Custom harvesters who pack clients' bunker silos or silage piles may want to check silage density levels — either to prove or improve their services.

The silage density of correctly packed bunker silos averages more than 14 lbs of dry matter/cu ft. If custom harvesters did that packing, they should emphasize the point to clients come contract time.

But if silage densities are less than 14 lbs/cu ft, custom operators and clients should look for ways to increase them the next time they pack. Low densities lead to high dry matter losses, says Brian Holmes, University of Wisconsin ag engineer.

“The problem with dry matter loss is that most of it isn't visible,” says Holmes, who has worked up several spreadsheets to help estimate and increase silage density.

“People can see a black layer on their bunker top and know that if they cover the bunker, they can significantly reduce top surface losses. But that's visible. A lot of the dry matter losses throughout the silo are sugars being decomposed and not leaving a lot of residue. It's hard for producers to envision these losses when they are not visible.”

Under good bunker silo management, dry matter losses typically can be 13-14%, he says. Loose packing holds in oxygen and lowers forage quality even further. Just a 10% dry-matter loss in hay silage totals $1,940/year for every 100 cows fed a 50-50 ration of hay and corn silage, he estimates.

To determine average density of silage stored in a bunker or pile, Holmes offers a new spreadsheet that can be found on the Web at www.uwex.edu/ces/crops/uwforage/StoredDensity1-28-05.xls.

If density is too low, forage packers need to change their practices, Holmes says. A second spreadsheet — that he and colleague Richard Muck created — lists ways to increase density. They include adding weight to the tractor(s), adding more pack tractors, slowing delivery rate from the field, adjusting forage moisture and packing silage in thinner layers. It's at www.uwex.edu/ces/crops/uwforage/bunkdensity_master1-13-02.xls.

“Packers might be able to change one, two or three of the variables, and if one of those is layer thickness, then they can go to a third spreadsheet. It helps them understand, based on the size and quantity of material in the wagon, what kind of filling surface slope they need to get the layer thickness they're looking for.” Log on to www.uwex.edu/ces/crops/uwforage/LayerThickness1-29-05.xls.

Layer thickness isn't easy to determine, Holmes adds.

“One of the things we have to assume is the density of the material after it's spread but before it's packed. We've done some work in the past looking at the density of material in a forage wagon. I'm assuming that, once it's spread, it's about the same density as it was in the forage wagon.”

The spreadsheet uses the volume of forage from each wagon or truck, and the bunker width and wall height, to determine how much to slope the filling surface to get a desired layer thickness.

“In the past, we have recommended a steep-sloped filling surface to minimize the surface area while packing in the progressive wedge technique. However, to achieve a thin layer, you will generally need a shallow slope on the filling surface,” Holmes says.