Weed population impacts using targeted herbicide applications with a precision sprayer in soybean over a three-year period

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Abstract Targeted herbicide applications have the potential to reduce herbicide use but they pose an inherent risk of missing late-season weed escapes. Furthermore, relying on targeted use of residual herbicides may increase weed emergence relative to broadcast applications. Research was conducted over a 3-yr period in Keiser, Arkansas, to compare traditional broadcast applications to targeted postemergence applications of various herbicides to soybean cultivars that are known to be resistant to glyphosate, glufosinate, and dicamba. The herbicide treatment protocol was consistent across treatments with a broadcast-applied preemergence residual, and a postemergence combination that included glufosinate + glyphosate + S -metolachlor followed by glufosinate + acetochlor, applied both via broadcast or targeted at the highest and lowest spray sensitivities of the John Deere See & Spray technology. The soil seedbank was similar at trial initiation across treatments, and there was no increase in the seedbank over 3 yr of broadcast and targeted applications at the highest spray sensitivity. Averaged over application timing, when herbicides were applied at the lowest spray sensitivity the weed density rose from 867 plants ha −1 to 2,870 plants ha −1 in Year 2, to 11,300 plants ha −1 in Year 3. This response is likely due to more Palmer amaranth escapes when the crop was harvested with an average of >1,000 plants ha −1 over the years compared with weed density after herbicides were applied with the highest spray sensitivity and via broadcast. Targeted applications improved profitability by reducing herbicide use and increasing application efficiency, providing an average savings of US$43.22 ha −1 to $129.19 ha −1 relative to broadcast postemergence cost of $227.22 ha −1 . The area sprayed was reduced by 20% to 90%, with an average spraying at early postemergence of 41.3% and 57.9% and at mid-postemergence equaling 48.1% and 49.3% when herbicides were applied with the lowest and highest spray sensitivities, respectively. The only difference in the area sprayed between sensitivity settings occurred early postemergence. Based on the results of this experiment, producers could apply postemergence herbicides targeted to weeds that grow with soybean to increase profitability, but the lowest sensitivity resulted in unacceptable increases to the weed seedbank, which could affect management in future years.