## 3 Results and Discussion
### Variable Rate Nitrogen Application Profitability
Besides, high-resolution UAV images provide over-resolved data which are not necessary for fertilization, as the working width of conventional spreaders is usually larger than 6 m. The use of UAVs can remain beneficial when high spatial resolution is required (e.g., leaf disease identification, estimation of wind damage) and if the mapping area is smaller than 77 hectares. These results were obtained considering an average advantage provided by variable rate nitrogen application on grains according to the meta-analysis of [PERSON] and [PERSON] (2018); however, a different result may be obtained considering a lower or higher profitability from variable rate nitrogen application [59]. Site-specific management may also be applied to herbicide application [60], irrigation [61] and harvest management [62]. These strategies may have a significant impact on economic analysis of site-specific management, but were not considered in this study, since their implementation in commercial farms is lower than variable rate nitrogen application. A similar study by [PERSON] et al. [63] found the break-even point between satellite and UAV acquired images at five hectares using RapidEye satellite images as reference. According to the result found in this manuscript, the break-even point between high-resolution satellites (such as RapidEye) and UAV acquired images was 13 hectares. Although a difference of seven hectares may be considered non-significant, this displacement may have the following reasons: (i) the economic analysis of [PERSON] et al. was performed on a limited number of price samples and field sizes (five and 50 hectares); (ii) that study was focused on comparison in terms of variability between sensors, and the economic analysis represented only a small part; (iii) that study was carried out on vineyards, while the current study is focused on grains. Although the aim of this study is to analyzed the economic benefits provided by remote sensing platforms for variable rate nitrogen application, such techniques can take advantage of proximal sensors [64]. In this latter case, sensors are located close to the vegetation and are often active. As a result, spatial resolution and SNR slightly affect the VI reading. The evaluation of the net returns for variable rate nitrogen application is strongly affected by within-field variability which may increase or reduce the variation in yield \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{**Break-Even Points**} \\ \hline
**Sensors** & **Hectares** & **EUR** \\ \hline MR & 2.52 & 83.1 \\ \hline HR & 13.2 & 434 \\ \hline Plane & 66.4 & 2191 \\ \hline VHR & 76.8 & 2536 \\ \hline UAV & n/a & n/a \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Break-even point in hectare and EUR for each sensor. and nitrogen requirement with variable rate management [65]. The reference value for profitability was extracted from a deep meta-analysis [52] which included studies with negative net return from variable rate nitrogen application. In these latter cases within-field variability may not be sufficient to be exploited by variable rate management, leading to a negative net return. Moreover, within-field variability and soil type may increase or decrease the potential benefits from nitrogen losses and the related subsidies from environmental policy [66]. The implementation of variable rate nitrogen application and the related net return should take into account the specificities of each field. Indeed, the placement of fields in nitrate vulnerable zones may increase the potential benefits of this practice [67].