They don't seem to like to crawl under fences, so if you put up a 4-5 foot field fence with a few 8 foot t-posts, you could leave a gap from the ground up (so you won't have to buy a taller and more expensive field fence) and turn a 4-5 foot fence into a 6 foot fence.
Hope this fancy diagram helps.

---------------------------------------
-------------------------------------- fence can go over the t-post
+-------------------------------------
+-------------------------------------
+------------------------------------
+-------------------------------------
+-------------------------------------
+-------------------------------------
+
+ gap from ground to bottom of fence
+ <--t-post

+__________________________________________ground