Mow the Yard or Move the Fence?

The original original purpose of our sheep was to keep the grass short on our farm.  We have about 15 acres that we have mowed for years.  And, as fuel prices have creeped up over $3/gallon, my father-in-law encouraged us to get sheep or goats.  We choose sheep and our flock has really grown!  However, the down side to our growing flock is that our portable fences need to be moved quite often.  Fortunately, my girls have grown very proficient at moving the fence.  We have two portable mesh electric fences that we can move to enclose an area of about 1/2 acre in size.  That area will last our sheep about 3-5 days.  We have been fortunate that this summer we have not needed to feed much hay.  It is in dry summers that our system does not work very well.