Spray Volume is Critical for Postemergence Herbicides

By Bob Hartzler, Professor of Agronomy, Iowa State University
Dr. H. Mark Hanna, Scientist II, Iowa State University

Proper herbicide application is critical to ensure products achieve their full potential for weed control. For postemergence herbicides, this includes application to appropriately sized weeds and equipping the sprayer to achieve uniform coverage of target weeds. Weed scientists at Purdue University recently reported on the influence of nozzle type and spray volume on target coverage within the soybean canopy. A traditional flat fan (XR) nozzle and three drift reduction nozzles were included in the study. The XR nozzle produced more driftable droplets than the other nozzles, and the two types of air induction nozzles had much less volume in small droplets than the XR and TTJ nozzles

Coverage of targets within the canopy was evaluated by placing water sensitive paper at different heights within 12” tall soybean planted in 15” rows. Averaged over nozzle types and spray volume, coverage declined by approximately 50 percent from the top of the canopy to targets placed 4” above the soil surface.

Table 1. Effect of nozzle type and size on production of driftable droplets. Legleiter and Johnson. 2016.

The TTI nozzle was less consistent than the other nozzles on coverage at the top and middle of the canopy. The TTI nozzle produces the largest droplet size of the nozzles evaluated, and thus would be expected to provide less uniform coverage than the others. However, nozzle type did not have a significant affect on coverage of targets at the bottom of the canopy. The authors speculated that large droplets were more efficient at penetrating the canopy than small droplets, therefore resulting in similar coverage by the different nozzles of targets placed at the base of the canopy.
Regardless of nozzle type or target placement, better coverage was achieved with the larger nozzle size that provided greater gallons per acre (GPA) spray volume. Averaged