BACKGROUND By 2050, weather is expected to become more variable with a shift towards higher temperatures and more erratic rainfall throughout the U.S. Corn Belt. The effects of this predicted weather change on pre‐emergence (PRE) herbicide efficacy have been inadequately explored. Using an extensive database, spanning 252 unique weather environments, the efficacy of atrazine, acetochlor, S‐metolachlor, and mesotrione, applied PRE alone and in combinations, was modeled on common weed species in corn (Zea mays L.). RESULTS Adequate rainfall to dissolve the herbicide into soil water solution so that it could be absorbed by developing weed seedlings within the first 15 days after PRE application was essential for effective weed control. Across three annual weed species, the probability of effective control increased as rainfall increased and was maximized when rainfall was 10 cm or more. When rainfall was less than 10 cm, increasing soil temperatures had either a positive or negative effect on the probability of effective control, depending on the herbicide(s) and weed species. Herbicide combinations required less rainfall to maximize the probability of effective control and had higher odds of successfully controlling weeds compared with the herbicides applied individually. CONCLUSIONS Results of this study highlight the importance of rainfall following PRE herbicide application. As rainfall becomes more variable in future, the efficacy of common PRE herbicides will likely decline. However, utilizing combinations of PRE herbicides along with additional cultural, mechanical, biological, and chemical weed control methods will create a more sustainable integrated weed management system and help U.S. corn production adapt to more extreme weather. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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Under drought conditions, numerous weed species exhibit increased competitiveness with maize (Patterson, 1995;Steckel & Sprague, 2004). Soil-active and foliar-applied herbicides are central to modern weed management systems in maize. In 2018, 97% of all US maize hectares received one or more herbicide applications (USDA-NASS, 2021). Between 2008 and 2012, global producers spent roughly $26 billion annually on herbicides (Atwood & Paisley-Jones, 2017).Herbicide efficacy, determined empirically using a "weed control" rating system during the growing season, ranging from 0% (no weed control) to 100% (complete weed control), varies by product, application rate and timing, weed species, and environmental conditions.For instance, weed control failure is high with inadequate precipitation shortly after application of soil-applied herbicides (Landau et al., 2021). High temperatures hasten the growth rate of important weed species and reduce the amount of time that foliar-applied herbicides provide control (Guo & Al-Khatib, 2003). Additionally, the rate of

Brassicaceae weeds can be problematic in canola varieties that have not been modified to resist specific broad-spectrum herbicides. The overall objective of this study was to evaluate the potential for increased rapeseed seeding rate as a management strategy for flixweed. To accomplish this objective, a field study was conducted to determine crop seeding rate effects on canopy light transmission and rapeseed yield characteristics, as well as a greenhouse study to determine morphological and photosynthetic responses of flixweed to decreasing irradiance levels. Results from the field study indicated that light transmittance through the canopy decreased linearly as crop seeding rate increased from 1.8 to 9.0 kg ha-1. Increasing crop seeding rate did not influence rapeseed aboveground biomass, seed yield, and harvest index, but negatively affected rapeseed seed oil content in one of two site-years. Greenhouse study results indicated that declining irradiance levels caused reductions in flixweed biomass, root allocation, and photosynthetic light compensation point. Flixweed leaf allocation, foliage area ratio, and specific foliage area increased in response to decreasing irradiance levels. Combined results of field and greenhouse studies suggest that increasing rapeseed seeding rate can suppress flixweed growth while not causing yield penalties from increased intraspecific competition. However, increased rapeseed seeding rate might not be an adequate control strategy on its own because flixweed displays characteristics of a shade-tolerant species.

Sweet corn (Zea mays L.) tolerance to dicamba and several other herbicides is due to cytochrome P450 (CYP) mediated metabolism and is conferred by a single gene (Nsf1). Tolerance varies by CYP genotypic class with hybrids homozygous for functional CYP (Nsf1Nsf1) being the most tolerant and hybrids homozygous for mutant CYP alleles (nsf1nsf1) being the least tolerant. The herbicide safener cyprosulfamide (CSA) increases tolerance to dicamba by stimulating the expression of several CYPs. However, the extent to which CSA improves the tolerance of different sweet corn CYP genotypic classes to dicamba is poorly understood. Additionally, the effect of growth stage on sweet corn sensitivity to dicamba is inadequately described. The objective of this work was to quantify the significance of application timing, formulation, and CYP genotypic class on sweet corn response to dicamba. Hybrids representing each of the three CYP genotypes (Nsf1Nsf1, Nsf1nsf1, nsf1nsf1), were treated with dicamba or dicamba + CSA at one of three growth stages: V3, V6, or V9. Across all timings the nsf1nsf1 hybrid was the least tolerant to dicamba, displaying 16% higher crop injury levels 2 weeks after treatment and 2.13 Mt ha-1 lower ear mass yields compared to the Nsf1Nsf1 hybrid. The V9 growth stage was the most susceptible time for dicamba injury regardless of genotypic class, with 1.89 and 1.75 Mt ha-1 lower ear mass yields compared to the V3 and V6 application timings, respectively. The addition of CSA to dicamba V9 applications reduced the injury from dicamba for all three genotypic classes; however, it did not eliminate the injury. The use of Nsf1Nsf1 or Nsf1nsf1 sweet corn hybrids along with herbicide safeners will reduce the frequency and severity of injury from dicamba and other CYP-metabolized herbicides.