Forage harvesters can be implements attached to a tractor, or they can be self-propelled units. In either configuration, they have either a drum (cutterhead) or a flywheel with a number of knives fixed to it that chops and blows the silage out a chute of the harvester into a wagon that is either connected to the harvester or to another vehicle driving alongside. Some larger machines also have paddle accelerators to increase material speed and improve unloading characteristics. Once a wagon is filled up, the wagon can be detached and taken back to a silo for unloading, and another wagon can be attached. Because corn and grass require different types of cutting equipment, there are different heads for each type of silage, and these heads can be connected and disconnected from the harvester. Grass silage is usually cut prior to harvesting to allow it to wilt, before being harvested from swathes with a collection header (windrow pickup). Maize and wholecrop silage are cut directly by the header, using reciprocating knives, disc mowers or large saw-like blades. Kernel processors (KP), a module consisting of two mill rolls with teeth pressed together by powerful springs, are frequently used when harvesting cereal crops like corn and sorghum to crack the kernels of these plant heads. Kernel processors are installed between the cutterhead and accelerator. In most forage harvesters, the KP can be quickly removed and replaced with a grass chute for chopping non-cereal crops.
The modern way of silage making is with a self-propelled machine with a tractor or lorry running along with the forager. Today's largest machines have multiple engines producing in excess of 1000 horsepower, are fitted with headers able to cut up to a 9 Meter swath of maize/corn in a single pass, and an output exceeding 2000 tons of silage per day. Silage made from grass, canola, oats or wheat are chopped in pieces 6 to 30 millimeters and treated with additives including bacterias, enzymes, mold inhibitors, and preservatives to accelerate the fermentation process. When silage is made of corn or sorghum additives are not necessary because of the high sugar level in the plant. Additives however are frequently added to corn and sorghum to augment their fermentation.