## 3 Technologies of Monitoring in Montado
### Precision Agriculture
#### 3.2.3 Pasture Monitoring
In addition, remote optical sensors can potentially detect physiological and biochemical changes in plants, in addition to the non-invasive detection of changes in photosynthetic energy conversion, which can help in decision making in an agricultural context [110]. Since both NDVI and capacitance present very similar and acceptable results for the characterization of pasture productivity, it is understandable that the active optical sensor \"OptRx\" will gradually replace the Grassmaster II (technology that allows for the estimation of pasture production). This eventual replacement is based on the advantages of this sensor concerning the capacitance probe, namely, regarding the possible speed of continuous monitoring of the pasture (on a mobile platform) without the operator's manual intervention at various points of the pasture [81]. However, in a study by [PERSON] et al. [118], a strong correlation was obtained between Grassmaster II readings and pasture biomass production (r\({}^{2}\) = 0.75). Thus, the total pasture biomass can be estimated directly through cutting and weighing or indirectly through capacitance meters ([PERSON] et al., 1990), cited by [85]. During the 1970s, many methods were evaluated. Some methods, such as the electronic capacitance probe (Grassmaster II), have been adapted commercially. Capacitance instruments generally consist of an electrical circuit that generates a signal at a specific frequency and then performs a capacitance measurement of the air-plant mixture [84]. This equipment makes it possible to automatically record and store the values of all the readings taken in each plot to be later downloaded to the computer and processed [38; 84]. This is a recognized advantage, since the operator does not need to interfere in recording the information, being able to sample a large area. According to [PERSON] [120], the measurements made by the capacitance probe vary depending on the type of plants that make up the pasture and changes in its structure. The Grassmaster II features 2 calibration equations developed on New Zealand pastures to estimate pasture DM production (kg/ha). these pastures consisted of a mixture of rye and clover, in a ratio of 80/20, respectively, with a DM content of 14-16% [38]. In a study carried out by [PERSON] et al. [121], in three Montado plots in Alentejo, to calibrate a capacitance probe (Grassmaster II) and to estimate pasture productivity in this ecosystem, robust correlations were obtained (r\({}^{2}\) = 0.94 and r\({}^{2}\) = 0.81 for DM in February and March, respectively). According to [PERSON] et al. [122] and [PERSON] et al. [123], biomass quantification is based on the fact that the capacitance of the air is low, while that of the vegetation is high, being necessary to calibrate the probe before being used. Therefore, it is necessary to carry out several readings quickly and effectively before taking readings in the intended locations. Capacitance probes have become a fast, accurate, and non-destructive technology for estimating vegetation production [124]. However, significant restrictions on using the capacitance probe include its inability to estimate the production of individual species ([PERSON], 1978), cited by [84]. When vegetation is more homogeneous and has fewer moisture content variations, estimates based on capacitance probe readings are more reliable [125]. However, this probe has great potential for estimating the production of GM and DM in cultures consisting of a single plant species [84]. [PERSON] et al. [38] found very high correlations in pastures composed of grasses (r = 0.90) and in heterogeneous pastures (r = 0.87) between pasture DM and Grassmaster II readings. As for pastures composed essentially of legumes, [PERSON] et al. [38] obtained a moderate correlation (r\({}^{2}\) = 0.48). In this context, [PERSON] et al.