UAV Biostimulant Spraying in Tall Sugarcane: Field Efficacy and Rotor–Canopy Airflow Modeling

Biostimulants can improve crop vigor, yet delivering them efficiently in tall, dense canopies remains challenging. We conducted a randomized field experiment in sugarcane (Saharanpur, India; 2023) to compare UAV spraying with manual knapsack application for a seaweed-based foliar biostimulant at matched active ingredient rates. A micro-UAV generated NDVI maps to schedule two spray windows (Grand Growth and Elongation). Three treatments were tested: T1 manual (150 L ha⁻¹, flat-fan), T2 UAV–coarse (30–40 L ha⁻¹, pressure nozzles, VMD ≈250–300 µm), and T3 UAV–fine/ULV (20–30 L ha⁻¹, rotary atomizers, VMD ≈120–150 µm). Deposition and penetration were quantified with water-sensitive cards and a fluorescent tracer at upper/mid/lower strata; canopy response was tracked by SPAD and NDVI at 0, 7, 14, and 28 DAT; and yield and operational metrics were recorded. To interpret deposition patterns, we added a lightweight rotor–canopy airflow model: a momentum-disk downwash coupled to a porous-media canopy (sugarcane leaf-area density 2–4 m² m⁻³) and a Lagrangian droplet settling–advection calculation for 120–300 µm droplets at 2.5–3.0 m AGL and 3–4 m s⁻¹. The model predicted higher residence time and interception for 120–150 µm droplets in the upper/mid canopy and guided flight-line overlap/height settings. Field results were consistent: UAV–ULV achieved more uniform mid/upper coverage at a ≥70% lower carrier volume versus manual, with comparable or improved NDVI/SPAD gains at 7–14 DAT, minimal off-target drift under 1–4 m s⁻¹ winds, and maintained or higher yield and net return. NDVI-guided, airflow-informed UAV biostimulant delivery appears technically feasible and operationally efficient for tall sugarcane canopies.