Abstract
Nitrogen and water are two key factors for wheat production due to their major roles in plant growth and development, photosynthesis, yield, and grain protein content. Plant uptake of water and N is fundamentally interactive. Our objectives were: (a) to analyze the effects of different irrigation (IR) and N rates on spring wheat (Triticum aestivum L.) yield and grain protein, and spectral indices (relative greenness [SPAD] and Normalized Difference Vegetation Index [NDVI]), and (b) to identify the optimum IR and N requirements for wheat grain production in semi‐arid conditions of Montana and Idaho. This article details the results from field experiments conducted at three locations for two growing seasons (6 site‐years). Relative greenness measured by SPAD chlorophyll meter was used to assess plant N status, whereas NDVI was used for both plant N status and estimation of wheat yield. Both SPAD and NDVI values increased as N and IR application rates increased. The SPAD and NDVI values explained 80 and 84% of the variation in wheat yield, respectively. We found that IR at 75% of evapotranspiration (ET) throughout the growing season is adequate to optimize wheat yield and grain protein. Nitrogen rate was not correlated with wheat yield at any of the site‐years. Based on this study's results, approximately 150 kg N ha−1 (total, soil residual N plus N added as fertilizer) may be sufficient to optimize yield and grain protein content of irrigated spring wheat in semi‐arid cropping systems.