There's no substitute for plastic on bunkers
by Ev Thomas and Bill Mahanna
Thomas is retired from the William H. Miner Agricultural Research Institute and president of Oak Point Agronomics Ltd.; Mahanna is with DuPont Pioneer and an adjunct professor at Iowa State University.
One of the most dreaded jobs on dairy farms is covering bunker silos and drive-over piles. For this reason, farmers have always been willing to try alternatives, even though most have wound up as either too expensive or not effective at preventing spoilage. There are a number of thoughtful concepts that haven't panned out.
These included a sprayed-on (and reportedly edible) black gunk that was tried some years ago at Miner Institute and other locations around New York state. It did a great job of preserving the silage but had to be custom-applied and cost about 10 times more than standard silo plastic.
Molasses has also been used on bunkers. But since it's water-soluble, precipitation leaches down through the silage, leaving a thick layer of spoilage. Likewise, ground limestone and sawdust have been put to the test. But, because they exclude neither air nor water, they are also inferior to plastic covers.
Farmers sometimes underestimate the cost of spoilage losses occurring from inadequate covering (or no covering at all). Each inch of black "gunk" originally began as 2 to 3 inches of fresh forage. There's also a transition zone of affected silage that can extend to 3 feet below the visible spoilage layer. Few farmers discard this silage which has already suffered a 20 to 30 percent dry matter loss - and in a typical bunker silo at least 20 percent of the silage is in the top 3 feet. Studies have found that a plastic silo cover returns about $8 for each $1 spent on the plastic and the labor required to apply it.
All plastics not equal
There are tremendous differences in the oxygen transmission rate (OTR) between standard 5- to 6-mil (millimeter) silo plastic and the low OTR plastics that have entered the market in the past 10 years or so. For instance, standard 5-mil "white on black" silo plastic has an OTR of about 1,800, while some of the oxygen barrier films, while less than 2 mils thick, have OTRs that are 60 times lower.
Some farmers use a double cover of standard 5-mil silo plastic, and while this is better than a single layer, it's not nearly as effective as one of the low OTR films covered by a second layer of silo plastic. (The OTR film prevents oxygen infiltration while the top layer protects the film from damaging sunlight.)
The low OTR plastic films are more expensive than standard 5-mil plastic, typically costing about twice as much. Add in the cost of a second layer of standard 5-mil plastic and the total material cost is about three times as much. Custom silo cover application prices in California suggest that the total cost of plastic plus the labor required for application is almost twice as much. The two layer, low OTR system can cost 40 cents per square foot compared to 22 cents for 5-mil plastic. However, research has found that the low OTR plastic does a much better job of preserving silage quality, with about a 10 percent reduction in silage organic matter losses.
Covering a bunker silo or drive-over pile is only half the job; the other half is weighting the silo plastic with tires, tire sidewalls, gravel bags or other ballast material. Often a combination of two or more of these materials is used on bunkers. Tire sidewalls are preferable to whole tires since they won't collect water which provides a habitat for mosquitoes and rodents.
Tire sidewalls work well for most applications but in windy areas may not provide enough ballast to prevent wind from getting under the silo plastic. It's important to provide about 6 feet of overlap between plastic sheets, and where wind is an issue, to protect the overlapped area with a double layer of tire sidewalls.
Gravel bags made from polyethylene mesh have grown in popularity. When filled with pea gravel, these 3-foot long bags are heavy enough to keep the silo plastic in place even during high winds. They're ideal when placed in a continuous row along the sidewalls of bunker silos to pin the silo plastic to the wall, or on the perimeter of drive-over piles, providing an essentially air-tight seal.
Many farmers also use gravel bags on top of the silo plastic, placed in parallel rows 15 to 18 feet apart. Properly cared for gravel bags should last for many years, resulting in a very reasonable annual cost.
For better protection
A good practice on bunker silos is to line the walls with a single layer of silo plastic, using enough to lap the plastic over the top sheet of plastic. The plastic should extend 2 to 3 feet onto the top of the silage and about the same distance along the bunker floor. This prevents water from seeping into the silage at the sidewall.