Brassinolides (BRs) are naturally-occurring phytohormones, which are essentially important to improve the crop adoptive capacity to various stresses. Spray volume (SV) and agrochemical application methods are associated with chemical deposition and field efficiency. The objective of this study was to compare the possible effects of 28-Homobrassinolide (HBL) dosages 18, 22, and 30 mg a.i. ha⁻¹ for unmanned aerial vehicle (UAV) sprayers (15 L ha⁻¹ and 30 L ha⁻¹) and 22 mg a.i. ha⁻¹ for Knapsack manual sprayers (KMS) (450 L ha⁻¹) at maize silking stage on droplets deposition distribution, photosynthetic parameters, grain filling process and yield. The results showed that the droplet deposition of UAV (15, 30 L ha⁻¹) was 47.04%, 8.89% higher than KMS. However, the UAV sprayer had a poor droplet deposition distribution. HBL significantly increased the photosynthetic parameters, grain filling rate, and yield. A UAV spray volume of 15 L ha⁻¹ with 22 mg a.i. ha⁻¹ significantly increased grains yield by 4.16–5.64%, 7.5–12.09% compared to KMS and CK in both years. Considering the high efficiency of the UAV sprayer and better effects of HBL on final yield, spraying 22~30 mg a.i. ha⁻¹ with UAV spray volume 15 L ha⁻¹ at the silking stage could be a better strategy.

Non-destructive acquisition and accurate real-time assessment of nitrogen (N) nutritional status are crucial for nitrogen management and yield prediction in maize production. The objective of this study was to develop a method for estimating the nitrogen nutrient index (NNI) of maize using in situ leaf spectroscopy. Field trials with six nitrogen fertilizer levels (0, 75, 150, 225, 300, and 375 kg N ha⁻¹) were performed using eight summer maize cultivars. The leaf reflectance spectrum was acquired at different growth stages, with simultaneous measurements of leaf nitrogen content (LNC) and leaf dry matter (LDW). The competitive adaptive reweighted sampling (CARS) algorithm was used to screen the raw spectrum’s effective bands related to the NNI during the maize critical growth period (from the 12th fully expanded leaf stage to the milk ripening stage). Three machine learning methods—partial least squares (PLS), artificial neural networks (ANN), and support vector machines (SVM)—were used to validate the NNI estimation model. These methods indicated that the NNI first increased and then decreased (from the 12th fully expanded leaf stage to the milk ripening stage) and was positively correlated with nitrogen application. The results showed that combining effective bands and PLS (CARS-PLS) achieved the best model for NNI estimation, which yielded the highest coefficient of determination (R²_val), 0.925, and the lowest root mean square error (RMSE_val), 0.068, followed by the CARS-SVM model (R²_val, 0.895; RMSE_val, 0.081), and the CARS-ANN model (R²_val, 0.814; RMSE_val, 0.108), which performed the worst. The CARS-PLS model was used to successfully predict the variation in the NNI among cultivars and different growth stages. The estimated R² of eight cultivars by the NNI was between 0.86 and 0.97; the estimated R² of the NNI at different growth stages was between 0.92 and 0.94. The overall results indicated that the CARS-PLS allows for rapid, accurate, and non-destructive estimation of the NNI during maize growth, providing an efficient tool for accurately monitoring nitrogen nutrition.

Exogenously applied phytohormones improve the endosperm cells and establish greater kernel sink capacity and grain filling, improving grain yield. In this study, 28-Homobrassinolide (HBR) dosages (20, 25, and 30 mg a. i. ha⁻¹) were applied separately at the silking stage under controlled conditions, and EDAH (a mixture of ethephon and diethyl aminoethyl hexanoate) dosage of 90 g a. i. ha⁻¹ was sprayed at the jointing stage to enhance the lodging resistance. Our objective was to investigate whether the application of HBR under controlled conditions or with EDAH could enhance the grain filling rate by regulating endogenous hormones. The results showed that HBR at the silking stage significantly increased endogenous hormones (ABA, IAA, Z+ZR), hampered leaf senescence, enhanced photosynthetic, improved dry matter accumulation in grains, and increased the grain-filling period, filling rate, and thousand-grains weight. Additionally, HBR 25 and 30 mg a. i. ha⁻¹ increased the final yield by 9.9% and 19.5% compared to the control (CK) in 2020 and 14.1% and 18.95% in 2021, respectively. There was no significant difference between the results obtained from HBR-controlled and EDAH treatments at the jointing stage. Thus, we conclude that spraying HBR 25~30 mg a. i. ha⁻¹ under controlled conditions may increase the grain yield under normal weather conditions. In adverse weather conditions and heavy wind, spraying EDAH 90 g a. i. ha⁻¹ at the jointing stage and HBR 30 mg a. i. ha⁻¹ at the silking stage can enhance maize production.