The mass of silage stored in bunker silos should not be considered homogeneous. The variation between layers of silage may be caused by climatic variations during filling and storage, variations in mechanical packing intensity during filling, variations in the maturity of corn silage brought to the silo, and the lack of an effective airtight, moisture-proof cover. Rain falling during the winter months of 1967 on an uncovered bunker silo in Glenn County greatly increased the moisture content of the upper 4 ft of silage and leached soluble nutrients into the lower layers and possibly out of the silage mass. The TDN content of the upper 2 ft was significantly reduced. Dry matter losses, representing loss in weight of silage, were severe in the upper 4 ft of the silage mass, with greatest losses occurring in the upper foot. Dry matter loss was less severe in the entire silage mass, but averaged 18.8 per cent of the dry weight of the original samples placed throughout the silo. The percentage of crude protein, crude fiber and ash apparently increases while the nitrogen-free extract component decreases in the upper layers of the silo, possibly due to leaching. However, in reality, the most severe loss in nutrients, and dry matter, in the entire silage mass resulted from surface spoilage through continued exposure to air, involving fermentation, and respiration by spoilage microorganisms. Covering a bunker silo with plastic did reduce losses of dry matter to less than 10 per cent in the 1965 and 1966 studies.