Bunk Silos Need To Be Covered

Posted: April 6, 2005

Exposure to air early in the fermentation process delays the drop in pH and prolongs the time needed to achieve stable silage. Exposure to oxygen any time after normal fermentation has been completed encourages growth of yeasts and molds that spoil silage. Even a densely packed silage mass can undergo aerobic spoilage if it is exposed to air. In addition to excluding air, covers prevent rain from entering the silage mass. Rainfall leeches nutrients such as water soluble carbohydrates, protein, and vitamins from the silo.

The amount of silage lost in an uncovered horizontal silo can be staggering. Consider the following results from a Kansas research trial. Silage stored in small bunker silos for 180 days was evaluated at three depths from the original surface. In covered bunker silos, almost 90 percent of the dry matter was recovered at all three depths (10 percent losses). However, in uncovered silos only 25 percent of the dry matter was recovered in the top 10 inches; that equals 75 percent storage loss in the top of the silo. Dry matter recovery was improved at greater depths, but even at 20 and 30 inches below the original surface, losses exceeded 25 percent.

To prevent these losses, cover the silage surface with plastic immediately after filling is completed. Use of the progressive wedge filling method allows silage to be covered with plastic as it is packed, which can be highly beneficial if harvest must be delayed due to rain. Plastic should be 4 to 6 mil thick and preferably contain ultraviolet blocking compounds. Since plastic covers can be punctured by rodents, livestock, dogs, cats, and small wild animals, regular inspection and patching of the cover is recommended. Mowing around the silo may help to discourage rodents.

Plastic covers must be weighted to prevent air infiltration under the plastic. All edges also must be secured to avoid billowing or flapping in the wind, which can pump air over the entire silage surface and greatly increase spoilage. Typically, used tires are placed closed together on the plastic and the edges are weighted with sandbags or soil. The number of tires needed to weight the plastic can be calculated at a rate of 0.25 tires per square foot, which will result in tires touching.

Although full-casing tires have been used to weight silo covers for years, they have some drawbacks. Full-casing tires are heavy and bulky, and they can hold water that increases their weight and provides a breeding ground for mosquitoes. The use of half tires or sidewall disks can reduce the number of tires needed, limit mosquito breeding grounds, and enable neater stacking when tires are not in use.

Many alternative covers have been suggested, including lime, sod, manure solids, straw, soil, limestone, and sawdust. Leaving a silo uncovered essentially uses the top layer of silage as a cover. Keep in mind that although visible spoilage (black layer) may be limited to one or two inches, that top layer may have been two to four inches of high quality forage when the silo was filled. In addition, spoilage research shows that losses are not limited to the top layer of silage. The transition layer between spoiled and unspoiled silage may be one to two feet deep and may undergo losses of 20 to 30 percent. Covering silage with plastic and tires requires time and labor, but it is the only method that has been shown to consistently reduce silage losses.

Coleen Jones, Dairy & Animal Science Research Associate and Jud Heinrichs, Dairy & Animal Science Extension