Objectives This study aims to optimize the zinc (Zn) and selenium (Se) fertilizer application timing and dosage by unmanned aerial vehicles (UAV) in typically Zn and Se deficient areas of the Loess Plateau, and provide theoretical basis and technical support for large area Zn and Se biofortification in wheat production.

Methods During the 2023−2025 wheat growing seasons, a field experiment was conducted at two representative sites in Shaanxi Province: a typical rainfed farmland (Yongshou County) and an irrigated farmland (Qishan County). A control group with no zinc or selenium fertilization was established. Treatments included foliar application of zinc (Zn) or selenium (Se) fertilizer via unmanned aerial vehicle (UAV) either once at the booting stage or twice at both the booting and flowering stages. For each application, three dosage levels were set: Zn at 780, 1560, and 2340 g/hm², and Se at 10.0, 20.0, and 30.0 g/hm². Wheat yield, Zn and Se concentrations in grains and flour, and available Zn and Se contents in soil were measured.

Results UAV-based foliar application of Zn or Se fertilizer had no significant effect on wheat yield but significantly increased Zn and Se concentrations in both grains and flour. The biofortification efficacy varied by location and year. Under high-dose Zn fertilization applied twice, the average grain Zn concentration over two seasons reached 34.7 mg/kg (rainfed) and 35.7 mg/kg (irrigated), and increased to 39.3 mg/kg and 51.3 mg/kg, respectively, in the precipitation-normal year of 2024, meeting the biofortification target. Flour Zn biofortification was more readily achieved in rainfed areas, where a single low-dose Zn application yielded an average flour Zn concentration of 15.6 mg/kg, whereas irrigated areas required two high-dose applications to reach 16.4 mg/kg. Compared with Zn, Se was more efficiently translocated to grains and flour. In 2024, with normal precipitation, the Se biofortification target in both grains and flour was achieved with a single high-dose application in rainfed fields and a single medium-dose application in irrigated fields. In the drought year of 2025, two high-dose applications in rainfed fields and two medium-dose applications in irrigated fields were required to meet the target. UAV Se fertilization did not significantly increase available soil Se content, whereas high-dose Zn fertilization significantly enhanced soil available Zn levels. The market price of Zn- and Se-biofortified flour was higher than that of non-biofortified flour, with an estimated economic benefit of 4296−11441 yuan/hm² from foliar spraying.

Conclusions UAV-based foliar Zn and Se fertilization is an effective approach for large-scale wheat biofortification on the Loess Plateau. In years with normal rainfall, applying Zn fertilizer at 2340 g/hm² at both the booting and flowering stages in rainfed areas, and at 1560−2340 g/hm² during the same stages in irrigated areas, can achieve the target Zn concentrations in both grains and flour. Applying Se fertilizer at 20−30 g/hm² at either the booting or flowering stage can meet the Se biofortification targets for grains and flour.