When building a new bunker silo, trying to shave construction costs by manipulating silo dimensions is a penny-wise, pound-foolish proposition, says Brian Holmes.

“Concrete walls represent a large cost of building a bunker,” says Holmes, a University of Wisconsin ag engineer. “We still see a lot of people trying to cut corners by building bunkers too wide and too short in attempts to save on construction expenses. In the end, that approach can cost you.”

Potential feed loss to spoilage is the major pitfall of an improperly sized bunker. In a bunker that's too wide, it's difficult to remove enough material from the silo face every day to stay ahead of spoilage.

A relatively straightforward pencil-pushing exercise will yield the information necessary for properly sizing a bunker. Start by multiplying the number of head being fed by pounds of forage/head/day to get an estimate of how much wet feed you'll need. To convert to dry matter, multiply the wet feed number by the percent dry matter.

Next, determine the cubic feet removed from storage each day. To do that, divide the dry matter number (from the previous calculation) by density of feed (pounds dry matter per cubic foot). For a range on density, figure about 12 lbs dry matter/cubic foot for poorly packed forage and 16 lbs dry matter/cubic foot for well-packed forage.

In most situations, it's probably reasonable to use 14 lbs/cu ft. For example, if you're feeding 3,000 lbs of dry matter/day out of a given bunker and the density is 14 lbs/cu ft, the volume removed daily is 214.3 cu ft (3,000÷14).

At this point, you can start calculating actual silo dimensions. Some considerations to keep in mind, according to Holmes:

  • Length. Estimate how much of a slice you'll need take off the silo face each day to keep the face nice and smooth and ensure that you're always feeding fresh feed. While 4" is a commonly recommended minimum removal rate, Holmes suggests 12" per day as a better target.

    Divide the number of days you're feeding by the slice size to come up with an estimate on silo length. As a practical management consideration, though, Holmes recommends a maximum length of 150'.

  • Height. Most recommendations call for a minimum sidewall height of 6' and a maximum height of 30'. Deeper silos facilitate denser packing and help minimize the surface area susceptible to top spoilage.

    The tradeoff is that higher sidewalls can create potential safety hazards like tractor rollovers during filling and/or silage avalanches during feed-out.

    “Consider the equipment you have on hand,” recommends Holmes. “If you can only reach up to a height of 12' with your loader, the silo shouldn't be 20' high.”

  • Width. Most discussions on bunker dimensions center on keeping the silo narrow enough to prevent spoilage problems during feed-out. But it's possible to build a silo that's too narrow, says Holmes.

“The silo needs to be wide enough for maneuvering loaders, tractors and trailers during filling and emptying,” he notes. “For example, if you have an 8'-wide tractor, the silo should be at least 16' wide. That will give you enough room to do a good job of packing.”

To calculate width, divide daily volume removed by height and feed-out rate. For example, if the removal volume is 214.3 cu ft, sidewalls are 10' high and the feed-out rate is 1'/day, the appropriate width would be roughly 22' (214.3 cu ft÷10'÷1').

“With that width, you could do a good job of packing the whole bunker surface,” says Holmes.

For a spreadsheet and other information on properly sizing bunker silos, check out the University of Wisconsin's Team Forage Web site: www.uwex.edu/ces/crops/teamforage (click on “Forage Resources,” then “Harvesting and Storage”).