Spray Tip Configurations with Pulse-Width Modulation for Glufosinate-Ammonium Deposits in Palmer Amaranth ( Amaranthus palmeri )

A commercial sprayer operated at a field speed of 24 km h -1 simultaneously applied glufosinate-ammonium through seven spray tip treatments spaced along a 30.5 m boom for measured foliar deposits of herbicide in 35 cm tall Palmer amaranth weeds and spray deposits on foliar-mounted water-sensitive paper (WSP). The experiment followed one that found increased herbicide deposits for dual tips with an adjacent, fore-aft mount, downward-pointed pre-orifice tip (Extremely Coarse) operated with blended pulse-width modulation (bPWM) and a pre-orifice tip (Fine) operated constant (non-bPWM) under moderate ambient wind velocities from 3.1 to 4.1 m s -1 . Additional dual-tip treatments were added to the dual-tip configuration for the current experiment to expand droplet Coarseness and to add dual tips operated constant to isolate bPWM effects. Tested treatments in common with the previous experiment included the original dual-tip bPWM and non-bPWM combination, Y-adapter fore-aft-mounted pre-orifice tips with diverging spray patterns both operated bPWM, and an air-induction extended-range tip operated constant. Palmer amaranth weeds, total spray volume rate of 93.5 L ha -1 , sprayer speed of 24 km h -1 , and test methods were similar between studies, except for negligible wind in the current experiment. Conditions were clear and sunny during spraying without indicators of a stable atmosphere. Overall mean glufosinate-ammonium deposits recovered from leaves were greatest for dual-tips operated constant at reduced droplet size (Very Coarse and Fine) due to reduced required tip size operated without bPWM, and for increased droplet size for Y-adapter-mounted pre-orifice tips (Extremely Coarse and Coarse) operated with bPWM, resulting in overall mean glufosinate-ammonium leaf deposits of 15.9 and 15.0 I¼g a.i. cm -2 , respectively. The combination of dual tips at reduced droplet size or the Y-adapter fore-aft spray pattern divergence of bPWM tips coupled with high sprayer speed enhanced droplet interception by Palmer amaranth plants under negligible wind conditions, since the collected deposits, even without summed integration over foliage height, significantly exceeded the applied rate of 8.2 I¼g a.i. cm -2 . An air-induction extended-range tip non-bPWM (Very Coarse) provided the next highest mean in overall glufosinate-ammonium deposit. One increased-droplet size dual-tip, pre-orifice tip bPWM and non-bPWM (Ultra Coarse and Coarse) resulted in a mean deposit that was not significantly different from the air-induction extended-range tip operated non-bPWM. Other dual-tip combinations with bPWM and non-bPWM, including the original dual-tip configuration in the previous study, resulted in significantly reduced mean herbicide deposits. Considering all tested tips, advantages of bPWM depended on spray tip droplet size classifications and Y-mounted fore-aft divergence of spray patterns. Overall mean WSP spot deposits were greatest for reduced droplet size (Very Coarse and Fine) dual pre-orifice tips operated non-bPWM, corresponding with the highest numerical overall mean of glufosinate-ammonium deposit. This correspondence of highest spot deposits and highest mean glufosinate-ammonium deposit also occurred in the previous study. Increased Palmer amaranth control correlated with increased glufosinate-ammonium deposit and decreased volume median diameter (D v0.5 ) determined with WSP electronic scans, with the air-induction extended-range tip operated constant and the Y-adapter pre-orifice tip operated as bPWM providing the highest weed control. Overall mean WSP spot deposits ranged from 42.3 to 81.1 spots cm -2 , compared to 14.0 to 47.0 spots cm -2 previously reported for similar spray conditions, with spot deposits attributed to negligible wind versus wind, respectively. Thus, the spray environment, particularly wind, exhibited effects on nozzle tip comparisons for foliar deposition and may offer some rationale for the conflicting published data beyond the examined treatments.