A system and related method of applying forage treatment material to harvested forage is provided. The system can include a mobile forage harvester having a chute, a vacuum source that produces a vacuum, and a bin that stores granular treatment material for dispensation. The chute can be joined with a catch tube having a catch opening. A tube can extend from the bin toward the catch tube, but can be separated from the catch opening by an open gap. The material can be dispensed from the tube into the open gap, and drawn across the open gap and into the catch opening with the vacuum so the material mixes with the harvested forage moving along the pathway in the chute under force of the vacuum. The bin, tubes and the open gap can be located in an interior compartment to protect them from the elements. A related method is provided.