Proper Packing is Essential to Good Silage Fermentation

Posted: March 6, 2005

Achieving a tight pack is one of the most important aspects of harvesting and storing silage.

Tightly packing silage forces oxygen out of the forage mass and limits air penetration at exposed surfaces during storage and feedout, which reduces dry matter losses. Better packing also increases the amount of forage you can fit into a silo, which reduces storage costs per ton. Silage density depends on several factors, including plant species, crop maturity, moisture content, length of cut, silo type, silo filling method, distribution, and compaction.

As forage is added to the silo, the weight of the material begins to force oxygen out of the forage mass. Gravity compacts forage naturally in deep silos, particularly in upright silos where density increases dramatically from top to bottom. In vertical silos bulk density (the density of wet forage) runs close to 20 pounds per cubic foot (lb/ft3) at the top surface and 60 lb/ft3 or more near the base. In horizontal silos the average bulk density can range from near 60 lb/ft3 for finely chopped, high moisture crops packed with heavy tractors in deeper silos, to 25 lb/ft3 or less for drier and longer-chopped hay crops in more shallow silos with moderate packing. Bulk density of 40 lb/ft3 in horizontal silos is considered ideal (this is a dry matter density of 14 lb/ft3).

Although some natural compaction occurs in deep horizontal silos, thorough mechanical packing is required to achieve adequate density and limit excess air infiltration. Research has shown that filling and packing methods greatly affect dry matter density. Fill horizontal silos using a progressive wedge technique (see figure), which exposes less surface area than horizontal layers and allows thin layers for better compaction than thick, vertical layers. Using the progressive wedge method, each load of silage is pushed up the silage face to form a slope with 30 to 40 percent grade and leveled into layers about 6 inches deep. These thin layers are one of the most important factors in properly packing the silo. Other important aspects of proper packing are the weight of packing tractors and the amount of time devoted to packing. In most cases, the packing tractor(s) should operate continually during filling.

Wisconsin researchers have developed a spreadsheet that allows users to estimate silage density based on the factors described above. The spreadsheet tool can be used to locate weak links in the silo filling process and experiment with solutions to increase silage density, such as slowing delivery rate, adding packing tractor weight, increasing the time spent packing, changing the thickness of layers to be packed, or increasing silage depth. This spreadsheet can be found at

Wheel-type tractors compact silage more than crawlertype tractors, because the wheels concentrate the tractor’s weight over a smaller area; this applies more pressure to the silage. It may be worthwhile to continue packing about one-half hour after the last load for the day has been put in the silo and to start packing again the next filling day about one-half hour before the first load is added.

To pack silage safely, keep a respectable distance from unsupported edges, use tractors with a roll over protection system, and wear a seat belt. To reduce the risk of rolling the packing tractor, avoid backing off the silage pile; instead, back onto the pile and drive forward off the pile.

To complete the silo filling, crown silage one-eighth of the silo width so precipitation can be diverted away from the silage mass and put somewhat higher moisture silage on the top layer to achieve a tighter pack.

You can estimate silage density by weighing the amount of silage removed over a known area. Start by marking the silage face on the bunker wall and measuring the average height of the face. Then measure the average silo width. Keep track of the total weight of silage (from that silo) added to your mixer wagon for the next 5 to 7 days. Return to the silo; mark the location of the new face on the wall and measure the new average face height. Then measure between your marks to determine the length of silage removed. Using average figures, calculate density as follows:

Density (lb/ft3) = Weight removed (lb) / [Width (ft) x Height (ft) x Length (ft)]

Your result will be a bulk density, so use the benchmark of 40 lb/ft3 to determine how well you are doing. Start planning your packing strategy for the upcoming year now by measuring silage density and using the Wisconsin spreadsheet to find ways to improve your current methods if necessary.

Bulk Silage - wedge filling methods

Using the progressive wedge filling method, thin layers are pushed up the silage face to build a slope of 30 to 40 degrees.

Coleen Jones, Dairy & Animal Science Research Associate and Jud Heinrichs, Dairy & Animal Science Extension