Optimal packing density: Harvest is the time to get it right!

By Barry Visser

• Density and dry matter (DM) content determine the porosity of the silage, which affects the rate at which air can enter the silage during storage and feedout.
• Thorough packing increases the amount of forage that can fit into a silo.  Higher densities generally reduce the annual cost of storage per ton of forage by increasing the amount of crop entering the silo and reducing crop losses during storage.

Ruppel (1992) measured the DM losses in alfalfa silage in bunker silos and developed an equation to relate these losses to the density of the ensiled forage (Table 1).  Researchers recommend a minimum packing density of 14 pounds per cubic foot DM (Muck and Holmes, 2000).  Many well managed forage producers are achieving or exceeding these numbers with goals of 16 to 18 pounds per cubic foot DM.

Table 1. Dry matter loss as influenced by silage density (Ruppel, 1992).

Factors affecting forage density
In the process of silo filling, as forage is added to the silo, the weight of the material begins to force oxygen out of the forage mass.  Gravity compacts forage naturally in deep silos, particularly in upright silos where density increases dramatically from top to bottom.  Some natural compaction occurs in deep horizontal silos, but mechanical packing is required to achieve adequate density and limit excess air infiltration.

Factors correlated with DM density included initial layer thickness, average packing tractor weight, packing time per ton of forage delivered, and DM content.

Producers can control several factors to achieve higher densities to minimize DM and nutrient losses during ensiling, storage, and feedout.  Here are several key items to focus on during forage harvest:

• Number of pack tractors:  Adding an additional packing tractor as delivery rate increases can help keep packing time in the optimum range of one to three minutes per ton of fresh forage.
• Pack tractor weight:  Adding weight to the front or side of the tractor or three-point hitch and filling the tires with fluid can increase pack tractor weight.
• Forage delivery rate:  Reducing the forage delivery rate is somewhat difficult to accomplish, as very few dairy producers or custom choppers are inclined to slow the harvest rate so that additional packing can be accomplished.  University of Wisconsin researchers suggest the fill rate (tons per hour) not exceed the tractor weight divided by 800.  For example, if using a 40,000-pound tractor, divide that weight by 800 to get 50 tons per hour.
• Silo filling methods:  Incoming forage should be spread in thin layers (6 to 10 inches).  In a properly packed bunker silo, the tires of the packing tractor should pass over the entire surface before the next forage layer is distributed.
• Filling silo to greater depths:  Greater silo depths will increase density, but you need to consider the practical limits and safety concerns.  These safety concerns relate both to filling and feedout.
• Other factors:  Forage maturity or DM, grain-to-stover ratio, chop length, optimal corn silage processing, storage unit type, and, in the case of bags, machine operators all have a huge influence on final densities achieved.

About the author:  Barry Visser is a dairy technical specialist based in Minnesota.  He grew up on his family’s dairy farm in northwest Minnesota.  Visser received his bachelor’s degree in animal science from the University of Minnesota in 1994.  He worked as a dairy nutritionist for four years before returning to graduate school to earn his master’s degree in dairy cattle nutrition.  His research focused primarily on transition cows and minimizing pre- and post-calving metabolic challenges.  While a graduate student, Visser also worked as an assistant herdsman for the University of Minneosta St. Paul dairy herd.  At Vita Plus, Visser works with fellow employee owners to troubleshoot farm nutrition, forage quality and management challenges.