Title : Assessment of spraying effectiveness using heavy-duty agricultural UAVs in high-growing crops: Spray deposition and canopy coverage efficiency
Abstract:
This study, conducted at the Institute of Soil Science and Plant Cultivation – State Research Institute (IUNG-PIB; Poland), investigates the operational efficacy of Unmanned Aerial Vehicles (UAVs) in precision agriculture.** The research evaluates spray deposition, coverage uniformity, and drift potential, comparing a high-growing crop model (*Nicotiana tabacum* L., cv. HYV 35) with a bare-ground control. The primary objective was to validate the hypothesis that rotor-generated downwash enhances canopy penetration and significantly mitigates off-target drift in high-biomass environments relative to open-field conditions.
Materials and Methods: Experimental trials were conducted using a DJI Agras T50 UAV platform equipped with rotary atomizers to ensure precise droplet size regulation. The study employed a randomized complete block design evaluating three variable operational parameters: flight altitude (1.5 m and 3.0 m above the target), application rate (30, 60, 150, and 300 L/ha), and droplet size spectra (ranging from fine at 100 μm to coarse at 350 μm).
Key Results and Discussion: Results indicated a statistically significant reduction in spray drift facilitated by the vegetative canopy. At an application rate of 30 L/ha (100 μm droplet size), drift measured 2.5 m from the swath edge was 7.87% on bare ground, compared to 3.40% within the tobacco canopy. This substantiates the canopy's function as an effective mechanical barrier, attenuating environmental contamination risk by over 50% in the immediate buffer zone. Reducing the flight altitude to 1.5 m induced pronounced rotor downwash, yielding divergent aerodynamic effects dependent on the surface matrix. In the vegetated environment, the lower altitude significantly enhanced the penetration of the lower canopy strata. At 300 L/ha (100 μm), a coverage inversion was recorded in the middle leaf tier, where abaxial (underside) coverage (19.5%) surpassed adaxial (upper) coverage (14.7%). This phenomenon provides substantial agronomic utility for targeting pests residing on abaxial leaf surfaces. Conversely, on bare ground, these identical parameters triggered a "ground effect," wherein the deflected airstream dispersed the spray cloud. Target zone coverage diminished to negligible levels (~3.7%), denoting a critical loss of deposition control in open terrain. Fine droplet spectra (100–150 μm) exhibited limited efficacy on bare ground due to acute susceptibility to drift and evaporation (yielding <3% coverage at 300 L/ha). In contrast, the crop canopy effectively intercepted fine droplets, resulting in satisfactory deposition. Coarse droplets (300–350 μm) afforded the highest operational safety margin. At 60 L/ha, increasing droplet size from 100 μm to 300 μm reduced near-field drift from 12.0% to <2.0%. Furthermore, coarse droplets (350 μm) applied at 150 L/ha achieved maximum target coverage efficiency (40.1%) under test-range conditions.
Conclusions:
- Canopy Interception: High-biomass canopies significantly attenuate spray drift. Consequently, operational parameters prone to off-target movement over open ground (e.g., fine droplet spectra) remain agronomically viable in dense vegetation due to enhanced foliar interception.
- Altitude Optimization: Low-altitude applications (1.5 m) are advisable exclusively for dense crop architectures to maximize abaxial leaf penetration. This approach is strictly contraindicated for bare or low-canopy environments due to severe drift exacerbation driven by ground-effect turbulence.
- Droplet Size Selection: For broad-spectrum UAV applications, medium-to-coarse droplets (200–300 μm) deliver an optimal equilibrium between coverage uniformity and drift mitigation. Fine droplet applications should be strictly confined to high-volume treatments in dense canopies necessitating deep structural penetration.
