In one aspect, an agricultural harvester may include a harvesting implement having a belt support assembly configured to support a conveyor belt of the harvesting implement as the conveyor belt moves relative to the belt support assembly. The system may also include a sensor provided in operative association the belt support assembly, with the sensor being configured to detect a parameter indicative of a force exerted on the belt support assembly as the conveyor belt moves relative to the belt support assembly. Furthermore, the system may include a controller communicatively coupled to the sensor, with the controller being configured to monitor the amount of the plant materials entering the harvester based on measurement signals received from the sensor.
