Dicamba and 2,4-D tolerance traits were introduced to soybean and cotton, allowing for over the top applications of these herbicides. Avoiding antagonism of glyphosate and clethodim by dicamba or 2,4-D is necessary to achieve optimum weed control. Three field studies were conducted in fallow fields with broadleaf signalgrass (Urochloa platyphylla) and Italian ryegrass (Lolium perenne ssp. multiflorum) pressure. A tractor-mounted dual boom sprayer was modified to spray one of three application methods: (1) two herbicides tanked-mixed (TMX); (2) two herbicides in separate tanks mixed in the boom line (MIL); and (3) two herbicides in separate tanks applied through separate booms simultaneously (SPB). One study compared the three application methods with sethoxydim applied with bentazon, the second compared clethodim applied with dicamba or 2,4-D, and the third compared glyphosate applied with dicamba or 2,4-D. In most cases over all three trials, there was a 7–15% increase in efficacy when using the SPB application method. Antagonism of all the herbicide combinations above was observed when applied using the TMX and MIL methods. In some cases, antagonism was avoided when using the SPB method. The separate boom application method increased efficacy, which allowed herbicides to be used more effectively, resulting in improved economic and environmental sustainability of herbicide applications.

Greenhouse studies were planted at the R.R. Foil Plant Science Research Center in Starkville, MS. In the efficacy trial, pots were seeded with barnyardgrass (Echinochloa crus-galli), broadleaf signalgrass (Urochloa platyphylla), and giant foxtail (Setaria faberi). In the lab detection trial, only barnyardgrass was seeded. Both studies consisted of 16 treatments with four replications per treatment. The treatments consisted of clethodim, glyphosate, dicamba, and 2,4-D applied singularly and in combination with each other. Each herbicide combination was applied with three application methods: tank mixture, sequential applications where the synthetic auxin was applied first (auxin applied first), and sequential applications where glyphosate or clethodim was applied first (auxin applied second). The auxin applied second method had higher visual estimations of control ratings and lower biomass weights compared to the other two methods. The auxin applied second method had more glyphosate and clethodim detected with the use of liquid chromatography tandem mass spectrometry.

No abstract

Residual weed control is influenced by herbicide-soil interactions. This study was conducted to determine whether herbicide sorption to soils and subsequent residual weed control can be manipulated with adjuvants included in tank mixtures. The effects of commercially available adjuvants on S-metolachlor and fluometuron sorption in the laboratory and residual control of barnyardgrass with these herbicides under field conditions were investigated on a Mantachie loam, Catalpa silty clay loam, and Marietta fine sandy loam. The addition of an adjuvant never increased herbicide sorption to soil but, rather, had no effect or decreased the sorption of S-metolachlor and fluometuron. In all tested soil types, sorption of S-metolachlor decreased by up to 17.6-fold. In silty clay loam and loam soil types, the adjuvant did not affect fluometuron sorption. However, in fine sandy loam soils, two of four experimented adjuvants decreased sorption of fluometuron to soil particles up to 1.6-fold. Moreover, no adjuvant influenced the residual control of barnyardgrass exhibited by either chemistry. These data indicate that the addition of an adjuvant to S-metolachlor and fluometuron mixtures will not increase their sorption to loam, silty clay loam, or fine sandy loam soils or alter the efficacy of these herbicides under field conditions.

A chloroacetamide herbicide by application timing factorial experiment was conducted in 2017 and 2018 in Mississippi to investigate chloroacetamide use in a dicamba-based Palmer amaranth management program in cotton production. Herbicides used were S-metolachlor or acetochlor, and application timings were preemergence, preemergence followed by (fb) early postemergence, preemergence fb late postemergence, early postemergence alone, late postemergence alone, and early postemergence fb late postemergence. Dicamba was included in all preemergence applications, and dicamba plus glyphosate was included with all postemergence applications. Differences in cotton and weed response due to chloroacetamide type were minimal, and cotton injury 14 d after LP application was less than 10% for all application timings. Late-season weed control was reduced up to 30 and 53% if chloroacetamide application occurred PRE or LP only, respectively. Late-season weed densities were minimized if multiple applications were used instead of a single application. Cotton height was reduced by up to 23% if a single application was made LP relative to other application timings. Chloroacetamide application at any timing except PRE alone minimized late season weed biomass. Yield was maximized by any treatment involving multiple applications or EP alone whereas applications PRE or LP alone resulted in up to 56 and 27% yield losses, respectively. While no yield loss was reported by delaying the first of sequential applications until EP, foregoing a PRE application is not advisable given the multiple factors that may delay timely POST applications such as inclement weather.