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Bags VS Bunkers VS Uprights
by Mike Hutjens, Extension Dairy Specialist, University of Illinois, Urbana

Storing forages is an important decision for Illinois beef and dairy managers. Several systems are available and should be evaluated by managers. The following factors should be considered.

  • Initial and annual costs to store forage
  • Herd size
  • Optimizing forage quality (harvested and stored)
  • Feed delivery system

Storage costs

University of Wisconsin agricultural engineers reported silage storage costs including capital investment and annual costs at various herd sizes. The analysis included hay silage stored in eight different systems (Table 1). Capital costs included structures and equipment used in filling, storing and emptying the hay silage. No transportation, harvesting, or moving feed to the animals were included. Silos and gravel pads had a life expectancy of 20 years while equipment was assumed to have a 10 years of life expectancy. Annual costs include capital costs, labor, plastic coverings, fuel, and dry matter lost during storage. Forage (hay equivalent basis) was valued at $85 a ton. Tractors were assumed to have other uses besides forage storage and allocated on a proportional basis to handle forage storage. Table 1 summarizes total capital and annual costs per ton of dry matter at two different quantities of stored dry matter (four amounts were calculated in original report).

Capital cost per ton of silage dry matter was highest for new steel oxygen-limiting structures compared to other systems. If refilling occurs with steel oxygen-limiting units (1.5 to 2 times annually), costs will be reduced. Used oxygen limiting and cast in place structures were similar. Silo bags, silage piles, and wrapped bales had the lowest investment. No significant economics of scale occurred above 758 tons of dry matter (other storage amounts evaluated were 1536 and 3072 tons). Capital cost per ton can be important on farms where capital is limiting due to expansion and/or existing debt load.

Table 1. Total capital cost and annual cost (in parenthesis) per ton of dry matter for 384 and 768 tons of stored dry matter (Holmes, 1998).

  384 tons DM 768 tons DM
Storage type --------------$/ton of dry matter----------------
Steel-glass oxygen limiting (new) 427 (82) 301 (60)
Steel-glass oxygen limiting (used) 268 (55) 187 (41)
Cast-in-place oxygen limiting 285 (58) 186 (41)
Concrete stave 192 (46) 138 (36)
Above ground bunker 152 (45) 103 (37)
Packed silage pile 63 (37) 41 (32)
Bagger 88 (38) 53 (32)
Wrapped bales 64 (36) 38 (32)

Annual costs reflect the yearly cost to store forage. Oxygen-limiting structures were highest while wrapped bales, bags, and packed silage were lowest. Again, no economics of scale were reported above 768 tons of dry matter. Good management is needed to achieve values in Table 1. Dry matter losses in storage were estimated to be six percent for oxygen limiting units; 10 percent for concrete stave and bags, and 13 percent for piles, bunkers, and wrapped bales. If dry matter losses in bunker silos increase to 18 or 24 percent, the annual cost increases to $49 and $55 per ton of dry matter for 384 tons of stored feed (similar increases occurred at 768 tons).

Herd size factors

After cost, herd size is the next important factor. If a herd size is less than 100 cows plus young stock, large storage structures not viable. Upright silos, bags, and wrapped bales are good choices. If forages are fed in a conventional barn, upright silos minimize weather-related risks and use of tractors to feed cattle. In-line stationary mixers and belt feeders favor tower structures. Bottom unloading structures can provide a constant and consistent supply of fermented forage to cows, but a layer of low quality forage can occur between each cutting or filling period.

Bags favor herds considering expansion with mobile TMR mixers. One major advantage with bags is the ability to segregate forage quality and feed smaller amounts. Smaller bag diameter does not force large amount of forage to be fed to maintain surface forage quality. Remove six inches of forage a day from the face and close the bag. Plastic disposal is an environmental concern.

Processing wrapped bales is a management consideration. Some TMR mixers can not handle, reduce forage length, and mix wet balage. Dairy manager must control intake as cow relish balage and will eat it exclusively if offered free choice in bale feeders. Using wrapper bales require solid management to achieve optimal moisture content (50 to 60 percent), avoid damage to the wrapped bale, and wrapping the bale properly.

Remove six inches of forage per day from the face of the bunker or pile to maintain quality and avoid secondary fermentation. When removing the forage, avoid disturbing the face exposing oxygen to the silage mass. Size bunkers and piles to match minimum feeding rates. Both units MUST be covered with plastic or another oxygen/air barrier.

  • Less than 100 cows: Tower silos, bags, and wrapped bales
  • 100 to 200 cows: Tower silos, bags, and narrow bunkers
  • Over 200 cows: Bags, bunkers, and packed piles

Forage Quality

All forage storage systems can be successful if matched to herd size to optimize feed out rate. Wetter forage lowers field loss. Excessive moisture (hay silage over 60 percent moisture and corn silage over 70 percent) can result in an undesirable fermentation and excessive seepage losses. Adding a research proven inoculant can improve fermentation characteristics, lower dry matter loss, increase digestibility, and optimize desirable VFA pattern (over 70 percent lactic acid of the total VFA produced). Rapid harvest and storage maintain forage quality from the field, reduce air exposure, and increase compaction.

Feed Delivery System

If TMR is or will be used, if a mobile mixer is or will be used, and rapid forage removal is desirable; packed piles, bunkers, and bags are obvious choices. If herd size is less than 100 cows, cows are housed inside a warm facility, and labor wants to work in a favorable environment, tower silos are one logical choice. Wrapped bales allows the use of existing hay harvesting equipment, minimizes labor requirements, and fits smaller herd sizes.


Holmes, B. J. 1998. Sizing and managing silage storage to maximize profitability. Four State Forage Feeding and Management Conference Proc. Pages 55-64.

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