Abstract
AbstractWeed size can nfluence herbicide performance and herbicide interactions in mixtures. To control a broad range of species in soybean or cotton, POST herbicide mixtures will likely be commonplace in Roundup Ready® XtendFlex® and Enlist™ technologies. The impact of weed size on herbicide interactions that could occur in Roundup Ready XtendFlex or Enlist crops was assessed in two field experiments conducted in 2015 and 2016 at the Northeast Research and Extension Center in Keiser, AR. Combinations of glufosinate, glyphosate, dicamba, and 2,4-D were applied to either 10-cm or 30-cm weeds and evaluated for percent weed control, height reduction, and density reduction, collected 5 wk after treatment. Colby’s method was used to analyze treatments for herbicide interactions for control of barnyardgrass, Palmer amaranth, and pitted morningglory. Antagonism was identified with at least one treatment on all species. Almost all treatments were antagonistic for percent weed control, height reduction, and density reduction on barnyardgrass. When glyphosate in mixture with 2,4-D or dicamba was applied to 30-cm barnyardgrass, control declined 9% for both mixtures relative to glyphosate alone. Glufosinate plus glyphosate was antagonistic when applied to both 30-cm pitted morningglory and barnyardgrass. Glufosinate plus dicamba provided less control and density reduction of Palmer amaranth than what was expected from Colby’s equation. Overall, antagonism was more likely to be identified when applications were made to 30-cm weeds compared with 10-cm weeds. The utility of a given herbicide mixture will depend on the species present in the field and the size of those species at the time of application.
Highlights
94% of soybean [Glycine max (L.) Merr.] and 91% of cotton (Gossypium hirsutum L.) hectares in the United States were planted to a variety containing an herbicideresistance trait in 2018 (USDA-NASS 2018)
2,4-D Experiment All herbicide mixtures were considered additive for control, height reduction, and density reduction 5 wk after treatment (WAT) (Table 5)
Dicamba Experiment Two mixtures were identified as antagonistic for Palmer amaranth control 5 WAT in the dicamba experiment when applied to 30-cm weeds: glufosinate plus glyphosate potassium (K) and glufosinate plus dicamba (Table 6)
Summary
94% of soybean [Glycine max (L.) Merr.] and 91% of cotton (Gossypium hirsutum L.) hectares in the United States were planted to a variety containing an herbicideresistance trait in 2018 (USDA-NASS 2018). Technology all contain a glufosinate-resistance trait, increasing the likelihood glufosinate will be applied in mixture with synthetic auxins or glyphosate. Evidence suggests glufosinate plus glyphosate and glyphosate plus a synthetic auxin are antagonistic when applied to various monocot species (Besançon et al 2018; Flint and Barrett 1989; Meyer et al 2017; O’Sullivan and O’Donovan 1980). Despite the prevalence of weeds across the United States that evolved resistance to glyphosate (Heap 2018), preserving the effectiveness of glyphosate on sensitive species is still of value. In multiple resistant-crop technologies (e.g., Enlist, Bollgard II XtendFlex) alternative herbicides to glyphosate, such as glufosinate, often require sequential applications or additional herbicides for effective control of grass species, including giant foxtail (Setaria faberi Herrm.) and johnsongrass [Sorghum halepense (L.) Pers.] (Meyer et al 2015a; Wiesbrook et al 2001). Effective management of glufosinate and glyphosate is needed to mitigate the likelihood of resistance, when antagonism in various mixtures may be present
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