Selection of effective herbicide strategies (i.e., one-pass versus two-pass and timing [preemergence versus postemergence]) is of great importance to corn growers. Field studies were conducted to evaluate overall end-of-season weed control efficacy of multiple herbicide strategies in conventional tillage corn production systems. These studies were carried out over six site-years at four locations in Wisconsin: Arlington (2018 and 2019), Brooklyn (2019), Lancaster (2019), and Janesville (2018 and 2019). Herbicide strategy treatments included one-pass preemergence, one-pass postemergence, two-pass preemergence followed by (fb) postemergence, and two-pass preemergence fb postemergence with layered residual herbicides. The weed species present at the experimental site-years included common lambsquarters, giant foxtail, giant ragweed, velvetleaf, and waterhemp. Except Arlington-2019, the herbicide strategy was not as influential for the site-years infested with common lambsquarters, giant foxtail, velvetleaf, and waterhemp species (e.g., Arlington-2018, Brooklyn-2019, Lancaster-2019), as effective overall end-of-season control (>90%) was achieved regardless of the herbicide strategy, and no significant differences were observed in the combined weed biomass across strategies. A two-pass strategy (e.g., preemergence followed by postemergence, or preemergence followed by postemergence with layered residual herbicides) was necessary for effective overall end-of-season control at the site-years infested with giant ragweed (Janesville-2018 and -2019). Weed interference reduced corn yield by 11% to 75% across site-years. Although certain weed communities can be effectively controlled by a one-pass herbicide strategy, two-pass strategies provided the greatest and most consistent overall end-of-season weed control and corn yield across all site-years, regardless of weed species composition and environmental conditions. Hence, a two-pass herbicide strategy is recommended for conventional-tillage corn production in Wisconsin to ensure effective end-of-season weed control while protecting yield potential of the crop, particularly in fields infested with moderate to high density of troublesome weeds such as giant ragweed.

Herbicide resistance in Palmer amaranth and waterhemp is on the rise and poses a great concern to growers in the United States. A multistate screening was conducted for these two weed species in the United States to assess their sensitivity to glufosinate, dicamba, and 2,4-D. The screening was designed to understand the weed sensitivity landscape and emerging trends in resistance evolution by testing each herbicide at its respective label rate and at half the label rate. A total of 303 weed seed accessions from 21 states representing 162 Palmer amaranth and 141 waterhemp seeds were collected from grower fields in 2019 and screened in greenhouse conditions. Statistical power of different sample sizes and probability of survivors in each accession were estimated for each species and herbicide treatment. Overall, the efficacy of glufosinate, dicamba, and 2,4-D against all these accessions was excellent, with greater than 90% average injury. The variability in herbicide injury, if any, was greater with half the label rate of 2,4-D in some Palmer amaranth accessions, while waterhemp accessions had exhibited variable sensitivity with half the label rate of dicamba and glufosinate. The study highlights the value of monitoring weeds for herbicide sensitivity across broader landscape and the importance of glufosinate, dicamba, and 2,4-D herbicides in managing troublesome weeds as part of a diversified weed control program integrated with other chemical, mechanical and cultural practices.

The distribution of herbicide-resistant weeds such as waterhemp has resulted in a greater need for a more integrated approach to weed management, especially in U.S. soybean production systems. Previous research has shown harvest weed seed control (HWSC) to be an effective method of reducing the amount of weed seed returning to the soil. One form of HWSC is the use of impact mills to destroy weed seed exiting the combine during harvest. In 2019 and 2020, we investigated the efficacy and operating costs of the Seed TerminatorTM impact mill in five Missouri soybean fields that contained significant waterhemp infestations. Results indicated that 22% to 40% of the available waterhemp seed in the field at harvest drops to the soil surface because of shatter whenever the combine reel contacts waterhemp plants. Across all locations, an average of 94% of waterhemp seed exiting the Seed Terminator™ was substantially damaged and considered nonviable. Consecutive seasons of use of the Seed TerminatorTM on the same field in two of the locations resulted in a 96% to 97% reduction of waterhemp in the soil seed bank the spring following the second harvest. The estimated increased operating cost of using a Seed Terminator™ was $14.18 ha–1 compared to harvesting with a conventional combine alone. Engine load increased by 12.5%, fuel consumption was 11.3 L h–1 and 1 L ha–1 greater with the Seed Terminator™, but there was no reduction in productivity when harvesting with a combine equipped with a Seed TerminatorTM compared to a conventional combine. The use of impact mills could play a significant role in reducing soil weed seed banks in soybean production systems in at least the Midwest region of the United States in the future.

Weed control of paraquat can be erratic and may be attributable to differing species sensitivity and/or environmental factors for which minor guidance is available on commercial labels. Therefore, the objectives of this research were to quantify selectivity of paraquat across select weed species and the influence of environmental factors. Experiments were performed under controlled conditions in the greenhouse and growth chamber. Compared with purple deadnettle (dose necessary to reduce shoot biomass by 50% = 39 g ai ha−1), waterhemp, Palmer amaranth, giant ragweed, and horseweed were 4.9, 3.3, 1.9, and 1.3 times more sensitive to paraquat, respectively. The injury progression rate over 3 d after treatment (DAT) was a more accurate predictor of final efficacy at 14 DAT than the lag phase until symptoms first appeared. For example, at the 17.5 g ha−1 dose, the injury rate of waterhemp and Palmer amaranth was, on average, 3.6 times greater than that of horseweed and purple deadnettle. The influence of various environmental factors on paraquat efficacy was weed specific. Applications made at sunrise improved control of purple deadnettle over applications at solar noon or sunset. Lower light intensities (200 or 600 μmol m−2 s−1) surrounding the time of application improved control of waterhemp and horseweed more than 1,000 μmol m−2 s−1. Day/night temperatures of 27/16 C improved horseweed and purple deadnettle control compared with day/night temperatures of 18/13 C. Though control was positively associated with injury rates in the application time of day and temperature experiments, a negative relationship was observed for waterhemp in the light-intensity experiment. Thus, although there are conditions that enhance paraquat efficacy, the specific target species must also be considered. These results advocate paraquat dose recommendations, currently based on weed height, be expanded to address sensitivity differences among weeds. Moreover, these findings contrast with paraquat labels stating temperatures of 13 C or lower do not reduce paraquat efficacy.

Sugarbeet growers only recently have combined ethofumesate, S-metolachlor, and dimethenamid-P in a weed control system for waterhemp control. Sugarbeet plant density, visible stature reduction, root yield, percent sucrose content, and recoverable sucrose were measured in field experiments at five environments between 2014 and 2016. Sugarbeet stand density and stature reduction occurred in some but not all environments. Stand density was reduced with PRE application of S-metolachlor at 1.60 kg ai ha–1 and S-metolachlor at 0.80 kg ha–1 + ethofumesate at 1.68 kg ai ha–1 alone or followed by POST applications of dimethenamid-P at 0.95 kg ai ha–1. Sugarbeet visible stature was reduced when dimethenamid-P followed PRE treatments. Stature reduction was greatest with ethofumesate at 1.68 or 4.37 kg ha–1 PRE and S-metolachlor at 0.80 kg ha–1 + ethofumesate at 1.68 kg ha–1 PRE followed by dimethenamid-P at 0.95 kg ha–1 POST. Stature reduction ranged from 0 to 32% 10 d after treatment (DAT), but sugarbeet recovered quickly and visible injury was negligible 23 DAT. Although root yield and recoverable sucrose were similar across herbicide treatments and environments, we caution against the use of S-metolachlor at 0.80 kg ha–1 + ethofumesate at 1.68 kg ai ha–1 PRE followed by dimethenamid-P at 0.95 kg ha–1 in sugarbeet.