Effects of Row Spacing and Nozzle Type on Spray Penetration Inside Soybean Canopy Under Various Wind Velocities

Adequate spray deposition and penetration of pesticides into the lower part of the soybean canopy can increase the chances of successfully protecting plants from diseases and insects. Only a small number of comprehensive studies have examined how spray application parameters (nozzle types, travel speed, droplet size, application rate, application equipment) affect droplet penetration into the inner and lower parts of the soybean canopy. However, the data obtained from replicated plots in these field experiments showed significant variability due to uneven soybean canopy characteristics and unpredictable wind speed and direction. To minimize variability in field studies, this study used a new methodology: conducting the experiment under controlled conditions in a wind tunnel. This research was conducted to evaluate the effect of increasing the distance between soybean rows on the spray coverage and deposition of different droplet size classes from various nozzles, delivering spray to the lower canopy in a wind tunnel. Four commercially available spray nozzles with droplet size classification from medium to extremely coarse were mounted on a spray boom with a spray controller to spray an application rate of 150 L ha−1 under laminar wind speeds of 0, 2.4, and 5.1 m s−1. Rectangular pots containing fully grown soybeans were placed in the test section of the tunnel at center-to-center distances of 0.38 and 0.76 m to replicate narrow and wide row spacings, respectively, commonly used by soybean growers. Eight points in each soybean row were selected to collect spray deposition and coverage with water-sensitive papers (WSPs) and acrylic plates (APs), respectively, at the top, middle, and lower layers of the canopy. Results showed that the top of the soybean canopy consistently received the highest amount of spray, regardless of application conditions, as expected, while the middle and lower layers of the canopy did not receive much spray. Nozzle types and wind speeds were not significant factors in increasing spray penetration into the middle to lower layers of soybean plants. Although wider row spacing improved the spray deposition in the lower part of the canopy, this improvement was not statistically significant. The main conclusions derived from this study indicate that even using wider row spacing configurations, spray penetration into the lower parts of the soybean canopy was limited due to denser canopy conditions and the effects of high wind speeds. Therefore, other advanced spray techniques, such as air-assisted spraying or using other mechanisms to expose lower parts of the canopy to the nozzles, may be needed to effectively overcome these limitations.