## 4 Discussion
### Discussion of Results  
The results demonstrate the reliable yield performance of the GFO fertilization system for both the multi-row winter barley variety (Meridian) and the two-row variety (Sandra) under the given trial conditions. The GFO fertilization system almost achieved the optimum yield (\(\rm{Y_{opt}}\)), and the optimum N-fertilize quantity (\(\rm{N_{opt}}\)) was also well matched. The frequently expressed criticism of the GFO system for ecological reasons--overestimation of fertilizer requirements, resulting in N over-fertilization [25]--cannot be confirmed for the present trial with winter barley. N fertilization of the GFO treatments was slightly above \(\rm{N_{opt}}\) only for Meridian in 2021, whereas for all other years, and for variety Sandra, it was below \(\rm{N_{opt}}\). [PERSON] [40] identified N fertilizer doses above the economic optimum as particularly susceptible to nitrate leaching. Yield losses compared to \(\rm{N_{opt}}\) were quite low. [PERSON] et al. [41] observed significant reductions in crude protein contents in Denmark because of strong fertilizer regulations. Crude protein contents in our trial were on average 12.1%, and therefore in range with standard crude protein contents in winter barley. Rather, at least for the experimental conditions applied here, it can be concluded that the GFO system is very well-adjusted and enables high yields without leading to excessive N surpluses. Therefore, there is no need to correct the fertilization algorithm. Quadratic regression models estimating the economic optimum showed that GFO performed close to the theoretical optimum. However, the LP model consistently recommends significantly lower \(\rm{N_{opt}}\), with differences of up to 55 kg ha\({}^{-1}\) for two-row barley and up to 43 kg ha\({}^{-1}\) for multi-row barley, depending on the year. The findings of this study highlight sensor-based fertilization systems [12; 42; 43] and BESyD as valuable alternatives to the established GFO strategy for N management. While the GFO remains a robust approach for achieving high yields and NUE, the sensor-based N 18 treatment performed comparably, achieving yields near the economic optimum and largely balanced N surpluses, demonstrating both environmental and agronomic benefits. Over the three-year period, GFO consistently outperformed BESyD and sensor-based systems in terms of NUE and N surplus reduction, particularly for the multi-row variety, Meridian. For instance, the sensor-based treatment Sensor 10 t ha\({}^{-1}\) achieved 92-94% of GFO's efficiency in certain years, while BESyD displayed even lower NUE values. The high NUE performance of GFO is further supported by its minimal or negative N surpluses, indicating a balanced N application approach. An important aspect of the sensor-based approach is its flexibility. The N 18 (Sensor 10 t ha\({}^{-1}\)) treatment, unrestricted by the GFO's N application ceiling, applied higher N rates in some cases and significantly lower rates in others. This dynamic adjustment highlights the potential of sensor-based systems to optimize NUE more precisely than GFO. In practical applications, however, GFO-compliant limits are legally binding, meaning that a sensor-based fertilization system operating within these limits (e.g., the N 28 treatment) could still lead to overall reductions in mineral N use while maintaining high yields. The study also tested fine-tuning adjustments to sensor-based fertilization (e.g., N 19 to N 22), but these variations did not improve performance. These findings align with observations from previous research, which frequently reported that sensor-based fertilization systems lead to reduced N application rates, higher NUE, and lower N surpluses, but rarely to significant yield increases compared to uniform field-level fertilization or GFO recommendations [18, 22, 30]. Despite their promise, sensor-based systems are not without challenges.