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This research examined a potential nuisance aspect of the use of the volatility-reducing agent (VRA) potassium carbonate when combined with glyphosate in spray-tank mixtures. A VRA is now required to be added to dicamba applications to reduce off-target movement from volatility. When no VRA potassium carbonate was added to the spray mixture, there was no pressure buildup. The addition of VRA potassium carbonate plus glyphosate (which lowers the pH) resulted in an observed pressure buildup. Although the gas produced was not identified, it would be expected to be carbon dioxide formed by the dissolution of the carbonate anion from the VRA. Source water pH range from 3.2 to 8.2 had no effect on pressure buildup. Pressure buildup was directly related to water temperature, with a linear response to temperature when the VRA was added last; in contrast, a less direct relationship of temperature to pressure buildup existed at temperatures >30 C when the VRA potassium carbonate was added first. There was no effect on the pressure increase from adding a defoamer or a drift control agent.
Field experiments were conducted in Louisiana and Mississippi from 2011 through 2013 to evaluate crop injury, weed control, and yield in field corn following pyroxasulfone applied PRE and POST. Pyroxasulfone PRE or POST did not injure corn at any evaluation. Barnyardgrass control was not improved with the addition of any POST treatment to pyroxasulfone alone or atrazine plus pyroxasulfone PRE; however, all POST treatments increased barnyardgrass control to at least 95% at all evaluations following atrazine PRE. All treatments that contained a PRE followed by POST application controlled browntop millet ≥90% at all evaluations. All POST treatments increased ivyleaf morningglory control to ≥92% following atrazine or pyroxasulfone alone PRE. However, control with atrazine plus pyroxasulfone PRE was similar or greater 28 d after POST than all treatments that received a POST application. In the absence of a POST treatment, pyroxasulfone or atrazine plus pyroxasulfone PRE controlled Palmer amaranth 93 to 96% at all evaluations, but atrazine alone PRE provided 84, 82, and 66% control 7, 14, and 28 d after POST, respectively. All programs that contained a PRE followed by POST herbicide treatment controlled Palmer amaranth >90% at all evaluations. Corn yield following all treatments except atrazine alone PRE and the nontreated were similar and ranged from 10990 to 12330 kg ha−1. This research demonstrated that pyroxasulfone can be a valuable tool for weed management in a corn weed management program.
Weed-free field experiments were conducted to evaluate soybean injury, growth, and yield following PRE or POST pyroxasulfone application. Soybean was injured 1 and 15% following pyroxasulfone PRE and POST application, respectively, 7 d after treatment (DAT). Injury following PRE and POST application was observed as delayed emergence and leaf necrosis and crinkling, respectively. Injury ranged from 0 to 6% following both application timings 14 and 28 DAT. Soybean was injured 5% or less following 60, 120, 180, 240, and 300 g ha−1 of pyroxasulfone. Soybean plant population, height, and yield were not affected by pyroxasulfone application timing. Only 300 g ha−1 of pyroxasulfone reduced soybean plant population to 90% of the nontreated 30 d after PRE. Pyroxasulfone rate did not influence soybean heights and yield. Data indicates that pyroxasulfone can safely be applied to soybean without a detrimental effect on plant growth or yield.
Four field experiments were conducted in Louisiana and Mississippi in 2009 and 2010 to evaluate POST herbicides treatments with tembotrione applied alone or as a prepackaged mixture with thiencarbazone for weed control in corn. Treatments included tembotrione at 92 g ai ha−1, thiencarbazone : tembotrione at 15 : 76 g ai ha−1, atrazine at 2,240 g ai ha−1, glufosinate at 450 g ai ha−1, glyphosate at 860 g ae ha−1, and coapplications of tembotrione or thiencarbazone : tembotrione with atrazine, glufosinate, or glyphosate. All treatments were applied to 26-cm corn in the V4 growth stage. Treatments containing thiencarbazone : tembotrione and those with tembotrione controlled barnyardgrass, browntop millet, entireleaf morningglory, hophornbeam copperleaf, johnsongrass, Palmer amaranth, and velvetleaf 85 to 96% and 43 to 97% 28 d after treatment and at corn harvest, respectively. Corn yield ranged from 9,200 to 10,420 kg ha−1 and was greater than the nontreated control following all herbicide treatments, except atrazine alone. Results indicated that thiencarbazone : tembotrione or tembotrione POST is an option for weed management in corn, and applications of thiencarbazone : tembotrione would be strongly encouraged where rhizomatous johnsongrass is problematic.
Johnsongrass populations that are resistant to 5-enolpyruvyl-3-shikimate synthase (EPSPS)–, acetyl coenzyme A carboxylase (ACCase)–, or acetolactate synthase (ALS)–inhibiting herbicides are increasingly common throughout the midsouth. Three trials were conducted in 2012, 2013, and 2014 in Fayetteville, AR and Alexandria, LA to evaluate strategies with and without ALS- and ACCase inhibitors for management of rhizomatous johnsongrass in the absence of glyphosate. Fluometuron or fluometuron plus pyrithiobac applied PRE followed by (fb) EPOST, MPOST, and LAYBY tank mixtures containing multiple effective mechanisms of action (MOA) controlled johnsongrass at least 90%. Simplifying the program by removing a herbicide or eliminating an application timing reduced control, and increased vegetative and sexual reproduction of johnsongrass. To manage severe infestations or escapes glufosinate plus clethodim fb glufosinate plus clethodim or clethodim plus pyrithiobac fb clethodim) effectively controlled 15-cm johnsongrass. However, johnsongrass control was reduced when ALS and ACCase inhibitors were tank mixed, especially for the second POST application, compared to ACCase inhibitors alone. Effective herbicide programs are available to growers to control johnsongrass in the absence of glyphosate, but the use of PRE herbicides followed by multiple applications of POST herbicides is critical for successful management.
Field studies were conducted in Louisiana and Mississippi in 2009 and 2010 to evaluate coapplications of glyphosate, pyrithiobac, and residual herbicides on growth and yield of glyphosate-resistant cotton. Treatments were a factorial arrangement of glyphosate (0 and 860 g ae ha−1), pyrithiobac (0 and 470 g ai ha−1), and two residual herbicides (pendimethalin [1,120 g ai ha−1], S-metolachlor [1,070 g ai ha−1], and no residual herbicide). Cotton injury was greatest 3 d after treatment (DAT) and decreased at each evaluation interval until 28 DAT when pyrithiobac was coapplied with glyphosate. Cotton injury ranged from 4 to 17% through 14 DAT when pyrithiobac was applied alone (no residual herbicide) or with pendimethalin, but injury decreased to ≤ 3% after 14 DAT. Cotton injury 3 to 21 DAT following pyrithiobac plus S-metolachlor ranged from 4 to 31%, but S-metolachlor alone injured cotton 1 to 7%. When pyrithiobac was included, cotton injury following S-metolachlor was 3 to 15% greater than that following pendimethalin from 3 to 14 DAT. Pendimethalin did not reduce plant height at 21 or 42 DAT compared with treatments receiving no residual herbicide, but S-metolachlor reduced plant heights 5 and 4% at 21 and 42 DAT, respectively. Although cotton injury was severe in some cases and persisted until 21 DAT, the injury did not cause reductions in yield. This indicates the early-season cotton injury was transient, and cotton was able to recover from the injury with no observed differences in yield.
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