Packing and Covering Bunker Silos


Bunker silos offer many advantages over tower silos for the storage of haylage and corn silage, but require careful management to minimize fermentation and spoilage losses. Along with proper harvest moisture, length-of-cut and the use of a research proven silage inoculant, additional attention must be applied to proper site selection, sizing, filling, packing, covering, rodent control and feed-out of horizontal silos. The invisible dry matter and quality losses as a result of poor packing and covering, in addition to the more visible spoilage losses are much more significant than we realize. With high production costs and forage values, these losses are expensive!

Fill Rapidly

Filling silos as rapidly as possible reduces silage exposure to air and rainfall. Bunker silos should be filled from back to front so that a “progressive wedge shape” (1:4 slope) is created, rather than filling from bottom to top. The exception would be in a high volume, rapid filling situation.

Pack, Pack, Pack

Packing is typically the weakest link in bunker silo management. When you see a bunker silo “settle”, that is actually fermentation dry matter losses due in part to poor packing. Dense packing reduces dry matter losses, heating problems and storage costs. The goal is to achieve more than 15 lbs of dry matter per cubic foot for corn silage and haylage. Silage densities can easily be measured using a special silage probe and a scale.

Pack in thin layers of no more than 15 cm (6 inches) in order to get good air exclusion and high silage density. Be sure to take precautions to prevent tractor rollovers. Excessive packing after the silo is filled does not compensate for packing in thin layers as the silo is being filled.

Often there is more spoilage and poorer quality silage at the top of the pile along the bunker wall. This is because the silage is less densely packed (difficult to get in there with tractor), more exposed to oxygen (closer to the surface) and sometimes too dry (harvested last).

Tractor Weight & Packing Time

Sufficient tractor weight and packing time is critical. Many custom operators are capable of extremely high delivery rates to the silo, which often far exceeds the capacity of the farmer to pack adequately. As a general rule of thumb, the number of minutes of packing required per ton of wet silage is equal to 18 divided by the tractor weight (tons). A 5 ton tractor requires 3.6 minutes per ton haylage or 10 ton tractor would require 1.8 minutes. A high delivery rate to the silo is desired, but this may mean using bigger packing tractors, adding more tractor weight, or more packing tractors in order to increase packing time per ton.

Cover & Seal

Covering and sealing horizontal silos quickly is essential to avoid spoilage and dry matter loss from both air infiltration and rainfall. Rainfall washes organic acids and soluble nutrients from the silage. Cover with an oxygen-barrier film and silage grade (UV protection) 6-mil white plastic. Dry matter losses can be 30 percent with an uncovered silo.

Plastic should be held in contact with the pile to keep air from moving under the plastic into the silage. Avoid situations where plastic flaps and acts like a bellows to increase air circulation over the surface. Old tires (whole, split or truck tire sidewalls) can be placed so that they touch each other. An alternative is a commercially available system of nylon bags filled with sand or gravel. Sealing the plastic edges can be done with soil, aglime or sandbags. Don’t put sandbags on the wall, because with “shrink” there will be an air gap under the plastic.

Keep Rainfall Runoff Out

It is important to prevent rainfall runoff from flowing between the silage and bunker walls. Silage at the bottom corners of the pile, against the wall, is often too wet when rain water flows from the top down along the bunker wall. This can result in too much butyric acid produced that results in poor palatability, high spoilage losses and subclinical ketosis in dairy cows.

One way to try to prevent this would be to crown the silage to drain over the walls, but this can be difficult. Another way would be to form flow channels several feet from the walls and sloping them to drain off the silo surface and away from the silo. An alternate method would be to fold the edges of the plastic back so that a channel is formed to carry the rainfall runoff away without reaching the wall.


The re-exposure of the silage to air at feed-out can result in the growth of moulds, yeast and aerobic bacteria, particularly with corn silage. Slower feed-out rates increase the likelihood of aerobic spoilage. The recommended feed-out rate is at least 15 cm (6 inches) per day. During hot, humid weather much larger feed-out rates are required to stay ahead of the spoilage. Bunker silos should be sized accordingly. Do not feed mouldy silage.

The silage face should remain tight and smooth to limit the penetration of air. Avoid fracturing the silo face by running at it with the front-end loader and using a lifting action. It is preferred to minimize fracturing by scraping down the face with the front-end loader and allowing the silage to fall to the floor. “Facers”, such as block cutters or shear buckets are other options. With high forage costs, they can easily pay for themselves. Uncover and loosen only as much silage as is required.

Fermentation Analysis

If you suspect nutritional problems caused by poor silage quality, consider a fermentation analysis. Fermentation analysis will measure pH, lactic, propionic, acetic and butyric acids, ethanol and ammonia-N. This can be useful in diagnosing silo management and animal performance problems. (Silage Fermentation Problems