I would not worry about the amount of arsenic that would leach out of a fence post. No. It is extremely unlikely it would hurt a cow based on the percentage of their total diet that would come from around the fence posts.
Arsenic is naturally occurring and the levels depend on the geological history of the area. The rocky Mountain region tends to have high arsenic levels.
As the Remedial Project Manager on the Butte Montana Priority Soils Superfund Project, Arsenic was one of the major contaminants of concern along with lead. On that site, the arsenic was the result of hard rock mining.
To set action levels for arsenic and other soil contaminants, site-specific risk assessments have to be conducted to determine the clean up levels. 90 % of the named responsible parties have enough money and resources that the EPA cannot pull cleanup levels out of the clear blue sky. They have to be demonstrated through risk assessments. Many times like in Butte, we fed contaminated soils to hogs to determine bioavailability and effects.
There are generally no specific legal levels set in statute. It all has to be proven in scientific study and evaluated by toxicologists.