The Origins of the Silo

Silos have often been described as the skyscrapers of the agricultural landscape. Traveling through prime farmland such as Lancaster County, Pennsylvania, silos are the most obvious single agricultural structure rising above the remainder of most farms. Since the introduction of silage into the diets of cows in the second half of the nineteenth century in the United States, silos have been a ubiquitous part of dairy farms. Silos have been constructed of a variety of materials including wood, stone, concrete, and metal, and in a variety of forms ranging from boxes, square in section, octagonal, and, most commonly, cylinders.

The purpose of a silo is to store winter fodder for cows. The word "silo" is derived from the Greek siros, an airtight pit for storing grain. Greek and Roman farmers used silos to store mature grain. Among the first published mention of a silo in American agricultural publications was in an article titled, "French Mode of Curing Forage," published in the 1875 Report of the Commissioner of Agriculture. Initially introduced from Europe, early silos were underground pits or long, low masonry buildings. Farmers in the United States adapted the form of this structure to make it more efficient and affordable. The silo was an air-tight receptacle for silage, usually corn, chopped and placed inside. Air particles in the corn caused fermentation until all the air was expended. Because the silage was stored in an air-tight container, further fermentation was prevented, thus preserving the silage.

Early American Silos

Among the earliest American silos were ones built c. 1873 on the Fred Hatch Farm in McHenry County, Illinois; at the Michigan Agricultural College in 1875 by Manly Miles; and one built by Levi P. Gilbert of Fort Atkinson, Wisconsin. The latter, following instructions contained in an 1875 USDA bulletin, was a pit, 32 feet long, 12 feet wide, and six feet deep, lined with straw. By 1917, there were an estimated 400,000 silos in the United States. Although silage has been used for feed for colts, horses, mules, lambs, and hogs, it has been most commonly used as feed for dairy cattle. The acceptance of silage as an alternative to hay and grain as feed for dairy cattle was attributable to several factors. Because silage was more palatable and nutritious than dry feed, cows responded with higher milk production. Farm labor needs were reduced by the use of a silo to store feed rather than the hay mow, and ensilage was a more concentrated form of nutrients than hay and grain. The earliest American silos were partially excavated masonry structures, but these were soon replaced by above-ground or partially above ground wooden silos, square or rectangular in cross-section.

Wood Silos

Background: The earliest wood silos were rectangular. Among the earliest such silos was an addition to an existing barn, square in section, constructed in 1884 by John Gould at Aurora, Ohio. By the end of the nineteenth century, square wooden silos were a common feature were common features of Ohio dairy farms, and square or rectangular silos continued to be constructed until about 1910. Such structures were prone to the bowing of walls, thus admitting air and allowing the wood lining to be rotted by acids in silage. University of Wisconsin agricultural physics professor W.H. King sought a solution by designing a round barn with a round silo in the center. The walls of this silo were strong enough to withstand pressure from fermenting silage, and the cylindrical form eliminated the problem of air pockets in the corners. The interior wall was separated from the outer by air space in the belief that adequate ventilation would reduce lining decay. In addition, studies discovered that the weight of silage itself in vertical silos could provide sufficient compression to exclude air, provide that the silo was at least 24 feet tall. The round vertical board silo that developed became known as the King or Wisconsin silo. A second type of wooden silo, the vertical stave, was introduced in the late 1890s. Stave silos used a single layer of wood, secured by circular iron bands. These silos worked well when full. However, when the dry staves contracted, the silos sometimes collapsed. The best staves were beveled, tongued and grooved to fit securely. An improvement was the double-walled wood stave silo that did not wrack and twist when empty. The use of wood stave silos was promoted by agricultural experiment stations. A Cornell Experiment Station bulletin reported that the wood stave silo was "the most practical and successful silo which can be constructed," while the Ottawa Experiment Station published the following statement:

From extensive observation and study of silos and silo construction, and from experience here with a number of different silos, it would appear that the stave silo is the form of cheap silos that for various reasons is most worthy of recommendation. It combines cheapness of construction with the requisite conditions to preserve the silage in the very best conditions for feeding.

In their 1911 publication, the Silver Manufacturing Company provided specifications for a 100-ton wood stave silo. The silo was to be set on a mortared fieldstone foundation and was set on bottom plates made of 2x4 lumber. The staves were described as follows: All staves shall be 26 feet long in two pieces, breaking joints, and made from clear, straight-grained cypress, 2x6 inches, beveled on edges to an outside radius of 8 feet, mill-sized to the exact dimensions and dressed on all side….The staves shall then be fastened together with two 2x4-inch battens cut on inside to an 8 ft. radius and bolted to each stave with two ¼-inch diameter carriage bolts with round head sunk on inside and nut on outside.

In his pamphlet on silos, J.R. McCalmont recommended the use of redwood, longleaf pine, white pine, cedar, and Douglas fir in addition to cypress for silo construction. He recommended that the silo be built of milled tongue-and-groove staves, creosoted before use.

The hoops were made from two pieces of 5/8-inch diameter round iron. Twelve hoops were specified, starting at the bottom six inches apart and increasing in distance 6 inches between each hoop. The roof was made in a gable roofed configuration.

Wood silos. Chenango Forks, New York.

A variation used hoops made of wood instead of metal. Hoops were typically made of four-ply, ½ inch by 4-inch white oak, elm, ash, pine or chestnut strip lumber which would bend to conform to the circle required. Short length of milled staves were used shorter than those in regular stave silos. Most of the hoops were placed 24 inches apart.

Timber Crib Silo

A timber crib silo was constructed of 2 by 4 or 2 by 6 inch lumber lined with boards or plaster. Rough-cut lumber was preferred to better hold the plaster. These silos had six or eight sides, each no more than six or seven feet wide.

Stone Silos

In Wisconsin, three methods were used to construct stone silos. The first involved use of rubble (fieldstone) as they were, held together with plenty of mortar. The second technique was to split the fieldstones and place the faced stones out with smaller stones for infill. The third method, developed in the 1870s, was to split all the fieldstone and lay it with a small amount of mortar. The principal problem with stone silos was that the mortar inside could become soft through contact with silage acids. Stone silos continued to be constructed until the 1910s when they fell out of favor as poured concrete became increasingly popular.

Stone silo, Minnesota.

Specifications for stone silos were included in F.H. King's book, A Text Book of the Physics of Agriculture published in 1907. King recommended that the underground portion of the silo be 10 to 13 inches to forestall the danger of freezing. The portion of the silo below the ground should be about two feet thick and laid in inexpensive cement. The upper portion of the silo wall need not be heavier than 18 inches, and it the size of stone permitted the outer face could be drawn in gradually to a thickness of 12 inches at top. King recommended the use of flat quarry rock such as limestone rather than boulders because the former bonded better. The inner face of the silo wall was recommended to be plastered. Iron rods should be bedded in the walls to prevent cracking between the doors.

Brick Silos

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Brick Silo

Concrete Silos

By the early twentieth century three types of concrete silos were in active use: monolithic concrete silos, both solid and hollow wall, and concrete block silos. These early types were soon joined by concrete stave silos which soon predominated.

Monolithic Concrete Silos. Monolithic concrete silos were constructed out of a solid mass of concrete. The common thickness of the wall is six inches, adequate for a diameter of 25 feet and a height of 50 to 60 feet. Typically such silos employ vertical and horizontal steel reinforcement.The silos were constructed using wood inner forms and galvanized sheet steel outer forms.

Monolithic concrete silo. Southern Wisconsin.

Concrete Block Silos. Concrete block silos were built from straight or curved block, with the latter preferable. The curvature of the block matches that of the silo. These blocks, typically, measuring 8 by 8 by 16 inches, are hollow with a dovetailed tenon at one end and a dovetailed mortise at the other end so that they interlock. The blocks were sometimes fabricated with a small groove near the upper outside edge so that a small reinforcing rods could be embedded every third or fourth course. Typically the outside face of the blocks displayed a rusticated finish.

Concrete Stave SIlos. Concrete stave silos are commonly constructed of staves that measure 30 inches long, 10 inches wide, and 2 ½ inches thick. A variety of edges were used to ensure a tight fit including tongue and grooved edge, concave and convex edges and interlocking devices. The staves are held in place with metal hoops. Concrete stave silos are usually crowned by hemispherical metal caps.

Ceramic Tile Silos

Sometimes erroneously referred to as brick silos, the hollow-tile silo is made of hollow, hard burned or glazed tile blocks reinforced typically by iron bands which fit into the mortar between the blocks or in grooves made in them. The curved blocks are laid on a concrete foundation. If the blocks are properly glazed, they are impervious to air and moisture. The hollow spaces in the blocks serve as protection against changes in temperature.

Tile silo. Smyrna, New York.

Steel Silos

Sheet steel silos. Sheet steel silos were first used in the early twentieth century. According to one source, there was considerable demand for this type of silo where dry seasons and strong winds seriously affected wooden silos. Because of the acidinty of silage, it was necessary to coat the inside wall of the silo with either plaster or a compound such as asphaltum, to prevent corrosion. Later, the use of better alloys and more durable coatings further reduced corrosion from silo juices.

Some early steel silos failed due to buckling at the base. To overcome this problem, vertical angle-reinforcing ribs were used on the lower rings of the silo.

Harvestore Silos. Many modern dairy farm landscapes are dominated by tall steel cylinders, cobalt blue in color, that are labeled, "Harvestore." The Harvestore silo was developed using technology originated by a Milwaukee industrial equipment manufacturing firm, the A.O. Smith Company, in which glass was fused to steel. After discussions with representatives of the University of Wisconsin College of Agriculture, the Smith Company introduced the silo at the Wisconsin State Fair in 1948. The glass lining of the Harvestore resists silage acids. Silage will not freeze in these structure and it can be filled at the top at the same time it was being unloaded from the bottom.

Trench Silos

Many farms have now converted to using above-ground silos. These silos are basically open trenches sometimes with a four-inch concrete floor slab. The walls are often stiffened with buttresses to withstand the lateral pressure of the silage. Other designs use heavy plank side walls. Typical sizes of these silos are about 12 feet wide and 8 feet high.

Trench silo. Smyrna, New York.

Additional Sources of Information

Beedle, Peggy Lee. Silos: An Agricultural Success Story. Madison: University of Wisconsin Extension, 2001.

Fish, N.S. History of the Silo in Wisconsin. The Wisconsin Magazine of History. 8:2(December 1924):160-170.

King, M.L. Silos: Construction and Service. St. Paul: Webb Publishing Co., 1913.

McCalmont, J.R. Silo Types and Construction. Farmers Bulletin 1819. Washington: U.S. Government Printing Office, 1938.

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