How to choose between silage bunkers, piles, and bags

Modern day dairy farms need to store and maintain an enormous amount of silage to feed their dairy cows adequately to keep milk production strong. Coming up with the right way to store the forage is a decision that every dairy farmer must make. This decision on how to store the forage can be driven by many factors and, depending on the situation, the answer is not always so simple. Some of the possibilities available today to store the silage are silage bunkers, silage bags, and silage piles. Each of these options has different benefits and limitations.

Silage Piles:

In order to create the pile, heavy farming equipment needs to move about, and at minimum, a pile needs to be created on top of a concrete slab. An asphalt slab can work, but just a few years of the large equipment moving over it, and the asphalt will have large sunken areas where the tractor tires moved over the asphalt. A concrete surface slab on the other hand has a much longer life over asphalt. Once the concrete slab is poured and ready, the silage pile is created on top of the slab, and covered with polyethylene silage sheeting. Assuming the pile has the recommended pitch roughly 1:3, a pile with a width of 20 ft. will have roughly 66 ft³ of feed per ft. of length. Based on this, a 20ft x 100ft pile for silage will have a volume roughly 6600 ft³. Future feed pile expansion is as simple as adding concrete to the existing base slab. Feed piles are easy to grow in both directions. The concrete slab can be expanded in either direction by pouring additional conrete. Below is a handy pile sizing chart based on the width that determines the height at the center, the volume of feed per ft. of length, and the necessary silage bunker cover width.

Silage Pile (without side walls) Attributes based on a 1:3 pitch
Pile Width	Feed Volume	Maximum Height	Minimum bunker cover width 2’ extra each side
20 ft.	66 ft³ per ft. of pile length	3’ 4”	25’
30 ft.	150 ft³ per ft. of pile length	5’	36’
40 ft.	266 ft³ per ft. of pile length	6’ 8”	46’
50 ft.	416 ft³ per ft. of pile length	8’ 4”	57’
60 ft.	600 ft³ per ft. of pile length	10’	67’
70 ft.	816 ft³ per ft. of pile length	11’ 8”	78’
80 ft.	1066 ft³ per ft. of pile length	13’ 4”	88’
90 ft.	1350 ft³ per ft. of pile length	15’	99’

Silage Bunkers:

Bunkers require the same equipment as piles to fill, but bunkers require the most infrastructure of any of the methods covered in this article. A bunker is has a poured concrete base, and either precast concrete walls, our poured in place concrete walls. The walls are either on 2 or 3 sides. Bunkers give you much more volume for the same footprint. Once you build your bunker, it is important that it can be used for many years to come so be sure to use 6 mil silage poly to keep the walls from acid breakdown. Bunker walls can be any height, but the most common heights are 8’ and 12’. These high bunker walls allow for greater compression. Studies have shown that greater compression reduces spoilage. The following is a chart based on the width that determines the height at the center, the volume per ft. of length, and the necessary silage bunker cover width.

Silage Bunker (8’ side walls) Attributes based on a 1:3 pitch
Bunker Width	Feed Volume	Maximum Height	Minimum bunker cover width 2’ extra each side
20 ft.	226 ft³ per ft. of bunker length	11’ 4”	25’
30 ft.	390 ft³ per ft. of bunker length	13’	36’
40 ft.	586 ft³ per ft. of bunker length	14’ 8”	46’
50 ft.	816 ft³ per ft. of bunker length	16’ 4”	57’
60 ft.	1080 ft³ per ft. of bunker length	18’	67’
70 ft.	1376 ft³ per ft. of bunker length	19’ 8”	78’
80 ft.	1706 ft³ per ft. of bunker length	21’ 4”	88’
90 ft.	2070 ft³ per ft. of bunker length	23’	99’

Silage Bunker (12’ side walls) Attributes based on a 1:3 pitch
Bunker Width	Feed Volume	Maximum Height	Minimum bunker cover width 2’ extra each side
20 ft.	306 ft³ per ft. of bunker length	15’ 4”	25’
30 ft.	510 ft³ per ft. of bunker length	17’	36’
40 ft.	746 ft³ per ft. of bunker length	18’ 8”	46’
50 ft.	1016 ft³ per ft. of bunker length	20’ 4”	57’
60 ft.	1320 ft³ per ft. of bunker length	22’	67’
70 ft.	1656 ft³ per ft. of bunker length	23’ 8”	78’
80 ft.	2026 ft³ per ft. of bunker length	25’ 4”	88’
90 ft.	2430 ft³ per ft. of bunker length	27’	99’

Silage Bags:

Silage bags have the least infrastructure necessary to store and maintain silage. The machinery needed for filling the bags is the only thing you’ll need. The feed bags can sit directly on soil. Bags hold the silage while the machine moves along as it is filling the bag. Bags come in different diameters and they get thicker as the diameter gets larger. Grain bags are slightly thicker than the silage bags, but using a grain bag for silage gets you extra strength for a little more cost. The following is a chart based on the diameter of the bag that determines the height at the center and the volume per ft. of length.

Silage Bags
Diameter	Volume	Maximum Height
8 ft.	50 ft³ per ft. of bag length	8’
9 ft.	63 ft³ per ft. of bag length	9’
10 ft.	78 ft³ per ft. of bag length	10’
12 ft.	113 ft³ per ft. of bunker length	12’
14 ft.	153 ft³ per ft. of bag length	14’

How do you choose the type of silage storage that is best for you?

There are many factors that will play a role in the decision as to whether you should build a bunker, pile, or use bags for your silage. Your estimated daily consumption, your space allocation, and the compaction you are looking for all play a roll.

The estimated daily usage may be the most important. Your volume of silage per foot cannot be greater than your daily usage, or you will get face spoilage. The rule of thumb is that at least 6” has to be removed every day or the face will spoil. To be safe, you should design to at least a foot. Multiplying the number of cows by the expected daily weight divided by the expected density and you get the daily volume. Your face area can be smaller, but if it is too big, the silage will spoil because your cattle may not consume the required 6" of feed. For smaller number of cows, bags are the clear choice. For a very large number of cows, if you don’t have enough face area, you will be moving through your silage very quickly. Every day, you will need to move the feed further and further, and efficiency will suffer.

If you have enough cows to justify the pile or bunker approaches, space considerations become the next factor. 2 sided bunkers can always be lengthened but are very difficult to widen. If you are limited in width, or are creating many side by side bunkers, then the bunker strategy is a very good one. Bunker costs are high, but they give you the absolute best silage due to the height of the pile. The best compaction happens under the weight of the silage, and the higher the pile the better.

Minimize spoilage:

Making sure that you use at least 6” per day of the face of the silage will keep spoilage from happening on the face. Everywhere else though, it is ultra-important that oxygen is kept out of the pile. Silage compaction is very important due to the fact that any oxygen left over after fermentation or oxygen that gets into the silage after fermentation promotes spoilage. According to a University study, the best compaction happens at the bottom of the pile. While driving over the pile can increase compaction, the height of the pile seems to be the most important factor. In addition, using plastic on the side walls and additional oxygen barrier on top the pile in addition to the silage cover will keep oxygen entry to a minimum. The standard silage cover is designed to reflect the heat of the sun away, and take the load of the tire side walls. Oxygen barrier has a different function. It is very thin and conforms to the pile better as it is flexible. Also, its impermeability to oxygen is much higher than the bunker cover. The oxygen barrier fills in gaps as the oxygen is used for fermentation, and keeps a vacuum from forming inside. This also keeps the oxygen from entering the silage.