Situations sometimes occur where we need to determine the amount of silage contained in a silo when the only information available is the size of a horizontal bunker or the diameter and depth of silage in a tower silo. Because silage moisture content, size of silage particles, fill method, and length of storage time all affect silage density (weight per unit volume), it is very difficult to get an accurate estimate of silage weight based only on silage volume. If the accuracy of the estimate is critical, it is always best to actually weigh and measure moisture content for individual loads of silage at the time the silo is filled or unloaded.
But if you can live with a rough estimate of silo capacity, published silo capacity tables are available. One of the best current sources of information on silo capacity is the 1997 edition of the MidWest Plan Service handbook Dairy Freestall Housing and Equipment, MWPS-7. Tables in chapter nine of this handbook provide approximate tonnage contained in different sizes of horizontal and vertical silos for silage, high moisture shelled corn, and ground ear corn. Click here for ordering information.
When you use the tables for tower silos, keep in mind that the silage at the bottom of a silo is more compacted, and therefore much more dense than silage at the top of the silo. Thus, the bottom 20 ft of a silo that was originally filled to a depth of 60 ft contains a greater weight of silage than does a silo that was only filled to a depth of 20 ft to start with. For example, Table 9-6 in MWPS-7 indicates that under typical conditions, a 20-ft diameter silo filled to a depth of 20 ft would hold about 95 wet tons of 65% moisture silage. A 20-ft diameter silo filled to a depth of 60 ft would hold about 455 wet tons and a silo filled to a depth of 40 ft would hold about 255 tons. To estimate the amount of silage remaining in a 20-ft diameter silo that originally held a 60-ft depth of silage after the top 40 ft of silage is removed, subtract the number of tons for a 40-ft depth from the number of tons for a 60-ft depth. In this case, 455 minus 255 equals about 200 wet tons of silage left in the bottom 20 ft of a 20 x 60 silo, which is more than twice the amount of silage contained in the silo that was initially filled to a 20-ft depth.
This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/