Spray Tip Effect on Glufosinate Canopy Deposits in Palmer Amaranth ( Amaranthus palmeri ) for Pulse-Width Modulation versus Air-Induction Technologies
Five nozzle tip treatments included nozzle tip designs and selected use of blended pulse-width modulation (bPWM), with tip selections based on a maxim of reduced spray drift potential, either through droplet size classification or spray entrainment of dual sprays. Spray nozzle technologies prioritized for this study included bPWM of pre-orifice tips in various configurations of single and dual tips, and air-induction tips operated in constant (non-bPWM) mode. A self-propelled sprayer equipped with a 30.5 m boom and bPWM applied glufosinate-ammonium herbicide simultaneously through the five different spray nozzle tip treatments. Ambient wind veleocities ranged from 3.1 to 4.1 m s-1 during application. Herbicide deposits on leaves and water-sensitive paper (WSP) coverage, spot density, and droplet size characteristics at top and middle canopy locations in Palmer amaranth were used to compare nozzle tip treatments. Treatment 1 (pre-orifice tips, bPWM and non-bPWM) produced significantly greater herbicide deposits than all tips except Treatment 2 (pre-orifice tip, bPWM). The latter was not significantly different from all other tips. The numerically lowest deposit level, based on the overall means, was produced by Treatment 5 (air-induction deflector tip, non-bPWM), although the leaf deposit was not significantly different from most tips. Deposits of glufosinate-ammonium for top canopy locations were greater than middle canopy location deposits for a given nozzle tip treatment. Summation of deposits across locations sometimes exceeded the application rate of 4.5 μg cm-2, attributed to spray cloud momentum due to the application speed of 24 km h-1. The addition of more weed plants or crop plants could possibly alter spray cloud momentum or serve as interceptors of droplets at the expense of deposit levels on Palmer amaranth. Glufosinate-ammonium leaf deposits were inversely proportional to and significantly correlated with canopy coverage of the ground, which supported this hypothesis. The highest mean WSP coverage occurring at top location for Treatment 1 was significantly greater than the mean WSP coverage at top locations for Treatment 4 (air-induction extended-range tip, non-bPWM) and Treatment 5, and greater than mean WSP coverages for all tip treatments at middle locations based on p-level observations. The highest mean WSP spot deposit for the top location of Treatment 1 (57.9 cm-2) was significantly greater than the mean WSP spot deposits for all other tip treatments and canopy locations. A wide range of droplet sizes resulting from the entrainment of a Fine spray into an Extremely Coarse spray may have contributed to high values of coverage and spot deposit for Treatment 1. In conclusion, use of a pre-orifice nozzle tip with bPWM plus a pre-orifice nozzle tip in constant (non-bPWM) mode that produced contrasting droplet sizes for spray entrainment between the two tips provided a means to increase glufosinate-ammonium deposits on leaves and increase coverage and spot density on WSP. On the other hand, Y-adapter mounted pre-orifice tips with bPWM and separate air-induction nozzle tips (extended-range and deflector designs) operated as non-bPWM applications did not result in high levels of glufosinate-ammonium deposits on leaves and did not result in high levels of coverage and spot density on WSP.