An irrigation system that includes multiple interconnected spans that are supported by multiple tower structures. Each intermediate tower structure and end tower structure may include a dual-controller drive assembly that may include both a fixed-speed drive controller controlling the speed of movement of the dual-controller drive assembly when the fixed-speed drive controller is selected and a variable-speed drive controller controlling the speed of movement of the dual-controller drive assembly when the variable-speed drive controller is selected, and one or more selector switches configured to either manually, based on manual operator input, or automatically, based on automatic sensor input, select between the fixed-speed drive controller and the variable-speed drive controller.
