Influence of the physical parameters of fields and of crop yield on the effective field capacity of a self-propelled forage harvester

Data for the effective field capacity of two self-propelled forage harvesters were collected from 163 fields located in Galicia, Northwest Spain, during the silage maize-harvesting season by using a telemetry system. The values obtained from the study were related to crop yield, field area, field longitudinal slope and cross slope, and to attributes of field shape: number of total vertices, number of acute vertices and shape index. Because of the lack of a shape index that reliably reflects the impact of field shape on the effective field capacity of harvesters, two new shape indices are proposed in this study. The results obtained using these two indices are analysed and compared with the results obtained using the conventional shape index (area/perimeter squared). The results of the analysis suggest a strong correlation between effective field capacity and both crop yield and field area. In addition, significant but weak correlations were found between effective field capacity and both longitudinal slope and shape index. The results obtained by considering all fields and just the largest fields (of more than 1 ha) were similar. Fields smaller than 1 ha showed a worse fit of the regression line to the data, which suggests the need to include new variables in the model. The results of the shape indices developed in this study were slightly better than the results obtained using the conventional index.