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1208 PD: Fencing systems for rotational grazing: Post selection

Tom Cadwallader and Dennis Cosgrove Published on 15 August 2008

It has been said, the first thing to plant in setting up a managed grazing system is fence posts.

Regardless of how you lay out your pasture system or the materials you use, the fencing system starts with setting posts in key spots around the property. Unlike woven wire or barb wire fences, high tensile electric fences are flexible systems that are allowed to move with changes in physical pressure on the fence caused by any number of things such as changes in temperatures or trees falling on the fence. Nearly all of the tension on these types of fences is on the ends and corners where fences start and change horizontal direction, and on hilltops and in valleys where they change vertical direction. All other posts just help hold the wires apart and off the ground.

Wood versus synthetics
As natural materials such as wood go up in price, new synthetic materials come in to take their place. Wood is still the most prevalent type of material used for posts in the critical structural portions of the fence, but fiberglass and some new plastic materials are becoming more common. One of the weaknesses of a fiberglass system is the lack of flexibility in construction. Wood can be nailed into, cut and shaped in any number of ways while fiberglass has to be put together as they are engineered.

Synthetic materials such as fiberglass also need to be manufactured so they resist breakdown from ultra-violet (UV) light. They may last for several years, but if they aren’t protected from UV rays, they can begin to break down and become more brittle. Also, if fiberglass posts aren’t solid they may not have the strength to absorb shocks.

Dug versus driven posts
Although post drivers can make the job easy, in some locations they may not work out very well. If there are lots of rocks to contend with and it’s important to put a post in a particular spot, posts can shift from one side to another in the driving process. They can also break if they hit large rocks or bedrock. Another reason to dig a post in would be the opposite reason. The ground is very loose and the posts will need some help to stay in the ground. But outside of those reasons, driven posts are much quicker and easier to put in and the driving process actually helps hold them in the ground better.

Treated versus untreated
Regardless of whether you are going to dig or drive a post in the ground, it’s important to put a post in the ground that will effectively resist rot for at least 20 years. Most of the above-ground materials on the market today can last at least that long, but having the system fail below ground is a waste of time and money. The natural resistance of several wood species to decay is listed in Table 1*.

If the decision is to go with treated posts the recommendation is to go with pressure-treated products. The pressure forces the treatment products into not only the sapwood but also the more difficult-to-penetrate heartwood area.

Natural versus mechanically rounded posts
Naturally rounded posts are basically small trees. Mechanically rounded posts are usually taken from larger trees that are quartered and then rounded off. Naturally rounded posts are generally preferred for a couple of reasons. First, the concentric rings of the natural posts add to the strength of the post. And secondly, round posts take preservative treatments more evenly. A quartered post generally has more heartwood on one side than the other and heartwood doesn’t take preservatives as well.

What about the density and grain?
Density means the mass of wood for the given dimensions. For posts that are dug-in, the denser the wood the better. Post strength can increase as there are more rings per inch. Even though posts can vary in strength from tree species to species, within a given species the more rings per inch the better. The additional rings make it a stronger post. If you’re using a machine to drive posts in, [you might use one that] is less dense but also more flexible. Posts that are more flexible will bend before they break if they are driven into ground that has rocks or are difficult to penetrate for one reason or another.

How long should they be?
The deeper you can put a post in the better, but if you’re limited to how deep you can put a post in the ground, then there is no sense wasting wood. For livestock fencing, 8-foot posts are the standard but 7-foot posts will do the job just fine if you can get the post in the ground at least 3 feet. In northern areas, the recommendation is usually to get the post below the frost line. Three feet is usually adequate but under the right conditions the frost can definitely go down deeper. For posts that are used as braces against horizontal forces use at least 8-foot posts.

How big around should the posts be?
Post diameter will depend on what you are asking it to do. A post should be sized in order to handle the tension that is being asked of it. For most fence systems, corners and ends should be at least 5 inches in diameter. Posts are usually sold by the diameter measurement at the top of a post. A post that is tapered with one end being 4 inches and the other 5 inches is called a 4” top post even though it runs from 4” at the top to 5” at the bottom. If you were going to bury this post you’d put the large end in the bottom of the hole to help hold the post in the ground. If you were going to drive the post in you’d do the opposite for the opposite reason. The post will get tighter as it is driven in.

Brace and line posts that aren’t being asked to carry the load of the ends and corners can be a smaller diameter. Four-inch top posts are adequate for either of these applications. The other possibility for horizontal or diagonal braces would be 4.5” landscape timbers.

Do they need to be straight?
Although it is common to select wood for straightness, posts with a bit of a bow may have places they can be used. But like any construction project, it’s much easier to start out with straight wood to work with.

Here’s what to look for in selecting posts for the fences foundation:

• naturally round posts
• pressure-treated
• at least 5-inch diameter tops for ends and corners
• at least 4-inch diameter tops for braces
• preferably straight
• 7 to 8 feet in length
• for dug-in posts, high density, tight grain
• for driven posts, lower density, open grain
• at least 8-foot posts for diagonal or horizontal braces  PD

—From University of Wisconsin Extension Forage Resources website

Table omitted but is available upon request to .

Tom Cadwallader
Agriculture Development Agent
University of Wisconsin

and

Dennis Cosgrove
Agriculture and Natural Resources Specialist
University of Wisconsin

See more articles like this at www.progressivedairy.com

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