Use of supervised and unsupervised approaches to make zonal application maps for variable-rate application of crop growth regulators in commercial cotton fields
Authors: Andrea Maria C. da S. , de Oliveira Cristiano F. , Mota Fabrícia C. M. , dos Santos Rafael C. , Rodrigues Junior Edilson F. , Bianchi Lucas M. , de Oliveira Rodrigo S. , de Gouveia Caio M. , Barbosa Victor G. S. , Bispo E Silva Marco A. 
Published: January 2025

Abstract

Background
Zonal application maps are designed to represent field variability using key variables that can be translated into tailored management practices. For cotton, zonal maps for crop growth regulator (CGR) applications under variable-rate (VR) strategies are commonly based exclusively on vegetation indices (VIs) variability. However, VIs often saturate in dense crop vegetation areas, limiting their effectiveness in distinguishing variability in crop growth. This study aimed to compare unsupervised framework (UF) and supervised framework (SUF) approaches for generating zonal application maps for CGR under VR conditions. During 2022–2023 agricultural seasons, an UF was employed to generate zonal maps based on locally collected field data on plant height of cotton, satellite imagery, soil texture, and phenology data. Subsequently, a SUF (based on historical data between 2020–2021 to 2022–2023 agricultural seasons) was developed to predict plant height using remote sensing and phenology data, aiming to replicate same zonal maps but without relying on direct field measurements of plant height. Both approaches were tested in three fields and on two different dates per field.

Results
The predictive model for plant height of SUF performed well, as indicated by the model metrics. However, when comparing zonal application maps for specific field-date combinations, the predicted plant height exhibited lower variability compared with field measurements. This led to variable compatibility between SUF maps, which utilized the model predictions, and the UF maps, which were based on the real field data. Fields characterized by much pronounced soil texture variability yielded the highest compatibility between the zonal application maps produced by both SUF and UF approaches. This was predominantly due to the greater consistency in estimating plant development patterns within these heterogeneous field environments. While VR application approach can facilitate product savings during the application operation, other key factors must be considered. These include the availability of specialized machinery required for this type of applications, as well as the inherent operational costs associated with applying a single CGR product which differs from the typical uniform rate applications that often integrate multiple inputs.

Conclusion
Predictive modeling shows promise for assisting in the creation of zonal application maps for VR of CGR applications. However, the degree of agreement with the actual variability in crop growth found in the field should be evaluated on a field-by-field basis. The SUF approach, which is based on plant heigh prediction, demonstrated potential for supporting the development of zonal application maps for VR of CGR applications. However, the degree to which this approach aligns itself with the actual variability in crop growth observed in the field may vary, necessitating field-by-field evaluation.