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Dairy Update Issue 109, January 1992

Bunker Silo Management

James A. Salfer, Extension Educator, Stearns County
James G. Linn, Extension Animal Scientist

The use of bunker silos for the storage of feedstuffs has increased in recent years in the Midwest. The increase in herd size and the desire for a relatively low cost storage system are some of the factors that have brought about this change. Generally, the cost for bunkers will be between 1/4 and 1/2 of that for tower silos. With proper management, spoilage can be kept to a minimum and the quality of feed can equal that of any other storage system.

Understanding the changes occurring in feedstuffs upon ensiling is important in understanding the factors that affect silage quality. Immediately after filling, aerobic bacteria use the sugars and the oxygen present to produce carbon dioxide (CO₂), water and heat. As the oxygen (CO₂) is depleted, anaerobic strains of bacteria begin multiplying. These anaerobic bacteria use the plant sugars to produce lactic acid which causes the pH of the silage mass to drop. When pH reaches 3.8-5.0, the bacteria die, fermentation halts and the silage is in a stable phase. It remains stable unless O₂ becomes available, at which time aerobic bacterial growth can begin again, causing heating and loss of nutrients from the silage. The key to high quality silage production is to get anaerobic fermentation as soon as possible and to reach a pH below 5.0 as rapidly as possible. The factors that influence this goal are similar, with the exception of a few modifications, for all storage systems. The factors as they specifically relate to bunker silos are as follows:

1. Fill as rapidly as possible. This minimizes the exposure time of the feedstuff to O₂.

2. Keep knives sharp and cut forage at correct length. Haylage should be chopped at 3/8 inch theoretical cut and corn silage at 1/4 inch cut. This should provide forage with enough effective fiber to prevent fat test depression and yet will allow effective packing.

3. Ensile at correct moisture. Bunker silos require forage and grain to be harvested at a higher moisture content than tower silos. This allows for better compaction and exclusion of air.

4. The silage should be spread in thin layers and PACKED!!! The silage should be spread in thin layers with a blade or loader. It is best if one person is responsible for packing the silage continuously while filling. Heavy tractors with rubber tires are best for packing in order to attain the maximum weight per cubic foot. The use of tractors with tracks should be avoided. Bunkers don't have the advantage of several feet of silage to aid compaction so it must be done manually. This is one of the key steps in producing high quality silage with a bunker.

5. Cover and seal carefully. The greatest potential loss from bunker silos is top spoilage. Cover the silo within a day of harvesting and keep it covered for at least 2 weeks if possible. Sealing at the edges is a top priority. The recommended plastic is 6 mil. or thicker. The entire silage surface should be weighted down to prevent whipping and tearing from the wind. Old tires are a low cost material often used. Inspect the plastic periodically for holes and fix them.

Table 1 shows the importance of covering immediately after filling. The uncovered silage had greater dry matter losses than the covered silage. The most dramatic difference was with the alfalfa. The uncovered alfalfa had only slightly more than 50% of the dry matter left in the top three feet, compared to over 90% left for the covered alfalfa. The loss in dry matter will more than pay for the cost of the 6 mil. plastic and labor.

¹ Each value is the mean of three silos/time/covering treatment.
² Each value is the mean of two silos/time/covering treatment.

Source: Dickerson et al.

6. Maintain the silo face and feed an adequate slice daily. It is important to maintain a smooth surface on the silo in order to minimize the amount of silage that is exposed to the air. The minimum size slice which must be fed from the surface varies with the ambient temperature. When the temperature is below freezing, it's possible to maintain quality silage while feeding only 2 inches per day. During extremely hot weather, however, it may be necessary to remove 6 inches per day.

Designing and Sizing Bunker Silos

Successful use of a bunker silo starts with the selection of the correct size and dimensions for your operation. Many factors need to be considered in sizing a bunker for a dairy operation.

1. The number of tons of storage that are needed. To compute this value, the number and class of animals being fed, the amount of feed that each class will be consuming per day and the number of days that the silo will be utilized must all be considered. The formula to use is:

To convert to tons of dry matter, multiply the tons of wet forage by the percent dry matter. This must be done for each class of livestock. Example:

2. The appropriate dimensions must be decided. The cubic feet of storage needed is determined by dividing the bulk density of the feed (lb per ft³) into the total storage desired. The bulk density (lb of feed per cubic foot) ranges from about 35 for poorly packed drier haylage to 50 for wet, finely chopped feed.

For most situations, a figure of 40 lb ft³ can be used with reasonable accuracy. In the example, 900 tons x 2000 = 1,800,000 lb divided by 40 = 45,000 ft³ of storage is needed.

The specific silo size (length x width x height) needs to be considered (Table 2). The length should correspond to the planned feeding schedule on your farm. The number of days of feeding along with the slice size needed to maintain fresh feed, should be considered. The current recommendation is to remove a minimum of 4 inches of silage daily. This corresponds to a length (ft) = feeding days/3. For year around feeding this is 360/3 = 120 feet. Moreover, factors that affect the height include available designs, equipment size, and safety considerations. Generally, the deeper the silo, the more dense it will pack. This also minimizes the surface area for top spoilage. The minimum recommended height is six feet. To minimize the danger of tractor rollover when filling and the collapse of huge chunks of silage when emptying, sidewalls should not be higher than 20 feet.

* Capacity is level-full volume for 30% DM.
** Length based on 4" daily removal x 360 days.

Another safety precaution to be aware of is the amount of crown and the angle of the slope at the edges of the silo. Silos should not be crowned more than 1/6 of their width.

The width of the silo must also be considered. The minimum width between the walls is determined by the amount of room needed to maneuver loaders, tractors and trailers or any other equipment used to fill or empty the silo and feeding rates. The best recommendation is to check the space needed to operate your equipment, but widths below 16 feet are discouraged as are widths greater than 30 feet.

Strong consideration should be given to including an end wall on the bunker. This decreases storage losses by decreasing the amount of surface area exposed. Bunkers with end walls are filled in a wedge method. The first material is packed back against the end wall with additional material filled in to make a ramp down into the silo. Wagons can be driven down from the back wall into the silo during filling. This method helps facilitate packing.

Other factors to be considered in using bunker silos are:

• How multiple fillings of the silo can occur if needed. Building two small silos sized to handle one cutting of forage each may be advantageous over one large silo.
  
  
• Consider drainage patterns and how snow drifting will affect silo usage.
  
  
• What future expansion plans are for the farm and how location of bunker(s) will be affected.
  

In conclusion, many farmers successfully use bunker silos in their feeding program. Conscientious planning to achieve the correct size and best design to maintain high quality feed throughout the whole feeding period is essential. This planning along with excellent management and attention to detail during the ensiling process can provide high quality forage at relatively low storage costs.

References

Bolsen, K. K., J. E. Baylor and M. E. McCullough. Field Guide for Hay and Silage Management in North America. 1991. NFIA.

Dickerson, J. T., K. K. Bolsen, C. J. Lin, R. Sonon, R. Suazo, and L. Paff. 1991. Rate and extent of top spoilage in horizontal silos. Kansas Agric. Exp. Stat. Rpt. of Prog. 623. Manhattan, KS.

Irish, W. W. 1989. Department of Agricultural and Biological Engineering Extension Bulletin, AEg-460. Cornell University, Ithaca, NY 14853.

Pitt, R. E. 1990. Silage and Hay Preservation. Northeast Regional Agricultural Engineering Service, Ithaca, NY 14853.

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