For years, recommendations for filling bunker silos have included: distribute feed in thin (6-inch) layers, pack steadily and uniformly with a heavy, single-wheeled tractor, and, finally, cover with uniformly weighted plastic. It was assumed that following these recommendations would produce dense silage (40 to 50 lb/cubic ft as fed, or 12 to 17.5 lb dry matter/cubic ft) with minimal loss of dry matter during fermentation, storage, removal, and feeding. Studies on farms, however, frequently show the recommended density is not achieved. Factors contributing to low density include:
Dry matter losses from inadequate packing could range from 5 to 20% of the original feed placed in storage. Using $100/ton of dry matter (DM) forage value and an additional 10% DM loss, a 30-ft x 10-ft x 100-ft bunker silo could have an additional $1,575 loss due to inadequate packing.
So how does a producer determine if “adequate packing” has been achieved? There are several methods of estimating silage density in a bunker silo. In the first method, mark the starting point of the silage face on the bunker silo wall, and then measure the average height of the silage face and the average width of the bunker. Record the weight of silage added to the total mixed ration (TMR) mixer for 5 to 7 days. Measure the average height of the face again and the length of bunker from which the silage was removed. Using average dimensions, calculate the density as:
Density (lb/cubic ft) = Weight removed (lb) /
[Width (ft) x Height (ft) x Length (ft)]
Another method is to use a silage probe similar to the one in Figure 1 (see page 2). Using a half-inch drill, force the probe into the face of the bunker to extract a sample. Release the probe from the adapter and use a stick to force the sample into a bucket of known weight. Weigh the bucket and sample, subtracting the bucket weight to obtain the sample weight. With a tape measure, measure the depth of the hole in the silage face. Using the following equation, calculate the silage density.
Density (lb/cubic ft) = Sample Weight (lb) /
{[(pi x D x D) ÷ 4] x L}
where: D = Probe inside diameter (ft)
L = Hole length or depth into silage face (ft)
pi = 3.14
When using the probe, more reliable results are obtained by averaging several values instead of relying on one sample. Probing at the center of the silage face (one half the depth) avoids low density in material at the top and more dense silage at the bottom of the bunker.
Any type of steel pipe can be used for the probe, but thin wall and smooth wall pipe will be easier to force into the silage. Most processes for building the probe can be performed in a farm shop. However, knurling the tip requires a lathe with a knurling tool. Tractor or auto repair shops might be able to knurl your probe for you. The knurled tip provides a serrated cutting edge which will not fill with fibers as will a toothed tip. Sharpen the probe by filing the inside surface of the tip. This will preserve the knurling and maintain the serrated edge longer.
The larger the probe diameter, the more accurate will be your results. However, a very large diameter probe will require a larger drill and much more force to push the probe into the silage.
Whichever method you select, the estimated silage density should be greater than 40 lb silage/cubic ft. If it is not, plan to make some changes the next time silage is packed.
Consider adding an additional packing tractor as an alternative to a larger tractor if adequate space allows for safe movement of both tractors. Remember safety when packing. Use tractors with rollover protection and fasten seat belts. Select mature operators who have experience with the type of tractor and blade being used. Avoid crowning the top surface too much, especially near the sides.
This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/