Wild_Spot - Mine itch themselves on the barb wire too...

That was my face the first time I watched that. Anyways, we've had barb wire for years with minimal problems. Of course, it's not ideal, but nothing in this world is.
My thought is if this filly has that much space there is something else going on with the fence. Either she's getting caught in something else (thorns, loose wire?), other horses/animals are running her into it, or possibly she's trying to go through the fence to get with other horses.