Whitewash Fences are the cheapest fences available on FarmVille.
They can be purchased in the Market for 
300 coins, or they can be received as a gift.
They normally occupy an area on the farm one square by five squares, but they can be overlapped at the end post,
so that two fences could occupy a space one square by nine squares (1x9),
three connected fences would occupy a space one square by thirteen squares (1x13).
Trivia
There seems to be a problem with its opacity, as animals sometimes seem to stand "inside" the fence.