Achieving a tight pack and attaining a high density are extremely important aspects of harvesting and storing silage. Reaching goals of high density are important for two main reasons. First, 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 feed out. Secondly, 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 of DM. Many well-managed forage programs achieve well above these numbers with goals of 16 to 18 pounds per cubic foot.

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

Density (lb. DM/ft3)    DM loss at 180 days
                (% of DM ensiled)
        10            20.2
        14            16.8
        15            15.9
        16            15.1
        18            13.4
        22            10.0

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. Initial layer thickness, average packing tractor weight, packing time per ton of forage delivered and DM content all impact forage density.
    Producers can control several factors to achieve higher densities to minimize DM and nutrient losses during ensiling, storage, and feed out. Here is a list of 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 1 to 3 minutes per ton of fresh forage.
    – Pack tractor weight: Filling the tires with fluid and adding weight to the front or side of the tractor or three-point hitch can increase pack tractor weight. Some farms have found value in pulling a packing wheel or device behind that pack tractor.
    – Keep pack tractors on the pile: Pack tractors need to stay on the pile when at all possible. They do no good when circling the pad off the silage. Tractors also need to be moving and not idle.
    – 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 additional packing can be accomplished. University of Wisconsin researchers suggest the fill rate (tons per hour) not exceed pounds of tractor weight divided by 800. For example, if the packing tractor weighs 40,000 pounds, the rate should not exceed 50 tons per hour. (40,000 ÷ 800 = 50).
    – 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 consider the practical limits and safety concerns, which relate to both filling and feedout.
    – Other factors: Forage maturity or DM content, grain-to-stover ratio, chop length, optimal corn silage processor, storage unit and – in the case of bags – machine operators all have a huge influence on final densities achieved.
    Now is a great time to explore improvements to forage density as you prepare for this upcoming harvest season. Finally, be safe. Corn silage harvest brings an increase in machinery traffic and long hours. As always, make safety your top goal this corn silage season.
    Barry Visser is a nutritionist for Vita Plus.