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August, 1999
WISCONSIN TEAM FORAGE
Attaining a high density in a silo is important for two primary reasons. First and most important, density and dry matter content determine the porosity of the silage. Porosity, in turn, sets the rate at which air moves into the silo and subsequently the amount of spoilage which occurs during storage and feedout. Second, the higher the density, the greater the capacity of the silo.
Wisconsin Team Forage reported on a study of a wide range of bunker silos where the density was measured and correlated with filling practices.
Silage densities were measured in over 160 bunker silos containing either corn or haycrop (largely alfalfa) silage. A survey was completed for each silo sampled, including: number of packing tractors, tractor weight, number of tires per tractor, tire pressure, tire condition, number of drive wheels, silage delivery rate, packing time per day, harvest time per day, filling time, filling technique, initial layer thickness, silo dimensions, maximum silage height, crop, crop maturity, and theoretical length of cut. A summary of the results is presented in the table below.
Based on the results of this survey, Wisconsin Team Forage reported that the use of rear duals or all duals on packing tractors had little effect on density. Other factors such as tire pressure, crop, and average particle size were not significantly correlated with density.
One practical issue raised in the study was packing time relative to crop delivery rate to the silo. Packing time per ton was highest (1 to 4 min/T As Fed) under low delivery rates (<30 T As Fed/h) and generally declined with increasing delivery rate. These results suggest that farmers using contractors to harvest their silage crops probably will need to pay particular attention to spreading the crop in a thin layer and would benefit from using several packing tractors simultaneously.
The following options for improving compaction and thus increasing silage density were suggested:
Decrease packing layer thickness from 12 inches to 6 inches.
Consider adding weight to the tractors by adding fluid to the tires, adding front end weights, adding steel wheel weights, or adding dual wheels with fluid and/or wheel weights.
Reduce delivery rate of silage to the bunker to increase the packing time per ton.
Increase dry matter content by allowing longer crop field drying time.
Increase depth of silage in the bunker silo.
Add more packing tractors. Use heavier rather than lighter tractors so the average weight is not reduced when adding a tractor.
Reduce packing layer thickness further.
Pack for additional time.
Summary of core samples collected from 168 bunker silos in Wisconsin (* SD = standard deviation).
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Haycrop Silage (87 silos) |
Corn Silage (81 silos) |
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Characteristic |
Average |
Range |
SD* |
Average |
Range |
SD* |
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Dry matter, % |
42 |
24-67 |
9.50 |
34 |
25-46 |
4.80 |
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Wet density, lbs/ft3 |
37 |
13-61 |
10.90 |
43 |
23-60 |
8.30 |
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Dry density, lbs/ft3 |
14.8 |
6.6-27.1 |
3.80 |
14.5 |
7.8-23.6 |
2.90 |
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Avg. Particle size, in. |
0.46 |
0.27-1.23 |
0.15 |
0.43 |
0.28-0.68 |
0.08 |
For a complete version of this article, and a related Excel
spreadsheet, see the Wisconsin Team Forage website:
http://www.uwex.edu/ces/crops/uwforage/h&s-fp.htm
Roger Brook – Abstracted from Minnesota/ Wisconsin Engineering Notes, Summer 1999.
