There are four types of dry matter loss – fermentation loss, loss from leaching, surface spoilage loss and feedout loss. Dr. Ken Griswold, Penn State Cooperative Extension
, conducted a study to measure losses from fermentation and leaching, with an objective to refine the relationship between density and loss.
Griswold worked with four relatively small bunker silos (180 by 42 by 8 feet) and two upright silos. He buried poly-weave nylon bags filled with fresh-chopped whole plant corn in each scenario. The bags were removed during feedout, the material was dried and losses were measured.
Both upright silos were filled slowly on an Amish operation. There was no significant difference in terms of loss throughout the nine bags in the silo. Overall, dry matter loss was a little under 3 percent.
The 36 to 42 bags used in the sidewall plastic sealed bunker silos did have some difference in their results. There was lower dry matter and lower density on the outside compared to the center, but no difference in dry matter loss from the edges to the center. When compared top to bottom, dry matter was significantly wetter on top. (It is important to note that this producer preferred to pack wet material on top as a means for better compaction.) Density in the bunker was much lower on top (9.5 pounds per cubic foot) than on the bottom (16.8 pounds per cubic foot). Because of the lower density, the top third of the bunk had greater dry matter losses (11.7 percent) than the lower third (5.6 percent). Dry matter losses were also greater toward the front of the bunker (9.2 percent) than at the rear (7.3 percent).
Griswold observed that ambient air temperature had an effect on loss in both the upright and bunker silos, and may have more of an impact than density. Bunkers packed in August and September recorded losses of 8.4 and 8.5 percent, respectively, while those packed in October and November had respective losses of 7.1 and 6.8.
He also noted that BMR corn silage packs differently than conventional corn silage. When the same operators and same equipment was used to pack each type in a bunker, there was no difference in dry matter loss, but a huge difference in density. BMR silage had a density of 14.0 pounds of dry matter per cubic foot compared to conventional at 12.9 pounds per cubic foot in the same year.
In short, Griswold concluded that losses were greater on the top level and front of the bunker silos. Silage dry matter loss was inversely related to dry matter density in the bunker silos, although the relationship was relatively weak. Ambient air temperature may have affected dry matter losses. Both dry matter density and dry matter loss may be affected by the type of corn (conventional versus BMR). Combining the dry matter content and density of silage cores may be a useful method to estimate corn silage shrink. However, estimates probably will not equal total dry matter loss.
Factors influencing density of silage
Given that density is inversely related to loss, it is important to know the factors that influence density so forage storage density can be increased and dry matter losses minimized. Griswold listed these factors:
- Delivery rate
- Depth of silage and wall height
- Packing layer thickness
- Packing time
- Dry matter content
- Corn maturity
- Particle size, processing
- Number of tractors
- Average tractor weight
- Tire pressure
to find an equation that will help you predict your packing power.
Over the past seven years, Griswold has performed density tests in 189 bunker silos at 75 different farms. Taking 12 samples per bunk, he received an average density range that was all over the place – 8.4 to 17.2 pounds of dry matter per cubic foot. In most cases the only time densities were close to 14 or 15 pounds per cubic foot were on the bottom layer of the bunker.
He found that even with more tractor weight and adequate delivery time of chopped corn a poor job of packing could still occur.
To best maximize bunker density, Griswold said it is important to know the limitations of the structure, the equipment and the operator. The wall slope and height can affect packing ability. The structure’s height dictates the silage height, and the bunker size may limit the size or number of tractors that can be used. Silage delivery rate should be a balance between chopper capacity and the packing tractor(s) capacity. An experienced operator is critical in keeping a thin layer, knowing when to stop and operating close to the walls.
Griswold also recommended an adequate use of plastic for sealing. Multiple layers of plastic on the top and sidewalls with adequate weighting can aid in achieving higher digestible silage.