Q: I want to install wood fence posts around my horse paddock. Would you recommend using treated wood?
A: Wood posts are a common and safe option for horse paddocks. However, wood has natural enemies including insects, mold, fungi, and bacteria. Some species have natural resistances, such as, cedars, junipers, locust, and redwood. Treated wood is more expensive than un-treated lumber, however, it will help extend the life of your wood fence, likely more than paying for the additional expenses. Current chemical treatments include copper cremated copper arsenate (CCA), ammoniacal zinc copper arsenate (ACZA), copper amines (copper azole, CBA-A & CA-B; alkaline copper quat, ACQ-B, ACQ-C, ACQ-D), and copper naphthenate (CU-Nap).
Pressure treated wood should last 30 to 35 years in Minnesota, compared to untreated wood, which generally lasts between 7 and 15 years. In drier climates, some posts can last longer. For example, cedar posts in western South Dakota can last longer than 100 years. In wet soils, filling the bottom 6 to 12 inches of the hole with a builders grade sand will increase the life of the post. Setting posts in concrete is not recommended because of the expensive and difficultly in replacing or moving the post.
Although horses do commonly chew on wood, I am not aware of any health problems (not counting dental issues) related to horses chewing on treated wood. If a horse is known to crib or chew, CU-Nap treated wood is the best option as no known health risk have been determined if ingested (maybe difficult to find). Another option is to install a single strand of electrified barbless wire, which will help keep the horses from both pushing on the fence and chewing on the wood; further extending the life of the fence and reducing maintenance costs.
CCA and ACZA treated wood has limitations because of arsenic, and has been band in the residential construction market; but still can be purchased in the agricultural sector. If CCA or ACZA is used, recommendations are to not have the CCA treated wood come in direct contact with feed; not used for bunks (support legs are OK), feed storage boxes, etc. The arsenic treated wood is also not recommended for use in playground equipment.
By: Chuck Clanton, PhD, University of Minnesota