Paying more attention to bunker silo packing can bring big improvements in silage density. But it may not be enough to curb excessive dry matter losses, especially near the tops and sides of bunkers.

It takes a tremendous amount of packing to achieve the recommended minimum silage density of 14 lbs of dry matter/cubic foot throughout a bunker, says Paul Craig, extension educator in Dauphin County, PA.

With help from several county agents, Craig and Penn State University agronomist Greg Roth conducted a two-year bunker density study on south-central Pennsylvania dairy farms.

A gas engine-powered drill and a 2"-diameter core sampler were used to drill into the face of each corn silage bunker at 12 locations. Silage density at each location, plus an average density for the bunker, was calculated.

Samples were collected at three levels of each bunker. The bottom level was approximately 4' above the base, the top level was roughly 3' below the surface, and the middle level was halfway in between. Four points were sampled across each level.

Of the 22 bunkers sampled in the winter of 2004, only seven had an average silage density greater than 14 lbs/cu ft, and eight averaged less than 12 lbs/cu ft.

Craig discussed the results with cooperating producers and reviewed bunker management practices during pre-harvest educational programs last year.

Participants did much better the second year. Of the 21 bunkers sampled in 2005, 10 exceeded 14 lbs/cu ft, and only three were below 12 lbs/cu ft. Most of the improvement came in the 14 bunkers that also had been sampled in 2004. On average, those bunkers improved by 1.3 lbs/cu ft.

However, many bunkers with average densities at or above 14 lbs/cu ft were below that level at the top and near the sidewalls.

“There's still room for improvement,” says Craig.

Sufficiently packing the outer 8-10' on both sides is challenging, because only one side of the packing tractor passes over those areas as it moves across the bunker. It takes a skilled driver to pack sufficiently in those areas, says Craig.

“Unfortunately, concrete walls aren't airtight, with many joints and cracks that can allow air to interface with a large area of silage that doesn't have a very dense pack.”

The top of the silage mass is the most difficult to pack, and is the area most likely exposed to air and moisture. To ensure adequate packing at the top, Craig recommends adding more packing equipment near the end of the filling process.

Avoid piling silage higher than the top of the walls. In the study, overfilled bunkers had significant declines in silage density above the walls. One bunker with 8' sidewalls was filled to nearly 14' at the highest point. Silage density was 16.1 lbs/cu ft at the bottom, 14.8 lbs/cu ft in the middle and 11.3 lbs/cu ft at the top.

The fact that the top and sides of the silage mass are inherently packed the least underscores the importance of a good bunker cover, says Craig.

“It's critical that the cover is put in place as quickly as possible and as well as possible,” he says.