For many years agricultural engineers have recommended that bunker silos be "packed well" and covered with an oxygen- and water-excluding material. While a graduate student at Cornell University, Kurt Ruppel studied 30 bunker silos to help answer the question "What is the best way to pack a bunker silo to know that it is packed well?" His data suggested that tractor weight, packing time, and either volume or surface area packed were important to characterize the packing process. However, the poor correlation between these factors and the density achieved did not give a great degree of confidence in the predictive relationships.
A study has been initiated in Wisconsin to try to improve the predictive relationship between packing method and density achieved. This study is supported by the UW-Madison Biological Systems Engineering Department, U.S. Dairy Forage Research Center, 23 Wisconsin county Extension agents, and Wieser Concrete Products. County agents are using a probe similar to the one described in the article "Probe for Silage Profits," published in the fall 1996 issue of Engineering Notes. The agents are collecting four probe samples from the face of each bunker silo as well as grab samples for particle size analysis. These samples, along with data collection sheets, are sent to the U.S. Dairy Forage Research Center for analysis. In addition to the samples, the agents have completed a three-page questionnaire about how each bunker silo was filled and compacted.
Preliminary results of the testing (with not all samples submitted) are presented in Table 1, along with the recommended values. The average values for dry matter density suggest that, on average, bunker silos are being packed to the recommended density. However, there are some bunker silos which are well below the recommended range. These silos run the risk of higher than desired dry matter loss during storage, especially during the feed-out phase. The average dry matter content for corn silage is on the high end of the recommended range. This suggests that producers are tending to put corn silage up too dry. However, there are some producers putting corn silage up too wet (less than 30% dry matter). They may experience seepage losses from expressed juice. In the case of hay silage, the average dry matter content is well outside the recommended range, with the tendency toward over-drying. Over-dried hay silage is susceptible to rain exposure for a longer period of time, has higher leaf loss at harvest, can be more difficult to pack, and is more likely to have heat damage during fermentation. Forage with more than 50% dry matter is susceptible to a silo fire if exposed to oxygen.
Those producers whose bunkers were tested are receiving information about the study results. Many of them are eager to learn of their effectiveness at silage packing. For those needing to make improvements in their management, the results of this study are very valuable.
During summer 1998, correlation between packing procedures and densities achieved will be analyzed to see if reliable, predictive relationships can be developed. By then, we hope to have the results of at least 150 bunker silo samples upon which to base the analysis.
Table 1. Preliminary results and recommended values for silage density
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Dry Matter (%) |
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Wet Density (lb/cuft) |
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Dry Density (lb/cuft) |
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Avg Particle Size (in.) |
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This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/