Effects of Long-Term Input of Controlled-Release Urea on Maize Growth Monitored by UAV-RGB Imaging

Maize is a critical crop for global food security, yet excessive nitrogen (N) application, while sustaining yields, leads to reduced nitrogen use efficiency (NUE), and the application of controlled-release fertilizer (CRF) is one of the effective options to achieve sustainable maize production while improving NUE. This study evaluated the long-term effects of CRF with varying N input rates on maize growth using low-cost UAV-RGB imaging. UAV-RGB images were captured in different growth stages, and the non-canopy background was removed using the maximum between-class algorithm (OTSU). Eleven vegetation indices were constructed from the images to analyze maize growth under different N treatments. The results indicated that a single application of CRF with an equivalent N input rate to conventional treatment yielded significantly better outcomes. The optimal controlled-release N ratio was 40% of the total N input, increasing maize yield by 6.73% and NUE by 15%. Indices such as NRI, NBI, ARVI, RGBVI, ExR, ExG, and ExGR effectively reflected plant N status, with R2 values exceeding 0.856 for yield estimation across growth stages. UAV-RGB imaging proved to be a viable method for rapid N status monitoring, aiding in the optimization of N management in maize production.