Optimizing Spray Technology and Nitrogen Sources for Wheat Grain Protein Enhancement

Increasing wheat (Triticum aestivum L.) grain protein concentration (GPC) without excessive nitrogen (N) inputs requires understanding the interactions between N source characteristics and application technology parameters. This study evaluated the effects of foliar N applications at anthesis on wheat grain yield and GPC across three locations over three growing seasons in Oklahoma. Treatments consisted of two N sources (urea-ammonium nitrate [UAN] and aqueous urea [Aq. urea]), three nozzle types (flat fan [FF], 3D, and twin [TW]), and two droplet types (fine and coarse). Late foliar applications increased GPC by 12% without affecting grain yield (0.5–5.8 Mg ha−1). During the 2020–21 growing season, a late season freeze during anthesis resulted in no significant differences in GPC across locations. UAN produced significantly higher GPC (13.7%) than Aq. urea (13.1%). Among nozzle types, the 3D nozzle consistently produced the highest GPC (13.8%), compared to FF (13.1%) and TW nozzles (13.2%). Two-way interactions revealed UAN with fine droplets achieved consistently high GPC (14.6%), as did Aq. urea with coarse droplets (14.5%) at Lake Carl Blackwell in 2021–22 as compared to Aq. Urea_Fine (13.8%). At Chickasha 2021–22 and Perkins 2020–21, a significant three-way interaction was observed, with the UAN_3D_Fine (13.2%) and UAN_3D_Coarse (12.2%) treatments producing the highest GPC, with 8% and 15% greater than the Aq. Urea_TW_Fine, respectively, which is lowest. These findings provide a foundation for precision agriculture approaches that optimize foliar N application parameters to enhance wheat quality while maintaining sustainable production practices.