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Silage Storage Structures1
R. P. Cromwell, J.W. Prevatt, and W. J. Becker2There are three types of silos: (1) the conventional upright or tower silo; (2) the oxygen-limiting tower silo; and (3) the horizontal silo. Horizontal silos can be either a pit, an above ground concrete bunker, or plastic bag. Silos differ in costs, convenience and storage losses. There is no best type for all farm situations. Table 1 gives the capacities of various types and sizes of silos.
HARVEST AND STORAGE LOSSES
Losses will vary depending on weather conditions, crops stored, moisture content of the crop stored, fineness of chop, type of silo used, and the level of management in handling the crop and facilities. Losses in forage making can be classified into two broad categories: harvest and storage. Harvest losses include plant respiration as well as mechanical losses from weathering, leaf shattering, and other wastage. Plant respiration refers to the use of nutrients by the plant before it completely dies. Storage losses include spoilage, seepage, fermentation, and respiration losses.A major factor influencing harvest and storage losses is the moisture content of the crop at harvest time. Expected losses, using various methods of forage making, are given in Table 2 . The hay crop considered in the table is alfalfa. Harvest losses that would occur when harvesting grass would be less than the amounts in the table because grasses do not have the leaf shatter problem that alfalfa does when it is harvested at relatively dry moisture levels.
Whole plant corn silage has relatively low total losses of dry matter because of its low storage losses. Corn is an excellent silage crop because of its high soluble carbohydrates and low protein content. The moisture level in corn silage stored in concrete tower and bunker silos should be in the 60 to 65% range because higher moisture causes excessive seepage losses and lower moisture will cause higher harvest losses.
The moisture content of the crop at ensiling influences the amount of dry-matter loss that occurs in various types of silo structures. Table 3 shows that storage losses for high moisture silage are less in horizontal silos because there is less seepage loss in this type of silo. Storage losses are greater in a horizontal silo when low moisture silage is made because it is difficult to pack relatively dry forage. The table indicates that at a moisture level in the 61 to 70% range the loss suffered in a concrete tower silo is only 1.3% greater than for an oxygen limiting silo. When drier forage is stored the difference tends to increase in favor of the oxygen limiting unit.
SELECTION OF A SILAGE STORAGE SYSTEM
When selecting a silage storage facility be sure to consider crop types, harvesting machinery, silo unloading, and feeding equipment. There are many choices to be made and they should be thought out carefully.
Tower silos offer ease of filling, low storage losses, and are adaptable to a totally automated feeding system. Generally tower silos are used on small- to medium-size dairies and feed lots. Tower silos are not used in very large operations because silage cannot be unloaded from tower silos as rapidly as it can be from large horizontal ones. This can be a very important consideration in large operations. However, tower silo unloaders have improved tremendously and the size of operation that can utilize a tower silo has grown with this improvement.
- Visit other farmers for ideas and opinions.
- Determine the availability of service for various pieces of equipment in your area.
- Consider how easy it would be to expand the system being considered.
- Determine whether the investment will pay off.
TYPES OF SILOS
There are three types of silos: tower, horizontal and plastic bag.Tower Silos
The types of tower silos are: (1) concrete stave; (2) poured concrete; and (3) oxygen limiting silos.Concrete Stave Silos
Stave silos are probably the most common silo. The staves have metal hoops wrapped around them to keep the structure from coming apart due to the pressure of the silage. The hoops are spaced closer near the bottom where the pressure is greatest. This type of silo is good for storing whole plant grain crop silages like corn and sorghum and also for relatively wet hay-crop silages stored at the 55 to 65% moisture level. Stave silos cost the least of all the tower silos. Stave silos are usually equipped with a top unloader.Silo costs and unloaders are covered more thoroughly later in this publication.
Poured Concrete Silos
This type of silo is made with a metal form. The concrete is strengthened with metal reinforcing rods embedded in the concrete. Poured silos are normally equipped with top unloaders.Oxygen Limiting Silos
Most of the oxygen limiting silos are made of steel panels that have glass fused to the steel to protect it from silage acids. However, poured concrete silos can be made to function as an oxygen limiting silo. This special type of poured silo is equipped with a bottom unloader and a reinforced concrete top rather than the conventional metal dome.Oxygen limiting silos are often used for storing low moisture hay crop silages (silage with a moisture content of about 50%) and high moisture grain (grain with a moisture content of 28 to 30%). There is a difference of opinion among researchers about the advantages of low moisture silage. Proponents of low moisture silage contend that cows eat more and perform better. However, many researchers feel that silage made at the conventional 60 to 65% moisture range compares favorably with the drier silage.
Oxygen limiting silos primarily use bottom unloaders. This type of unloader allows filling from the top while removing silage from the bottom of the silo.
Horizontal Silos
Horizontal silos can be either an above-ground bunker, a trench, or a sealed plastic envelope. Above-ground bunkers can be made by merely mounding up soil on the ground or by constructing a silo of concrete.A trench silo is formed by digging a long trench in a well-drained location. The trench will often have earthen walls and in some instances an earthen floor. Having an earthen floor in any type of horizontal silo is usually a mistake. Unloading the silage churns up the wet floor creating a bog.
Horizontal silos are adaptable to large-scale operations. Silage can be unloaded from a horizontal silo faster than is possible with most tower silo unloaders. Silage can be unloaded with a front end loader or by special horizontal silo unloaders. Horizontal silos are best suited to crops that yield a high tonnage in one harvest like corn or sorghum that allow rapid filling. Filling with small amounts over a long period leaves a lot of the silage surface exposed and causes excessive spoilage.
Wilted hay-crop silage, such as pangola or bermuda grass, can be stored in a horizontal silo, but it shouldn't be drier than 60 to 70% moisture. Hay-crop silages drier than 60% do not pack well. Poorly packed silage will suffer heat damage because of the oxygen remaining in the silage.
Plastic Bag Silos
Storing chopped silage in large plastic bags is a relatively recent practice in comparison to the other types of silos discussed. This system requires a machine that forces the chopped material into the bag.Advantages of Bag Silos
- The system is very flexible. A farmer can produce and store the amount of silage needed for any particular year.
- Hay-crop silages yield relatively small amounts of silage per cutting. Bags are suited to these crops because whatever amount is harvested can be sealed. In contrast, most silos that have the capacity to store a full season of grass silage will have a number of spoilage layers depending on the number of cuttings required to harvest the crop.
- Good silage can be made at moisture levels ranging from 50 to 75%. Harvesting can be spread over a relatively long period if necessary.
- Storage losses are comparable with tower silos and better than most other horizontal silos if the bags do not tear.
Disadvantages of Bag Silos
- The machine that stuffs the silage into the plastic bag is expensive. Prices range from $10,000 to $25,000.
- The plastic bags are not reusable.
- Varmints rip holes in the plastic causing a zone of spoiled silage around the opening. This is more of a problem with grain crop silages than with hay-crop silages.
- If the feeding of the silage isn't managed properly, the losses can be higher than average for other types of storage.
Roll Bale Silage
Making silage by storing rolls of hay-crop material at 50 to 65% moisture level in a sealed plastic envelope is presently being investigated at various universities. The rolls are being stored in individual plastic bags, plastic tubes that hold approximately 15 rolls, and under a sealed plastic cover draped over a large stack of wet forage rolls. This method of silage making shows promise of being useful under certain circumstances, but additional research must be conducted before it can be recommended with the same degree of confidence as the more proven methods.DETERMINING SILO CAPACITY NEEDED
The silo capacity needed depends on how the silo is to be used and the number and class of livestock to which the silage is fed. Some operations would use the silo to carry a herd over a winter for 120 to 150 days while others would feed out of the silo throughout the year. Year-around feeding of silage is popular with some farmers because more forage can be harvested from a given area than with pasturing. Also, forage quality can be more uniform and the amount and kind of ration can be controlled.Warm weather can cause exposed silage to spoil. In the hot humid Southeast, a farmer should remove at least 4 inches and preferably 6 inches per day from the exposed surface of the silo. More than 4 to 6 inches can be removed, but based on removing 4 inches or 1/3 of a foot, a farmer should have a minimum of 122 feet of packed silage for a 365-day feeding program. This could be two 70-foot-tall tower silos or a 140-foot-long bunker silo. More than the exact 122 feet should be provided in order to allow for settling. Refilling a silo can reduce this requirement. However, a farmer needs to be conservative when figuring how many times he will refill the silo.
Table 4 and Table 5 can be used to size the silo to the required daily volume of silage.
Silo Unloaders
The machinery used to unload tower and horizontal silos differ considerably. Tower silo unloaders are specialized machines. Tower silo unloaders are either top unloaders or bottom unloaders. Horizontal silos can be unloaded with a multi-purpose front-end loader mounted on a farm tractor or with specialized silage unloaders.Top Unloaders
This type of unloader loosens silage from the surface of the silage and conveys it to the center of the silo. The silage enters a blower at the center and is blown through a chute that is directed at an open door in the side of the silo. The silage impacts a metal tunnel built around the line of doors in the side of the silo. It falls down the tunnel, and is collected in a feed wagon or on a conveyor that automatically distributes the silage along a feed bunk.One of the most inconvenient things about top unloaders is that the chute has to be changed from one window to the next lower window as the unloading proceeds. The frequency that someone has to climb up the ladder rungs on the silo doors inside of the tunnel to lower the chute depends on the rate that silage is being fed. This is a far bigger problem in the early stages of unloading a tall silo because of the height that must be climbed.
Bottom Unloaders
There are at least two designs of bottom unloaders used. The silage slips down the silo onto the rotating conveyor that scrapes silage off and delivers it to the center of the unit. The silage is then conveyed out of the silo by a conveyor located in a tunnel in the foundation of the silo. This type of unloader does not require periodic climbing up the silo like the top unloader does.Another type of bottom unloader uses weights on the end of chains that are attached to a shaft at the center of the silo. The shaft rotates causing the chains to extend horizontally. The weights at the ends dislodge silage from the cylinder of material as it slips down the silo. The loosened silage falls through an opening in the bottom.
Horizontal Silo Unloaders
Silage is normally unloaded from horizontal silos with a front-end loader or with a machine designed specifically for the job.The front end loader is chosen by many farmers because it is often mounted on a tractor which has many other uses on the farm. The drawbacks to using a front-end loader for unloading silage are:
The machines designed specifically for unloading horizontal silos can fill a feed wagon very fast, and the face of the silage is left smooth. The smooth face helps to keep spoilage losses low because the surface exposed to the air is less than when the face is ragged. There were at least two manufacturers of specialized bunker silo unloaders in the past. Unfortunately both manufacturers have ceased production on these machines. Unless another company begins to manufacture a similar machine, farmers will have to try to find a used one or use a front end loader.
- Unloading with a front-end loader is a relatively slow operation because of the maneuvering required unless large construction type unloaders are used.
- The face of the silage is left ragged and is more prone to spoil.
Economic Comparison of Silage Storage Facilities
Deciding whether or not to invest in a silage program and in particular which silage storage facility is not a simple decision. Presently, livestock operators make this decision while considering numerous alternative storage facilities and in an unpredictable economic environment. Therefore, the following cost comparison of silage storage facilities will describe the factors that affect the cost of storing silage.Investment costs vary considerably due to the type and size of the silage storage facility, as shown in Table 6 . The metal and concrete oxygen-limiting facilities are the most expensive, while the concrete bunker is the least expensive.
The annual ownership costs of silage storage facilities expressed as a percentage of investment costs are shown in Table 7 . The ownership cost rates of interest and taxes were identical among the silage storage facilities and equipment since these ownership costs are not directly affected by the length of use. However, the depreciation rate is affected by the length of life of the capital asset. Repair rates as a percentage of investment cost were similar except for the bagger and equipment associated with the facilities. Insurance percentages were identical for the silo facilities, but varied for the concrete bunker, bagger, and equipment. The concrete bunker did not have an insurance cost because it was considered impractical to insure this facility against fire loss. The total ownership costs as a percentage of investment cost for the silage storage facilities ranged from 16.5% to 24.5%, with the bagger having the largest total percentage.
The annual ownership costs and ownership cost per dry matter ton of silage for the silage storage facilities are reported in Table 8 . The metal and concrete oxygen-limiting facilities resulted in the largest annual ownership costs and ownership cost per dry matter ton of silage. The concrete bunker had the lowest annual ownership cost and ownership cost per dry matter ton of silage. The ownershhip cost per dry matter ton of silage for the concrete bunker was from one-third to one-fourth of the metal oxygen-limiting facilities.
The percent of dry matter loss increased the ownership cost per dry matter ton for each storage facility, as shown in Table 9 . Dry matter loss increased the ownership cost from 5% to 14% for the storage facilities. The dry matter loss for the metal oxygen-limiting facility (20 ft x 70 ft) accounted for the largest dollar increase in ownership cost per dry matter ton which totaled $7.57 per dry matter ton or $1,491 increase in annual ownership cost. The dry matter loss of a facility is only a small part of the total dollar loss when the cost of producing and harvesting silage is included.
Based on the annual ownership costs and ownership cost per dry matter ton, a wide variation among silage facilities is apparent. These variations are due to a combination of different levels of investment costs, ownership cost percentages, size of the facility, and the level of dry matter loss for the respective facilities. Additional fillings per year would substantially reduce the ownership cost per dry matter ton. The bagger facility particularly lends itself to multiple fillings on an annual basis since storage is in detachable large plastic bags, and the bagger is mobile.
The use of the ownership cost comparisons developed is intended to provide livestock operators with a method to evaluate the cost of silage storage facilities. The ownership cost of the facilities coupled with the production, harvesting and feeding cost of silage will provide cattlemen with the necessary information to select a facility.
Tables
Table 1. Approximate silo capacities (Penn State University).
Silo type and size (ft)
Silo capacity (tons)/moisture level
Tower
Dry Matter
70%
60%
50%
20 x 50
115
385
290
232
20 x 60
141
470
352
282
20 x 70
171
570
428
342
30 x 50
264
880
660
528
30 x 60
324
1,080
810
648
30 x 70
383
1,275
958
766
30 x 80
444
1,480
1,110
888
Horizontal
10 x 30 x 96
150
500
375
300
12 x 40 x 112
300
1,000
750
600
14 x 50 x 112
450
1,500
1,125
900
14 x 60 x 128
600
2,000
1,500
1,200
Table 2. Expected dry matter losses in forage harvesting and storing (Penn State University).
Dry matter losses (%)
Preservation method
Storage
Total
Hay (alfalfa), Small bale
No rain
3.6
21.0
Rained on
4.0
36.6
Hay, Large bale stored outside
Field Cure
14.2
39.2
Acid treated
10.7
25.7
Silage
Hay-crop silage
70 or more % moisture
21.2
23.2
60-69%
10.1
15.1
under 60%
8.2
19.7
Corn silage
70 or more %
13.4
17.4
60-69%
6.3
11.3
under 60$
6.3
22.5
Table 3. Expected dry matter losses in hay-crop forages ensiled in various types of silos (Penn State University).
Dry matter losses (%)
Moisture level (%)
Concrete
tower
Oxygen
limiting
Horizontal
71 and over
21.2
19.1
13.4
61 - 70
10.1
8.8
14.0
60 and under
8.2
5.5
16.7
Table 4. Horizontal silo capacities (ton/foot of length).
Depth (ft)
Bottom width (ft)*
20
30
40
50
60
70
80
8
3.4
5.0
6.5
8.1
10.0
11.3
13.0
10
4.3
6.2
8.4
10.2
12.2
14.2
16.2
12
5.2
7.5
10.0
12.3
14.6
17.0
20.0
14
6.0
8.7
11.5
14.3
17.0
20.0
22.7
16
7.0
10.0
13.1
163.0
20.0
22.7
26.0
*Sidewalls slope out 1' in 8' of height.
Note: Capacities are based on 70% moisture silage weighing 40 lb/cubic ft.
Table 5. Pounds of silage in a 4-inch layer of silage for circular silos and the animals fed at two feeding rates.
Number of animals fed
with a 4-Inch layer
Silo
diameter
(ft.)
Volume of dry matter
4" (cubic ft) In 4" Layer
12 lbs DM or
40 lbs @ 70%
20
104
5,232
87
22
127
6,332
105
24
151
7,531
125
26
177
8,844
147
28
205
10,250
171
30
235
11,770
195
Table 6. Investment cost of silage storage facilities.
Investment cost**
Silo type and size
Unloader/
blower
Tractor
& loader
Total
---------------($)---------------
Concrete stave
20 x 70
7,200
4,500
30,950
30 x 80
8,000
4,500
53,250
Poured concrete
20 x 70
7,200
4,500
38,950
30 x 80
8,000
4,500
74,250
Metal oxygen-limiting
20 x 80
17,800
4,500
82,300
25 x 90
19,800
4,500
121,300
31 x 90
46,000
4,500
200,500
Concrete oxygen-limiting
20 x 72
*
4,500
69,850
30 x 80
*
4,500
118,250
30 x 100
*
4,500
128,750
Concrete bunker
10 x 30 x 96
8,000
21,000
12 x 40 x 112
8,000
25,500
14 x 50 x 112
8,000
28,500
Bagger***
5 bags
8,000
34,625
15 bags
8,000
37,875
25 bags
8,000
41,125
*The investment cost of the unloader and blower was included in the silo investment cost as a "turn key" price.
**Tractor and front-end loader investment costs were assumed to be $18,000 and $2,000, respectively. One-fourth of tractor investment cost was allocated to the concrete stave, poured concrete, metal oxygen-limiting and concrete oxygen-limiting silos. The entire investment cost of the front-end loader and one-third of the tractor investment cost was allocated to the concrete bunker and bagger.
***The investment cost for the bagger includes the cost of the bags.
Table 7. Annual ownership costs of silage storage facilities as a percentage of investment cost.*
Item
Depreciation
Interest
Repairs
Taxes
Insurance
Total
--------------------(%)--------------------
Concrete stave
5
7
2
1.5
3
18.5
Poured concrete
5
7
2
1.5
3
18.5
Metal oxygen-limiting
5
7
2
1.5
3
18.5
Concrete oxygen-limiting
5
7
2
1.5
3
18.5
Concrete bunker
5
7
3
1.5
0
16.5
Bagger
10
7
5
1.5
1
24.5
Unloader & blower
10
7
5
1.5
3
26.5
Front-end loader
10
7
5
1.5
1
24.5
Tractor
10
7
10
1.5
1
29.5
*The estimated life of the concrete stave, poured concrete, metal oxygen-limiting, concrete oxygen-limiting, and concrete bunker was 20 yr.; while the bagger, unloader and blower, front-end loader and tractor were assumed to have an estimated 10 yr. life.
Table 8. Annual ownership costs and ownership cost per ton of silage dry matter for silage storage facilities.*
Silo type
and size
Dry
Matter**
(ton)
Investment
cost
($)
Annual
ownership
cost***
($)
Ownership
cost/ton
dry matter
($/ton)
Concrete stave
20 x 70
171
30,950
6,796
39.75
30 x 80
444
53,250
10,375
23.37
Poured concrete
20 x 70
171
38,950
8,276
48.40
30 x 80
444
74,250
14,871
33.49
Metal oxygen-limiting
20 x 80
197
82,300
17,144
87.03
25 x 90
346
121,300
24,519
70.87
31 x 90
532
200,500
41,267
77.57
Concrete oxygen-limiting
20 x 72
176
69,850
13,417
76.23
30 x 80
444
118,250
22,371
50.39
30 x 100
560
128,750
24,313
43.42
Concrete bunker
10 x 30 x 96
150
21,000
4,405
29.37
12 x 40 x 112
300
25,500
5,147
17.16
14 x 50 x 112
450
28,500
5,642
12.54
Bagger
5 bags
248
36,625
10,010
40.44
15 bags
743
37,875
13,260
17.86
25 bags
1,238
41,125
16,510
13.34
*Storage investment cost includes the capital outlay for the silo, unloader and blower, tractor, front-end loader and bags. The ownership costs per ton of silage dry matter provides a measurement for unit-cost comparison. **The dry matter capacity, measured in DM units, was estimated for silage. When storing other feedstuffs, such as haylage, a different dry matter capacity may be appropriate.
***The annual ownership cost is the sum of the depreciation, interest, repairs, taxes and insurance for the various capital items, as described in Tables 10 and 11.
Table 9. Dry matter loss and ownership cost per ton dry matter for silage storage facilities.
Silo type and size
Dry matter
Loss
Before loss
After loss
(%)
-------------($/ton)------------
Concrete stave
20 x 70
10
39.75
44.17
30 x 80
10
23.37
25.97
Poured concrete
20 x 70
10
48.40
53.78
30 x 80
10
33.49
37.21
Metal oxygen-limiting
20 x 80
8
87.03
94.60
25 x 90
8
70.87
77.03
31 x 90
8
77.57
84.32
Concrete oxygen-limiting
20 x 72
8
76.23
82.96
30 x 80
8
50.39
54.77
30 x 100
8
43.42
47.20
Concrete bunker
10 x 30 x 96
14
29.37
34.15
12 x 40 x 112
14
17.16
19.95
14 x 50 x 112
14
12.54
14.58
Bagger
5 bags
5
40.44
42.57
15 bags
5
17.86
18.80
25 bags
5
13.34
14.04
Footnotes
1. This document is DS54, one of a series of the Animal Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date September, 1992. Reviewed July, 2002. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.2. Associate Professor, Agricultural Engineering Department; Associate Professor, Gulf Coast Research and Education Center; Associate Professor, Agricultural Engineering Department; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.
U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
Copyright Information
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