Everyone in the silage business acknowledges that sealing (covering) a horizontal silo (i.e., bunker, trench, or stack) is not the most fun job on the farm, nor does it usually receive the highest priority by many producers. Is it important to cover the bunker? Does it pay?
When silage is not tightly covered, air and moisture can easily enter the silo and adversely affect both the ensiling process and the quality of silage during storing and feeding. This creates a great potential for excessive dry matter (DM) and nutrient losses, moldy feed and other problems. The extent of these losses in the top 2 to 4 feet is far greater than most people realize.
Several studies at Kansas State University have reported at least a 30 percent loss in DM from the top 3 feet of silage in uncovered bunker silos compared to bunkers covered with plastic sheeting weighted down with tires. In a 12 foot high x 80 foot wide x 140 foot long bunker, the top 3 feet of silage contains approximately 672 tons (as fed) of silage at a density of 40 lbs/ft3. Thirty percent loss in this top three feet would equal 201.6 tons of as-fed silage. If corn silage is worth $60 per ton as-fed, the total savings potential, in terms of lost corn silage is at least $12,096. This does not take into account any negative effects that the top 3 feet of spoiled material might have on DM intake, milk production, or reproduction. Nor does it consider that additional silage is lost on the sides and closed end of uncovered bunkers. These losses can easily amount to 2-3 percent of the total silage volume. If we assume a 2 percent loss on the entire pile, this additional loss in our example would be 53.8 tons of silage worth another $3,226.
Although future technology might bring a more environmentally and user-friendly product, polyethylene (e.g., 6-mil plastic) is currently the most effective material for covering silage. After it is placed over ensiled forage, the plastic sheet must be weighted down. Tires are the most commonly used weights. They should be placed close together so they touch (about 20 to 25 tires per 100 sq. ft.). To reduce the number of tires needed, and water pooling inside the tires, tires should be cut in half and placed with the open side down. Cost of plastic is usually in the range of $0.115 to $0.120/sq. ft., or about $1,344 (includes a 5 percent overlap) for a 12 foot X 80 foot X 140 foot bunker silo. If we assume it takes about 25 hours of labor at $12/hour, the total labor is $300/year. Initial cost of tires to cover this silo would be about $250 to $500. If we assume $350 initial tire cost with a 10-year expected useful life, this only amounts to $35/year.