Biofuels play an important role in the reduction of greenhouse gas emissions, but their production results in greenhouse gases such as nitrous oxide (N₂O), mainly from nitrogen (N) fertilization. Brassica carinata (carinata) is an unexplored winter crop in the world’s main cropping areas, with multiple applications (cover crop, jet biofuel, and animal feed, among others). We analyzed a set of on-farm fertilization experiments (2016–2018) in southern South America using quantile regression to establish a baseline for both seed yield (Y_SEED) and the different components of nitrogen-use efficiency (NUE). Maximum Y_SEED for the 50th and 90th percentiles ranged from 2.5 to 3.5 Mg ha⁻¹, with a N availability of 150–160 kg ha⁻¹ (soil + fertilization). The NUE ranged from 3 to 13 kg seed per kg of N available. Carinata, in the absence of other limiting factors, had a high N uptake (N_UPT) capacity (1.0 to 1.5 kg ha⁻¹ N_UPT per kg ha⁻¹ N available [soil + fertilization]). The explored N fertilization rates had no significant influence on oil concentrations (455 to 517 g kg⁻¹) and protein concentrations (192 to 253 g kg⁻¹). The region has a high potential for carinata production, with a high capacity to take up available N.

Improving nitrogen (N) use efficiency in Brassica carinata is essential to sustain yield and lower biofuel carbon intensity under variable field conditions. We hypothesized that integrating physiological indicators, such as the nitrogen nutrition index (NNI) and the critical nitrogen uptake curve (CriNUC), into N management frameworks would better discriminate crop N status and improve the diagnostic evaluation of fertilizer performance in B. carinata under temperate field conditions. A 2‐year, multi‐site field experiment in Uruguay evaluated conventional split urea (CONTROL), slow‐release, and nitrification‐inhibited fertilizers (ENTEC 150 and Sulfammo‐NPRO) at 0, 90, and 150 kg N ha ⁻¹ . Crop responses were assessed through yield components, nitrogen uptake, nitrogen use efficiency (NUE) metrics (agronomic efficiency of nitrogen [AE _N ], recovery efficiency of nitrogen [RE _N ], and IE _N ), and physiological indicators (NNI and CriNUC). ENTEC 150 increased grain yield by 70% relative to the CONTROL and consistently maintained N sufficiency from elongation to flowering. Across treatments, NNI and CriNUC provided complementary, in‐season insights that were not captured by static NUE indices, particularly under environmental stress. Neither RE _N nor AE _N correlated with NNI at flowering, highlighting potential physiological or structural constraints (e.g., lodging) that limit predictability under field conditions. Treatments with NNI > 1 showed increased protein and reduced oil concentration, confirming trade‐offs between productivity and grain composition. While results are exploratory due to the use of a generic dilution curve, our findings suggest that combining enhanced‐efficiency fertilizers with crop‐based diagnostics supports more precise and adaptive nitrogen management in B. carinata . Further calibration is needed to refine these tools and validate their utility for decision‐making across diverse environments.