When Marvin Helbig built a bunker silo three years ago, it was dirt cheap.

That's because soil instead of concrete forms the sides and closed end.

Earlier that year, Helbig, an Oakdale, IL, dairy farmer, had dug a large pond to supply water for his dairy barn. That left a huge mound of soil behind the facility. To make the bunker, he simply dug a 50 × 200' corridor into the mound, then poured 8" of concrete for the floor.

With silage piled 2' above the 12'-high sides, the silo's capacity is about 3,500 tons. The cost: about $18,000, not including the expense of creating the mound, which is charged to Helbig's water supply.

“I priced a very similar concrete structure that I was going to have built, and they were talking approximately $70,000,” he says.

“It works great for corn silage,” he adds. There's no spoilage on the sides and end because the clay-based soil was packed tight by the heavy excavating equipment that created the mound.

“It's almost as hard as concrete,” says Helbig.

He's impressed with the bunker's safety aspect, too. The danger of packing-tractor rollovers is minimal, because the outer walls taper to ground level. “If you have a young or inexperienced person helping with packing, you don't have to worry about him falling off the side.”

The tapered walls are also handy when the filled structure is sealed. A truck driver drives up and down the sides, spreading ground limestone.

Helbig built the bunker after seeing a similar one on another southern Illinois dairy farm. His worked so well that he put a second one in the same mound last summer. He used that one for haylage, but will store corn silage in both structures this year.

He recommends earthen bunkers, but warns that they might not work as well in other soil types. Cutting into a hillside to make a bunker won't work as well, either. It needs to be above ground, away from natural drainage, says Helbig.

Mike Hutjens, University of Illinois extension dairy specialist, calls Helbig's earthen bunkers “a neat idea.” But he might design them somewhat differently.

He likes the tapered outer walls, saying they offer both safety and packing advantages. With less rollover risk, tractor operators likely will drive closer to the edge for better silage compaction near the walls.

Hutjens says packing could be further improved if the tapered outer walls were only 3-6' high. Then tractors could drive across the bunker as well as lengthways, yet there wouldn't be spoilage around the edges like with silage piles.

He also prefers to see both ends of the bunker open so filling and feed-out can be done from either end.

“You could feed out of one end, then when the new crop comes in you could start feeding out of the other end so you always have fermented feed,” says Hutjens.