Factors Affecting Bunker Silo Densities

High densities in bunker silos minimize losses and reduce storage costs; however, the guidelines to attain high densities are based on relatively little research. The objective of this study was to determine those practices or factors most correlated with bunker silo density. Density was measured in 175 bunker silos across Wisconsin using core samples collected at chest height (1.13 m, 3.70 ft, on average) across the feed-out face. Silo filling practices were surveyed and correlated with density. Most silages sampled were alfalfa or corn. Dry matter densities ranged from 106 to 434 kg/m3 (6.6 to 27.1 lb/ft3). The core densities were correlated with the height of silage above the core, indicating the effect of selfcompaction. To adjust for this, all densities were corrected for the median depth below the surface (2.16 m or 7.09 ft) using the equations of Pitt (1983) for density with height for the center of tower silos. The adjusted dry matter densities were most strongly correlated with how thinly a load was spread (L), tractor weight (W), packing time per tonne as-fed (T), and dry matter content (D). These four factors were combined into a packing factor [W (TD)1/2 L–1] that explained 18.2% of the variation in dry matter density. Additional factors such as the use of dual wheels, etc. did not significantly improve the prediction of dry matter density. An equation was developed to predict average density in a bunker silo based on the packing factor plus crop height in the silo.