Electrostatic spraying systems can improve the pesticide application efficiency by enhancing droplet deposition and coverage within crop canopies. This study evaluated the droplet size spectra and charge-to-mass ratio (CMR) of five electrostatically charged hollow-cone nozzles and one flat-fan nozzle paired with an electrode. Each nozzle was mounted on a moving boom in a wind tunnel and operated with the electrode and voltage that produced the highest CMR. Their effects on the spray coverage and deposition inside boxwood shrubs at wind speeds of 0 and 2.24 m s⁻¹ were assessed. The nozzles operated with the optimized electrode had average improvements in the canopy deposition and canopy coverage of 1.33 µg cm⁻² and 4.4% at a wind speed of 0 m s⁻¹ and 0.26 µg cm⁻² and 0.9% at a wind speed of 2.24 m s⁻¹. The airborne drift measurements at various heights above the wind tunnel floor showed an average 0.50 µg cm⁻² reduction in the drift at 0.1 m, variable results at 0.35 m, and minimal changes at heights of 0.7 m and above at a downwind distance of 2 m. These findings highlighted the potential of optimized electrostatic spraying systems to enhance pesticide deposition inside the crop canopy under various wind speeds while reducing the spray drift potential.

Environmental factors such as rainfall occurring soon after herbicide spraying can decrease weed control efficiency. The objective of this study was to evaluate the effect of simulated rain on the rainfastness of an herbicide mixture composed of 2,4-D + glyphosate, used to control Amaranthus spp., Commelina benghalensis, and Glycine max L. Merril soybean at two developmental stages. The herbicides used were dimethylamine salt (DMA806 BR® 670 g L-1) and glyphosate potassium salt (Roundup Transorb R®, 480 g L-1). The mixture was sprayed using a TTI11002 nozzles (300 kPa) at an application rate of 150 L ha-1. The plants were subjected to simulated rain at 5, 15, 30, 45, 120, and 240 minutes after herbicide application. Control efficiency assessments were performed visually at 35 days after application. Rainfall up to 240 minutes after 2,4-D + glyphosate mixture application reduced the control efficiency for G. max and C. benghalensis weed species. The control of Amaranthus spp. was satisfactory after 30 minutes without rainfall. Therefore, the decreased control efficiency of the 2,4-D + glyphosate herbicide mixture is influenced by the interval between rainfall and plant species and developmental stage.

Rainfastness is the ability of products applied to the target to resist the effect of rainfall after application. This study aimed at evaluating the effect of simulated rainfall on the rainfastness of the mixture of the herbicides dicamba + glyphosate in the control of the species Amaranthus spp., Commelina benghalensis, and Glycine max L., in two developmental stages. The herbicides used were: dicamba (Atectra® 480 g L-1) and glyphosate potassium salt (Roundup Transorb R®, 480 g L-1). The mixture was sprayed using a TTI 11002 (300 kPa) nozzle with an application rate of 150 L ha-1. After application, the plants were subjected to simulated rainfall at different times: 5, 15, 30, 60, 120, and 240 minutes. The results indicated that when rainfall occurred up to 240 minutes after the application of the mixture of dicamba + glyphosate, the control of the species Amaranthus spp. and C. benghalensis reduced, while G. max showed satisfactory control regardless of the occurrence of rainfall. It is concluded that the control efficiency of the mixtures of the herbicides dicamba + glyphosate depend on the precipitation interval after application, the plant species found at the site, and the developmental stage of the species. Glycine max showed satisfactory control regardless of the precipitation interval after spraying the dicamba plus glyphosate tank mix. In contrast, a four-hour precipitation-free period after spray application was not sufficient for effective control of Amaranthus spp. and Commelina benghalensis (plants taller than 0.15 m and more than ten leaves).

The application of plant protection sprays using drones or remotely piloted aircraft is a key advancement in the digital technological automation of agriculture. However, the optimal spray application rate and droplet size required to minimize the impact of weather conditions on spray coverage and deposition remain to be elucidated. This study aimed to evaluate how weather conditions, spray application rate, and spray droplet size affect spray coverage and deposition when applied using remotely piloted aircraft (RPA). The experiment was performed in a 2 × 2 × 4 factorial design (time of day, application volume, and droplet size). An irregular 5 × 10 m grid was established with 20 sampling points for the distribution of collectors for spray deposition and coverage analyses. The application of sprays as medium-sized droplets resulted in higher deposition levels, whereas coarse-sized droplets provided greater coverage than achieved with other droplet sizes. Weather conditions and droplet size classes are crucial factors affecting spray coverage and deposition during spraying operations with remotely piloted aircraft. The results obtained in this study revealed a clear distinction between the effects of mediumand coarse-sized droplets on spray coverage and deposition, regardless of spray application rate and weather conditions. Spray application rate directly impacts deposition uniformity, with higher application rates leading to reduced variation than observed at lower application rates.

The addition of adjuvants to herbicides to improve the effectiveness of spray solutions for weed control has been widely adopted. This research aimed to determine the spread, contact angle, and evaporation time of glufosinate, with and without adjuvants, on the leaf surfaces of four weed species Digitaria insularis (L.) Fedde, Cenchrus echinatus L., Amaranthus viridis L., and Bidens pilosa L.). We used a completely randomized design and a 4 x 4 factorial scheme, with ten replications. The surfaces of four weed species were the first factor and the applied solutions (distilled water, glufosinate herbicide, glufosinate + surfactant, and glufosinate + mineral oil) were the second factor. Concerning droplet deposition, the wide-leaved weeds showed a broader spread and a greater contact angle in contrast to the narrow-leaved ones. The droplet spreading area was negatively correlated with evaporation time and contact angle. We conclude that the spreading area and evaporation time of herbicide droplets depend on the characteristics of each surface area and vary among different weed species. The addition of adjuvants to spray solutions is essential for glufosinate herbicide, particularly on leaf surfaces with higher wax content.

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.