Submitted to: Applied Engineering in Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/19/2003
Publication Date: 3/1/2004
Citation: Muck, R.E., Savoie, P., Holmes, B. 2004. Laboratory assessment of bunker silo density, part 1: Alfalfa and Grass. Applied Engineering in Agriculture. 20(2):157-164. Interpretive Summary: For farmers making silage in bunker silos, achieving a high density when putting the crop in the silo has several benefits: better preservation of the silage in the silo, a silage that is less likely to heat and spoil during feeding, and reduced overall storage costs. However, most of the strategies for achieving a high density are based on experience or the results of silo surveys. Consequently, it is not known which factors are truly most important. We have begun to study several of these factors (pressure, time/layer, layer thickness) in alfalfa and grass in a pilot-scale compactor that allows us to compress up to six feet of compacted forage. Dry matter density in these hay crop silages was affected most by pressure, dry matter content, crop species and chop length. Short compaction times and thick layers reduced density but the effects were smaller than those of pressure. Our next step is try similar experiments on field-scale bunker silos in order to confirm the results. If confirmed, the results will be incorporated in a spreadsheet model that will help farmers decide the best way to make high density silage in their particular circumstances.
Technical Abstract: Previous models to predict density in bunker silos were developed from empirical data and a limited number of variables such as crop moisture, packing tractor weight, time of compaction, and layer thickness. To develop a more general relationship between bunker silo density and these and other variables, a laboratory apparatus was developed to simulate pressure, time of compaction and layer thickness as applied in a bunker silo. Chopped alfalfa or grass was placed in layers of 0.15, 0.30 and 0.45 m in a rectangular container 482 mm by 584 mm simulating the footprint of a tractor tire. Pressure between 20 and 80 kPa was applied to the forage by a platen. The most frequently used pressure of 40 kPa corresponded to the weight of a 4600 kg tractor spread over four tires. The total time of compaction varied between 2 and 10 s; the most frequently used time of 5 s was equivalent to two tires passing four times at a speed of 3.4 km/h. A total of 23 tests were conducted: 17 with alfalfa, 3 with grass and 3 with mixed alfalfa-grass. The crop dry matter ranged between 20 and 54%. The pre-compressed density of the first layer (0.30 m high) averaged 72 and 55 kg DM/m^3 for alfalfa and grass, respectively. The highest compressed density ranged between 138 and 339 kg DM/m^3 with an average of 220 kg DM/m^3. After releasing pressure, the relaxed density of the first or uppermost layer ranged between 81 and 152 kg DM/m^3 with an average of 127 kg DM/m^3. After 6 layers, the average relaxed density was 181 kg DM/m^3, 18% lower than the average highest compressed density. As successive layers were added, the cumulative DM density increased according to a logarithmic model. The model suggested that density would continually increase, slowly but without reaching a plateau, as the silo height increased. Within the experimental range, parameters of the logarithmic model were not significantly affected by layer thickness or time of compaction. They were significantly affected by pressure, dry matter content, crop species and chop length. More laboratory data are needed to understand interactions between the variables while field validation is necessary to extrapolate results for deep bunker silos.