Silos are filled using blowers, augers, conveyors, or heavy equipment, depending on whether the silo stores chopped forage (silage) or dry grain. The method also depends on the silo’s shape: tall tower silos, wide bunker silos, grain bins, and plastic bag silos each require different filling techniques. Here’s how each one works.
Tower Silos: Blowers and Pipes
The classic upright silo, often 40 to 80 feet tall, is filled from the top using a silo blower. A tractor powers the blower through its power take-off shaft, spinning a large fan inside the machine. As chopped forage is unloaded from a wagon into the blower’s hopper, an auger or conveyor feeds the material into the spinning fan, which launches it upward through a series of connected pipes running along the outside of the silo.
The energy required is significant. A 40-foot silo can be filled with a tractor in the 40-horsepower range, though it will run slowly. Most operators prefer 60 or more horsepower to keep things moving at a reasonable pace, and large blowers paired with tall silos can demand well over 100 horsepower. Filling a 72-foot silo, for example, has been done with high-capacity blowers running off tractors with several hundred horsepower.
Once inside, the forage piles up and gradually settles under its own weight. Some tower silos have mechanical distributors at the top that spread the material evenly. As the silo fills over several loads, the weight of new material compresses the layers below, squeezing out oxygen and creating the airless environment that allows fermentation to preserve the forage.
Grain Bins: Augers and Bucket Elevators
Dry grain like corn, wheat, or soybeans goes into metal grain bins using a different approach. Portable augers, essentially long tubes with a rotating spiral inside, carry grain from a truck or wagon up into the top of the bin. These augers come in diameters from 6 to 10 inches and lengths from 20 to 60 feet, powered by tractor PTO, electric motors, hydraulic motors, or gasoline engines. A 10-inch auger handling dry shelled corn can move around 3,650 bushels per hour, while a 6-inch model handles roughly 1,760 bushels per hour.
Larger commercial operations often use bucket elevators instead of augers. These are vertical chains of small buckets that scoop grain from the bottom and dump it at the top, handling higher volumes with less grain damage.
One critical detail in grain bin filling is how the grain distributes inside. When grain simply drops from a central spout, broken kernels and fine particles concentrate heavily in the center, reaching concentrations as high as 25 percent directly under the spout while the edges stay around 2 percent. This uneven distribution creates airflow problems during drying and storage, because air takes the path of least resistance and bypasses the dense, tightly packed center. Mechanical grain spreaders solve this by scattering the grain more evenly as it enters the bin, reducing center fines to under 10 percent and creating a more uniform mass that dries and stores predictably.
Bunker Silos: Trucks and Packing Tractors
Bunker silos are long, open-topped structures with concrete walls on the sides, sometimes just trenches cut into a hillside. Filling them looks nothing like filling a tower silo. Trucks or wagons dump chopped forage directly into the bunker, and then heavy tractors drive back and forth over the pile to pack it down.
Packing density is everything. The minimum target is 14 pounds of dry matter per cubic foot to squeeze out enough oxygen for proper fermentation, though densities above 20 pounds per cubic foot are achievable and produce better results. Operators use a rule of thumb called the “800 rule” to match the total weight of their packing tractors to the rate forage is being delivered. If forage arrives faster than the tractors can pack it, the silage ends up too loose, traps oxygen, and spoils.
The forage is spread in thin layers, typically 6 inches or less, before being driven over repeatedly. Once the bunker is full, the entire surface is sealed with heavy plastic sheeting weighted down with tires or gravel bags to maintain the airless environment during fermentation.
Silage Bags: Compression Tunnels
Silage bags are an increasingly common alternative, especially for operations that need flexible, expandable storage. A specialized bagging machine sits at the open end of a long plastic tube, usually 8 to 12 feet in diameter. Chopped forage is fed into the machine, where a rotor compacts the material through a press tunnel and into the connected plastic bag. As more material is forced in, the bag fills from back to front, and the machine slowly pushes itself away from the growing bag.
The compression creates the same oxygen-free environment as a packed bunker or a settled tower silo, allowing fermentation to begin almost immediately. The bags can be placed on any relatively flat surface, making them popular on farms that have outgrown their permanent silo capacity.
Monitoring Fill Levels
Knowing how full a silo is matters for both safety and efficiency. Modern operations increasingly use ultrasonic or radar sensors mounted at the top of silos and grain bins. Ultrasonic sensors send sound pulses downward and measure how long the echo takes to bounce back from the surface of the grain or silage. The travel time translates directly into a distance measurement, telling operators exactly how much space remains. Radar sensors work on the same principle using microwave signals instead of sound, which makes them more reliable in dusty conditions or when moisture builds up on the sensor.
These sensors feed into automated systems that can trigger augers to start or stop, alert operators when bins are nearly full, and track inventory across multiple storage structures from a single control panel.
Gas Hazards During and After Filling
Filling a silo, particularly a tower silo with fresh forage, creates a serious and invisible danger. As the plant material begins to ferment, it produces toxic gases that are heavier than air and settle inside the silo and in connected spaces like chute rooms. The first week after filling is the most dangerous period. The standard safety guidance is to avoid entering a silo for three weeks after it has been filled. If entry is absolutely necessary, the blower should run for 20 to 45 minutes before anyone goes inside, and it should keep running the entire time a person is in the silo.