For more than thirty years, recommendations for managing bunker silos have included covering them as soon after filling as possible. The specific recommendation was covering with plastic weighted to maintain close contact with the silage and arranged to shed water.
Throughout this period, producers have sought easier and less time-consuming and less difficult alternatives. The process of covering with plastic and adding weights requires much hand labor for covering and uncovering, and can be a wet and dirty job. For these reasons, there are many stories about producers using alternative covers with "pretty good" success.
Visual observations of the silo top frequently reveal a two- to eight-inch layer of spoiled (black) feed. Producers consider the loss of this thin layer to be a small sacrifice. The blackened material removed prior to feeding is referred to as visible waste. This visible waste is only a portion of the total loss occurring in the silo.
Precipitation which percolates through the silage and juice from the harvested plants are called effluent. This effluent carries soluble organics which represent a dry matter loss. Percolating water can also transport oxygen into the silage. The oxygen supports microbes that deteriorate the silage causing a gaseous dry matter loss. The effluent and gaseous dry matter losses are not visible to producers, so they are not aware of their magnitude.
Minson and Lancaster [1965] conducted research in New Zealand to determine the impact of silo covers on dry matter loss. They harvested grass at about 80% moisture and placed it into bunker silos to a depth of 3.5 feet. Precipitation onto the bunker silos was about 20 inches during the 167- to 224-day storage period. Effluent was collected from the bottom of all but one of the silos. Spoiled feed was weighed as it was rejected by the workers. Gaseous losses were determined by subtracting effluent and visible waste losses from the total dry matter loss, Table 1.
Cover type None Roof Sawdust Soil* Limestone Plastic
Visible waste (in.) 3.0 4.0 3.0 2.0 2.0 0.0
Cause of loss Two-year Average DM Loss (%)
Visible waste 5.6 10.0 4.2 6.3 5.8 0.8
Effluent 7.5 3.0 6.5 5.0 ** 2.5
Gaseous 21.1 19.6 19.3 13.8 -- 8.6
Total 34.2 32.6 30.0 25.1 23.6 11.9
Moisture content 82.0 78.6 81.6 79.4 80.4 78.6
at recovery (%)
*Vegetation grew during last 60 days.
**Leak resulted in failure to collect effluent.
The gaseous loss was lowest for the plastic cover. Movement of oxygen through the other surface covers by diffusion and/or percolation contributed to significant losses. Total losses for the alternative covers were 2-2.9 times greater than for the plastic cover.
What does this mean to producers in the upper Midwest who place hay silage valued at $100/per ton of dry matter (TDM) in an 8.5-foot deep bunker silo? Assume adequate wilting so that the 2.5% effluent loss from plant juices does not occur. Let's compare covering with plastic and tires to not covering. Assume the bottom five feet of silage experiences a gaseous loss of 8.6% in both cases. If the silage is packed to a density of 40 lb/cubic foot (14 lbs dry matter per cubic foot) in a bunker silo 25 feet wide by 100 feet long, the bunker will contain 148.8 TDM worth $14,880. The average loss in the plastic- covered silo is 8.9% and the average loss in the uncovered bunker is 18.1%. The dry matter loss in the plastic-covered bunker is 13.3 TDM valued at $1330, while the loss in the uncovered bunker is 26.9 TDM valued at $2690. The $1360 difference in lost value can defray the cost of the plastic. If the plastic costs $100 and the labor to cover and uncover the bunker is 20 hours, the payment for investing this labor is $63/hr, which is a good wage rate for a good manager!
For a more complete discussion of covering silos, request a copy of the article entitled You Can't Judge a Bunker Silo By Its Cover from the University of Wisconsin Biological Systems Engineering Department at (608) 262-3310, or the University of Minnesota Biosystems and Agricultural Engineering Department at (612) 625-9733.
Minson, D. J. and R. J. Lancaster. 1965. The efficiency of six methods of covering silage. New Zealand J. of Agric. Res.
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