Different from domain ontologies, task ontologies must describe the knowledge from its structural and behavioural views, considering aspects as sequence of execution, conditional deviation, external expected and unexpected events interference, pre and post conditions, task granularity, agent participation, geographic localization, resource consummation, production and change. Although the use of conceptual models is well accepted to formally describe domain ontologies, there is little research about conceptual models for complex task ontologies. This paper describes the ongoing research on the Agriculture Operations Task Ontology (AGROPTO) where OntoUML is used to develop conceptual models to describe complex task’s aspects and possible modelling solutions based on Unified Foundation Ontology (UFO). An extension of the E-OntoUML, a language for modelling task ontologies, is suggested to describe methods for modelling task objectives, external event interference, pre/post conditions and task execution state modifications in order to guide future research.