In agriculture, fences are used to keep animals in or out of an area.

The earliest fences were made of available materials, usually stone or wood. In areas where field stones are plentiful, fences have been built up over the years as the stones are removed from fields during tillage and planting of crops. The stones were placed on the field edge to get them out of the way. In time, the piles of stones grew high and wide.

In other areas, fences of timber were constructed. Zigzag log fences were constructed in newly cleared areas by stacking logs.

In older times, livestock would roam and were fenced out of areas, such as gardens and fields of crops, where they were unwanted. This prevails yet today in sparsely populated areas. States in the American West that follow this tradition are called "fence out" states, in contrast to Midwestern states which have "fence in" laws where livestock must be confined by their owners.

The Industrial Revolution brought the first barbed wire fences, which were widely used after their introduction in the mid 19th century. This technology made it economically feasible to fence rangeland for the first time. The introduction of barbed wire contributed to the range wars of that century, as it exacerbated tensions between landowners seeking exclusive control over large tracts of land, and traditional transient users of that land.

Barbed wire was made by many manufacturers in an almost endless variety of styles. For the most part these were functionally identical. The differences reflected peculiarities of each manufacturing process rather than deliberate design of the end product. Sections of unusual barbed wire are collected by some enthusiasts.

The traditional barbed wire used from the late 19th through most of the 20th century was made from two mild steel wires, usually of about 12 or 14 gauge, with about 15-30 twists per metre. Steel barbs were attached every 10-20 cm. Barbs had either two or four points, with the two point design using somewhat heavier and longer barbs. The relative merits of two point vs. four point wire are the subject of deeply held views among many farmers and ranchers, to the extent that both types are still made today.

Barbed wire is typically run on wooden posts, either commercially grown in plantations or, perticularly in the American West, often posts that have been cut from nearby trees. Wire is attached to the posts using fencing staples. Typically four or five strands of barbed wire make up a fence. Posts are usually spaced 3-6 m apart.

In areas where there is more rainfall, either rot-resistant wooden posts or steel posts are used. Wood with natural rot resistance, such as oak and juniper, was often used until it became in short supply in the 1950s. Then, chemically treated pine and spruce posts became prevalent. Creosote, pentachlorophenol, and chromated copper arsenate were all widely used.

In the 1970s, high-tensile barbed wire became available. It is lighter in gauge (usually 16 gauge) but, due to higher carbon content, just as strong as the traditional mild steel. Advantages include lighter weight and lower cost.

Barbed wire is effective for cattle and horses, but not for pigs, sheep, or goats. Where these animals are to be fenced, woven wire is used instead, often with one or several strands of barbed wire at the top. For swine, a ground-level barbed wire strand is used as well to prevent digging. Woven wire is costly to purchase and time-consuming to install.

Electric fencing became available in the 1950s and has been widely used for temporary fences and as a means to improve the security of fences made of other materials. It is made using lightweight steel wire (usually 14-17 gauge) attached to posts with insulators made of porcelain or plastic. A fence charger places an electrical pulse from ground to the wire about once per second. The pulse is narrow and usually around 5-20 kV. Animals receive a painful but harmless shock when contacting the wire, and learn to stay away from it.

High tensile (H-T or HT) fencing using smooth, heavy gauge (usually 12.5 gauge, 2.5mm) steel wire was introduced in the 1980s and has slowly gained acceptance. It permits the use of wider post spacings and offers improved stock safety compared to barbed and woven wire fences. It can be insulated and electrified. Trellising for horticultural purposes is generally constructed from HT wire as it is able to withstand a higher cropping load. Joining HT wire is difficult because of its stiffness and its reduction in strength when bent to form a knot.

Fences of wood, stranded cable, and pipe are used where cost is not a consideration, particularly on horse farms, or in areas like corrals where stock are likely to challenge the fence.

Cattle and horses are strong enough to go through most types of fence by main force, and occasionally do so when frightened or motivated by hunger, thirst, or sex drive.

See also

• Electric fencing