A major cause of dry matter (DM) loss in silage is deterioration of readily available carbohydrates by aerobic microorganisms. That can result from a too-loose pack or from a feed-out face that is too large. These aerobic microorganisms need a supply of oxygen to grow, and when they get it they cause heating and deterioration.

Commonly you'll see these results from aerobic damage:

1. Increased fiber and crude protein contents

2. In extreme cases, moldy gray- to black-colored silage

Silage experts often refer to the density of pack, therefore the oxygen-exclusionary potential of the silage storage, by measurements of "bulk density" and "porosity." Specifically, both terms measure the amount of voids between the solid particles of silage material. The pore space can be filled with fluids and/or gasses. The more porous it is, and the more continuous the openness of spacing, the more likely oxygen is to penetrate the pile. Also, higher porosity means lower bulk density. Of course, the preferred gas in stored silage is carbon dioxide. In high concentration, carbon dioxide displaces oxygen and helps preserve the silage.

To minimize silage porosity, forage should be packed to a bulk density which is as high as practical, say Brian Holmes, University of Wisconsin Madison, and Richard Muck, U.S. Dairy Forage Research Center at Madison.

At the recommended range of moisture content for hay crop silage of 60 to 65%, and 65 to 70% for whole-corn-crop silage, the researchers recommend porosity be kept below 40%. This is a bulk density greater than 44 pounds per cubic foot. "The higher the bulk density, the lower the porosity and the slower the rate at which oxygen can penetrate through the silage," Holmes and Muck wrote in a recent paper on silage storage and pack. Of course, this universally tight pack is sometimes easier said than done. A study in Pennsylvania several years ago showed significant decreases in silage densities of samples taken from the outside edges, within 8-10 feet of the walls. These areas consistently had lower compaction values and therefore would have higher aerobic spoilage than tighter-packed areas.

"The lower densities measured along walls or the outside edges of piles result from the challenge of operating equipment in these areas," says Gregory Roth, agronomy professor at Penn State University. He adds that packing along walls with large equipment requires an experienced operator to achieve the desired pack, but it needs to be done. Roth says adding more tractors and heavier tractors can be an important management tool too. He also says put extra traffic in wall areas in bunkers, if possible. Roth further suggests reducing the layer thickness of silage you are trying to compress with each pass to increase densities. It can be done. In the Pennsylvania study operators were able to improve packing density by an average of 13% by applying these and other management tips.

Remember, too, at feeding time that the degree of aerobic deterioration at the feed-out face is a function of oxygen infiltration rate and the time of exposure to oxygen, say Holmes and Muck.

Silage heating may be evident at the feed-out face, but it may be more problematic at some distance behind the feed-out face where active aerobic deterioration is penetrating. Therefore, temperature monitoring for heating at the feed-out face surface may be deceiving.

Oxygen has been measured up to three feet behind feed-out faces in well-packed bunkers, the researchers say. This tells us we feed two or perhaps three feet per day off the silage face, which would be ideal. That requires some calculation and planning to get the right bunker shape and size.