Forage harvest season is well under way. While many factors contribute to forage quality, letting too much air remain in the forages during filling and packing can significantly impact the quality of forages fed to your herd.

For proper fermentation to occur, you need to remove air so a rapid drop in pH can occur. However, when air enters the forage mass during filling and packing, it slows the production of lactic acid - the acid primarily responsible for decreasing silage pH, says Limin Kung, professor of ruminant nutrition and microbiology at the University of Delaware. Thus, if you delay filling - such as dropping a load of forage at the bunker at 5 p.m. and not packing it until 8 a.m. the next day or filling the silo too slowly - it hampers fermentation.

According to a study presented at the 2000 American Dairy Science Association Joint Annual Meeting, chopped barley silage exposed to air for 24 hours prior to packing had a pH of 4.61 compared to a pH of 3.98 for silage that was chopped and immediately packed into the silo.

The barley silage with the higher pH also had a greater concentration of butyric acid and ammonia-nitrogen, two compounds produced by clostridial fermentation, an undesirable fermentation process, adds Kung.

Proper packing also is one of the keys to putting up high-quality silage. University of Delaware research showed that the pH of tightly-packed alfalfa forage - forage ensiled at a density of 14 to 15 pounds of dry matter per cubic foot - declined more quickly than forage packed at a density of 11 to 12 pounds of dry matter per cubic foot. In short, tightly packed alfalfa ensiled more quickly than loosely packed alfalfa. And, it developed less yeast and molds during storage.

Bottom line: Rapid filling and careful packing excludes air in the forage going into your storage structure. That, in turn, helps the forage pH drop rapidly, thus encouraging proper fermentation and the production of high-quality feed.

In order to minimize air entry into your forages, Kung suggests using the following guidelines for filling and packing your bunkers:

  • Fill your storage structure as quickly as possible.
  • During filling, distribute the forage evenly in the storage structure.
  • Use the progressive wedge technique - a strategy that fills the bunker at an angle - when filling bunker silos.
  • Pack forage in bunkers or drive-over piles in layers that are no more than 6 to 8 inches thick.
  • Aim for a high packing density - at least 14 to 16 pounds of dry matter per cubic foot.
  • Monitor the density of silage bags with a density gauge (available from the silage bag manufacturer) or by monitoring the diameter of the bag during filling.

In addition to these guidelines, a Microsoft Excel spreadsheet is available to help you achieve high density when filling a bunker. You can access the spreadsheet at the University of Wisconsin forage extension Web site located at:

Once there, scroll down and click on "Bunker Silo Density Calculator." The spreadsheet allows you to input factors such as silo dimensions, tractor weight, forage delivery rate, forage dry matter and packing time.