate was applied alone to velvetleaf ≤20 cm tall compared with 47% to 100% control applied to ≤12-cm-tall plants. Dicamba applied alone to ≤20-cm-tall velvetleaf provided <75% control and <87% biomass reduction 28 DAT compared with ≥90% control with dicamba at 560 g ae ha−1 + fluthiacet at 7.2 g ai ha−1 or glyphosate at 1,260 g ae ha−1. Dicamba at 280 g ae ha−1 + glyphosate at 630 g ae ha−1 applied to ≤20-cm-tall velvetleaf resulted in 86% control 28 DAT compared with the expected 99% control. The interaction of dicamba + fluthiacet + glyphosate was additive for velvetleaf control and biomass reduction regardless of application rate and velvetleaf height.

Tolpyralate is a new 4-hydroxyphenyl-pyruvate dioxygenase (HPPD)–inhibiting herbicide for weed control in corn. Previous research has reported efficacy of tolpyralate + atrazine on several annual grass and broadleaf weed species; however, no studies have evaluated weed control of tolpyralate + atrazine depending on time-of-day (TOD) of application. Six field experiments were conducted over a 2-yr period (2018, 2019) near Ridgetown, ON, to determine if there is an effect of TOD of application on tolpyralate + atrazine efficacy on common annual grass and broadleaf weeds. An application was made at 3-h intervals beginning at 06:00 h with the last application at 24:00 h. There was a slight TOD effect on velvetleaf, pigweed species, and common ragweed control with tolpyralate + atrazine; however, the magnitude of change throughout the day was ≤3% at 2, 4, or 8 wk after application (WAA). There was no effect of TOD of tolpyralate + atrazine on the control of lambsquarters, barnyardgrass, and green foxtail. All weed species were controlled ≥88% at 8 WAA. There was no effect of TOD of tolpyralate + atrazine application on corn yield. Results of this study show no evidence of a TOD effect on weed control efficacy with tolpyralate + atrazine.

Narrow-windrow burning has been a successful form of harvest weed seed control in Australian cropping systems, but little is known about the efficacy of narrow-windrow burning on weed seeds infesting U.S. cropping systems. An experiment was conducted using a high-fire kiln that exposed various grass and broadleaf weed seeds to temperatures of 200, 300, 400, 500, and 600 C for 20, 40, 60, and 80 s to determine the temperature and time needed to kill weed seeds. Weeds evaluated included Italian ryegrass, barnyardgrass, johnsongrass, sicklepod, Palmer amaranth, prickly sida, velvetleaf, pitted morningglory, and hemp sesbania. Two field experiments were also conducted over consecutive growing seasons, with the first experiment aimed at determining the amount of heat produced during burning of narrow windrows of soybean harvest residues (chaff and straw) and the effect of this heat on weed seed mortality. The second field experiment aimed to determine the effect of wind speed on the duration and intensity of burning narrow windrows of soybean harvest residues. Following exposure to the highest temperature and longest duration in the kiln, only sicklepod showed any survival (<1% average); however, in most cases, the seeds were completely destroyed (ash). A heat index of only 22,600 was needed to kill all seeds of Palmer amaranth, barnyardgrass, and Italian ryegrass. In the field, all seeds of the evaluated weed species were completely destroyed by narrow-windrow burning of 1.08 to 1.95 kg m−2 of soybean residues. The burn duration of the soybean harvest residues declined as wind speed increased. Findings from the kiln and field experiments show that complete kill is likely for weed seeds concentrated into narrow windrows of burned soybean residues. Given the low cost of implementation of narrow-windrow burning and the seed kill efficacy on various weed species, this strategy may be an attractive option for destroying weed seed.