Yes, you can - and should - depreciate the fence on the farm land you purchased. Normally, you would use the Fair Market Value (FMV) of the fencing at the time you purchased it as your cost. You will need to separate the value of the fencing and any other depreciable items from the purchase price of the land.
When determining FMV , replacement cost (which properly includes the cost of the fence and its installation) is often used as the starting point. Then the cost figure must be discounted to reflect the age and condition of the fence. Note that the resulting figure may not necessarily coincide with the undepreciated (or adjusted) basis of the fence in the hands of the person you bought it from. The fair market value of the fence is ultimately a question of fact to be determined based on all of the facts and circumstances relevant to the fence in question.
I have included a link from the Iowa State University Center for Agricultural Law and Taxation that addresses the subject of Allocation
of Value to Depreciable Items
