Malting barley (Hordeum vulgare) requires precise nitrogen (N) fertilizer management to achieve a narrow range of grain protein content (≈9–10.5%) while maintaining yields, but practical tools to accomplish this are lacking. This study hypothesized that canopy reflectance (Normalized Difference Vegetation Index (NDVI)) measured at tillering (Feekes 2–3) and expressed as a sufficiency index (SI), can estimate the likelihood of a site-specific response to in-season N fertilizer in malting barley. Canopy reflectance was measured from plots at tillering with a GreenSeeker and unmanned aerial vehicle (UAV) borne multispectral cameras in trials across heterogeneous California agroecosystems. Field experiments included a range of N fertilizer application rates (0–168 kg N ha^-1) and timings (pre-plant, tillering, or evenly split), and resulted in a range of crop N sufficiency/deficiency. NDVI-based SI measurements were categorized into one of three quantitative categories (low, medium, and high) without additional experimental context using Gaussian mixture modeling. Despite that 85% of variation in protein yield was due to site-year, the reflectance-based categories indicated whether N fertilizer applied in-season would increase protein yield (p < 0.01).  Nitrogen application at tillering increased yield and protein for plots in the “low” and “medium” SI categories (45 and 4% for yield and 16 and 12% for protein, respectively) (p < 0.05), while “high” SI plots had neither yield (p = 0.23) nor protein (p = 0.26) increases. Importantly, the broader agronomic conditions of a site primarily determined whether response to in-season N manifested as increased yield or protein.

The .Rmd file details how the data was analyzed and used in the manuscript.