# Defining the Ideal Phenological Stage for Estimating Corn Yield Using Multispectral Images
## 4 Discussion
The behavior of electromagnetic radiation in the visible range is mainly 347 determined by chlorophyll. The mesophyll pigments tend to absorb wavelengths in the blue and red range [49]. Thus, the radiation from the green channel is reflected in a greater incidence 349, which may justify the presence of the green band and the vegetation indices such as GNDVI and CI-G in the 350 models of the phenological stages of VE and V5. The visible vegetation indices also comprise, in larger numbers, the models of the phenological stages of V8. In a study conducted by [48], the VARI and TGI indices presented the potential for estimating yield in corn crops; the authors generated models based on the NN algorithm, using six IV as the input parameters, among which were TGI and VARI. The models arising from the phenological reproductive stages mark the presence of the near-infrared range for the first time among the input parameters used. It is possible to verify its presence in the R1, R2, R3, R4, R5, and R6 stages. The radiation coming from the near-infrared range tends to be sensitive to vegetation due to low absorption and backscattering when in contact with the internal cellular structures of the plant [49]. The reproductive stages mark the development of the corn ear [41]. In R1, the growth of the stalk and internodes are finalized. At this stage, the stigmatyle continues its growth to be pollinated. Thus, the process of fertilization of the ovule can continue [42]. At R2, there is an accumulation of soluble sugars in the endosperm of the grain, which helps to increase the mass of grain. During this period, the corn ear is near the final fully developed size [41]. These internal changes may favor the greater reflectance from the near-infrared region and the vegetation indices derived from it. The phenological stage of R6 is characterized by the physiological maturity of the grains. In this period, the senescence process in the leaves becomes more visible as they gain a \"drier\" appearance due to chlorophyll degradation. This stage is commonly destined to harvest once the grains are independent of the mother plant. In this context, the presence of the SAVI index and its variations of MSAVI and OSAVI, which are among those selected for the constitution of the R6 model, stand out. The results displayed in Table 4 present an oscillation between the evaluation metrics used. In general, the possibility of estimating yield in the initial phenological stages is explained by the sensitivity of the bands and indices with macro and micronutrients of the exposed soil due to poorly developed crop canopy. Studies [50; 51] have demonstrated that the amount of macro and micronutrients at the time of planting influence and have a high correlation with the final yield of the crop [52]. From the yield distribution maps, the treatments' effect were observed primarily in the corn plant trials in all sections of the study areas, where yield alternated according to natural crop development. The challenges in implementing the models in areas neighboring the study area require indexing of more yield data from other agricultural regions, that is, corn growing areas subject to different management techniques and environmental conditions. In relation to the spatial distribution pattern of yield classes, an irregular geometric pattern can be observed; that is, the map does not significantly reflect any spectral model sensitivity to the rectilinear geometry defined by the different irrigation sectors. Thus, the spatial distribution of yield is more conditioned to the biotic and abiotic factors of the crop itself; that is, the yield classes do not exhibit the influence of the anthropic actions applied in the experimental area.