Canopy reflectance informs in-season malting barley nitrogen management: an ex-ante classification approach data

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.